• Determines what organs are infected, thus determining the clinical signs of disease.

• Viruses bind to receptors normally expressed on host cell membranes and these receptors are used to infect target cells.

• Determines the utilization of cellular biologic processes, thus determining the characteristics of cellular dysfunction or death.

• Viruses hijack cell organelles, genomes, and biologic processes to complete the viral replication cycle.

• Determines if the cell will live or die, thus determining the clinical signs of the disease.

• Viruses that escape from cells by killing or lysing cells create more severe disease processes.

Parvovirus Enteritis (Parvovirus, Nonenveloped DNA Virus): Parvovirus enteritis is a general name used to group two closely related strains of parvovirus that cause canine parvovirus enteritis and feline panleukopenia (feline parvovirus enteritis). The mechanism of injury is death of crypt epithelial cells and lymphocytes, including those in the bone marrow. Specificity for these mitotically active cells occurs because parvoviruses require a host cell–derived duplex transcription template, which is only available when cells divide during the S-phase of the cell cycle. Parvoviruses are unable to turn on DNA synthesis in host cells, so they must wait for host cells to enter the S-phase of the cell cycle before infecting these cells. Gross lesions include segmental areas of the mucosae that are rough and granular (enterocyte necrosis, villus atrophy) with areas of hemorrhage, acute inflammation, and fibrin exudation (see Fig. 7-160).

Dogs and cats encounter parvoviruses in fomites from body fluids contaminated with virus-infected fecal matter through direct contact with virus-infected animals. The virus is inhaled or ingested; deposited on mucosae of the oral, nasal, and pharyngeal cavities; and trapped in the mucus layer. It has not been determined if and how virus penetrates the mucus layer to gain access to mucosal epithelial cells, mucosal macrophages, lymphocytes, and/or dendritic cells. Virus probably infects macrophages or dendritic cells migrating in the mucus layer and on the surface of mucosae. Virus replicates in these cells and is then spread via leukocyte trafficking to the lamina propria of the tonsils. Here, additional macrophages and lymphocytes are infected and spread the virus via leukocyte trafficking in lymphatic and blood vascular systems to regional lymph nodes and systemically to the spleen, thymus, lymph nodes, bone marrow, and mucosa-associated lymphoid nodules such as Peyer’s patches of the small intestine. Virus may also be spread as a cell-free viremia in lymph via lymphatic vessels to regional lymph nodes.

In these diseases, the majority of virus-infected intestinal epithelial cells are found in crypts neighboring Peyer’s patches in the small intestine. Experimental studies demonstrate that virus arrives at Peyer’s patches before it reaches contiguous crypt enterocytes. Although not yet shown with canine or feline parvovirus, other similar viruses spread from Peyer’s patches to M cells in Peyer’s patches. Morphologically, M-cell processes extend into the mucosa and are contiguous with crypt enterocytes forming intestinal crypts. Additionally, virus entry into intestinal epithelial cells has a polarized pattern in which entry is restricted to the basolateral areas of crypt enterocytes, the areas nearest Peyer’s patches and M cells. Collectively, these findings suggest that virus initially spreads to the intestine via the blood vascular system and not in ingesta via peristalsis. It is unclear whether a virus arrives as a cell-free viremia or within cells of the monocyte-macrophage and/or lymphoid systems; however, (1) virus infects such cells in the oral, nasal, pharyngeal mucosa, and tonsil and regional lymph nodes and (2) leukocyte trafficking is commonly used by other viruses to spread virus systemically to lymphoid and other organ systems, suggesting that parvovirus is spread to the intestine via leukocyte trafficking.

Infection is initiated through capsid-mediated attachment to one or more glycosylated receptors on target cell membranes and is followed by entry via receptor-mediated endocytosis. Parvoviruses appear to use coreceptors for the attachment and entry processes. Attachment receptors may assist in aggregating virus near the cell membrane, and entry receptors may assist the virus in penetrating the cell membrane. In the dog, this process requires capsid proteins to bind to transferrin receptors, whereas in the cat, the process requires capsid proteins to bind to neuraminic acid and transferrin receptors. These receptors appear to determine which cells and which species of animal are infected by parvovirus strains. Parvoviruses are released from infected crypt enterocytes when the cell is killed after the replication cycle is completed. Because of this outcome, parvovirus enteritis causes an osmotic malabsorption-maldigestion diarrhea. Diarrhea occurs because of the failure of replacement of absorptive enterocytes covering villi that are lost through normal turnover (≈48-hour lifespan). As a result, affected villi collapse, are atrophic, and all absorptive and digestive surfaces are lost; thus dietary carbohydrates are available for fermentation by intestinal bacteria. Under normal conditions, enterocytes covering villi are replaced by dividing crypt epithelial cells that move up and cover the villus. The loss of enterocytes covering villi also functionally opens a barrier system that normally prevents endotoxins from being absorbed by capillary beds in the lamina propria of the villi. Endotoxic shock and DIC can result and kill the affected animal. Panleukopenia also occurs because of virus-induced cytolysis of rapidly dividing stem cells in the bone marrow. The effects of parvovirus on organs of the lymphatic system are discussed in the section on Bone Marrow, Blood Cells, and Lymphatic System.

Bovine Viral Diarrhea and Mucosal Disease (BVD Virus, Pestivirus, Enveloped RNA Virus): The range of diseases caused by BVD virus is diverse and complex. Some of these diseases will be discussed in this chapter and other chapters of this book. Bovine viral diarrhea and mucosal disease as discussed herein refers to the disease that affects mucosae of the alimentary system from the oral cavity to the small intestine. The mechanism of injury in bovine viral diarrhea and mucosal disease is dysfunction and death of mucosal epithelial cells of the oral cavity and esophagus (stratified squamous epithelium) and of the small intestines (enterocytes) preceded by dysfunction and death of submucosal lymphocytes in MALT, such as those in Peyer’s patches. Gross lesions include erosion, ulceration, and hemorrhage of mucosae of oral/nasal cavities and pharynx, esophagus, and small intestine (see Figs. 7-3, 7-142, 7-143, and 7-144).

The typical pathogenesis of mucosal disease involves two forms of BVD virus, a noncytopathic form and a cytopathic form, acting synergistically to cause lesions. The noncytopathic form of the virus is likely introduced into the herd in new stock, commingling of cattle, semen, or other management practices that allow contact with carrier cattle. The cytopathic form of the virus commonly arises from a noncytopathic form that exists in the herd through mutations of its viral genome or it is introduced into the herd as discussed above. Under the proper conditions as discussed below, the noncytopathic form makes cattle immunotolerant to cytopathic forms of BVD virus. Mucosal disease occurs when immunotolerant cattle are exposed to a cytopathic form of the virus (1) most likely originating through mutations on the same farm or (2) introduced into the herd by means discussed above. When cattle with a normal adaptive immune response (i.e., not immunotolerant) are exposed to the cytopathic form of the virus, they are usually able to prevent or limit the severity of mucosal disease that occurs unless the viral strain has several highly pathogenic virulence determinates.

Immunotolerant calves occur when fetuses of naïve (normal immune responses, unvaccinated, no prior exposure) pregnant cows are infected with the noncytopathic form of the virus. Infected cows are asymptomatic, but functionally serve as a means for the virus to infect the fetus and establish “persistently infected (PI)” calves (noncytopathic form of the virus) in the herd. These calves, present in small numbers, usually die before a year of age, but serve as farm reservoirs for noncytopathic virus as they constantly shed virus in body secretions (saliva, tears) and feces into the environment.

For convenience, let’s begin the mechanistic sequence of events that ultimately leads to mucosal disease with the exposure of pregnant cows to the noncytopathic form on the farm and in utero transplacental spread of this form from infected cows to their calves. Cows encounter the noncytopathic form in fomites from contaminated body fluids or wastes through direct contact with PI calves or carrier animals. The noncytopathic form is inhaled or ingested and deposited on mucosae of the oral, nasal, and pharyngeal cavities; especially favored are mucosae overlying the tonsil. It has not been determined if and how virus penetrates the mucus layer to gain access to mucosal epithelial cells or submucosal macrophages, lymphocytes, and/or dendritic cells, but this process could be facilitated via phagocytosis in the mucus layer by mucosa-associated macrophages, lymphocytes, and/or dendritic cells migrating through the mucosae. Noncytopathic virus probably infects and replicates in monocytes, macrophages, lymphocytes, and dendritic cells and is spread via leukocyte trafficking in lymphatic vessels from tonsil and submucosal lymphoid nodules to regional lymph nodes and then systemically to the caruncular side of placentomas. Trophoblasts in the placentoma can be infected by noncytopathic virus, likely allowing virus to complete a replication cycle, migrate into and through the cotyledons, and infect fetal macrophage-like cells that enter the fetal vascular system and spread to the fetus. Additionally, noncytopathic virus can infect and spread within the allantoic and amniotic membranes and then infect the fetus, but it is unclear which cells facilitate this spread.

Bovine fetuses, infected in utero, become immunotolerant (see Chapter 5) to the noncytopathic form of the virus. They also do not recognize antigens from cytopathic forms of the virus as foreign, and as a result they fail to develop an effective adaptive immune response. When exposed to cytopathic virus, mucosal disease ensues in these calves. Mechanistically, the sequence of events that ultimately leads to mucosal disease begins when these immunotolerant calves inhale or ingest cytopathic virus and it is deposited on mucosae of the oral, nasal, and pharyngeal cavities, and the tonsil. The mechanism of infection and spread of the virus from the mucus layer systemically to MALT of the alimentary system, especially Peyer’s patches, is similar to that described above for the noncytopathic virus. Herein, the cytopathic virus infects follicular dendritic cells and B lymphocytes in MALT and spreads to, infects, and kills overlying stratified squamous epithelial cells and/or crypt enterocytes, resulting in mucosal erosions, ulcerations, and hemorrhage. In the small intestine because of the death of crypt enterocytes, there is a failure of replacement of sloughed villus enterocytes after normal enterocyte turnover at the villus tip. This outcome may in part explain the mucosal lesions and initiate the formation of ulcers. Hemorrhage occurring with the ulcers could be the result of exposure of capillary beds to endotoxins or other toxic molecules absorbed through an open intestinal barrier system (cell junctions). Diarrhea could also occur secondary to the absorption of large quantities of endotoxins into the lamina propria and deeper supporting stroma that contains the enteric nervous system, resulting in an acquired dysautonomia (see Chapter 14). It has been recently reported that certain molecules released from lymphocytes and/or monocytes infected with cytopathic virus can initiate apoptosis in bystander lymphocytes and monocytes not infected with virus. The role of apoptosis in ulceration of mucosae has not been determined. Additionally, a vasculopathy involving arterioles and small arteries in submucosal tissue of Peyer’s patches has been reported and is characterized by segmental necrosis of vascular walls and lymphohistiocytic perivasculitis. Potentially, such lesions could cause endothelial injury and occlusive thrombi, resulting in infarction of the mucosal enterocytes overlying Peyer’s patches. Lymphoid cells in Peyer’s patches initially proliferate when infected, but infection is followed by massive death of lymphocytes as part of the viral replication cycle, likely caused by a virus-induced apoptotic mechanism.

