Function

The spleen filters blood and removes foreign particles, bacteria, and erythrocytes that are senescent, have structural membrane abnormalities, or are infected with hemotropic parasites. Its immunologic functions are those of a secondary lymphoid organ and include activation of macrophages to process and present antigen, B lymphocyte proliferation and production of antibody and biologic molecules, and the interaction of T lymphocytes and antigens. In some species, it stores significant quantities of blood (Box 13-6). These functions are best considered on the basis of the two grossly visible components of the splenic parenchyma: red and white pulp and the anatomic systems associated with them: (1) the monocyte-macrophage system, red pulp vascular spaces, and hematopoiesis within the red pulp and (2) the B and T lymphocyte systems within the white pulp.

Responses to injury

The responses of the spleen to injury (Box 13-7) include acute inflammation, hyperplasia of the monocyte-macrophage system, hyperplasia of lymphoid tissues, atrophy of lymphoid tissues, storage of blood or contraction to expel reserve blood, and neoplasia. These responses are reflections of the spleen’s three chief anatomic components: (1) monocyte-macrophage system, (2) lymphoid (immune) system, and (3) vasculature, chiefly the red pulp vascular spaces and their supporting reticular stroma (reticulum). Because the spleen is examined on the basis of red and white pulp, this approach is used in the following description.

Structure

The three dimensional structure of a lymph node is complex and difficult to fully appreciate in a two-dimensional histological slide or diagram. It is convenient from the point of view of understanding the pathological response of the lymph node to consider them based on its anatomic components (Fig. 13-41), which are as follows:

Function

The lymph nodes are one of the first lines of defense against lymph and blood borne infectious and non-infectious agents. The functions of the lymph node are to filter lymph of particulate matter and microorganisms, and to facilitate the surveillance of incoming antigens and their interaction with B and T lymphocytes and to produce B lymphocytes. It is helpful to consider the paths taken by particles, molecules, and cells arriving at a lymph node. This is accomplished via two possible routes: (1) via the arterial blood supply or (2) via lymphatic afferents. T lymphocytes arriving by the arterial route migrate out of the HEVs into the T lymphocyte areas adjacent to the follicle (interfollicular portion of the deep cortex) (see Chapter 3).

Material arriving in the lymph can be subdivided into the following three types:

Particles and molecules in the lymph arriving via the afferent lymphatic vessels enter the subcapsular sinus, which has been likened to a swamp. Hydrostatic pressure is low, particles tend to settle, and reticular fibers crossing the sinus impede the flow. All these factors facilitate phagocytosis by the sinus macrophages. Larger molecules and particles flow from the subcapsular into the trabecular sinuses and then into the medullary sinuses, where particles and bacteria are phagocytosed by the sinus histiocytes. This is well illustrated by the concentration of carbon in the medulla of the tracheobronchial lymph nodes of animals with anthracosis (see Fig. 13-40).

Smaller molecules, such as free antigens and chemokines, cytokines and other soluble products of the immune and inflammatory responses, penetrate the floor of the subcapsular sinus and are carried along reticular fibers to the outer cortex adjacent to the HEVs, the site of migration of B and T lymphocytes into the cortex. Thus these molecules are brought into close association with B and T lymphocytes and dendritic cells, and this process facilitates the activation of antigen specific B lymphocytes, which develops into antibody secreting plasma cells.

Antigens can also reach the regional lymph node in antigen-presenting cells such as dendritic cells. These cells have been activated by uptake and processing of antigen and migrate via lymphatic vessels to the subcapsular sinus where they exit and migrate along the reticulum to the outer cortex where they present the antigen to the follicular B lymphocytes (see Chapter 5). The classic example is the intraepidermal dendritic cell (Langerhans’ cell) draining from the skin.

Responses to Injury

Responses to injury are listed in Box 13-8, and the responses are discussed on the basis of the following systems: monocyte-macrophage system, outer cortex, inner cortex, medullary sinuses, and medullary cords.

Structure and Function

Although hemal nodes are often considered to be unique to ruminants, they have also been found in horses and primates. They are small, dark red-to-brown nodules. Their architecture resembles that of a lymph node with lymph follicles and sinuses, except that the hemal node sinuses are filled with blood (Web Fig 13-4). It was once thought that they had no afferent or efferent lymphatic vessels, but these have been identified. As erythrophagocytosis can be present, it is presumed that hemal nodes can filter blood and remove senescent erythrocytes, but their functional importance is not clear.

Structure

Histology books use the following terms: diffuse lymphatic tissue and solitary and aggregated lymphatic nodules to describe these forms of lymphatic tissue, but the immunology and pathology literature uses mucosa-associated lymphoid tissue (MALT). The importance of these three types of MALT is frequently overlooked because most of these tissues in the normal animal, apart from aggregated lymphatic tissue of the tonsil and Peyer’s patches, are visible only microscopically. The actual mass is significant, and MALT is essential for mucosal immunity. It is a site where antigen is phagocytosed and from which immune cells and antigen flow via afferent lymphatic vessels to regional lymph nodes. MALT consists of three distinct morphologic types:

The solitary lymph nodules and aggregated lymphatic tissues are classified by their anatomic locations and form the MALT. These lymphatic tissues include (1) the bronchus-associated lymphatic tissue (BALT), which is often at the bifurcation of the bronchi and bronchioles), (2) a ring of lymphoid tissue (pharyngeal and palatine tonsils [see below]) and lymph nodules (NALT, LALT, and ATALT [see below]) that surround the oral and nasal pharynges and encounter infectious microorganisms during ingestion and inhalation, and (3) the gut-associated lymphatic tissue (GALT), of which the most important and most obvious components are Peyer’s patches and the pharyngeal and palatine tonsils. Lymph nodules can also be present in the mucosa of the conjunctiva-associated lymphatic tissue (CALT) (Fig. 13-46), mammary gland, urinary bladder mucosa, nose (NALT), larynx (LALT), auditory tube (ATALT), and genital tract.

