11 Parasitic infections
Pathogenesis and immunity
Key points
• Parasites cause disease by diverse mechanisms including mechanical damage, physiological disturbance, tissue destruction and immunopathology.
• Some parasitic worm infestations are associated with raised levels of IgE and eosinophilia. These responses may provide some protection.
• Some parasites may evade immune responses by seclusion away from the immune system, by antigenic variation, by acquiring host-derived molecules or by immunosuppression.
• Despite major efforts, particularly those directed towards malaria, there are no available effective vaccines against parasitic infections.
By convention the term ‘parasitic diseases’ refers to those caused by protozoa, worms (helminths) and arthropods (insects and arachnids). Such parasites affect many hundreds of millions of people in tropical parts of the world and are responsible for many severe and debilitating diseases (see Chs 62–64).
These infections are associated with a broad spectrum of effects. Some are due to the parasites themselves and others are a consequence of the host response to the invader. The nature and extent of the pathological effects are dependent upon the site and mode of infection, and also on the level of the parasite burden.
The development of protective immunity to such parasites is more complicated than that to bacteria and viruses because of the complicated life cycles of the parasites involved.
As with other infectious agents, the site occupied by a parasite is important. A host will survive with a large number of lung flukes (Paragonimus spp.) in the lungs but infection in the brain may cause far more serious effects. The severity of disease depends not only on the degree of infection but also on the physiological state of the host. A lowering of general health, due to malnutrition for example, predisposes to more serious consequences following infection by parasites.
Physical obstruction of anatomical sites leading to loss of function can be a major component of the diseases caused by parasites. The intestinal lumen can be blocked by worms such as Ascaris lumbricoides or tapeworms, and filarial parasites (Wuchereria bancrofti and Brugia malayi) can obstruct the flow of lymph through lymphatics.
Intestinal infection with the tapeworm Taenia solium is usually of little consequence, but the eggs may develop into larvae (cysticerci) in humans, causing cysticercosis. The cysticerci can be found in muscle, liver, eye or, most dangerously, the brain. Hydatid cysts, the larval stage of the dog tapeworm (Echinococcus granulosus) in man, may reach volumes of 1–2 L, and such masses can cause severe damage to an infected organ.
Large numbers of Giardia spp. covering the walls of the small intestine can lead to malabsorption, especially of fats. Competition by parasites for essential nutrients leads to host deprivation. Thus depletion of vitamin B12 by the tapeworm Diphyllobothrium latum sometimes leads to pernicious anaemia. Other forms of anaemia result from blood loss, especially in hookworm infection, and from red blood cell destruction in malaria.
Some parasites produce metabolites that may have profound effects on the host. Trypanosoma cruzi secretes a neurotoxin that affects the autonomic nervous system. Malaria parasites are thought to produce a metabolite with vasoconstrictor activity.
The presence of parasites can result in the release of proteolytic enzymes that damage host tissues. Ulceration of the intestinal wall occurs in amoebic dysentery, and trophozoites (the active, motile forms of a protozoan parasite) can penetrate deep into the wall of the intestine to reach the blood and hence the liver, lungs and brain where secondary amoebic abscesses may occur. The skin damage caused by skin-penetrating helminths, such as Strongyloides stercoralis and hookworms, can also permit entry of other infectious agents.
The host reaction to parasites and their products can evoke immunological reactions that may lead to secondary damage to host tissues. This is seen in schistosomiasis, where the host response to parasite eggs in tissues leads to the formation of a granuloma with subsequent tissue destruction through fibrosis. Other types of hypersensitivity reaction can be generated in various parasitic infections (see below).
The large size of parasites means that they display more antigens than bacteria or viruses to the immune system. When the parasite has a complicated life cycle, some of these antigens may be specific to a particular stage of development. Parasites have evolved to be closely adapted to the host, and most parasitic infections are chronic and show a degree of host specificity. For example, the malaria parasites of man, birds and rodents are confined to their own particular species. An exception to this is Trichinella spiralis, which is able to infect many animal species.
