Parasitology 1 Host-Parasite Interactions PDF

11/5/2022 Lebanese University Faculty of Agricultural & Veterinary Sciences PARASITOLOGY 1 DR WALID DARWICHE Ho...

11/5/2022 Lebanese University Faculty of Agricultural & Veterinary Sciences PARASITOLOGY 1 DR WALID DARWICHE Host-Parasite Relationships 1 HOST–PARASITE INTERACTIONS DR. WALID DARWICHE 2 2 Dr. Walid Darwiche 1  11/5/2022 INTRODUCTION  The effects of the host on the parasite are inseparable from the effects of the parasite on the host.  The host tries to get rid of its parasite.  The goal of this cohabitation is that the interaction remains stable, and the system remains in equilibrium.  In the opposite case, there is the eradication of the parasite, or the death of the host.  There is then a dynamic and lasting interaction which depends on the following points: ❖ the action of the parasite on the host ❖ host defenses ❖ the parasite's defenses against host reactions  These three points lead to the co-evolution of the host and the parasite. DR. WALID DARWICHE 3 3  The effects of the parasite on the host depend on many factors which may be: related to the host as the age, the sex, the diet, the genetic factors, the immunity... or related to the parasite such as the size (especially when it migrates into the brain, for example), the number, the virulence (term used especially in bacteriology, different between bacteria and protozoa), the mobility, the migration (cycle in the tissues), the food (blood-sucking or not), the target organs (brain, mesentery).... It is all these factors that determine the action of the parasite on the host, they are all independent of each other. DR. WALID DARWICHE 4 4 Dr. Walid Darwiche 2  11/5/2022  The harmful effects of parasitism (malnutrition, vitamin deficiency, nervous disorders, etc.) result from direct or indirect effects (secretion of toxins for example).  2 infected individuals will not necessarily have the same reactions.  Sometimes the host's defense against the parasite is overdone and can make the host sick.  This leads us to observe the importance of a long co-evolution.  Indeed, we see often a mutual host-parasite adaptation.  This co-evolution leads to a decrease in the pathogenicity of the parasite. DR. WALID DARWICHE 5 5 Example:  The amoeba Entamoeba invadens is a parasite of herbivorous turtles but is not pathogenic, it feeds among other things on starch which it needs to encyst in it.  Indeed, it can remain free and multiply or become encysted.  On the other hand, it is pathogenic in carnivorous snakes and causes a mortality of 100% because there is no encystment by default of starch.  The parasite then continues to multiply at the expense of the intestinal mucosa.  The longer the host and the parasite have lived together, the less pathogenic the parasite is to the host. DR. WALID DARWICHE 6 6 Dr. Walid Darwiche 3  11/5/2022 I. PARASITE SPECIFICITY IN RELATION TO HOST SPECIES 7 A. INTRODUCTION  Parasite specificity or Host specificity is the natural adaptability of a species or groups of parasite to certain species or groups of host and is dependent upon the compatibility of a host to a parasite.  In natural host parasite relationships, the parasite must be precisely adapted to the structural and physiological conditions that characterize the host species.  This adaptation, which develops over long periods of evolutionary change, is the basis for the phenomenon of host specificity or parasite specificity. DR. WALID DARWICHE 8 8 Dr. Walid Darwiche 4  11/5/2022  Parasite specificity is usually defined in terms or establishment or failure to establish in a host.  However, a range of parameters, such as establishment, number, size, developmental stage of worms, duration, level of egg production and duration of infection can give indication of the degree of adaptation to a particular host.  For example, the eggs of the human ascarid, Ascaris lumbricoides can hatch in a variety of mammalian hosts but can develop into adults in humans alone.  However, in some cases, the restriction is absolute or total as occurring in Eimeria species or the restriction may be very loose and parasite can undergo development in and be transmitted between, a wide variety of hosts as occurring in the nematode Trichinella spiralis. DR. WALID DARWICHE 9 9  Parasite specificity may be supra specific where, groups of parasites are associated with groups of hosts or infra specific where a specific parasite is associated with a specific species of host.  Parasite specificity determines the host range of a parasite and accordingly, the parasite may have a narrow or wider host range.  Certain parasites such as Haematopinus suis infest only pigs (narrow host range), while some such as Trypanosoma evansi, have a wider host range and infect many hosts. DR. WALID DARWICHE 10 10 Dr. Walid Darwiche 5  11/5/2022  Specificity may also vary between larval and adult stages of the same parasite to hosts.  