Rinderpest (Cattle Plague, Morbillivirus, Enveloped RNA Virus): Because of similarities in clinical presentations, lesions, causative virus, and mechanisms of infection and spread between Rinderpest and other viral diseases, the following materials should be reviewed: (1) morbilliviruses—local, regional, and systemic infection and spread and their target cells in the section on canine distemper; (2) bovine viral diarrhea-mucosal disease—clinical presentation and lesions; and (3) parvoviruses—mechanisms used to infect and spread between cells.

The mechanism of injury in Rinderpest is dysfunction and death of mucosal epithelial cells, dendritic cells (Langerhans’ cells [oral cavity]), M cells, lymphocytes, and macrophages of the alimentary system from the oral cavity to the small intestine. Gross lesions include erosions, ulcerations, and hemorrhages of the oral cavity, including the gums, lips, hard and soft palate, cheeks, and base of the tongue, the esophagus, and the small intestine over Peyer’s patches (Fig. 4-38). Lymph nodes, especially mesenteric nodes, are enlarged, hemorrhagic, and edematous.

Cattle (and likely sheep and goats) encounter the virus in fomites from body fluids and wastes, such as nasal-ocular fluids, salvia, urine, and feces, through direct contact with virus-infected cattle. Virus is inhaled, deposited on, and trapped in mucosae of the conductive and exchange components of the respiratory system through centrifugal and inertial turbulence. It has not been determined if and how virus penetrates the mucus layer to gain access to mucosal epithelial cells, mucosal macrophages, and/or dendritic cells. Virus probably infects and replicates in mucosal macrophages and dendritic cells as they migrate through the mucus layer and mucosae and then is spread by these cells locally through leukocyte trafficking to the submucosa where they infect and replicate in tissue macrophages, lymphocytes, and dendritic cells. These cells then spread virus via leukocyte trafficking through lymphatic vessels to regional lymph nodes. Herein, similar cells are infected and used to spread the virus systemically via lymphatic vessels, the thoracic duct, and the blood vascular system systemically to lymph nodes, and other organ systems, including the alimentary and respiratory systems. Systemically, primary cell targets for infection include those cells in Peyer’s patches of the small intestine and in lymphoid nodules including Langerhans’ cells of the Malphigian layer of stratified squamous epithelium of the oral cavity and esophagus.

Erosive lesions in the oral-pharyngeal-lingual mucosae begin in the Malpighian layer (stratum basale [germinativum], stratum spinosum, and stratum granulosum). Langerhans’ cells (dendritic cells) are located in the Malpighian layer and are sentinel cells that migrate in and out monitoring for foreign antigens. Although unproved, Langerhans’ cells are likely infected with Rinderpest virus via encounters with virus-infected macrophages migrating through these mucosae. Infected oral Langerhans’ cells also spread virus to contiguous squamous epithelial cells. Herein, the virus replication cycle results in lysis of infected squamous epithelial cells (oral-pharyngeal-lingual mucosal ulceration) and release of virus into the alimentary system. Erosive lesions in intestinal mucosae likely occur via a similar mechanism facilitated by the infection and migration of macrophages, monocytes, and dendritic cells systemically and into and through Peyer’s patches and then to enterocytes. The entry of Rinderpest virus into mucosal enterocytes has a polarized pattern restricted to the their basolateral areas, the areas nearest Peyer’s patches and M cells. The virus replication cycle results in lysis of infected enterocytes (small intestine mucosal ulceration) and release of virus into the alimentary system.

Similar to distemper virus, the Rinderpest virus has envelope and hemagglutinin/fusion surface glycoproteins for attachment and fusion, respectively, to host cell membrane glycoprotein receptor CD150 (signaling lymphocyte activation molecule [SLAM]). SLAM has been demonstrated in membranes of lymphocytes, monocytes, and macrophages and of epithelial cells of the respiratory, alimentary, and integumentary systems.

Feline Infectious Peritonitis (Feline Infectious Peritonitis Virus; Nonenveloped RNA Virus): See the section on the Cardiovascular System and Lymphatic Vessels.

Bluetongue (Orbivirus, Nonenveloped RNA Virus): The mechanism of injury in bluetongue is dysfunction and death of endothelial cells. Gross lesions include systemic hemorrhage, edema, and vasculitis. Such lesions are more severe in sheep when compared to cattle, apparently because there are species differences in the susceptibility of endothelial cells to infection and the severity of endothelial injury. Bluetongue is a noncontagious disease of sheep, cattle, and other ruminants (deer). The virus is encountered in fluids from hematophagous Culicoides (biting midges), which is the insect vector for the virus. After skin penetration, virus gains access to cutaneous blood and fluids, as well as cutaneous dendritic cells (Langerhans’ cells), monocytes, and tissue macrophages. Although unproved, virus probably infects these cells, and virus-infected monocytes and macrophages migrate to local lymph nodules and/or aggregates, then to regional lymph nodes via lymphatic vessels. Here, virus infects lymphocytes and additional dendritic cells, monocytes, and macrophages. Macrophages then enter the blood vascular and lymphatic systems (blood vascular via the thoracic duct) and migrate in the vascular system to all organ systems. Herein, they adhere to, migrate through, and reside in the walls of blood vessels and thus are in direct contact with endothelial cells. Virus lyses and escapes from these macrophages and binds to receptors on endothelial cells.

Bluetongue virus has two attachment proteins, capsid structural proteins (VP2 and VP5). These proteins bind to glycosaminoglycans in host cell membranes and facilitate attachment and penetration of virus into macrophages and likely into endothelial cells. Systemically, the attachment of virus-infected macrophages to endothelial cells is likely facilitated by molecules of the leukocyte adhesion cascade (see Chapter 3). Virus in infected monocytes/macrophages that are adhered to endothelial cells escape (via cell lysis) from these monocytes/macrophages and adhere to, infect, and replicate in endothelial cells, leading to endothelial cell injury and death (necrosis-vasculitis). Depending on the severity of endothelial cell injury, vasculitis can be followed by hemorrhage and edema (increased vascular permeability) affecting the lung, vascular thrombosis leading to oral mucosal ulcerations, tissue infarction, and DIC, which kills the affected animal.

Transmissible Gastroenteritis (Coronavirus, Enveloped RNA Virus): The mechanism of injury in transmissible gastroenteritis is dysfunction and death of epithelial cells (villus enterocytes) covering tips and sides of intestinal villi (Fig. 4-39, A). Gross lesions include congestion and thinning of the wall of the small intestine and shortening (atrophy) of villi (Fig. 4-39, B, see 7-148, 7-149, and 7-150). Piglets encounter virus in feces through direct contact with virus-infected pigs. Virus is ingested and by swallowing and peristasis it is carried through the oral pharynx, esophagus, and stomach to the small intestine where it is trapped in the mucus layer. The mucus layer has mucins and mucin-like glycoproteins that contain sialic acid. The viral envelope expresses S protein, which binds to sialic acid in the mucus layer, but it has not been determined how the virus penetrates the mucus layer to gain access to enterocytes. When in contact with cell membrane, S protein also binds to a glycoprotein receptor, aminopeptidase N, which is expressed on apical surfaces of villus enterocytes. This interaction facilitates the attachment and entry of virus into villus enterocytes where the virus replicates. Virus then lyses villus enterocytes and escapes into the lumen of the small intestine to be passed in the feces. Injured and killed villus enterocytes are sloughed, resulting in collapse (atrophy) of the villus. Basement membranes are not injured, and crypt enterocytes divide and migrate up the denuded villus to cover exposed basement membrane. Early in the reparative process, these migrating cells are flattened squamous-like cells stretched over the basement membrane. As the cells increase in density and maturity, they regain a more columnar morphology. Additionally, the loss of enterocytes allows endotoxins and other potentially harmful molecules in the digesta to gain access via diffusion to the capillary and lymphatic vessels in the lamina propria of villi and through absorption cause systemic cardiovascular and hemodynamic effects. A malabsorption osmotic diarrhea also occurs because of the loss of intestinal epithelial cells and the failure to digest carbohydrates (impaired hydrolysis) and other molecules in the digesta. This outcome leads to bacterial fermentation of substrates like glucose and an osmotic diarrhea.

Rotavirus Enteritis (Rotavirus, Nonenveloped RNA Virus): The mechanism of injury, pathogenesis, and clinical outcomes of rotavirus enteritis are similar to those of transmissible gastroenteritis, but the pathogenicity (virulence determinates) of rotavirus is much less severe (see Fig. 4-39). Viral capsid attachment proteins, VP4 and VP7, appear to be involved in the attachment and entry of virus into villus enterocytes through a multistage receptor-mediated process by binding to host cell membrane proteins such as sialic acids, integrins, heat shock proteins, and gangliosides located on apical surfaces. The replication of rotavirus in villus enterocytes results in the production of NSP4, an enterotoxin that (1) induces a secretory diarrhea, (2) stimulates the enteric nervous system and causes intestinal hypermotility, and (3) increases the concentration of intracellular calcium, disrupting the cytoskeletal system and tight junctions, and results in increased mucosal permeability. NSP4 appears to cause dysfunction of cell membrane systems modulating electrolyte and water movement such as calcium ion-dependent chloride secretion, sodium-glucose transport proteins, brush-border membrane disaccharidases, and calcium ion-dependent secretory reflexes.