In the natural host there is no single defence mechanism that acts in isolation against a particular parasite. In turn the parasite will have evolved a number of strategies to evade elimination. In general terms, cell-mediated immune mechanisms are more effective against intracellular protozoa, whereas antibody, with the aid of certain effector cells, is involved in the destruction of extracellular targets. Again, because of the life cycles of some parasites, either cell-mediated or humoral immunity may be of greater importance at different states of their development.
Several of the innate or natural defence mechanisms that are active against bacteria and viruses are also effective against parasitic infections. The physical barrier of the skin protects against many parasites but is ineffective against those transmitted by a blood-feeding insect. In addition, other parasites, such as schistosomes, have evolved active mechanisms for penetrating intact skin. Certain individuals are genetically less susceptible to certain parasites. Thus, individuals with the sickle cell trait have a genetic defect in their haemoglobin that causes a mechanical distortion in their erythrocytes. This somehow leads to the destruction of intracellular malaria parasites. The Duffy blood group antigen is the attachment site for one of the plasmodium parasites. Individuals who lack this determinant are therefore protected from malaria caused by Plasmodium vivax.
Several other non-specific host defence mechanisms are involved in the control of parasitic infections. These include direct cellular responses by monocytes, macrophages and granulocytes, and by natural killer cells. The byproducts of acquired immune reactions enhance the antiparasitic activity of these cells. For example, certain protozoan parasites infect macrophages. In particular, Leishmania spp. are obligate parasites of mononuclear phagocytes, and are completely dependent upon macrophages, where they survive in the phagolysosome. The parasite appears to be able to survive within non-stimulated resident macrophages, whereas they are destroyed in activated macrophages.
Complement, through activation by the alternative pathway, is active against a number of parasites, including adult worms and active larvae of T. spiralis and schistosomula of Schistosoma mansoni. The spleen is thought to be active in the elimination of intracellular parasites, as its filtering of infected erythrocytes is thought to remove intracellular plasmodium.
Macrophages play an important role in the elimination and control of parasitic protozoa and worms. They secrete monokines, such as interleukin (IL)-1, tumour necrosis factor and colony-stimulating factors that affect not only T cells and antibody production but also granulocytes. However, other monokines, for instance prostaglandins, are immunosuppressive. Macrophages are also phagocytic cells and function as such in the elimination of parasites. In the same way as in the eradication of other infectious agents, opsonins greatly enhance this process. Once internalized, the parasite is killed, by oxygen-dependent and -independent mechanisms, and digested. Many of the molecules produced by macrophages are cytotoxic, and when produced in close proximity to a parasite will kill it. Specific antibody, immunoglobulin (Ig) G and IgE, can mediate the attachment of the macrophage to the surface of parasites that are too large to internalize but are vulnerable to antibody-dependent cell-mediated cytotoxicity. Acting as antigen-presenting cells, they can aid elimination by helping in the initiation of an immune response.
In addition to lymphokines some products of parasites themselves, such as those produced by Tryp. brucei and the malaria parasite, can cause macrophage activation. These may be direct effects or result from the production of monokines such as tumour necrosis factor.
In some parasitic infections the immune system is unable to eradicate the offending organism. The body reacts by trying to isolate the parasite within a granuloma. In this situation there is chronic stimulation of those T cells specific for antigens on the parasite. The continual release of lymphokines leads to macrophage accumulation, release of fibrogenic factors, stimulation of granuloma formation and, ultimately, fibrosis. Granuloma formation around schistosome eggs can occur in the liver, and this response is thought to benefit the host by isolating host cells from the toxic substances produced by the eggs. It can, however, lead to pathological consequences if the damage to the liver leads to loss of liver function.
Neutrophils and eosinophils are thought to play a role in the elimination of protozoa and worms. The smaller parasites can be phagocytosed and destroyed by both oxygen-dependent and -independent processes. The phagocytic capacity of neutrophils is superior to that of eosinophils. Both cell types possess receptors for the Fc portion of immunoglobulin and for various complement components, so the presence of opsonins increases phagocytosis. Extracellular destruction of large parasites can occur by antibody-dependent cell-mediated cytotoxicity.