In some, the larval stages have strong host specificity to intermediate hosts (snails) and less for definitive hosts as in trematodes while in some such as Toxoplasma gondii, the asexual stages have a wider host range and infect a wide variety of intermediate hosts while the sexual stages infect only one host.  In some parasites, as in Eimeria, the host specificity is less at the generic level where they infect a wide variety of hosts but very strong at the individual species level with very high niche specificity in hosts. DR. WALID DARWICHE 11 11 B. FACTORS INFLUENCING PARASITE SPECIFICITY Parasite specificity is also governed by anatomical, physiological and nutritional, beside ecological factors. The anatomy of the host animal may prevent the establishment of a parasite. The intestinal villi or crypts may be too short or intestinal movements too rapid to prevent the establishment of a parasite. Physiological factors such as composition of the bile, dissolved carbon dioxide in hosts, pO2, redox potential and the normal temperature of a host may also regulate host specificity. In unnatural hosts, the unsuitable body temperature may fail to provide the right stimulus for the parasite to establish in hosts. DR. WALID DARWICHE 12 12 Dr. Walid Darwiche 6  11/5/2022 C. PARASITE SPECIFICITY IN RELATION TO HOST SPECIES Host animals are susceptible to some parasites and resistant to others. For example, majority of the helminths of cattle are incapable of infecting sheep and goats. The normal resistance of various species of animals to various pathogens is due to the presence of antibodies on their erythrocytes called isohaemagglutinins. Many parasites do not develop in hosts other than their natural hosts. A good example for host specificity is Eimeria sp. The red worms (Strongylus sp) of horses are specific to equine hosts and cannot infect cattle, buffalo, sheep and goat. DR. WALID DARWICHE 13 13  The nematode Ancylostoma caninum is a parasite of both dogs and cats.  However, the strain adapted to dog survives better and produces more eggs in dog, as compared to the strain found in cat.  Similarly, the cat adapted strain develops better and produces more eggs in the cat as compared to dogs.  Limited degree of development of parasite occurs in unnatural host in some cases as in the case of larvae of Ostertagia ostertagi (cattle parasite) in sheep where only few reach to adult stage or the dog parasite,  Toxocara canis which undergoes limited development in children causing the condition visceral larval migrans. DR. WALID DARWICHE 14 14 Dr. Walid Darwiche 7  11/5/2022 D. PARASITE SPECIFICITY IN RELATION TO SEX Some parasites affect only the females and not the males as in the case of the trematode, Prosthogonimus species which is mostly found in the oviduct of female gallinaceous birds. The influence of sex on helminth burden appears to be largely hormonal. In animals whose estrus cycle is seasonal, parasites tend to synchronize their development with that of the host. For example, ewes show a spring rise in fecal egg counts after lambing and onset of lactation. DR. WALID DARWICHE 15 15 Similarly, in the case of bitches above six months old, development of Toxocara canis is influenced by hormonal changes. When bitches become pregnant, the dormant larvae are activated and transported to the fetus and so puppies are born with Toxocara canis infection. However, in the case of male dogs older than 6 months, dormant larvae are not activated but become calcified. Therefore, older male dogs serve as dead end hosts for Toxocara canis. DR. WALID DARWICHE 16 16 Dr. Walid Darwiche 8  11/5/2022 E. PARASITE SPECIFICITY IN RELATION TO BREEDS Different parasites have varied susceptibility to different breeds. Indigenous breeds of cattle suffer less from tick infestation and tick- borne diseases in comparison to cross breeds and exotic breeds. N’Dama & Mutura breeds of cattle in Africa and their crosses are tolerant to trypanosomes. DR. WALID DARWICHE 17 17 F. PARASITE SPECIFICITY IN RELATION TO LOCATION IN HOSTS Each parasite has a specific predilection site or a location in hosts. The best example for site specificity is Eimeria species which parasitize only certain areas of the intestine. Other examples include the nematode Dictyocaulus species which affect only the lungs in ruminants and the trematode, Paragonimus species in lungs of dogs. DR. WALID DARWICHE 18 18 Dr. Walid Darwiche 9  11/5/2022 G. PARASITES IN RELATION TO LOCALITY When parasites are limited to certain ecological or geographical areas as in the case of African animal trypanosomes and human trypanosomes that are restricted to Western, Central and Eastern Africa, it is referred to as ecological or geographical restriction. DR. WALID DARWICHE 19 19 II. EFFECTS OF PARASITES ON HOSTS DR. WALID DARWICHE 20 20 Dr. Walid Darwiche 10  11/5/2022 A. UTILIZATION OF HOST'S NONNUTRITIONAL MATERIALS  In some cases, parasites also feed on host substances other than stored or recently acquired nutrients.  