Function

The primary function of MALT is to protect mucosal barriers and thus they are strategically placed to act as sentinels, but they can also become a portal of entry for bacteria and this is particularly true of the tonsils and Peyer’s patches. Peyer’s patches in the ileum are overlaid by a special epithelium known as the follicle-associated epithelium (FAE), in which there are numerous M cells. These M cells deliver macromolecules (including antigens), particles, pathogenic bacteria, and viruses across the epithelium to an area rich in dendritic cells, which are able to transport the material into the GALT. Table 13-4 lists the functions of the MALT with different microorganisms.

Responses to Injury

The responses of MALT (diffuse lymphatic tissue and solitary and aggregated lymph nodules) to injury (Box 13-9) are similar to those of other lymphatic tissue: hyperplasia, atrophy, and inflammation.

Thymus

Viruses and toxins, such as aflatoxins, enter the thymus hematogenously via the arteries.

Spleen

Infectious agents, such as bacteria, enter the spleen by two routes: via the blood vasculature or by direct penetration. The spleen has no afferent lymphatic vessels and efferent lymphatic vessels drain only the capsule and trabeculae. The splenic capsule is thick, and thus inflammation from an adjacent peritonitis is unlikely to penetrate it. Direct penetration is extremely rare, but foreign bodies occasionally do penetrate as a sequela to traumatic reticulitis in cattle. Contraction of the reticulum extrudes a foreign body, usually a nail or wire through the wall of the reticulum. Most of these objects travel cranially to penetrate the diaphragm and pericardium, causing traumatic reticulopericarditis. Less frequently, some enter the visceral surface of the ventral extremity of the spleen, often causing a splenic abscess. In horses, on rare occasions, splenic abscesses can develop secondary to perforation of the gastric wall. Also, small pieces of wire have been found in these splenic abscesses, presumably originating from the GI tract. Equine splenic abscesses can be secondary to gastric ulcers (Gasterophilus intestinalis) or by extension on the granulomatous inflammation around Habronema spp. in the wall of the stomach.

Lymph nodes

Bacteria, viruses, and antigens enter the lymph node by two routes: afferent lymphatic vessels and via the blood vasculature (hematogenously).

Lymph nodules (MALT)

Lymph nodules do not have afferent lymphatic vessels and thus antigens arrive hematogenously, in migrating macrophages or across mucous membranes. M cells of Peyer’s patches can transport infections agents. Live-attenuated Salmonella spp. bacteria induce dendritic cells in intestinal submucosal tissues to migrate to lymph follicles (B lymphocytes) and parafollicular (T lymphocytes) areas of Peyer’s patches. Other pathogenic bacteria known or believed to enter via the M cells from the intestine include Listeria monocytogenes, Mycobacterium avium ssp. paratuberculosis, and Yersinia pseudotuberculosis. The scrapie agent also accumulates in Peyer’s patches.

Thymus

Because the thymus involutes after sexual maturity, evaluating whether it is smaller than normal is difficult unless the change is extreme or age-matched control animals are available. Before sexual maturity, the thymus is easily identified as a lobular organ, white to gray with a thin capsule. After sexual maturity, the gland is often grossly indistinguishable from adipose connective tissue within the cranial mediastinum, although microscopic remnants may remain. An extremely small thymus in a neonatal animal should be considered abnormal and an indicator of a possible underlying primary or acquired immunodeficiency. Enlargement of the thymus is almost always caused by tumors. The cut surface should be examined for tumors and hematomas.

Spleen

There is wide variation in location (ruminants versus the others), size, shape, and appearance of the cut surfaces between the spleens of domestic animals. At necropsy, the spleen is dissected free, and in the case of nonruminants, the gastrosplenic ligament is checked for torsion. The spleen is evaluated for size (splenomegaly, normal size, or atrophy) and the surface of its serosa and capsule for smoothness, nodularity, and ruptures.

Spleens vary widely in size within the same species and among the different species of domestic animals. Some pathologists wryly state “spleens come in two sizes: too large and too small.” The spleen can be enlarged (splenomegaly), normal in size, or small, and its surface can be smooth or nodular. The color of the capsular surface of the spleen also varies among species of domestic animals and depends on the opacity or translucence of the splenic capsule, which determines the visibility of the red pulp. The degree of opacity of the capsule is a function of its thickness and the amount of collagen. In the pig, dog, and cat, the splenic capsule is thin, and the surface of their spleens are red. The splenic capsules of horses and ruminants are thicker and usually appear gray because the color of the red pulp is not visible through the capsule. However, if the capsule has thick and thin areas, it is mottled gray and red. The tenseness of the capsule of normal and diseased spleens depends on how much the splenic parenchyma is distended, for example, by stored blood or infiltrating inflammatory cells or neoplasms. Storage spleens devoid of blood usually have a wrinkled surface. The appearance of the cut surface of the spleen in normal animals depends on the amount of stroma (e.g., trabeculae are prominent in ruminants); the size and visibility of the white pulp, which reflects the amount of lymphoid tissue; and whether the red pulp is congested from blood. The cut surfaces are examined for bulging (usually an indication of distention), visibility of the white pulp, and focal or diffuse lesions.

Enlarged spleens are classified into congested and noncongested. The cut surface of severely congested spleens may be red to bluish-black and exude blood and are often termed bloody spleens. Enlarged spleens that are not congested but firm are often called meaty spleens because of their firmness and texture. Little blood oozes from their cut surfaces, and the color of this surface depends on how much of the normal red pulp has been replaced by inflammatory cells, stored materials, or neoplastic cells (see the section on Splenomegaly and Table 13-5). The spleen may be measured and weighed but because of the wide variation in the dimensions and weight of normal spleens and the amount of blood stored, these data can be difficult to interpret.

In standard necropsy technique, the spleen is sliced transversely at 4 to 6 mm intervals, depending on whether it is small or large. The cut surfaces are checked for lesions and at this time specimens are taken for those tests that require fresh tissue (e.g., bacteriologic and virologic examinations). Cross-sections are placed in fixative (10% buffered neutral formalin). However, if the cut surface has bulged during fixation, the bulged tissue that is the best fixed is discarded at trimming. One solution is to trim off and discard the bulged area after 1 to 2 hours fixation in 10% BNF, trim the tissue to no more than 15 × 15 × 4 mm, and then replace it with the fixative.