Neutrophils are attracted to sites of inflammation and will clear the offending parasite. They have been reported to be more effective than eosinophils at eliminating several species of nematode, including T. spiralis, although the relative importance of the two cell types may depend on the class of antibody present.
Eosinophilia and high levels of IgE are characteristics of many parasitic worm infections. It has been suggested that eosinophils are especially active against helminths, and IgE-dependent degranulation of mast cells has evolved to attract these cells to the site where the parasite is localized. The eosinophilia is T cell dependent and the lymphokines that induce production of the cells also cause an increase in their activation state. These effector cells are attracted to the site by chemotactic factors produced by mast cells (see below). Once at the site they degranulate in response to perturbation of their cell membrane induced by antibodies and complement bound to the surface of the parasite. The toxic molecules are therefore released on to the surface of the target and cause its destruction.
The mediators stored and produced by mast cells play an important role in eliminating worm infections. Parasite antigens cause the release of mediators from mast cells; these molecules induce a local inflammatory response. Chemotactic factors are produced and attract eosinophils and neutrophils. Thus the IgE-dependent release of mast cell products helps in expulsion of the worm. The number of mucosal mast cells rises during a parasitic worm infestation due to a T cell-dependent process.
Platelet activation results in the release of molecules that are toxic to various parasites, including schistosoma, Toxoplasma gondii and Tryp. cruzi. The release process does not require antibody, although IgE-dependent cytotoxicity is possible, but seems to involve acute-phase proteins. The cytotoxic potential of platelets is enhanced by various cytokines, including γ-interferon and tumour necrosis factor.
An individual with a parasitic infection mounts a specific response against the invading parasite. These immune reactions generate antibody and effector T cells directed against specific parasite antigens. Memory B and T cells are also produced. For a number of reasons, described below, much acquired immunity is ineffective in protecting the host against recurrent infection. However, in certain cases, such as amoebiasis and toxoplasmosis, immunity to reinfection is fairly complete. In schistosomiasis, the presence of surviving adult forms protects against further infections. However, this may be an effect of the parasite and not of the host.
The specific immune response to parasites leads to the production of antibody. Infection by protozoan parasites is associated with the production of IgG and IgM. With helminths there is, in addition, the synthesis of substantial amounts of IgE. IgA is produced in response to intestinal protozoa, such as Entamoeba histolytica and Giardia lamblia.
In addition to these specific T-dependent responses, a non-specific hypergammaglobulinaemia is present in many parasitic infections. Much of this non-specific antibody is the result of polyclonal B cell activation by released parasite antigens acting as mitogens. This response is ineffective at counteracting the parasite and can enhance the pathogenicity by causing the production of autoantibodies, and may actually lead to a diminished specific response due to B cell exhaustion. It has also been reported that some parasite molecules are T cell mitogens. This could lead to the generation of autoreactive T cells or activation of suppressor responses.
There are a number of mechanisms by which specific antibody can provide protection against and control parasitic infections (Table 11.1). As with viral infections, antibody is effective only against extracellular parasites and where parasite antigens are displayed on the surface of infected cells. Antibody can neutralize parasites by combining with various surface molecules, blocking or interfering with their function. The binding of antibody to an attachment site stops the infection of a new host cell. The agglutination of blood parasites by IgM may occur, leading to the prevention of spread, as in the acute phase of infection with Tryp. cruzi. Toxins and enzymes produced by certain parasites add to their pathogenicity, and antibodies that inhibit these molecules protect the host from damage and also affect the infection process directly. Intracellular parasites have evolved a number of mechanisms that allow them to survive in this environment. Antibodies against the molecules that aid the parasite in these activities, for instance to escape from endosomes or inhibit lysosomal fusion, lead to removal of the intruder by the phagocyte. Parasitic worms are multicellular organisms with defined anatomical features that are responsible for functions such as feeding and reproduction. Antibodies that block particular orifices (e.g. oral and genital) interfere with critical physiological functions and may cause starvation or curtail reproduction.