The endo- and ectoparasites that feed on the host's blood are examples.  It is extremely difficult to estimate the amount of blood any organism can take from its host.  The table below lists some estimated amounts taken in by a few blood-feeding species.  From the data presented, it should be obvious that the blood lost through parasitic infections can constitute an appreciable amount over a period of time. DR. WALID DARWICHE 21 21 DR. WALID DARWICHE 22 22 Dr. Walid Darwiche 11  11/5/2022 B. MECHANICAL ACTION  The obstruction can be caused by parasites themselves. This concerns tubes (blood vessel, urinary tract, etc.). For example, Ascaris or Taenia can form balls and block the digestive tract. The arrival of food in the intestine is then compromised. DR. WALID DARWICHE 23 23  The tissue reaction to the presence of parasites (inflammation). The body produces large nodules which will block the duct and thus itself creates a pathogenesis responsible for its disease. We can cite the case of esophageal nodules formed during spirocercosis or that of the formation of thrombi in the presence of a large quantity of Babesias or Piroplasms (the red blood cells agglutinate to form a thrombus). Endoscopic appearance of the esophagus in a dog with spirocercosis. DR. WALID DARWICHE 24 24 Dr. Walid Darwiche 12  11/5/2022  The existence of compressions. Encystment of parasites can cause compression of nearby organs. One can quote the Echinococcus whose hydatid cyst can lodge in the lungs, the liver, or the brain (it can grow up to the size of a soccer ball in the liver ! ). More dramatically, it compresses the brain in the skull. Heavily infected sheep liver with hydatid disease DR. WALID DARWICHE 25 25  Trauma occurs when a parasite digs in order to move (migration) or attaches itself to tissue in the body. It can then make the intestines communicate with the abdominal cavity, pierce blood vessels… For example, the hookworm attaches itself to the blood capillaries of the intestinal mucosa and voraciously sucks blood, it can then cause hemorrhages. Fluke larvae pass through the liver parenchyma to migrate into the bile. Hookworm DR. WALID DARWICHE 26 26 Dr. Walid Darwiche 13  11/5/2022  The presence of movement irritation. The several meters long Taenia will irritate the intestinal mucosa its undulating movements. Likewise, a liver fluke has spikes on its wall and thus irritates the bile ducts. DR. WALID DARWICHE 27 27 C. TOXIC ACTION  The parasite produces substances which, once released, can prove to be toxic to the host.  This action is found locally (saliva of biting arthropods which emit toxins preventing blood clotting) and systemic level (tropical ticks produce a powerful anticoagulant toxin, sometimes as strong as cobra venom, causing a reaction similar to that of cobra.  Example : tick toxicosis w fi kamen tick paralysis, w bas nshil l tick byrja3 l pet b sir mnih DR. WALID DARWICHE 28 28 Dr. Walid Darwiche 14  11/5/2022 D. ANTIGENIC ACTION This consists of an immunological reaction that may be harmful for the host (risk of runaway immune system). We distinguish two types of parasitic actions related to the presence of antigens (Ag):  The somatic Ag (or of constitution): everything that is constituent of the body of the parasite is an antigen. In general, the host organism only has access to surface Ag. But if the parasite is killed then all of the somatic Ags are released, and not just the surface Ag, in sensitive areas. For example, if a fly larva is eliminated during its migration through the spinal cord of cattle, then the presence of all its Ags can cause the death of the host. Similarly, during a dirofilariasis in dog, the worm involved is in the heart of the animal , which poses the same problems. Surface Ag do not trigger an effective immunity, but they have a real diagnostic interest because they trigger an immune response in general.  Metabolic Ags: they are secreted when the parasite is alive , at a precise moment, and they are essential for its functioning. They are of great importance for diagnosis and vaccination. DR. WALID DARWICHE 29 29 E. ETHOLOGICAL ACTION  The presence of a parasite has been clearly shown to influence behavior from the host.  These behavioral changes promote the transmission of the parasite. Example 1: Dicrocoelium lanceolatum (small liver fluke, in sheep).  The cycle of this parasite goes through a larval stage in the ant (intermediate host).  The parasitized ants climb to the top of the blades of grass and grip to them and no longer move.  