Contagious Ecthyma (Orf, Sore Mouth, Pustular Dermatitis: Parapoxvirus; Enveloped DNA Virus): The mechanism of injury in contagious ecthyma is (1) dysfunction and death of squamous epithelial cells of oral mucosae and/or skin by viral replication and cytolysis and (2) exuberant hyperplasia (proliferation) of squamous epithelial cells of oral mucosal and/or skin through modulation of regulatory activities in the cell-division cycle by virulence determinates expressed in the viral genome. Gross lesions include (1) macules, papules, vesicles, pustules, scabs, and scars and in cases having extensive injury resulting from vesicles and pustules (2) a reparative response with proliferation of mucosal squamous epithelial cells resulting in a thickened and granulation tissue–like appearance of affected mucosae (see Figs. 7-8 and 17-48). Lesions are most easily observed on wool-free or hair-free areas such as the muzzle (lips and mouth) and udder (teats) but also can occur in the skin of the perineum, groin, prepuce, scrotum, axilla, and vulva. This disease is zoonotic.

Sheep and goats encounter the virus in fomites of fluids from ruptured macules, vesicles, and pustules and from skin debris and scabs through direct contact with virus-infected animals. The virus can also be spread through mechanical contact with contaminated clothing, instruments, and clippers. Virus gains access to the Malpighian layer of the squamous mucosae through traumatic abrasions, lacerations, or burns and infects Langerhans’ cells (dendritic cells) and capillary endothelial cells. Infection of additional Langerhans’ cells occurs when these dendritic cells migrate through the subcutis of the Malpighian layer. Infection of endothelial cells may be facilitated by the migration of dendritic cells through the capillary wall. Virus appears to use F1L envelope protein as an attachment protein to bind to glycosaminoglycan heparin sulfate receptor proteins on the surface of host cells. Endothelial cells are injured and lysed (killed) by virus, and injury is accompanied by vascular dilatation, leakage (edema), and active hyperemia, likely contributing to formation of macules, vesicles, and papules. Reparative and regenerative responses contribute to proliferative lesions (hyperplasia) of squamous mucosae and skin. Hyperplasia is apparently caused by (1) synthesis of vascular endothelial growth factor molecules from virus-infected capillary endothelial cells, (2) proliferation of new capillaries as occurs in angiogenesis, and (3) the concurrent proliferation of mucosal epithelial cells much like in the formation of granulation tissue. Virus also infects cells of the stratum basale (germinativum) that are regenerating (actively dividing [mitotic]) as a reparative response to the initial injury of mucosae; however, the relationship between infection of these epithelial cells and the proliferative response that ensues is unclear.

Bovine Papular Stomatitis (Parapoxvirus, Enveloped DNA Virus): The pathogenesis and mechanism of injury in bovine papular stomatitis are similar to those of contagious ecthyma. The disease occurs primarily in cattle and also in sheep and goats (see Figs. 7-6 and 7-7).

Vesicular Stomatitis (Vesiculovirus, Enveloped RNA Virus): The mechanism of injury in vesicular stomatitis is cell dysfunction and death leading to intercellular edema (vesiculation) of mucosae and skin, followed by rupture of the vesicles and subsequently erosion and ulceration. Gross lesions include vesicles, erosions, and ulcerations on the mucous membranes and skin of the tongue, oral cavity, hoof coronary bands and interdigital skin, and teats. The pathogenesis has not been determined to an extent that results in an understanding of the chronologic sequence of events leading to disease. The virus, an arbovirus, is spread to cattle, horses, and pigs primarily by sandflies and blackflies and rarely by instruments or equipment. Animals encounter the virus through the bite wounds of these insects, where the biting process injures blood vessels and capillaries, resulting in virus being deposited directly into plasma of blood vessels and/or into interstitial fluids (also containing plasma leaked from vasopuncture) within vascularized submucosal and subcutaneous ECM (connective) tissues. It appears that vesicular lesions occur at or near the sites of insect bites, suggesting that the virus infects target cells locally and there is no systemic spread of virus to mucosae or skin through leukocyte trafficking or viremia. Squamous epithelial cells of mucosae and skin are the primary target cells for viral infection, but Langerhans’ cells (dendritic cells) and cells of the monocyte-macrophage system, although likely target cells for infection with virus, have not been clearly identified as target cells. In addition, it is likely but unproved that local migration of dendritic cells and cells of the monocyte-macrophage system spread virus to additional local target cells as described below. Lesions suggest that epithelial cells of the stratum basale and/or spinosum must be targets for virus infection, replication, and escape (through cell lysis). Death of these cells results in the formation of intercellular spaces that fill with fluid and form vesicles. Trauma likely ruptures the vesicles and leads to erosion/ulceration of the overlying mucosa or skin; however, acute inflammation may also contribute to the process. Virus appears to use envelope glycoprotein G as an attachment protein to bind to host target cells; however, receptors on host cells have not been clearly identified.

Swine Vesicular Disease (Enterovirus, Nonenveloped RNA Virus): The mechanism of injury in swine vesicular disease is cell dysfunction and death leading to intercellular edema (vesiculation), rupture of vesicles, and subsequent erosion and ulceration of mucosae and skin. Gross lesions include vesicles, erosions, and ulcers on the mucosae and skin of the snout, mouth, tongue, hoof coronary bands and interdigital skin, and teats (see Figs. 7-2 and 7-3). Pigs encounter virus through (1) contact with infected vesicular fluid, (2) contact with contaminated clothing or instruments, or (3) ingestion of contaminated pig offal, by-products, or meat products. It appears that the virus can enter the body through inhalation, ingestion, or contact with abraded skin.

Through inhalation or ingestion, the virus encounters oronasal-pharyngeal mucosae, especially of the tonsil. It has not been determined if and how the virus penetrates the mucus layer to gain access to mucosal epithelial cells, mucosal macrophages, and/or dendritic cells. The role of mucosal epithelial cells in infection is unclear. The virus probably infects and replicates in mucosal macrophages, lymphocytes, and/or dendritic cells as they migrate through the mucus layer and mucosae and then is spread by these cells locally through leukocyte trafficking to the submucosa where they infect and replicate in submucosal tissue macrophages, lymphocytes, and dendritic cells of limpid nodules and aggregates. From here, the virus spreads via lymphatic vessels to regional lymph nodes and infects similar cells, spreads systemically in these cells to other organ systems, including mucosae and skin via lymphatic vessels, the thoracic duct, and the blood vascular system.

Through ingestion, the virus encounters mucosae of the small intestine especially overlying Peyer’s patches. Although unproved, the virus likely infects M cells, which spread virus to tissue macrophages, dendritic cells, and other cells in Peyer’s patches. Here, similar cells are infected and then migrate to spread virus via lymphatic vessels to regional lymph nodes and then systemically to other organ systems, including mucosae and skin.

Finally, it has been suggested that virus can infect Langerhans’ (dendritic cells) or other cells of the Malpighian layer if the skin of the coronary band of the hooves is traumatized and epithelial cells of the stratum basale and/or spinosum are exposed to the environment. Virus can replicate in these squamous epithelial cells and also likely in Langerhans’ cells. Thus they may serve as a site of local infection, followed by spread to regional lymph nodes via lymphatic vessels, and systemic spread to other organ systems, including mucosae and skin.

No matter the route used to establish, sustain, and amplify the systemic infection, it appears that virus can infect, injure, and lyse squamous epithelial (mucosae) and dendritic cells of the skin, resulting in vesicle formation. The mechanisms involved in vesicle formation have not been identified but could be similar to those used in poxvirus infections and swine vesicular disease. It is unclear whether spread is via a cell-free viremia or leukocyte trafficking; both mechanisms of virus spread have been demonstrated in enterovirus infections. Virus appears to use capsid proteins, VP1-4, as attachment proteins to bind to glycoprotein receptors, such as ICAM, expressed on the surface of host cells. When interacting with cell receptors, viral capsid proteins undergo conformational changes leading to fusion of virus to cell membrane and internalization of virus within the host target cell. The diversity of ICAM receptors expressed on a variety of host cell membranes probably determines target cell specificity. Additionally, coxsackievirus-adenovirus receptor and sulfated glycosaminoglycans, such as heparin sulfate, may also be used as receptors on host cells.

Vesicular Exanthema of Pigs (Calicivirus [Waikavirus], Nonenveloped RNA Virus): The pathogenesis and mechanisms of injury in vesicular exanthema of pigs are likely similar to those of swine vesicular disease. Capsid proteins used by the virus to attach and bind to host target cells and receptors for virus on host cells have not been clearly identified. Vesicles are shown in Figs. 7-2 and 7-3.

Foot-and-Mouth Disease (Aphthovirus, Nonenveloped RNA Virus): The pathogenesis and mechanisms of injury in foot-and-mouth disease in cattle and pigs (less common in sheep and goats) are likely similar to that of swine vesicular disease and vesicular exanthema of pigs. In summary, virus encounters target cells through inhalation or ingestion; establishes a local infection in the oronasal-pharyngeal mucosae especially of the tonsil and in submucosal lymphoid cells, macrophages, and dendritic cells; spreads via lymphatic vessels to regional lymph nodes to sustain and amplify the infection; and then spreads systemically to infect, replicate in, and lyse epithelial cells of the stratum spongiosum of mucosae and skin resulting in vesicles (Fig. 4-40). Capsid proteins used by the virus to attach and bind to host target cells appear to include VP1-4 attachment proteins, whereas α integrins (Vβ1, Vβ3, and Vβ6) expressed on host cells are used are receptors.

Infectious Canine Hepatitis (Canine Adenovirus Infection, Canine Adenovirus Type 1, Nonenveloped DNA Virus): The mechanism of injury in infectious canine hepatitis is cell death (cytolysis) affecting epithelial cells of the liver and kidney and endothelial cells of all organ systems. Gross lesions include randomly distributed white-gray foci (≈1 mm in diameter) of miliary necrosis, as well as mucosal and serosal hyperemia and hemorrhage, and edema of multiple organ systems, including the liver, kidney, lymph nodes, thymus, gastric serosa, pancreas, and subcutaneous tissues (see Fig. 8-74). Edema of the gallbladder wall is prominent and is likely the result of injury of vascular endothelial cells leading to changes in permeability. Tonsillar enlargement, characteristic of the disease, probably results from proliferation of lymphocytes as part of the innate and/or adaptive immune responses against virus-infected cells through hyperplasia of uninfected lymphocytes in response to inflammatory mediators or through recruitment of lymphocytes from other lymphoid tissues and organs.