Lymph Nodes

Lymph nodes should be dissected free of fat and connective tissue and any firm attachment to adjacent tissues noted because this attachment can be an indication of neoplastic infiltration through the capsule. Gross examination includes determining the size (measurement or weight), the shape, and whether the capsule is intact. The cut surface is examined for bulging, edema, hyperemia or congestion, exudate, and color (see the section on Pigmentation); the ratio of cortex to medulla; changes in or obscuring of the normal architecture; and masses such as abscesses, granulomas, and neoplasms.

Microscopic Examination

In live animals, microscopic examination is easily accomplished by cytologic evaluation of superficial lymph nodes. Aspirates provide excellent cellular detail, but for the study of architectural derangements, removal of a node for histologic evaluation is necessary.

The surgeon must handle lymph nodes carefully to minimize artifacts. Any compression (e.g., squeezing with forceps) causes artefacts in nuclei, usually pyknosis or “streaming” of nucleic acids. One end of the node should be removed and used to prepare impression smears. Depending on size, the remainder of the node or selected transverse slices are placed in fixative (10% BNF) for 1 hour before slicing. This period of fixation hardens the node and prevents artifactual bulging of the tissue from the cut surface. The lymph node, if large, should then be sliced transversely into parallel 3 to 5 mm thick sections to allow examination and ensure penetration of the fixative. To avoid compression artifacts and facilitate slicing, some pathologists prefer not to incise small lymph nodes before fixation. Lymph nodes that are either pathologically enlarged or normally large as those in large domestic animals need to be incised to allow fixative to penetrate rapidly.

Cross-sections of lymph node are preferred for histologic examination because they are usually sufficiently small to allow a full cross–section, including both the cortex and medulla to fit onto a microscope slide. The exception is the porcine lymph node with its different anatomic arrangement. Because the location and amount of cortex and medulla vary at different sites of the transverse cut, a longitudinal plane section is recommended as being the most representative.

Location of Affected Lymph Nodes

Enlarged lymph nodes can be distributed in several different patterns in the body. First, all lymph nodes throughout the body (systemic or generalized) may be enlarged (lymphadenomegaly). This pattern is generally attributed to systemic infectious, inflammatory, or neoplastic processes. If a regional lymph node (i.e., a lymph node draining a specific area such as one of the limbs or one side of the oral cavity) is enlarged, then the area drained by that node must be checked for infectious, inflammatory, or neoplastic processes. Finally, enlargement of the mesenteric lymph nodes or GALT suggests that the infectious, inflammatory, or neoplastic process originates within the gut, although they can be infected hematogenously. Normal mesenteric lymph nodes are continuously receiving and responding immunologically to barrages of antigens and bacteria received via their afferent lymphatic vessels from the intestinal tract. This role is reflected in their appearance. They are larger and have numerous well-developed lymphoid follicles (follicular lymphoid hyperplasia) with active germinal centers. Often the sinuses contain histiocytes (sinus histiocytosis), presumably in response to the need to phagocytose incoming material.

Multiple lymph nodes of a chain may be enlarged because infection or metastases spread via the lymphatic vessels, from one lymph node to the next in the chain. Thus it is important to know the area drained by specific lymph nodes; for example, all lymph from the head ultimately drains to the medial retropharyngeal lymph nodes, and lymph from the foreleg drains to either the axillary or superficial cervical (prescapular) lymph node. However, there are some surprises. Besides the thoracic cavity, the caudal sternal and caudal mediastinal lymph nodes drain the cranial abdomen and may be enlarged in the absence of thoracic lesions. Similarly, the celiac lymph nodes of dogs, pigs, and horses receive some afferent lymphatic vessels from thoracic organs and lesions in them do not necessarily mean that disease originated in the abdomen.

Congenital Disorders

Congenital disorders of the thymus are discussed in detail in Chapter 5. Summaries of the gross and microscopic morphologic changes are described in the sections on Disorders of Horses and Disorders of Dogs.

Thymic cysts can be found within the developing and mature thymus and in thymic remnants in the cranial mediastinum. These cysts are often lined by ciliated epithelium and represent developmental remnants of branchial arch epithelium and are usually of no significance.

Inflammatory and Degenerative Disorders

Thymitis, an infrequent lesion, is seen in porcine circovirus 2 (PMWS) infection (see Disorders of Pigs and also Chapter 4), enzootic bovine abortion (EBA; see Chapter 18), and salmon poisoning disease of dogs (see Chapter 7). Variable degrees of acquired immunodeficiency may be caused by infectious agents (viruses), toxins, chemotherapeutic agents and radiation, malnutrition, aging, and neoplasia.

Infections that most commonly injure lymphoid tissues do so because they infect lymphocytes and destroy the lymphoid component of the thymus. Viruses, which can damage the lymphoid component of the thymus and are often found in other lymphoid tissues, include canine distemper virus, EHV-1 in aborted foals (Fig. 13-47), canine and feline parvovirus, FIV, BVDV, and hog cholera virus. In kittens infected with FIV, thymic lymphoid depletion is an early lesion, and in kittens with progressive FIV infection, ultimately there is complete loss of thymic architecture with only minimal thymic tissue remaining in the connective and adipose tissue of the mediastinum.

Environmental toxins, such as halogenated aromatic hydrocarbons (e.g., polychlorinated biphenyls and dibenzodioxins, lead, and mercury) have a suppressive effect on the immune system. Fumonisins B₁ and B₂, which are secondary fungal metabolites produced by members of the genus Fusarium, cause lymphocytolysis in the thymic cortex. In weaned and grower pigs, aflatoxin causes thymic atrophy because of lymphocyte depletion. In the case of the halogenated aromatic hydrocarbons, there is a genetic basis for susceptibility that is mediated through a complex composed of the aromatic hydrocarbon receptor and the aromatic receptor nuclear transporter. In general, the end result is that these chemicals cause severe atrophy of the primary and secondary lymphoid organs. Other environmental toxins, such as the metals lead and mercury, cause toxic effects through the interaction of the metal ion with enzyme systems, cell membranes, or organelles. Similarly, chemotherapeutic agents in general specifically target enzyme systems and cellular components essential for cell replication. Thus they can cause inhibition of a cellular function rather than cause a morphologic change.