Table 11.1 Humoral defence mechanisms against parasite infections
| Mechanism | Effect | Parasite |
|---|---|---|
| Neutralization | Blocks attachment to host cell | Protozoa |
| Acts to inhibit evasion mechanisms of intracellular organisms | Protozoa | |
| Binding to toxins or enzymes | Protozoa and worms | |
| Physical interference | Obstructs orifices of parasite | Worms |
| Agglutination | Protozoa | |
| Opsonization | Increases clearance by phagocytes | Protozoa |
| Cytotoxicity | Complement-mediated lysis | Protozoa and worms |
| Antibody-dependent cell-mediated cytotoxicity | Protozoa and worms |
Antibodies can bind to the surface of parasites and cause direct damage, or interact with complement leading to cell lysis. Antibody also acts as an opsonin and hence increases uptake by phagocytic cells. In this context, complement activation leads to enhanced ingestion due to complement receptors. Macrophage activation leads to the expression of increased Fc and complement receptors, so phagocytosis is enhanced in the presence of macrophage-activating factors. Phagocytes play an important role in the control of infections by Plasmodium spp. and Tryp. brucei.
Antibody-dependent cell-mediated cytotoxicity has been shown to play a part in infections caused by a number of parasites, including Tryp. cruzi, T. spiralis, S. mansoni and filarial worms. The effector cells – macrophages, monocytes, neutrophils, eosinophils and natural killer cells – bind to the antibody-coated parasites by their Fc and complement receptors. Close apposition of the effector cell and the target is necessary because the toxic molecules produced are nonspecific and may damage host cells. A major basic protein from eosinophils damages the tegument of schistosomes and other worms, causing their death. It appears that different cell types and immunoglobulin isotypes are active against different developmental stages of parasites. Eosinophils are more effective at killing newborn larvae of T. spiralis than other cells, whereas macrophages are very effective against microfilariae.
The importance of T cells in counteracting protozoan infections has been shown using nude (athymic) or T cell-depleted mice, which have a reduced capacity to control trypanosomal and malarial infections. The transfer of spleen cells, especially T cells, from immune animals gives protection against most parasitic infections. The type of T cell that is effective depends on the parasite. CD4+ T cells transfer protection against Leishmania major and L. tropica, and may be necessary for the elimination of other parasites. The intracellular parasite of cattle, Theileria parvum, is destroyed by cytotoxic T (TC) cells.
CD4+ T cells may act by providing help in antibody production, but they also secrete various lymphokines that interact with other effector cells. CD8+ cells may be cytotoxic in certain situations, but these cells also produce a variety of lymphokines. IL-2 production has been shown to be deficient during parasitic infections, such as malaria and trypanosomiasis. Administration of IL-2 to mice infected with Tryp. cruzi reduces parasitaemia and increases survival.
Colony-stimulating factors (e.g. IL-3 and granulocyte–monocyte colony-stimulating factor) are also produced by activated T cells. These molecules act on myeloid progenitors in the bone marrow, causing increased production of neutrophils, eosinophils and monocytes. They also increase the activity of these cells; the monocytosis and splenomegaly in malaria are caused by these T cell-derived molecules. The accumulation of macrophages in the liver as granulomata in schistosomiasis and the eosinophilia that is characteristic of worm infestations are also T cell-dependent phenomena.
In certain cases the production of lymphokines may have adverse effects. Leishmania infects macrophages, and the release of molecules that stimulate the production of more host cells may potentiate the infection.
γ-Interferon does not inhibit or kill parasites directly, although multiplication of the liver stages of the malaria parasite is inhibited by γ-interferon, possibly through interaction with its receptor on the surface of hepatocytes. γ-Interferon is a potent macrophage activation factor and is probably involved in the resistance and elimination of intracellular parasites, such as Toxoplasma gondii and Leishmania spp. Activated macrophages are more effective killers and can destroy intracellular parasites before they establish themselves within the cell.
All animal pathogens, including parasitic protozoa and worms, have evolved effective mechanisms to avoid elimination by host defence systems (Table 11.2).