This will promote transmission of the parasite to sheep, which has a better chance of eat the ant at the same time as the blade of grass. DR. WALID DARWICHE 30 30 Dr. Walid Darwiche 15  11/5/2022  Example 2: Toxoplasma gondii (in congenitally infected mice). "The basic toxoplasma cycle is Tom and Jerry". Mice with this parasite (cysts in the brain) that are put in test boxes "in open field" spend a lot more time in the middle of the open space that normal mice that tend to be closer to the edges. Parasitized mice are therefore more likely to be eaten by the cat (definitive host) than their healthy counterparts. The toxoplasma, once ingested, uncysts and multiplies, then it forms eggs which will be released in the feces of the cat. Thus, it can be ingested by a new mouse.  These small behavioral differences will allow the parasites to improve their transmission. DR. WALID DARWICHE 31 31 F. ROLE OF VECTORS  Parasites are preferred vectors for the transport and inoculation of certain pathogens.  This is the case with ticks, mosquitoes (malaria). DR. WALID DARWICHE 32 32 Dr. Walid Darwiche 16  11/5/2022 G. PREDISPOSING INFECTIONS  Parasites are sometimes at the origin of lesions which will promote infections, or even cause them.  Indeed, parasites also carry bacteria on their surface.  Example: the nematodes that live in the grass do not wash their feet before entering the body, so they bring bacteria with them. DR. WALID DARWICHE 33 33 III. THE HOST'S REACTIONS TO THE PARASITE DR. WALID DARWICHE 34 34 Dr. Walid Darwiche 17  11/5/2022  The goal of the host is to exclude the parasite from its organism.  It therefore sets up various systems aimed at achieving this result.  Hosts have evolved many behavioral and other strategies to reduce the risk of succumbing to parasitism.  The most powerful form of defense, however, is the immune system. This comprises a series of chemical and cellular weaponry used to combat invasive organisms.  Immune reactions may completely or partially disable the attacker or they may alleviate the clinical consequences of infection. DR. WALID DARWICHE 35 35  Ideally, immunity should protect against reinfection after the invading parasites have been eliminated.  This is called ‘sterile immunity’. It can last for a lifetime but often wanes with time.  Sometimes, however, such protection persists only as long as a few parasites survive to continually boost the immune processes. This is known as ‘premunity’.  In some cases, parasite evasion has gained an evolutionary advantage that renders host immunity relatively ineffective, so the host remains vulnerable despite being repeatedly exposed to infection (e.g. sheep with liver fluke).  Some immune reactions directed at a parasite can produce collateral damage to host tissues. Hypersensitivity and allergy are well known examples. DR. WALID DARWICHE 36 36 Dr. Walid Darwiche 18  11/5/2022 A. BEHAVIORAL ACTIONS  The host will modulate his behavior in order to reduce the parasitism of which he is target.  Evolution selects individuals who defend themselves against parasites. Example 1: Antelopes and Ticks Some ticks attach themselves in large numbers to the flanks and legs of antelopes for their blood meal (around a hundred at the end of the day). The researchers made the hypothesis that the existence of interdental spaces allow antelope to "comb" with their teeth, and so reduce the number of ticks attached. The cleaning behavior had already been observed. DR. WALID DARWICHE 37 37 Example 2: The choice of the sexual partner in birds. (Hamilton and Zuk hypothesis) We leave the choice to a female in front of 3 males. Male 1 has no parasite and has nice plumage. Male 2 has no parasite but has stunted plumage (unfavorable genetic factors). Male 3 has parasites and therefore damaged plumage. The female will choose male 1, this choice is therefore made according to the plumage and the presence or absence of parasite. It thus selects an individual who in terms of evolution has the best chance of giving his descendants genes for resistance to the parasite, which in a way corresponds to a standard to be reached. Example 3: Herbivores Herbivores, for example, will not eat the lush grass close to a fecal deposit where the greatest concentration of infective worm larvae occurs (the ‘zone of repugnance’). DR. WALID DARWICHE 38 38 Dr. Walid Darwiche 19  11/5/2022 B. IMMUNE REACTION DR. WALID DARWICHE 39 39 1. INNATE IMMUNITY  The innate (or nonspecific) immune response is the body’s first line of defense.  It functions similarly whatever the nature of the invader and whether or not the host has experienced similar attack before.  It comprises a series of natural physical, chemical and cellular barriers that are either permanent features (such as the integrity of skin and mucosae or the acidity of the stomach) or that can be quickly mobilized.  The latter include a variety of cell-types with different modes of attack as well as humoral factors such as complement.  