Anticancer drugs, radiomimetic anticancer drugs, or ionization radiation used in cancer therapy target dividing cells and destruction of lymphocytes can result in immunosuppression (Fig. 13-48). Lymphocytes are very sensitive to these agents. Most cytotoxic drugs used to treat cancer inhibit cell division by mechanisms that are centered on the function and activity of nucleic acids. Purine analogs (e.g., azathioprine) compete with purines in the synthesis of nucleic acids, whereas alkylating agents like cyclophosphamide cross-link DNA and inhibit the replication and activation of lymphocytes. Cyclosporin A specifically inhibits the T lymphocyte signaling pathway by interfering with the transcription of the IL-2 gene. Methotrexate, a folic acid antagonist, blocks the synthesis of thymidine and purine nucleotides. As chemotherapeutic drugs, they have the beneficial effect of targeting cancer cells and the detrimental effect of also targeting noncancerous immune cells, resulting in immunosuppression, the proverbial double-edged sword. Because of the desirable effect of these and other drugs, they are also used to prevent allograft rejection after transplantation. Corticosteroids are considered to be immunosuppressive, although the degree of suppression is highly variable among species, and in general, humans and rodents are corticosteroid sensitive, whereas domestic species are considered corticosteroid resistant.

Ionizing radiation targets components of the cell’s DNA and through nonlethal mechanisms such as the generation of highly reactive free radicals can damage and be lethal to lymphoid tissues and cells (see Fig. 13-48).

Malnutrition and cachexia impairs thymic function because they induce lymphoid deletion and resultant thymic atrophy, with resultant reduction in thymic hormone output. The result is a decrease in the number of circulating T lymphocytes in the blood, depletion of T lymphocytes from the secondary lymphoid organs, and impairment of T lymphocyte functions. Consequently, blood concentration of lymphocytes is decreased in starvation and immunosuppression. This effect of starvation is mediated by the hormone leptin, whose concentration in the blood is proportional to the mass of fat in the body.

Aging

As part of the general effects of aging in cells (see Chapter 1), all lymphoid organs decrease in size (atrophy) with advancing age and thus have reduced lymphocyte populations. In the case of the thymus, this reduction in size occurs normally after sexual maturity and is more appropriately termed thymic involution. The term involution should be reserved for normal physiologic processes in which an organ either returns to normal size after a period of enlargement (e.g., postpartum uterus) or regresses to a more primitive state (e.g., thymic involution) (see the section on Methods of Gross and Microscopic Examination of the thymus). Atrophy, although most commonly used to denote a pathologic decrease in the size of a cell, tissue, or organ, can also be used to indicate a decrease in size of an organ that depends on hormonal stimulation. Stated simply, involution should be reserved for nonpathologic physiologic processes, whereas atrophy may or may not imply a pathologic process.

Neoplasia

Because the thymus has both lymphoid and epithelial components, it is possible for either or both components to serve as cells of origin of neoplasm. Thymic lymphoma is a T lymphocyte neoplasm of young animals, particularly in cats and cattle and is less frequently seen in dogs (Fig. 13-49) (see the section on Hematopoietic Neoplasia). Clinical findings reflect the presence of a large mass in the cranial mediastinum. This mass can often be forced into the thoracic inlet with gentle compression of the cranial ribs. Thymomas are usually benign neoplasms that occupy the cranial mediastinum, usually of older animals. They are significantly less common than thymic lymphoma and are only distinguishable microscopically by the presence of neoplastic epithelial cells. Variable numbers of lymphocytes are present within thymomas but are not neoplastic. Nonneoplastic lymphocytes often outnumber the neoplastic epithelial cells. Thymomas have been associated with myasthenia gravis (which may be accompanied by megaesophagus) and polymyositis (immune-mediated) in dogs. A rare condition, thymic hyperplasia, which results from the formation of B lymphocyte follicles within the thymus, has also been reported in association with myasthenia gravis in dogs and cats.

Miscellaneous disorders

Congenital Disorders

Asplenia or the failure of a spleen to develop in utero occurs occasionally in animals. This condition is usually incidental, because congenital asplenia is so rare, it has not been possible to be certain of the effect on the animal’s immune status. Splenic aplasia is well known in certain strains of mice, but as these are usually maintained under either germ-free or specific pathogen–free (SPF) conditions the effect of asplenia cannot be evaluated. However, congenitally asplenic mice have a high mortality rate to experimental plasmodium infection. Splenectomy in the adult results in an increased susceptibility to pathogenic bacteria, particularly those with polysaccharide-rich capsular antigens such as pneumococci.

Congenital immunodeficiency diseases have been described in the discussions on the thymus in the sections on Disorders of Horses and Disorders of Dogs.

Splenomegaly

The procedure for gross examination of the spleen is described in the discussion in the section on Methods of Gross and Microscopic Examinations; this examination involves deciding whether the spleen was enlarged (splenomegaly), normal, or small. Enlarged spleens are classified as congested and noncongested. The cut surface of severely congested spleens may be red to bluish-black and exude blood; these spleens are often termed bloody spleens. Enlarged spleens that are not congested but firm are often called meaty spleens because of their firmness and texture. Little blood oozes from their cut surfaces, and the color of this surface depends on how much of the normal red pulp has been replaced by inflammatory cells, stored materials, or neoplastic cells (see the section on Splenomegaly and Table 13-5). The spleen may be measured and weighed but because of the wide variation in the dimensions and weight of normal spleens and the amount of blood stored, these data can be difficult to interpret. The diseases and disorders associated with splenomegaly are discussed using the following categories:

Table 13-5 lists the common causes of uniform splenomegaly.

Small Spleens (Splenic Hypoplasia and Atrophy)

The most common diseases or conditions associated with small spleens are (1) developmental anomalies, (2) aging changes, (3) wasting and/or cachectic diseases, (4) splenic contraction, (5) radiation of the spleen, and (6) splenic trauma.

Miscellaneous Disorders of the Spleen

Lymph Nodes

At necropsy the pathologist has to evaluate the size of lymph nodes, thus disorders are discussed on that basis.