Dogs encounter the virus in fomites from body fluids such as saliva, urine, or feces. Virus enters the body through ingestion and likely inhalation and is trapped in the mucus layer of oral and pharyngeal mucosae, especially of the tonsils. It has not been determined if and how virus penetrates the mucus layer to gain access to mucosal epithelial cells, mucosal macrophages, and/or dendritic cells. Virus probably infects and replicates in mucosal macrophages and dendritic cells as they migrate through the mucus layer and mucosae. It is then spread by these cells locally through leukocyte trafficking to the submucosa and tonsil where they infect and replicate in additional tissue macrophages, lymphocytes, and dendritic cells and then spread via lymphatic vessels to regional lymph nodes and infect similar cells. A cell-free viremia has also been proposed as a mechanism of spread.

Although undetermined, virus may also be swallowed and through peristalsis encounter and infect M cells and spread and infect macrophages, dendritic cells, and lymphocytes in Peyer’s patches and then be spread to regional mesenteric lymph nodes. A virus capsid protein called fiber protein has been identified and may serve as an attachment protein that binds to host cell receptors such as coxsackievirus-adenovirus or integrin receptors.

Using either the inhalation or ingestion route of infection and spread, virus spreads from regional lymph nodes, systemically to infect endothelial cells and their contiguous epithelial cells in many organ systems, including liver, kidneys, spleen, and lungs. Infection of, replication in, and release from these cells cause their lysis and subsequent necrosis. The attachment of virus or virus-infected macrophages to endothelial cells is likely facilitated by molecules of the leukocyte adhesion cascade (see Chapter 3). Virus infects and replicates in endothelial cells leading to endothelial cell injury and death (necrosis-vasculitis). Depending on the severity of endothelial cell injury, vasculitis can be followed by hemorrhage and edema (increased vascular permeability) and DIC. Infection of epithelial cells of the liver and kidney are likely facilitated by ligand-receptor interactions, although none have been identified, Infection and death of lymphocytes in lymphoid tissues and likely bone marrow may account for the leukopenia occurring early in the infection.

Wesselsbron Disease (Flavivirus, Enveloped RNA Virus): The mechanism of injury in Wesselsbron disease is disruption and death of hepatocytes affecting young to very young sheep, cattle, and goats (ruminants). Gross lesions include an enlarged yellow to orange-brown liver (hepatomegaly) with randomly distributed white-gray foci (≈1 mm in diameter) of miliary necrosis of hepatocytes. Ruminants encounter this arbovirus through bite wounds from virus-infected mosquitoes; domestic herbivores likely serve as the animal reservoir. Seasonal variations in temperature and precipitation influence the population density of mosquitoes and thus the occurrence of disease. Virus can enter the circulatory system through direct penetration of a blood vessel during a bite wound, in which virus infects monocytes. It can also be deposited in vascularized ECM (connective) tissues, in which virus gains access to cutaneous blood and fluids, as well as Langerhans’ cells (dendritic cells) and trafficking tissue macrophages.

Through either route, virus-infected monocytes, macrophages, and/or dendritic cells spread virus via lymphatic vessels to regional lymph nodes where similar cells are infected. These cells then spread virus systemically via lymphatic vessels and the thoracic duct or postcapillary venules to the blood vascular system and other lymphoid tissues, such as the spleen, and other organ systems, such as the liver and in this example use hepatocytes and Kupffer cells (part of the monocyte-macrophage system) as target cells. Hypertrophy and hyperplasia of Kupffer cells has been reported experimentally; however, their role in the pathogenesis of Wesselsbron disease has not been determined. The virus may also possibly spread via cell-free viremia. Although unidentified at this time, viral envelope glycoproteins probably serve as attachment proteins for receptors expressed on specific populations of host target cells, thus probably determining cellular tropism for the virus.

Rift Valley Fever (Phlebovirus, Enveloped RNA Virus): The pathogenesis and mechanisms of injury in Rift Valley fever are similar to those of Wesselsbron disease.

Infectious Bovine Rhinotracheitis (Bovine Herpesvirus, Alphaherpesvirus, Enveloped DNA Virus): Bovine herpesvirus targets cells of the respiratory system, but can also infect cells of the nervous system. The mechanism of injury in infectious bovine rhinotracheitis is death of nonciliated and ciliated (mucociliary apparatus) epithelial cells of the oral, nasal, pharyngeal, and respiratory mucosae (Fig. 4-41). Gross lesions include active hyperemia, hemorrhage, edema, and necrosis leading to large areas of mucosal erosions and ulcers often covered with a fibrinous membrane (see Figs. 9-14 and 9-18).

Cattle encounter bovine herpesvirus in fomites of body fluids contaminated with virus through direct contact with virus-infected animals. Virus can be inhaled or ingested and deposited on and trapped in the mucus layers of oral, nasal, and pharyngeal mucosae. Virus can also be inhaled and deposited and trapped in the mucus layer of the mucosae of the conductive component of the respiratory system through centrifugal and inertial turbulence. Additionally, virus can be deposited on conjunctiva. It has not been determined if and how virus penetrates mucus layers of these mucosae to gain access to epithelial cells or if mucosal macrophages and/or dendritic cells are involved in carrying virus to the membranes of epithelial target cells. Viral envelope glycoproteins B, C, and D are used to attach to and enter a variety of host target cells via an array of glycosaminoglycan receptors, such as herpesvirus entry mediator A, nectin-1 and -2 (herpesvirus entry proteins C and B), and 3-O-sulfated heparin sulfate, most commonly expressed on mucosal epithelial cells and also on the sensory nerve endings that innervate the mucosa. These receptors have a polarized pattern of expression and are present only on apical and lateral surfaces of mucosal epithelial cells above junctional complexes; therefore inhalation of virus provides it with optimal opportunities for interactions with appropriate receptors. Virus enters nonciliated and ciliated mucosal epithelial cells, completes its replication cycle in these epithelial cells, and causes cell death through cytolysis as a means of releasing progeny virus into the environment.

Bovine herpesvirus can also gain access to sensory nerve endings in the respiratory mucosae. It infects, replicates, and spreads in the trigeminal and olfactory nerves via retrograde axonal transport and spreads via this mechanism to other neurons in the CNS. Neurons serve as reservoir cells in which virus establishes a latent infection. Additionally, because neurons express no MHC class II molecules and low concentrations of MHC class I molecules, they are less likely, even when infected with virus, to be recognized by and acted on by cytotoxic and helper T lymphocytes, macrophages trafficking through the nervous system, and by resident microglial cells. During latency, viral genomes are present in the nucleus of infected neurons, but no viral proteins (antigens) are synthesized. With activation, the virus reestablishes its replication cycle and through axonal transport mechanisms spreads back to nerve endings in mucous membranes to be released and potentially infect adjacent mucosal epithelial cells and transmit the disease. Bovine herpesvirus produces proteins that (1) disrupt the synthesis of interferon, (2) block the recognition of virus-infected cells by cytotoxic T lymphocytes, and (3) block the homing of T lymphocytes to virus-infected cells. Virus can also infect and induce high levels of apoptosis in T helper lymphocytes, thus suppressing the adaptive immune response to the virus. It is likely that a combination of these immunosuppressive mechanisms and disruption of the mucociliary apparatus through death of virus-infected ciliated mucosal epithelial cells make affected animals more susceptible to many secondary bacterial diseases of the respiratory system, such as Pasteurellosis or Mannheimiosis, that follow an outbreak of infectious bovine rhinotracheitis.

Equine Viral Rhinopneumonitis (Equine Herpesvirus, Alphaherpesvirus, Enveloped DNA Virus): The pathogenesis and mechanisms of injury in equine viral rhinopneumonitis are similar to those of infectious bovine rhinotracheitis.

Feline Viral Rhinotracheitis (Feline Herpesvirus, Alphaherpesvirus, Enveloped DNA Virus): Feline viral rhinotracheitis and feline calicivirus infections often occur concurrently and thus are termed the feline upper respiratory disease complex. The pathogenesis and mechanisms of injury in feline viral rhinotracheitis are similar to those of infectious bovine rhinotracheitis and equine viral rhinopneumonitis. Additionally, virus also infects mucosal macrophages and spreads to and infects similar cells in regional lymphoid nodes and then systemically via leukocyte trafficking or cell-free viremia to infect bone, eye, and lung, resulting in death of osteoblasts and osteocytes in the turbinates, necrosis of conjunctival and corneal epithelial cells, and necrosis of alveolar macrophages, respectively. Viral envelope glycoprotein G has been shown to attach and bind to chemokine receptors on host target cells.

Feline Calicivirus (Calicivirus, Nonenveloped RNA Virus): The pathogenesis and mechanisms of injury in feline calicivirus infection are similar to those of feline viral rhinotracheitis, infectious bovine rhinotracheitis, and equine viral rhinopneumonitis. Although the mechanism of injury is probably necrosis and cell death, experimental studies have suggested that synthesis of caspases can be induced in virus-infected cells, resulting in apoptosis of these cells. Virus infects and replicates in mucosal epithelial cells and likely mucosal macrophages and then spreads in lymphatic vessels to regional lymph nodes via leukocyte trafficking (or cell-free viremia) to infect additional lymphocytes and macrophages. These cells then spread virus systemically to infect synovial macrophages and pulmonary alveolar macrophages leading to synovitis and probably interstitial pneumonia, respectively. Ligand-receptor interactions are probably involved in tropism for specific cell types. It is unclear whether interstitial pneumonia (1) results from direct infection of mucosal epithelial cells and alveolar macrophages of mucosae of the conductive and O₂-CO₂ exchange components of the respiratory system via inhalation and infection of apical membranes of these cells, (2) is the result of leukocyte trafficking of virus-infected lymphocytes, macrophages, and monocytes back to the lung after infecting and being amplified in regional and systemic lymph nodes and lymphoid organs, or (3) is attributable to a combination of both mechanisms.

Recently, a syndrome termed virulent systemic feline calicivirus infection has been recognized. In addition to epithelial cell tropism, this virulent strain has acquired tropism for endothelial cells. It causes systemic vascular injury, microthrombosis, DIC, and death attributable to multiple-organ failure. This change in viral pathogenicity likely occurred through reassortment of viral capsid genes leading to enhanced virulence determinates that modulate attachment and entry and likely replication in endothelial cells. Furthermore, an additional virulence determinate appears to contribute to an exuberant host cell cytokine response as a defense mechanism against virus-infected epithelial and endothelial cells. Thus vascular lesions may, in part, be immune-mediated.