Table 11.2 Parasite escape mechanisms
| Escape mechansim | Organisms |
|---|---|
| Intracellular habitat | Malaria parasites, trypanosomes and Leishmania spp. |
| Encystment | Toxoplasma gondii and Trypanosoma cruzi |
| Resistance to microbicidal products of phagocytes | Leishmania donovani |
| Masking of antigens | Schistosomes |
| Variation of antigen | Trypanosomes and malaria parasites |
| Suppression of immune response | Most parasites (e.g. malaria parasites, Trichinella spirolis and Schistosoma monsoni) |
| Interference by antigens | Trypanosomes |
| Polyclonal activation | Trypanosomes |
| Sharing of antigens between parasite and host (molecular mimicry) | Schistosomes |
| Continuous turnover and release of surface antigens of parasite | Schistosomes |
Many parasites inhabit cells or anatomical sites that are inaccessible to host defence mechanisms. Those that attempt to survive within cells avoid the effects of antibody but must possess mechanisms to avoid destruction if the cell involved is capable of destroying them. Plasmodium spp. inhabit erythrocytes, whereas toxoplasmas are less selective and infect non-phagocytic cells as well as phagocytes. A number of different ways of avoiding destruction in macrophages have evolved. Leishmania donovani amastigotes are able to survive and metabolize in the acidic environment (pH 4–5) found in phagolysosomes, and Tox. gondii is able to inhibit the fusion of lysosomes with the parasite-containing phagosome.
L. major has a similar escape mechanism by attaching to a phagocyte complement receptor (CR1) that does not trigger the respiratory burst. Activation of the complement system by protozoan parasites seems to be a common mechanism for achieving attachment to target cells. Tryp. cruzi trypomastigotes can infect T cells of both the CD4 and CD8 subsets, and may be similar to retroviruses in using receptor molecules on the T cell surface for penetration.
The effectiveness of macrophages in the elimination of Tryp. cruzi depends upon the stage of development of the parasite. Trypomastigotes are able to escape from the phagocytic vacuole and survive in the cytoplasm, whereas epimastigotes do not escape and are killed. Macrophages are also the preferred habitat of Leishmania spp., which multiply in the phagolysosome where they are resistant to digestion.
In an immune host these evasion mechanisms are less effective because of the presence of antibody and lymphokines. The ability to resist complement destruction also appears to be important. For example, L. tropica is easily killed by complement and causes only a localized self-healing lesion in the skin, whereas a disseminating, often fatal, disease is seen with L. donovani, which is ten times more resistant to complement killing. Large parasites such as helminths cannot infect individual cells; however, they can still achieve anatomical seclusion. T. spiralis larvae avoid the immune system by encysting in muscle; intestinal nematodes live in the lumen of the intestine.
Parasites may avoid recognition by:
African trypanosomes have the capacity to express more than 100 different surface glycoproteins. By producing novel antigens throughout their lives, these parasites continuously evade the immune system. By the time the host has mounted a response against each new antigen, the parasite has changed again. Plasmodia pass through several discrete developmental stages, each with its own particular antigens. A similar situation is seen in certain helminths such as T. spiralis. As a result, each new stage of the lifecycle is seen by the host as a ‘new’ infective challenge.
A number of parasites are known to adsorb host-derived molecules on to their surface. This is thought to mask their own antigens and enable them to evade immunological attack.
Parasitic protozoa and worms also use devices to avoid immune destruction. Certain parasites retain a surface coat, or glycocalyx, that blocks direct exposure of its surface antigens.
Parasites are not always able to evade detection and many have evolved mechanisms to suppress or divert immune reactions. Some parasites produce or generate molecules that act against cells of the immune system. Thus, the larvae of T. spiralis produce a molecule that is cytotoxic to lymphocytes, and schistosomes can cleave a peptide from IgG, thereby decreasing its effectiveness. During many parasitic infections a large amount of antigenic material is released into the body fluids, and this may inhibit the response to or divert the response away from the parasite. High antigen concentrations can lead to tolerance by clonal exhaustion or clonal deletion. The immune complexes formed can also inhibit antibody production by negative feedback via Fc receptors on plasma cells. Many of these released molecules are polyclonal activators of T and B cells. This leads to the production of non-specific antibody, impairment of B cell function and immunosuppression. It has also been proposed that many parasites can cause unresponsiveness by activating immune suppressor mechanisms.