Pigmentation of Lymph Nodes

Red: This color may be caused by erythrocytes draining blood from a hemorrhagic or acutely inflamed area, or from hyperemia of the lymph node in acute lymphadenitis. Because of the peculiar anatomy of the porcine lymph node, with large sinuses under the capsule, blood in these nodes is very obvious. It is frequently seen in acute septicemias, in which endotoxin-induced vasculitis or DIC have caused hemorrhages. Histologically, initially there are erythrocytes in the sinuses, and these rapidly undergo erythrophagocytosis by sinus histiocytes, which rapidly increase in number (sinus histiocytosis). If the animal lives, hemosiderin deposits are present within 7 to 10 days in the perisinusoidal macrophages, which will give the lymph node a grossly brown color. Uniformly reddened lymph nodes can also been seen in dependent areas in postmortem hypostatic congestion.

Black: Carbon pigment from pulmonary anthracosis (see Chapter 9) drains to the tracheobronchial lymph nodes and from skin tattoos drains to the regional lymph node. The tattoo ink used in black animals is often green. These pigments are usually visible in the medulla, the location of most of the sinus macrophages.

Brown:

Green: Rarely, lymph nodes have a greenish tinge. Causes are green tattoo pigment, usually used in black animals, ingestion of green-blue algae, massive accumulation of eosinophils, for example, in metastatic bovine mast cell tumor and in mutant Corriedale sheep with a genetic defect for the excretion of bilirubin by hepatocytes. As chlorophyll breakdown products, such as phylloerythrin, share the same excretory pathway as bilirubin, if this is defective, they are not excreted into the biliary canaliculi and are circulated in the blood. It is thought that the green color, which can be intense in these sheep, is caused by one of these products.

Miscellaneous colors: Animals injected intravenously with a dye (e.g., methylene blue or trypan blue) may have generalized discoloration of lymph nodes, or if the injection is subcutaneous (e.g., drug), then the discoloration may be in the regional lymph node. Icterus lymph nodes are stained similarly to other tissues. Mesenteric lymph nodes of sheep draining intestine affected with Johne’s disease caused by the pigmented strain of Mycobacterium avium spp. paratuberculosis may be orange.

Miscellaneous Lymph Node Disorders

Lymph Nodules (MALT)

MALT is involved in a variety of ways with bacteria and viruses, and these are summarized in Table 13-4. These interactions include being a portal of entry for pathogens, such as Salmonella spp., Rhodococcus equi, and BVDV; a site of replication for viruses (e.g., BVDV); occasionally a site infected hematogenously (panleukopenia virus and parvovirus); and a site of gross or microscopic lesions in some diseases caused by viruses. Bovine coronavirus, BVDV, Rinderpest virus, malignant catarrhal fever (MCF) virus, feline panleukopenia virus, and canine parvovirus all cause lymphocyte depletion. Bacteria such as Salmonella spp., Rhodococcus equi, Yersinia pestis, Mycobacterium bovis, M. avium ssp. paratuberculosis (goats), and Listeria monocytogenes infect Peyer’s patches and cause disease.

Severe combined immunodeficiency (SCID)

SCID in Arabian foals is a hereditary disease in which affected foals lack T and B lymphocytes and therefore is characterized by notable lymphoid aplasia or hypoplasia of primary and secondary lymphoid tissues (Fig. 13-81). Reflecting the immunodeficiency, necropsy findings are often severe bronchopneumonia and a small thymus, spleen, and lymph nodes. The thymus may be difficult to identify or may consist of a few isolated lobules within the mediastinal fat and microscopically, it usually consists of a few islands of lymphocyte-like cells and Hassall’s corpuscles. Grossly, spleens are exceptionally small, firm, and pale red. The spleen is smaller than normal because of a marked reduction in the white pulp owing to absence of germinal centers (see Fig. 13-81) and periarteriolar lymphoid sheaths. Microscopically, they lack lymphoid follicles and PALS, and there are few to no plasma cells. Grossly, peripheral lymph nodes and internal lymph nodes may be small and difficult to identify because of the absence of lymphocytes. Congenital immunodeficiency diseases are also discussed in detail in Chapter 5.

Strangles

Streptococcus equi ssp. equi, the etiologic agent of equine strangles, causes lesions typical of an acute bacterial infection. The classic lesion of strangles is a nasopharyngitis with lymphadenitis of the regional lymph nodes, usually the mandibular and retropharyngeal (see Fig. 13-74). This lymphadenitis can progress to the formation of abscesses and ultimately the abscesses may rupture and discharge pus through a sinus to the skin surface. If the organism becomes bacteremic, it commonly causes abscesses in liver, kidney, synovia, mesenteric and mediastinal lymph nodes, and occasionally in the spleen. Bastard strangles is the term given to the form of the disease characterized by Streptococcus equi ssp. equi abscesses anywhere in the body other than the pharyngeal area (see the discussion on acute infectious diseases in the section on Nodular Spleens with Firm Consistency).

Rhodococcus equi infection

The intestinal and pulmonary lesions have been described in Chapter 7 and 9, respectively. Rhodococcus equi enters the intestinal wall at Peyer’s patches, causing a granulomatous inflammation and abscess formation and necrosis, which results in ulceration. The organisms drain to the regional lymph node (mesenteric, colonic, and cecal), causing a granulomatous lymphadenitis with granulomas and abscesses. Intestinal mucosal lesions are multiple and over sites of GALT. The diffuse lymphatic tissue in the lamina propria has increased numbers of macrophages and giant cells, many of which contain phagocytosed Gram-positive bacteria.

Equine lymphoma

Equine lymphoma is divided into three anatomic types on the basis of the location of the majority of the lesions. These types are alimentary, which affects the rostral (upper) small intestine and its regional lymph nodes: abdominal with lesions in widespread lymph node involvement, large intestine and its regional lymph nodes, spleen, liver, peritoneum, and segmentally in the small intestine; multicentric, which can be a mediastinal mass that may extend into the heart and lungs and peripheral lymph nodes and abdominal cavity; and cutaneously (see Chapter 17).