Equine Influenza (Orthomyxovirus, Enveloped RNA Virus): The mechanism of injury in equine influenza is death of epithelial cells of the oral, nasal, pharyngeal, and respiratory mucosae. Gross lesions include active hyperemia, hemorrhage, edema, and necrosis leading to mucosal erosions and ulcers often covered with a fibrinous membrane.

Horses encounter virus in fomites from body fluids contaminated with virus through direct contact with virus-infected animals. Virus is inhaled and deposited on and trapped in the mucus layer of mucosae of the nasal and pharyngeal cavities and of the conductive component of the respiratory system through centrifugal and inertial turbulence. Virus must penetrate the mucus layer to gain access to ciliated epithelial cells (mucociliary apparatus); however, mucus contains glycoprotein receptor molecules that bind to virus and prevent it from attaching to these cells. This defense mechanism allows virus to be removed via the mucociliary apparatus and phagocytosis and killing by mucosal macrophages. To counteract this defense mechanism, virus expresses a viral neuraminidase that destroys receptors that mimic viral glycoprotein receptors in the mucus. However, it has not been determined how virus penetrates the mucus layer to gain access to mucosal epithelial cells. When virus encounters these cells, hemagglutinin and neuraminidase glycoproteins in its viral envelope bind to host cell membrane receptors composed of sialyloligosaccharides. This ligand-receptor binding allows virus to attach to and enter ciliated epithelial cells. The overall structure of sialyloligosaccharide receptors, in part, determines the target specificity at the cellular and the species levels. Early in the encounter before cell death affects function, the mucociliary apparatus spreads virus to additional target cells. This mechanism of spread becomes less effective as virus kills ciliated and nonciliated mucosal epithelial cells and the physiologic continuity of the mucociliary apparatus is disrupted. It also appears that virus can spread in lymphatic vessels to regional lymph nodes via cell-free viremia or leukocyte trafficking and infect lymphocytes and macrophages. It is likely that death of lymphoid cells (immunosuppression) and ciliated epithelial cells and disruption of the mucociliary apparatus makes horses more susceptible to secondary bacterial diseases of the respiratory system. Antigenic drift, creating antigenically heterologous viruses, occurs commonly in influenza viruses, allowing them to evade vaccinal immunity. Recently, viruses closely related to equine influenza virus have caused outbreaks of severe respiratory disease in racing greyhounds and English foxhounds. Experimentally, it has been shown that canine and equine respiratory epithelium express similar sialyloligosaccharides. This finding suggests that receptors recognized by equine influenza virus are expressed on canine respiratory epithelial cells; nevertheless, subtle differences in receptor specificity may exist.

Bovine Influenza (Orthomyxovirus, Enveloped RNA Virus): The pathogenesis and mechanisms of injury in bovine influenza are similar to those of equine influenza.

Swine Influenza (Orthomyxovirus, Enveloped RNA Virus): The pathogenesis and mechanisms of injury in swine influenza are similar to those of equine influenza.

Canine Influenza (Orthomyxovirus, Enveloped RNA virus): The pathogenesis and mechanisms of injury in canine influenza are similar to those of equine influenza.

Porcine Reproductive and Respiratory Syndrome (Mystery Swine Disease; PRRS Virus; Arterivirus; Enveloped RNA Virus): The mechanism of injury in porcine reproductive and respiratory syndrome (PRRS) is death of all cell populations in the lung and associated regional lymph nodes secondary to acute inflammation (interstitial pneumonia) and its mediators and degradative enzymes. Gross lesions include lung lobules distributed at random throughout all lung lobes that are firm (consolidation) and red tan to beige with septal edema. Lymph nodes, especially those draining the lungs, are enlarged, firm, and edematous and have a beige-white cut surface that bulges. These lesions may, in part, be attributable to secondary infection with a bacterium like Pasteurella multocida.

PRRS occurs in two sequential stages: an acute stage followed by a persistent stage. In the acute stage, pigs encounter virus in fomites from body fluids through direct contact with virus-infected pigs. It is inhaled, deposited on, and trapped in the mucus layer of the mucosae of the conductive and O₂-CO₂ exchange components of the respiratory system through centrifugal and inertial turbulence. Pulmonary alveolar macrophages probably phagocytose the virus in the mucus layer and then spread it to BALT and to tissue macrophages of alveolar septa, pneumocytes of alveoli, and epithelial cells of bronchioles via leukocyte trafficking, where infection and replication occur and acute inflammation (acute interstitial pneumonia and alveolitis) ensue. In this context, virus appears to be able to escape being killed in these cells. Potential mechanisms are discussed below. Concurrently, virus-infected macrophages migrate via lymphatic vessels to regional lymph nodes (tracheobronchial) and infect macrophages and lymphocytes. It has also been suggested, but unproved, that porcine dendritic cells may be infected and spread the virus. Infection causes hypertrophy and hyperplasia of these cells, leading to enlarged lymph nodes; production of proinflammatory cytokines, resulting in cell death, acute inflammation, and edema; and the initiation of an adaptive immune response. Spread to other systemic organ systems (potentially the reproductive system) occurs at this stage, but it is unclear if spread is cell free or cell associated in macrophages (the latter being the most likely). In these organ systems, virus also likely infects cells of the monocyte-macrophage system.

In the persistent stage, virus via leukocyte trafficking establishes reservoirs in tissues, such as the tonsil, spleen, lymph nodes, and lung and in cells of the monocyte-macrophage system, such as alveolar macrophages. In part, infection of cells of the monocyte-macrophage system is likely determined by ligand-receptor interactions and may be related to the presence of sialoadhesin, a glycoprotein macrophage-specific receptor, expressed on cells of monocyte-macrophage lineage as well as scavenger receptor CD163 and heparan sulfate receptors. As an enveloped virus, PRRS virus escapes from cells without causing cell death, but infected alveolar macrophages release proinflammatory cytokines, leading to acute inflammation and the recruitment of additional inflammatory cells followed by cell death attributable to mediators and degradative enzymes of the inflammatory response. As a result, alveoli become filled with neutrophils, necrotic cell debris arising from killing of cells by the degradative activities of inflammatory cell enzymes, and edema fluid. Acute inflammation may also cause limited injury of the mucociliary apparatus leading to increased opportunities for pulmonary infections caused by other bacteria.

It appears that PRRS virus, possibly mediated via nucleocapsid proteins, has both suppressive and stimulatory activities on cells it infects in the immune system. On the one hand, it is able to alter functions of the innate and adaptive immune systems, specifically of cells of the monocyte-macrophage system, by suppressing the ability of these cells to (1) kill virus infected cells, (2) phagocytose, kill, and present antigens to effector cells, (3) stimulate other effector cells, and (4) secrete cytokines such as IFN-α, and TNF-α that are necessary to implement an effective immune response. On the other hand, during the acute phase of infection virus is able to stimulate cells to significantly increase the production of IL-10 from infected cells. This cytokine is immunosuppressive and interacts with a wide array of immune cells, including the cells of the monocyte-macrophage system and lymphocytes, resulting in inhibition of innate and adaptive immunity, particularly the cell-mediated immune responses.

Canine Infectious Tracheobronchitis (Canine Parainfluenza Virus, Enveloped RNA Virus): Canine infectious tracheobronchitis is an example of a disease in which there is primary injury caused by canine parainfluenza virus leading to increased susceptibility secondarily to infection with Bordetella bronchiseptica (or other bacteria). Other viruses (canine adenovirus type 2, canine respiratory coronavirus, reovirus, canine herpesvirus, canine distemper virus) and bacteria (Mycoplasma spp., Streptococcus zooepidemicus) have been implicated in canine infectious tracheobronchitis, thus the phrase canine infectious respiratory disease complex has been used to categorize this multifactorial pathogenesis. The mechanism of injury is dysfunction and death of ciliated epithelial cells of the mucociliary apparatus primarily from virus-induced cytolysis and secondarily from acute inflammation (bronchitis/bronchiolitis) and its mediators and degradative enzymes. Gross lesions include active hyperemia and granularity (necrosis) of the respiratory mucosae and concurrent inflammation of mucosae and submucosae (see Fig. 9-88).

Dogs encounter parainfluenza virus in fomites of oronasal-pharyngeal fluids through direct contact with virus-infected dogs. It is inhaled, deposited on, and trapped in the mucus layer of the mucosae of the conductive component of the respiratory system through centrifugal and inertial turbulence, but it has not been determined if and how virus penetrates mucus layers of these mucosae to gain access to epithelial cells or if mucosal macrophages and/or dendritic cells are involved. Virus infects and replicates in all epithelial cells; however, ciliated mucosal cells are the primary target cells. It appears that virus attaches to and enters cells via viral attachment glycoproteins (HN and F glycoproteins) that bind to sialic acid receptors on host cell membranes. When the mucociliary apparatus is dysfunctional, its cells are more susceptible to secondary infection with bacteria, especially Bordetella bronchiseptica. It is inhaled, deposited on, and trapped in the mucus layer of the mucosae of the conductive component of the respiratory system through centrifugal and inertial turbulence. The bacterium colonizes ciliated epithelium via fimbrial and nonfimbrial adhesins such as filamentous hemagglutinin and pertactin. It has not been determined if and how it penetrates mucus layers to gain access to epithelial cells or if mucosal macrophages and/or dendritic cells are involved. Once ciliated cells are colonized, Bordetella bronchiseptica releases exotoxins, such as adenylate cyclase-hemolysin and dermonecrotic toxin and endotoxins, that further impair function of the mucociliary apparatus, allowing for additional colonization of the mucosae at new sites. This damage results in an acute inflammatory response that further injures the mucosa. Toxins may also disrupt phagocytosis and/or killing of bacteria by alveolar macrophages and neutrophils and suppress cellular and humoral immune responses. Bordetella bronchiseptica can also invade epithelial cells, evade immunologic defense mechanisms, and establish a persistent infection.

Ovine Progressive Pneumonia, (Maedi; Maedi-Visna Virus [Ovine Lentivirus]; Enveloped RNA Virus): The mechanism of injury in ovine progressive pneumonia is dysfunction and death of cells of the respiratory system from infection with virus and from chronic-active granulomatous interstitial inflammation and its mediators and degradative enzymes. Ovine lentivirus persistently infects monocyte precursor cells, systemic monocytes, and alveolar and tissue macrophages. Gross lesions include dense, rubbery, and enlarged lung lobes that are uniformly affected and grayish-yellow to grayish-blue (see Figs. 9-61 and 9-79). Cut surfaces bulge and are rubbery and are not edematous or exudative, but excessive mucus may be present in airways.