In many cases the immunosuppression has been attributed to macrophage dysfunction associated with antigen overload or the presence of intracellular parasites. In addition to the non-specific immunosuppression there can be parasite-specific effects. Mice infected by Leishmania spp. show antigen-specific depression of lymphokine production. As this genus inhabits macrophages and is partly controlled by activation of these cells by lymphokines, the effect is a diminished response against the pathogen.
Schistosomes have a receptor for part of the antibody molecule. They also release several proteases that cleave antibody molecules and release products that prevent macrophage activation. A schistosome-derived inhibitory factor suppresses T cell activity and is believed to allow other parasites to survive the effects of T cells and may explain the inefficiency of cytotoxic T cells in damaging the parasite.
When tested in a lymphocyte proliferation assay, peripheral blood lymphocytes of patients infected with Plasmodium falciparum are unresponsive to antigen prepared from the parasite, and in nearly 40% of the patients this persists for more than 4 weeks. Patients infected with P. falciparum show a suppression of lymphocyte reactivity that is not related to the degree of parasitaemia or severity of the clinical illness. The depressed lymphocyte reactivity is associated with a loss of both CD4+ and CD8+ lymphocytes from the peripheral blood. Once the parasite has been cleared, the response returns to normal. An even more sophisticated strategy has been evolved by Leishmania mexicana and L. donovani, which use IL-2 to stimulate their own growth. Mammalian epidermal growth factor has also been shown to stimulate the growth of certain trypanosomes in vitro.
The immune response to parasites is aimed at eliminating the organisms, but many of the host reactions have pathological effects.
The IgE produced in parasitic worm infections can have severe effects on the host if it stimulates excessive mast cell degranulation (type I hypersensitivity). Anaphylactic shock can occur if a cyst ruptures and releases vast amounts of antigenic material into the circulation of a sensitized individual. Asthma-like symptoms occur in Toxocara canis infections when larvae of worms migrate through the lungs.
The polyclonal B cell activation seen with many parasitic infections can give rise to autoantibodies. In trypanosomiasis and malaria antibodies against red blood cells, lymphocytes and deoxyribonucleic acid (DNA) have been detected. Host antigens incorporated into the parasite, as an immune evasion mechanism may stimulate autoantibody production by giving rise to T cell help and overcoming tolerance. In Chagas’ disease about 20% of individuals develop progressive cardiomyopathy and neuropathy of the digestive tract that is believed to be autoimmune in nature. These effects are thought to result from cross-reactivity between antibody or T cells responsive to Tryp. cruzi and nerve ganglia.
Immune complex-mediated disease occurs in malaria, trypanosomiasis, schistosomiasis and onchocerciasis. The deposition of immune complexes in the kidney is responsible for the nephrotic syndrome of quartan malaria.
Enlargement of the spleen and liver in malaria, trypanosomiasis and visceral leishmaniasis is associated with increased numbers of macrophages and lymphocytes in these organs. The liver, renal and cardiopulmonary pathology of schistosomiasis is related to cell-mediated responses to the worm eggs. Symptoms similar to those seen in endotoxaemia induced by Gram-negative bacteria are found in the acute stages of malaria.
The non-specific immunosuppression discussed above may explain why individuals with parasite infections are especially susceptible to bacterial and viral infections.
No effective vaccine for humans has so far been developed against parasitic protozoa and worms, mainly because of the complex parasite life cycles and their sophisticated adaptive responses. As protection depends in many cases on both antibody and cell-mediated reactions, a vaccine must induce long-lived B and T cell immunity. In addition, because recognition by T cells is genetically restricted, the vaccine preparation must stimulate T cells from most haplotypes, preferably without suppressor epitopes. Because of the immunopathology seen in many parasite infections, antigens that induce a potentially damaging response must be avoided.
A much better understanding of the biological mechanisms underlying the natural history of parasitic diseases is required before it will be possible to control these globally important diseases.