Johne’s disease

As the lesions of Johne’s disease have been described in detail in Chapter 7, only those affecting the lymphatic system are mentioned here. Bovine Johne’s disease is characterized by a chronic diffuse noncaseating (in contrast to sheep and goats) granulomatous enteritis affecting the submucosa and mucosa of the ileum, cecum, and colon. Secondary to this, there is a similar type of granulomatous lymphadenitis in the regional lymph node (see Web Fig. 13-7). Unlike bovine Johne’s disease, which has a diffuse noncaseating (lepromatous) granulomatous inflammation in the intestine and lymph node, sheep and goats have caseous granulomas that may mineralize. Also, granulomas are scattered in other lymph nodes; in organs, such as the liver, lung, and spleen; and throughout the body.

Anthrax

Anthrax is caused by Bacillus anthracis and is primarily a disease of ruminants, especially cattle and sheep. It is discussed in the section on Uniform Splenomegaly of Bloody Consistency in this chapter and also in Chapter 4.

Bovine Virus Diarrhea

Lymphocytolysis and necrosis of the germinal centers of the mesenteric lymph nodes are found in the follicles of the GALT. Lesions are grossly striking in Peyer’s patches where a fibrinonecrotic pseudomembrane covers the damaged GALT and outlines Peyer’s patches (see Chapters 4 and 7).

Bovine lymphoma

The two forms of bovine lymphoma are a BLV-associated type and a sporadic type that is not associated with BLV (see the section on Bone Marrow), and the distribution of the lesions varies with the two types. In the BLV-associated type, lymphomas are located in the superficial, pelvic, and abdominal lymph nodes (see Fig. 13-80) and in the abdominal wall, extradurally in the vertebral canal, kidney, heart (right atrium), retrobulbar space, uterus, liver, and spleen (splenomegaly). The non-BLV lymphoma, designated sporadic form, is subdivided into thymic, cutaneous, multicentric, and calf form. The calf form affects calves at birth and up to 6 months of age and is rapidly progressive, with grossly visible infiltrations of neoplastic lymphocytes into all lymph nodes, liver, spleen, kidneys, and bone marrow. Bovine thymic lymphoma most often occurs in beef cattle 6 to 24 months of age and is characterized by massive thymic enlargement. The cause is unknown, and the occurrence of a concurrent leukemia is unusual. Microscopically, lymphomas are dominated by lymphocytes, which are homogeneous in size, shape, nuclear morphology, and nuclear-cytoplasmic ratio. The multicentric type has widespread symmetric lymph node enlargement and lesions in the liver spleen, kidneys (diffuse), and sometimes in skeletal muscle (see the section on Bone Marrow).

Splenic abscesses

Splenic abscesses can be the result of direct penetration by a foreign body from the reticulum (see the sections on Spleen and Portals of Entry).

Caseous lymphadenitis

Caseous lymphadenitis, a chronic suppurative lymphadenitis, is a disease of sheep and goats caused by Corynebacterium pseudotuberculosis. In sheep, the bacterium enters the skin through wounds, such as shearing cuts, and then drains to the regional lymph node. This node is usually either the superficial cervical (prescapular) or the subiliac (prefemoral) lymph node because the cuts are frequently on the legs, and these are a portal of entry for C. pseudotuberculosis. A suppurative lymphadenitis develops. Initially, there are multiple microabscesses with numerous eosinophils in the sinuses. These microabscesses coalesce and caseate and become encapsulated by fibrous tissue. However, they continue to enlarge, a process that results in the characteristic concentric laminations, which can often be seen on a cross-section of an old abscess. On gross examination, the pus in the abscess is initially greenish (because of the eosinophils) and semifluid (see Fig. 13-75), but it becomes caseous with age (see Fig. 13-76), loses its green color, and becomes inspissated. Old abscesses can reach a diameter of 4 to 5 cm. Similar abscesses may be found in the lungs, especially in older sheep. The abscesses in goats are usually more numerous and frequently involve lymph nodes of the head and neck.

Postweaning multisystem wasting syndrome

PMWS, or porcine circovirus 2, is also discussed in Chapters 4 and 9. The major findings at necropsy are poor body condition, generalized lymphadenopathy, and interstitial pneumonia. In the lymphatic system, the disease is characterized by B and T lymphocyte depletion and a granulomatous response. The lymphocyte depletion affects all lymphatic tissues—spleen, lymph node, MALT (tonsil and Peyer’s patches), and thymus. Microscopically, in the lymph node the characteristic lesions are granulomatous lymphadenitis and lymphocyte depletion. The increase in macrophages is chiefly in the follicles but also to a lesser degree diffusely throughout the deep cortex. The microscopic lesions in the follicles include depletion of lymphocytes, germinal center necrosis, and proliferation of macrophages with the formation of syncytial cells. Many of the macrophages contain inclusion bodies—intracytoplasmic, basophilic, round, and often multiple. Similar lesions are seen in the Peyer’s patches. In the spleen, the T lymphocyte depletion affects the PALS and in the thymus there is a diffuse infiltration of macrophages.

Porcine Reproductive Respiratory Syndrome

Porcine reproductive respiratory syndrome (PRRS) is caused by a virus transmitted by contact with body fluids—saliva, mucus, serum, urine, and mammary secretions and from contact with semen during coitus. The virus infects macrophages at the site of contact. Infected macrophages migrate to lymphoid tissues (nasal or tonsillar in oral infections), infect and replicate in similar cells in these tissues, and then disseminate in macrophages throughout the body. The result is a reduction in the phagocytic and functional capacity of macrophages of the monocyte-macrophage system and as a consequence, reduction in resistance to common porcine bacterial and viral pathogens. The major lesions are interstitial pneumonia, generalized lymphadenopathy, and lymphocytic infiltrates into a wide variety of organs. Gross lesions are most likely in the tracheobronchial and mediastinal lymph nodes, which may be enlarged, pale, and firm. Microscopically, the lesions in the lymph nodes, tonsils, and spleens consist of follicular and deep cortical (T lymphocyte) hyperplasia, apoptosis of cells in the follicular germinal centers, and sometimes multinucleated macrophages.

Porcine jowl abscess

In jowl abscess, Streptococcus porcinus colonizes the oral cavity, which results in infection of the tonsils and the regional lymph nodes. The mandibular lymph nodes are the most often affected, but the retropharyngeal and parotid lymph nodes may be involved (Fig. 13-82). Multiple abscesses 1 to 10 cm in diameter may be present in the lymph nodes.