Sheep most likely encounter virus in fomites from respiratory fluids through direct contact with virus-infected animals. In the fluid, virus may be free or in macrophages. However, any condition that facilitates mechanical transfer of infected blood to the circulatory system or mucosae of uninfected animals can also serve to spread virus. It is inhaled, deposited on, and trapped in the mucus layer of the mucosae of the conductive component of the respiratory system through centrifugal and inertial turbulence. Free virus is likely phagocytosed by alveolar macrophages in the mucus layer, whereas virus-infected macrophages likely release virus and/or die (or are killed) and release virus into mucus and mucosae; migrate to local lymphoid tissues (BALT) and release virus and/or die (or are killed) and release virus; or are killed by host immune cells in mucosae and release virus that is phagocytosed by host alveolar macrophages. Virus-infected alveolar macrophages migrate to local lymphoid tissues (BALT) and then via leukocyte trafficking in lymphatic vessels to regional lymph nodes where additional macrophages are infected. Macrophages then spread virus via leukocyte trafficking to immature monocyte precursor cells (monoblasts or promonocytes) in the bone marrow (and likely similar cells in spleen and lymph nodes) where small numbers of infected precursor cells serve as biological reservoirs for the distribution (via the circulatory and lymphatic systems) of virus-infected monocytes back into the blood. Virus likely uses envelope glycoproteins to attach, bind to, and fuse with alveolar macrophages and other target cells that express small ruminant lentivirus receptors A or B or some other membrane receptor. All cells infected with virus are permanently infected (persistent infection) because virus inserts its genome into chromosomal DNA of host cells.

The ability of virus to replicate in a cell is directly related to the maturity of the permanently infected cell. In bone marrow, virus integrates into precursor monocytes (monoblasts or promonocytes) and persistently infects a very small number of such cells. It is not able to replicate in these cells. These cells differentiate into monocytes, migrate to tissues and organ systems that utilize the services of the monocyte-macrophage system, and then differentiate into macrophages. Virus-infected monocytes in the peripheral blood do not produce virus. When infected monocytes mature and differentiate into macrophages in tissues, virus is able to replicate in these cells and express viral proteins and proinflammatory chemokine and cytokine molecules leading to inflammation. These macrophages can then infect and activate other susceptible and differentiated tissue macrophages, such as alveolar macrophages, and this interaction initiates and sustains the chronic active granulomatous inflammatory process. The lifespan of tissue macrophages ranges from 6 to 16 days. Thus the effective lifespan for virus-infected mature tissue macrophages capable of sustaining the infection is shortened because a portion of the lifespan must include time for integration of its genome, replication, and assembly of virus in tissue macrophages. As tissue macrophages infected with virus die off, a cyclic process evolves in which infected monocytes arrive from bone marrow to infect naïve tissue macrophages that replicate locally or arrive and differentiate from uninfected blood monocytes. This cyclic process, often characteristic of lentiviral-induced diseases, combined with genetic variation is used to sustain and enhance the severity of the interstitial pneumonia. Virus-infected monocytes travel throughout all tissues and organ systems of the body; however, chronic-active inflammation only occurs in specific tissues. It appears that selectivity and specificity of lung, brain, mammary gland, and synovia occur in tissues where macrophages are permissive to genome integration. Kupffer cells in the liver are not permissive and do not allow transcription of viral RNA, and the liver does not develop lesions. Based on this mechanism, Maedi-visna should occur in the same sheep at the same time; however, this outcome is not common. The mechanism for this outcome is unknown.

Interstitial pneumonia results from virus-infected alveolar macrophages expressing high concentrations of proinflammatory chemokine, IL-8, that recruits inflammatory cells (not infected with virus) into the lung. These uninfected and recruited lymphocytes, plasma cells, macrophages, and neutrophils produce additional proinflammatory cytokines capable of sustaining inflammation and propagating the interstitial pneumonia. Thus a small number of virus-infected alveolar macrophages, responding to molecules through specific cell-membrane receptors, utilize a cascade of membrane, cytoplasmic, and nuclear messenger systems to control and sustain a large inflammatory response. Additionally, several studies suggest that lesions in ovine progressive pneumonia are in part immune mediated and that cytotoxic T-lymphocytes may be important effector cells. Virus-infected macrophages present viral antigens to T lymphocytes, and activated T lymphocytes in turn release cytokines that lead to differentiation of monocytes to macrophages and recruitment of additional inflammatory cells. Host defense mechanisms are ineffective in ending virus infection because (1) the viral genome becomes part of the host cell genome; (2) viral infection of cells of the monocyte-macrophage system results in dysfunction of this system and an ineffective adaptive immune response (see Chapter 3); and (3) the parental virus can modify its progeny through repeated cycles of gene reassortment (genetic variation) so that these progeny are able to escape an effective adaptive immune response (cyclical [recurring] infection).

Caprine Pneumonia (Caprine Arthritis-Encephalitis Virus, Enveloped RNA Virus): The pathogenesis and mechanism of injury in caprine pneumonia are similar to those of ovine progressive pneumonia (Maedi) and caprine encephalitis of goats.

Bovine Respiratory Syncytial Virus Pneumonia (Pneumovirus, Enveloped RNA Virus): The mechanism of injury in bovine respiratory syncytial virus pneumonia is dysfunction and death of cells of the respiratory mucosa, including ciliated cells of the conductive system and alveolar type II pneumocytes from infection by virus and from acute inflammation and its mediators and degradative enzymes. Gross lesions include active hyperemia, interstitial edema and inflammation (proliferative and exudative bronchiolitis), and subpleural and interstitial emphysema. Syncytial cells with intracytoplasmic inclusion bodies are observed in the microscopic lesions (see Fig. 9-69).

Cattle encounter bovine respiratory syncytial virus in fomites from body fluids contaminated with virus through direct contact with virus-infected animals. It is inhaled, deposited on, and trapped in the mucus layer of the mucosae of the conductive component of the respiratory system through centrifugal and inertial turbulence, but it has not been determined if and how virus penetrates mucus layers of these mucosae to gain access to epithelial cells or if mucosal macrophages and/or dendritic cells are involved. Virus infects and replicates in all epithelial cells; however, ciliated cells are the primary target cells. When virus encounters ciliated cells, it attaches and binds to membrane glycosaminoglycan receptors via heparin-binding domains on envelope glycoprotein G (attachment protein) and enters via envelope glycoprotein F (fusion protein). It has also been shown that virus can infect and replicate in lung dendritic cells and alveolar macrophages and cause the synthesis of interferons and interleukins. Infection of all of the cell types as previously mentioned appears to induce the synthesis of a cascade of proinflammatory chemokines and cytokines that recruit neutrophils, lymphocytes, and macrophages and lead to tissue injury. Additionally, TLR3 and TLR4 may initiate this cascade. In tissue culture experiments, virus appears to cause little or no injury to ciliated epithelial cells, suggesting that lesions may result in part from host defense mechanisms such as those modulated by the innate and adaptive immune responses. Bovine respiratory syncytial virus is part of bovine respiratory disease complex (shipping fever). This complex is characterized chronologically by (1) environmental or management stressors that suppress protective mechanisms in the respiratory system such as the production of protective mucus, (2) a primary viral infection that injures structural protective mechanisms such as the mucociliary apparatus, and (3) a secondary bacterial infection that causes severe inflammation often with fibrin exudation.

Inclusion Body Rhinitis–Porcine Cytomegalovirus Infection (Herpesvirus-Cytomegalovirus; Enveloped DNA Virus): The pathogenesis of porcine cytomegalovirus infection has not been studied in sufficient detail to provide an evidence-based discussion of the chronologic sequence of events characteristic of the disease. The mechanism of injury is probably dysfunction and death of epithelial cells of the nasal and respiratory mucosa via infection with virus, especially the epithelial cells that form the mucous glands of the nasal cavity and from acute inflammation and its mediators and degradative enzymes. Gross lesions can include active hyperemia, congestion, and excessive mucus covering mucosal surfaces of the nasal septum and turbinates.

Pigs encounter porcine cytomegalovirus in fomites from body fluids contaminated with virus through direct contact with virus-infected animals. It is inhaled, deposited on, and trapped in the mucus layer of the mucosae of the conductive component of the respiratory system through centrifugal and inertial turbulence, but it has not been determined if and how virus penetrates mucus layers of these mucosae to gain access to epithelial cells or if mucosal macrophages and/or dendritic cells are involved. The conjunctiva may also be a source of infectious virus that later infects the nasal turbinates and septum via the lacrimal duct. Envelope attachment and fusion glycoproteins and cell membrane receptor proteins are likely involved in host cell infection, virus replication, and spread of virus to other cells and tissues. It is unclear how virus spreads systemically from the nasal cavity to other organ systems; however, in other animal models and humans, leukocyte trafficking and cells of the monocyte-macrophage system are involved in the spread of similar viruses.

Feline Infectious Peritonitis (Feline Infectious Peritonitis Virus; Nonenveloped RNA Virus): The mutation of feline enteric coronavirus to feline infectious peritonitis virus is central to the pathogenesis of this disease. The mechanism of injury is chronic-active pyogranulomatous inflammation (vasculitis and perivasculitis) and its mediators and degradative enzymes. Gross lesions include gray-white nodules of varied sizes that have a perivascular pattern of distribution and in some cases a linear pattern following blood vessels in serosa and mesenteries (see Figs. 7-161, 11-75, and 14-105). Body cavities may contain a thick yellow exudate containing fibrin and pyogranulomatous inflammatory cells (see Fig. 7-161).

Cats encounter feline enteric coronavirus by ingestion of virus-contaminated fomites through two routes: (1) contact with virus-contaminated feces in litter boxes and (2) contact with carrier cats, usually queens. Fomites from saliva or respiratory droplets probably serve as a source of the virus to infect naïve cats via ingestion; therefore grooming behavior increases the likelihood that virus will enter the oral cavity. Feline enteric coronavirus is swallowed and moved to the alimentary system via intestinal peristalsis where it gains access to mucosae. The replication of enteric coronavirus is primarily restricted to mature intestinal epithelial cells (terminally differentiated cells with a lifespan of 3 to 8 days); however, the virus can enter a carrier state and persist in unidentified cells of the intestinal mucosa. These cells are likely progenitor cells (i.e., crypt stem cells) with infinite lifespans, so cell death through normal enterocyte turnover does not affect the carrier state of the virus. Feline enteric coronavirus spreads from enterocytes and carrier cells into the lamina propria and then to macrophages in Peyer’s patches. It has not been determined if and how virus penetrates the mucus layer to gain access to mucosal epithelial cells or if mucosal macrophages, dendritic cells, or M cells are involved. Leukocyte trafficking to the submucosa by these cells would explain how virus spreads to macrophages in Peyer’s patches. Feline enteric coronavirus likely uses capsid proteins, such as S protein, and other glycoproteins to bind to feline aminopeptidase-N, a cell membrane receptor on monocytes and macrophages. In mucosal macrophages of Peyer’s patches and in blood monocytes, feline enteric coronavirus mutates into feline infectious peritonitis virus. Thus the genome of each new feline infectious peritonitis virus variant is unique to an individual cat. When feline enteric coronavirus mutates to feline infectious peritonitis virus, feline infectious peritonitis virus acquires virulence determinates that allow it to infect and replicate in cells of the monocyte-macrophage system, resulting in rapid dissemination of the virus throughout the body.

Monocytes and macrophages infected with feline infectious peritonitis virus spread from Peyer’s patches to regional lymph nodes via leukocyte trafficking in lymphatic vessels and infect additional macrophages. They then migrate in lymphatic vessels through the thoracic duct into the circulatory system and to all tissues of the body and infect additional populations of free and fixed tissue macrophages. Virus-infected macrophages appear to target small and medium-sized veins of serosal membranes and tissues, cause damage to endothelial cells, and are recognized as foreign by the cat’s innate (inflammation) and adaptive (cell-mediated and humoral) defense mechanisms (see Chapters 3 and 5). This process likely involves activation of the leukocyte adhesion cascade and binding of macrophages and monocytes to endothelial cells facilitated by ligand-receptor interactions and the activation of acute inflammation via proinflammatory cytokines released from activated macrophages and monocytes. All of these processes result in injury of vascular and perivascular tissues (vasculitis). Cats with a strong cell-mediated response do not develop feline infectious peritonitis. Cats with a weak cell-mediated response have the dry (noneffusive) form; cats with no cell-mediated response have the wet form (effusive). An effective humoral response appears to increase the severity of disease. Tissue macrophages provide a source of viral antigens in and around venules, and if adequate antibody is present, antigen-antibody complexes form and a type III hypersensitivity response ensues. Where immune complexes are formed (i.e., basement membrane of endothelial cells) or whether they are free or cell associated is not clearly understood. These complexes activate complement resulting in chemotaxis and accumulation of neutrophils via the leukocyte adhesion cascade. Additionally, they also activate tissue macrophages, leading to the secretion of a variety of proinflammatory cytokines that act on endothelial cells to increase neutrophil and mononuclear cell chemotaxis into the area and open tight junctions of endothelial cells (increased permeability), thereby allowing leakage of plasma and fibrin into body cavities. These mechanisms result in the vasocentric pyogranulomas and pyogranulomatous inflammation, fibrinous effusions, and fibrinous polyserositis (feline coronaviral polyserositis) so characteristic of feline infectious peritonitis. A type IV hypersensitivity reaction may be involved in the pathogenesis of some pyogranulomas. It appears that the commonly used categories of wet and dry forms and type III and type IV hypersensitivities are based more on clinical characteristics and immunologic tests, respectively, than on any morphologic criteria. Experimental studies have shown that there are no distinct histopathologic lesions that distinguish wet from dry cases, type III from type IV hypersensitivities, or acute/subacute cases from chronic cases.

Parvovirus Myocarditis (Parvovirus, Nonenveloped DNA Virus): See parvovirus enteritis in the section on the Alimentary System and the Peritoneum, Omentum, Mesentery, and Peritoneal Cavity for information on the pathogenesis of viral spread and replication before spreading to the heart. The mechanism of injury in parvovirus myocarditis is cell death (necrosis of rhabdomyocytes) attributable to infection with virus. Gross lesions include gray-white areas of varied sizes distributed in the myocardium (see Fig. 10-82). It is likely that virus spreads via leukocyte trafficking or cell-free viremia in lymphatic or blood vessels from Peyer’s patches to regional lymph nodes and then systemically in the circulatory system to capillaries and rhabdomyocytes in the heart. Endothelial cells are dividing cells, and studies suggest that in the heart, virus initially infects and replicates in these cells and then spreads to infect contiguous cardiac rhabdomyocytes. Rhabdomyocytes are actively dividing cells in dogs under 15 days of age; therefore they can be infected with virus and are lysed with release of virus. This outcome results in necrosis of rhabdomyocytes and ectopic irritable foci, cardiac arrhythmias, and unexpected death. If dogs survive this stage, healing mechanisms cause cardiac fibrosis that can contribute clinically to dysfunction of the conduction system and contraction of the cardiac musculature later in life. Specificity for these mitotically active cells occurs because parvoviruses require a host cell–derived duplex transcription template, which is only available when cells divide during the S-phase of the cell cycle. Ligand-receptor interactions are also probably involved.

Canine Herpesvirus Infection (Canine Herpesvirus Type 1, Enveloped DNA Virus): The mechanism of injury in canine herpesvirus infection is death of endothelial cells systemically and epithelial cells of multiple organ systems (pantropic). Gross lesions include mucosal and serosal hemorrhage and randomly distributed white-gray foci (≈1 mm in diameter) of miliary necrosis within organ systems, especially the kidneys (see Fig. 11-74). Miliary necrosis can also be observed in spleen, lymph nodes, lung, and liver.

Puppies ingest and inhale virus in fomites from body fluids of the birth canal or nasal-oral cavity of bitches through grooming. It is deposited on mucosae of the nasal and oral pharynx, especially those of the tonsil, and is thought to infect mucosal epithelial cells. It has not been determined if and how virus penetrates the mucus layer to gain access to mucosal epithelial cells. Although it appears that virus infects lymphocytes in the tonsil, it has not been determined if and how virus spreads from mucosal epithelial cells to lymphocytes or if leukocyte trafficking or dendritic cells are involved. It is also unclear if or how virus migrates to regional or systemic lymphoid nodes, thymus, or spleen before it spreads systemically to infect endothelial and epithelial cells of other organ systems; however, virus appears to be spread systemically in lymphocytes via leukocyte trafficking. It has not been satisfactorily determined (1) how virus-infected lymphocytes interact with endothelial and epithelial cells or if this interaction is facilitated by molecules of the leukocyte adhesion cascade (see Chapter 3); (2) how virus infects and replicates in endothelial cells, leading to injury and cell death; and (3) if injury results in vasculitis and vascular thrombosis, leading to tissue infarction and DIC. Canine herpesvirus expresses envelope glycoproteins B, C, and D; however, their role in attaching and binding to target cells has not been clearly defined. Heparin sulfate may serve as a target cell receptor for canine herpesvirus.

Equine Viral Arteritis (Arterivirus, Enveloped RNA virus): The mechanism of injury in equine viral arteritis is death of endothelial cells, myocytes, and pericytes of small muscular arterioles and venules of multiple organ systems. Gross lesions include (1) congestion, edema, and hemorrhage in subcutaneous tissues of the limbs and abdomen; (2) hydroperitoneum, hydropericardium, and hydroabdomen; and (3) edema and hemorrhage in lymph nodes and intestines.

Horses inhale virus in fomites of body fluids, most commonly urine, through direct contact with virus-infected animals. It is deposited on mucosae of the conductive and O₂-CO₂ exchange systems through centrifugal and inertial turbulence and trapped in the mucus layer. Virus is likely phagocytosed by bronchiolar and alveolar macrophages as they migrate through the mucus layer and mucosae and spread locally through leukocyte trafficking to the submucosa (BALT) where they infect additional tissue macrophages. From here, macrophages spread virus to regional lymph nodes via lymphatic vessels where additional macrophages are infected. Ligand-receptor interactions are probably involved in tropism for specific cell types. Virus expresses envelope glycoproteins and a nucleocapsid protein; however, their role in binding to target cells has not been clearly defined. Additionally, membrane receptors for the virus have not been identified. Macrophages leave regional lymph nodes and enter the circulatory system via postcapillary venules or lymphatic vessels and the thoracic duct. During vascular migration, infected macrophages encounter endothelial cells, myocytes, and pericytes of small arterioles (and venules) and their interactions are facilitated by molecules of the leukocyte adhesion cascade (see Chapter 3). Monocytes containing viral antigens have been observed adhering to endothelial cells, and virus appears to spread from monocytes to endothelial cells, myocytes, and pericytes of these vessels.

Migrating monocytes also encounter and infect hepatocytes, adrenal cortical cells, seminiferous tubular cells, and thyroid follicular cells. Virus-induced vascular injury leads to cell death characterized by endothelial swelling, degeneration, and necrosis, acute and chronic (lymphomonocytic) inflammation, necrosis of myocytes, and thrombus formation, leading to edema and hemorrhage in many tissues and organs. Virus replication occurs in endothelial cells, and the expression of envelope glycoproteins in endothelial cells likely activates acute inflammation, fibrinogenesis, the complement cascade, and the recruitment of neutrophils into vascular intima and tunica media and in severe cases results in fibrinoid necrosis and vasculitis. The role of proinflammatory chemokines and cytokines in vascular injury has not been defined. A lymphomonocytic inflammatory cell population is also commonly found in the tunica media and adventitia suggesting that cytolytic T lymphocytes could induce cytolysis of virus-infected endothelial cells. Why death of endothelial cells and myocytes dominates over death of epithelial cells, such as those in renal tubules, is unknown. However, infection and death of renal tubular epithelial cells with release of virus into urine appears to be the mechanism by which virus is spread to naive horses.