Lymphoma

Lymphoma is a frequent tumor in pigs. There are two forms: multicentric in lymph nodes (more common in visceral lymph nodes) and in spleen, liver, kidney, and bone marrow and the mediastinal form, which tends to affect pigs less than one year of age.

X-linked severe combined immunodeficiency

X-linked SCID (XSCID) has been reported in basset hounds, Jack Russell terrier, and Welsh corgi breeds of dogs. The thymus of these dogs is small and often obscured by mediastinal fat. Tonsils, lymph nodes, and Peyer’s patches usually cannot be identified at necropsy. Microscopically, the thymic tissue consists of small dysplastic lobules with a variable number of Hassall’s corpuscles. Congenital immunodeficiency diseases are also discussed in detail in Chapter 5.

Thymic hematomas

Thymic hemorrhage and hematomas have been reported in dogs, especially young dogs. Many of these dogs die unexpectedly from hypovolemic shock as a result of massive thymic and mediastinal hemorrhage. A variety of causes have been implicated. These include rupture of dissecting aortic aneurysms, trauma from automobile accidents or excessive pulling on a collar, and ingestion of anticoagulant rodenticides (warfarin, dicumarol, diphacinone, and brodifacoum). In the last type, hemorrhage causes expansion of thymic lobules and interlobular septa and appears to originate in the medulla.

Gastrosplenic Volvulus

See the section on Uniform Splenomegaly with Bloody Consistency.

Splenic myeloid metaplasia

Splenic myeloid metaplasia, with histiocytosis and hypersplenism, in the dog is characterized by severe, diffuse, and persistent splenomegaly, and the splenic capsule may be smooth or have multiple and confluent nodules. There is often random vascular thrombosis, producing grossly visible splenic infarcts. Microscopically, extramedullary hematopoiesis is present with interspersed foci of prominent macrophages (PAMS). The process effaces the splenic red pulp and is responsible for the splenic enlargement. This disease may be rather benign, not involving other organs, in which case splenectomy is generally curative. However, EMH may be systemic, involving bone marrow and liver, and this is indicative of a poor prognosis.

Splenic hematomas, hemangiomas and hemangiosarcomas, and Acute splenic infarcts

For more discussion on splenic hematomas induced by lymphoid hyperplastic nodules, splenic hematomas induced by splenic vascular neoplasms, splenic hematomas induced by lymphoid hyperplastic nodules, hemangiomas and hemangiosarcomas, and acute splenic infarcts, see the section on Nodular Spleens with Bloody Consistency.

Splenic nodular hyperplasia

Splenic nodular hyperplasia is most commonly seen in the spleen of older dogs and is often an incidental finding. This lesion has also been called canine nodular splenic hyperplasia and splenoma. Nodules are formed by hyperplastic lymphoid cells or mixed accumulations of hyperplastic erythroid, myeloid, and megakaryocytic cells (EMH) with lymphoid cells. The nodules can be a single discrete nodule or multiple coalescing firm nodules protruding from the surface but covered by the splenic capsule. The capsular surfaces and cut surfaces often have a mottled red-white pattern because of the intermingling of erythrocytes and leukocytes. Hyperplastic nodules are usually hemispheric and up to 2 cm or larger in diameter (see Fig. 13-63) and on cross-section have intermixed areas of red and white (see Web Fig. 13-5). Nodules have no deleterious effect unless they interfere with the drainage of blood from the adjacent marginal sinus and cause a large hematoma, which can rupture and cause hemoperitoneum. Rupture is usually caused by trauma or even a misjudged jump from a couch. These masses must be distinguished from other types of nodules in the spleen, including those of hematoma, and fibrohistiocytic nodules, hemangioma, hemangiosarcoma, and primary or metastatic neoplasms.

Fibrohistiocytic nodules

Fibrohistiocytic nodules are splenic masses in dogs that are typically solitary and spheric. They are composed of a mixed population of histiocytoid spindle cells, hematopoietic cells, plasma cells, and lymphocytes. These nodules likely represent a continuum between lymphoid nodular hyperplasia and malignant fibrous histiocytoma. A higher lymphoid-to-fibrohistiocytic cell ratio is associated with better long-term survival. Increased mitotic figures and fibrohistiocytic proportion are associated with metastasis and mortality, likely representing transformation into malignant fibrous histiocytoma.

Malignant fibrous histiocytomas

Malignant fibrous histiocytomas in canine spleens are considered a continuum of proliferations of fibrous and histiocytic cells (fibrohistiocytic nodule) normally found in the splenic reticular meshwork of the red pulp (see Nodular Spleens with Firm Consistency.

Accessory spleens

Accessory spleens can be acquired after rupture of the spleen from traumatic injury in dogs (see Fig. 13-70). Fragments of spleen may be implanted onto peritoneal surfaces or embedded in the pancreas. Here, they become vascularized and functional. Implanted fragments grossly and histologically resemble normal splenic tissue. They have red and white pulp areas and a thick fibromuscular capsule. These features are important in differentiating accessory spleens from peritoneal implanted fragments (metastases) of hemangiosarcoma, which have a thin shiny serosal covering and a poor prognosis. It is thought that hyperplastic splenic nodules are more friable than normal spleen and thus are more likely to rupture and be a source of accessory spleens.

Siderocalcific plaques and Splenic Rupture

See the discussion on miscellaneous disorders in the section on the Spleen.

Histoplasmosis

Histoplasmosis caused by Histoplasma capsulatum is a diffuse disease of the monocyte-macrophage system and causes a marked proliferation of macrophages in a wide variety of tissues including spleen, lymph nodes, liver, lungs, and intestine. The dimorphic fungus Histoplasma capsulatum grows as a mold in soil and as a yeast in animal tissue. The fungus is distributed throughout the world, in major river valleys, and in temperate and tropical climates; Histoplasma capsulatum grows especially well in soil enriched by bird feces. The greatest incidence of disease is in dogs; the incidence is lower in cats.

In most animals, the organism is inhaled and results in mild self-limiting infections in the lungs, but causes enlargement of tracheobronchial lymph nodes. Dogs and cats are usually asymptomatic. Because the fungus is confined to monocytes and macrophages, its spread beyond the respiratory tract is assumed to occur by hematogenous and lymphogenous dissemination of infected cells. Disseminated histoplasmosis in dogs and cats results in GI or hepatic disease of long duration.