Bovine Malignant Catarrhal Fever (Ovine Herpesvirus-2 and Alcelaphine Herpesvirus-1 [γ-Herpesviruses], Enveloped DNA virus): The mechanism of injury in bovine malignant catarrhal fever is dysfunction and death of vascular endothelial cells and hyperplasia, dysfunction, and death of lymphocytes in lymphoid tissues. Gross lesions include (1) erosive, ulcerative, and hemorrhagic lesions of mucosae of gingiva, tongue, oral papillae, hard and soft palate, oral pharynx, esophagus, turbinates, trachea, rumen, reticulum, and omasum (see Fig. 7-4); (2) enlargement of lymphoid organs and tissues followed by atrophy; and (3) increased size of visceral organs and tissues resulting from perivascular accumulations of lymphocytes (lymphoproliferative vasculitis). Worldwide, sheep are the reservoir animal for sheep-associated ovine herpesvirus-2 (OvHV-2) that causes malignant catarrhal fever in cattle, bison, pigs, and deer. In Africa, blue wildebeest are the reservoir animal for wildebeest-associated Alcelaphine herpesvirus-1 (AlHV-1) that causes malignant catarrhal fever in cattle. These viruses persist in carrier animals without ill effects. The chronologic sequence of events leading to malignant catarrhal fever have not been clearly determined.

Animals probably encounter these viruses through inhalation and ingestion of fomites from oronasal-pharyngeal-ocular fluids (also seminal fluid) from reservoir animals that are actively shedding virus. In reservoir animals, OvHV-2 is shed predominantly through nasal secretions derived from turbinates and shedding episodes are stress-induced and occur more frequently in lambs than adult sheep. Virus is deposited on mucosae of the oral, nasal, and pharyngeal cavities; the conjunctiva; or the conductive component of the respiratory system through centrifugal and inertial turbulence. It is trapped in the mucus layer and apparently phagocytosed by mucosal macrophages and spread to submucosae and BALT via leukocyte trafficking. It has not been determined if dendritic cells are involved. In submucosae, virus infects lymphocytes (possibly B lymphocytes), macrophages, and monocytes and spreads in CD8⁺ T lymphocytes via leukocyte trafficking to regional lymph nodes and then systemically to other organ systems and lymphoid tissues. Ligand-receptor interactions are probably involved in tropism for specific cell types, but viral envelope glycoproteins or cell receptors have not been identified.

Virus-infected CD8⁺ T lymphocytes are distributed in intimal, medial, and adventitial tissues of blood vessels (vascular and perivascular pattern) in organ systems. This tropism may be determined by (1) ligand-receptor interactions or (2) permissiveness of specific vascular cells to viral infection and replication. As part of this tropism, virus-infected CD8⁺ T lymphocytes produce proinflammatory cytokines and express viral glycoproteins in their cell membranes. Proinflammatory cytokines can act as cytotoxic molecules and may injure and kill bystander cells, such as those in the vasculature, and viral glycoproteins may recruit lymphocytes, macrophages, and monocytes and lesser numbers of neutrophils and plasma cells into perivascular and vascular tissues, leading to lymphoproliferative necrotizing vasculitis and vascular wall necrosis. Atrophy of lymphoid tissues after viral infection is not likely caused by virus-induced cell death. Because lymphocytes are short-lived effector cells, atrophy is likely the result of normal cell aging and turnover that follows massive proliferation. It is not known if virus can infect, injure, and kill endothelial cells directly. Additionally, virus-infected large granular lymphocytes and other recruited cytotoxic lymphocytes and macrophages may play roles in vascular injury because they have been shown to be cytotoxic for vascular endothelial cells. The cause of the erosive, ulcerative, and hemorrhagic lesions has not been determined; however, infarction of mucosal blood vessels secondary to thrombosis induced by necrotizing vasculitis could potentially lead to these outcomes. Apparently, there is a lack of spread between susceptible animals because they are dead-end hosts for these viruses. Virus spread appears to require cell-free virus in body fluids, and in susceptible animals, the virus replicates in a cell-associated manner (lymphocytes, macrophages, monocytes) and cell-free virus is not produced. Because virus-infected host cells do not produce infectious virus during the virus replication cycle, these species are unable to transmit virus to other animals (see carrier animals above).

Classical Swine Fever (Hog Cholera, Pestivirus, Enveloped RNA Virus): The mechanism of injury in classical swine fever is death of endothelial cells of multiple organ systems and of hemopoietic cells. Gross lesions include a red-blue discoloration of the skin, hydropericardium, hydrothorax, and hydroperitoneum; hemorrhage and necrosis of the palatine tonsil; and petechial and ecchymotic hemorrhages in most organs of the body, especially the kidney (Fig. 4-42).

Pigs encounter virus through (1) ingestion and likely inhalation of fomites from body fluids, body waste, or offal or other virus-contaminated pork products and (2) mechanical transfer on virus-contaminated vehicles, clothes/boots, instruments, and needles. Virus is deposited on mucosae of the oral and nasal pharynx, especially of the tonsil, where it infects and replicates in epithelial cells of tonsillar crypts. Although unknown, virus likely buds from basal surfaces of tonsillar epithelial cells and infects subjacent mucosal macrophages in lymphoid nodules. It has not been determined how virus penetrates the mucus layer to gain access to mucosal epithelial cells or if mucosal macrophages or dendritic cells phagocytose virus in the mucus layer and spread it through leukocyte trafficking to the submucosa. E^rns and E2 envelope glycoproteins and other envelope glycoproteins appear to be involved in binding to and entering mucosal epithelial cells and macrophages via cell surface glycosaminoglycan receptors such as heparin sulfate. Infected macrophages migrate via leukocyte trafficking in lymphatic vessels to regional lymph nodes such as the submandibular and pharyngeal. Here, they infect and likely recruit additional macrophages and induce lymphoid hyperplasia through release of proinflammatory chemokines and cytokines. Lymph nodes become edematous and hemorrhagic because of injury (necrosis) to endothelial cells after infection with virus from tissue macrophages. Subsequently, macrophages leave regional lymph nodes and enter the circulatory system via postcapillary venules or lymphatic vessels and the thoracic duct to migrate systemically to other organ systems.

Infected macrophages likely interact with endothelial cells of these organs by adhering to and migrating through the endothelium, likely by activating the leukocyte adhesion cascade (see Chapter 3). Virus spreads from macrophages and infects and replicates in endothelial cells, resulting in direct injury and inducing an acute inflammatory response. Vascular lesions are probably lytic and characterized by endothelial swelling, degeneration, and necrosis; acute and chronic (lymphomonocytic) inflammation; necrosis of myocytes; and thrombus formation leading to edema and hemorrhage in many tissues and organs. This pattern of injury serves as the basis for hemorrhage observed in the kidney. Macrophages also spread virus to lymphoid tissues and bone marrow, where it infects and kills these cells, resulting in severely impaired adaptive immune responses with decreased neutralizing antibody production, decreased numbers of phagocytes, and decreased cellmediated immune responses. The impairment or loss of these defense mechanisms makes pigs more susceptible to other infectious diseases.

African Swine Fever (Asfivirus, Enveloped DNA Virus): The pathogenesis, mechanisms of injury, and clinical outcomes of African swine fever are very similar to those of classical swine fever, but the pathogenicity (virulence determinates) of and thus the disease and lesions caused by African swine fever virus are much more severe (Fig. 4-43). Additionally, African swine fever virus can gain access to the blood vascular system and directly infect macrophages through bites of ticks. Virus envelope glycoproteins p12, p54, and p30 appear to be involved in attaching and binding to and entering host target cells via cell receptors. Target cell receptors have not been clearly identified. DIC, leading to collapse of the circulatory system and shock, is likely the cause of death in virus-infected pigs.

African Horse Sickness (Orbivirus, Nonenveloped RNA Virus): The pathogenesis and mechanism of injury in African horse sickness are similar to those of bluetongue disease. The mechanism of injury is endothelial cell barrier dysfunction and virus-induced dysfunction and death of endothelial cells. There are four clinical forms of African horse sickness; however, in each form the gross lesions are characteristic of vascular (endothelial cell) injury and include edema (pulmonary, systemic, subcutaneous, intramuscular, supraorbital fossae, eyelids, lips, cheeks, tongue, intermandibular space, and larynx), active hyperemia, petechial and ecchymotic hemorrhages (serosal [epicardial, endocardial], subcapsular [spleen], cortical [kidney], and mucosal [intestines]), hydrothorax, hydropericardium, and ascites, and rhabdomyocytic necrosis (Fig. 4-44). The expression of these forms may be related to differences in viral tropism for different types of vascular endothelial cells within organ systems of the body or the permissiveness of different types of endothelial cells in allowing the virus to replicate efficiently or in large numbers. African horse sickness virus also infects cells of the dendritic, lymphoid, and monocyte-macrophage systems. African horse sickness is a noncontagious disease of horses, donkeys, and mules.

Animals encounter virus in bite wounds from midges. After skin penetration, virus can enter the circulatory system or be deposited in vascularized ECM (connective) tissues. If a blood vessel is penetrated, virus can enter the circulatory system and infect macrophages and lymphocytes or be carried cell free to systemic lymphoid tissues. If deposited in connective tissue, virus gains access to cutaneous blood and fluids, as well as cutaneous dendritic cells (Langerhans’ cells) and tissue macrophages. Although unproved, it is likely that virus infects these cells and virus-infected macrophages or dendritic cells spread the virus via leukocyte trafficking and lymphatic vessels to regional lymph nodes. Here, virus infects lymphocytes and additional dendritic cells and macrophages. African horse sickness virus has two attachment proteins, capsid structural proteins (VP2 and VP5). These proteins bind to glycosaminoglycans on host cell membranes and facilitate attachment and entry of virus.

From regional lymph nodes, virus spreads systemically in macrophages via leukocyte trafficking to the circulatory system, through postcapillary venules and/or lymphatic vessels and the thoracic duct, to infect, injure, and kill vascular endothelial cells in the lungs, heart, spleen, lymph nodes, liver, and kidney. Infected macrophages likely interact with endothelial cells of these organs by adhering to and migrating through the endothelium, likely by activating the leukocyte adhesion cascade (see Chapter 3). Virus spreads from macrophages and infects and replicates in endothelial cells, resulting in direct injury and inducing an acute inflammatory response. Vascular lesions are probably lytic and characterized by endothelial swelling, degeneration, and necrosis and depending on the severity of injury, vasculitis can be followed by hemorrhage and edema (increased vascular permeability) affecting the lung and vascular thrombosis leading to tissue infarction. Necrosis of rhabdomyocytes in the heart has been attributed to the release of endogenous catecholamines, but experimental findings suggest that necrosis is caused by microthrombosis of myocardial capillaries likely resulting in myocyte ischemia. Rarely, DIC has been reported in African horse sickness. Additionally, NS3, a viral protein inserted in the host cell membrane, may be cytotoxic (acting as a viroporin that alters host cell membrane permeability) and involved in the release of virus from infected endothelial cells and membrane damage.