Disseminated histoplasmosis is characterized by neutrophilia and monocytosis in some animals. Nonregenerative anemia is common because of the chronic inflammation. Nonspecific changes as a result of the damage to the liver are elevated serum alkaline phosphatase activity and hyperbilirubinemia. The total serum protein may be low, normal, or increased, depending on factors such as extent and duration of the diarrhea and emaciation.

Cytology is useful for the diagnosis of histoplasmosis. The least invasive procedures include examination of cells of body fluids, tracheal wash preparations, and aspirates of bone marrow and lymph nodes. The organisms are often visible in macrophages (Web Fig. 13-9).

Dogs dying of this disease are emaciated. The large bowel is thickened with mucosal corrugations caused by infiltration of the submucosa and lamina propria by macrophages, lymphocytes, and plasma cells. Lymph nodes are uniformly enlarged (Fig. 13-83), and normal architecture may be obscured (Fig. 13-84). In contrast to lymphoma, the nodes are firm when incised.

Histologically, in the lymph nodes, coalescing granulomas replace the normal cortical lymphoid tissue. Typical yeast organisms, which are 2- to 4-mm-wide hematoxylinophilic dots surrounded by a clear halo, are present in variable numbers in epithelioid macrophages (see Fig. 13-84). The spleen and liver are enlarged and firm, and the liver is diffusely gray. Affected organs can be imprinted on glass slides for cytologic evaluation.

Leishmaniasis

Leishmaniasis is a disease of the monocyte-macrophage system caused by protozoa of the genus Leishmania. It occurs in dogs and other animals and is endemic, in parts of the US, Europe, Mediterranean, Middle East, Africa, and Central and South America. It may be seen in any part of the world in dogs that have resided in endemic areas. The protozoa proliferate by binary fission in the gut of the sand fly and become flagellated organisms, which are introduced into mammals by insect bites; they then assume a nonflagellated form in macrophages.

The two types of lesions are cutaneous and/or visceral. The cutaneous lesions in dogs are ulcers at the site of insect bites. These ulcers are directly attributable to proliferation of the organism within macrophages; inflammation with the accumulation of neutrophils, lymphocytes, and plasma cells; and focal disruption of the dermis and epidermis. In the visceral form of the disease, dogs are emaciated and have general enlargement of abdominal lymph nodes. Dogs have a nonregenerative anemia and a polyclonal hypergammaglobulinemia with total protein concentrations that may exceed 100 g/L (10 g/dL).

At necropsy, dogs with visceral leishmaniasis are emaciated and have an enlarged liver, spleen, and lymph nodes (see Web Fig. 3-15). Histologically, the lymph node sinuses are filled with macrophages (sinuses histiocytosis), which are filled with intracytoplasmic organisms (see Web Fig. 3-15). Lymph node aspirates contain macrophages, in which there are organisms. Imprints made of these enlarged organs have macrophages containing numerous round organisms approximately 2 µm in diameter. They have a vesicular nucleus and a small kinetoplast, which aids in distinguishing them from Histoplasma capsulatum (Web Fig. 13-10). The bone marrow is usually hyperplastic. Late in the disease, there is usually atrophy of the lymph nodes and spleen.

Canine distemper

Canine distemper virus preferentially infects lymphoid, epithelial, and nervous cells (see Chapter 14). It spreads by aerosols of secretions, and the virus is phagocytosed and transported by macrophages to the adjacent lymphoid tissue—tonsil or tracheobronchial lymph nodes and disseminated to the spleen, bone marrow, and distant lymph nodes, where it causes lymphoid necrosis. The cortices of lymph nodes of dogs infected with canine distemper are depleted of lymphocytes 6 to 9 days after exposure. Accompanying this is a profound lymphopenia a result of the depression of lymphopoiesis in the bone marrow.

Canine parvovirus

Parvovirus is cytolytic to dividing cells and attacks these in the bone marrow, liver, kidney, heart (myocytes) vessel, intestinal epithelium, and lung in neonatal dogs. The most striking lesions are in the dividing cells in the crypts of the small intestine. Hemorrhage from these drains to the regional lymph nodes. Because of a lack of supply of lymphocytes from the bone marrow, there is lymphoid depletion, followed by atrophy in the thymus and depletion of lymphocytes in the B and T lymphocyte areas in lymph nodes and MALT. Also in lymph nodes and MALT, follicular lymphocytes are lost from lymphocytolysis.

Lymphomas

Lymphomas are common tumors in dogs, and there are five anatomic types: multicentric with generalized lymph node enlargement and often with splenic and hepatic involvement; alimentary (Fig. 13-85); thymic (see Fig. 13-49); cutaneous; and epidural (Fig. 13-86). The histologic appearance of the cell types is discussed in the section on Bone Marrow.

Thymomas

Thymomas are usually benign neoplasms that occupy the cranial mediastinum, usually of older animals. They are significantly less common than thymic lymphoma and are only distinguishable microscopically by the presence of neoplastic epithelial cells. Variable numbers of lymphocytes are present within thymomas but are not neoplastic, and these nonneoplastic lymphocytes often outnumber the neoplastic epithelial cells. Thymomas have been associated with myasthenia gravis (which may be accompanied by megaesophagus) and polymyositis (immune-mediated) in dogs. A rare condition, thymic hyperplasia, which results from the formation of B lymphocyte follicles within the thymus, has also been reported in association with myasthenia gravis in dogs and cats.

Feline panleukopenia (parvovirus)

Parvovirus causes lymphocytolysis of proliferating cells, including those in the bone marrow. As the thymus solely depends on bone marrow for the supply of lymphocytes, the result is thymic atrophy and collapse of the cortex. Similarly, in all secondary lymphatic organs such as spleen, lymph node and MALT (Peyer’s patches), B lymphocyte (follicles) and T lymphocyte (inner cortex) areas become hypocellular. Also, follicular lymphocytes are destroyed by lymphocytolysis and microscopically only the pale staining stromal and stem cells remain.

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Tests to Evaluate Platelet Function Or Immune-Mediated Thrombocytopenia

Tests for Evaluating the Coagulation System