A spectrum of communication molecules (cytokines and chemokines) released by white blood cells (leukocytes) enables the innate immune system to interact with the acquired immune system. DR. WALID DARWICHE 40 40 Dr. Walid Darwiche 20  11/5/2022  Humoral: a word used to describe aspects of immunity mediated by macromolecules in the blood or other body fluids (as opposed to cell-mediated immunity).  Complement: a biochemical cascade of small plasma and membrane-bound protein molecules that assist in the destruction of some invading organisms. One such cascade is a nonspecific innate response (the ‘alternative pathway’) while another is antibody-dependent (the ‘classical pathway’).  Cytokines: signaling molecules that cells use to communicate with each other. The term includes the interleukins (with names such as IL-2 and IFN-γ) that serve to modulate immune responses.  Chemokines: a specific class of cytokines that attract cells towards each other (chemotaxis), e.g. immune cells to the site of infection. "btaarfuwun" DR. WALID DARWICHE 41 41 2. ACQUIRED IMMUNITY  Acquired (also called ‘adaptive’ or ‘specific’) immune responses come into action more slowly than innate reactions as they are tailor-made to combat the particular nature of each new challenge.  A quicker response occurs when an animal is subsequently re-exposed to the same pathogen as the system is already primed for that specific reaction.  Acquired immunity starts with the detection of foreign molecules (antigens) and the processing of these by antigen-presenting cells.  This process generates two forms of adaptive response which are strongly linked to each other: a cellular response characterized by T-lymphocyte participation, and humoral immune reactions mediated by B-lymphocytes and antibody-producing plasma cells. DR. WALID DARWICHE 42 42 Dr. Walid Darwiche 21  11/5/2022 3. IMMUNITY TO ARTHROPODS  Most parasitic arthropods are ectoparasites.  The degree of contact they have with body tissues and the time they spend on the host vary greatly – from a mosquito’s fleeting visit to mites that burrow into the superficial epidermis. botfly  A few, like warble fly larvae, are true endoparasites, penetrating much more deeply into the body.  Thus, opportunity for host detection of arthropod antigens varies accordingly, influencing both the nature and effectiveness of the subsequent immune responses.  In cases where contact is intimate and prolonged, as with some mange mites, a cell- mediated and partially protective immunity often develops.  But where the antigens presented to the host are confined to those in the saliva injected during transient feeding behavior (e.g. biting insects), immune responses may be limited to a local hypersensitivity. DR. WALID DARWICHE 43 43  Such reactions do little to discourage further flies from biting and can become very itchy (pruritic).  This may be of benefit if it encourages animals to move away from infested land or to adopt a more effective grooming behavior (e.g. in flea or louse infestations), but pruritus can also provoke excessive scratching, rubbing and biting.  Ixodid ticks are rather different as, although they are temporary parasites, they remain attached to their host for several days while taking a blood meal.  This provides greater opportunity for immune attack and, over time, parasitized hosts can develop a partially effective species-specific immunity.  This acts by interrupting blood-sucking processes, thereby reducing the well-being and reproductive capability of the tick. DR. WALID DARWICHE 44 44 Dr. Walid Darwiche 22  11/5/2022 4. IMMUNITY TO PROTOZOA  Parasitic protozoa that establish in extracellular positions within the body are exposed to humoral immune responses and are thereby susceptible to destruction by membrane disruption or ingestion by phagocytes.  Those that have adopted an intracellular lifestyle will be shielded from such attack (except when moving between host cells) and cellular immune mechanisms are then more likely to be effective. DR. WALID DARWICHE 45 45 5. IMMUNITY TO HELMINTHS  In contrast to protozoa, helminths are multicellular, relatively large and have a less intimate relationship with host tissues.  Generally, they are extracellular and do not multiply within the host. Consequently, it is more difficult for the host to respond effectively.  This is especially true for the many helminths that live in the lumen of the gastrointestinal tract as they are not in direct contact with any body tissue.  Immune attack has to be multifaceted and is often aimed at securing the parasite’s death by long-term slow destruction rather than rapid killing. DR. WALID DARWICHE 46 46 Dr. Walid Darwiche 23  11/5/2022  Expulsion of nematodes from the gastrointestinal tract is a complex two-stage process. Firstly, the mucosal lining has to become permeable to macromolecules so that specific antibodies (e.g. IgA) can ‘leak’ into the lumen at the site of parasitism. During this process goblet cell hyperplasia results in excess mucus formation. This helps to dislodge some helminths while others exploit it as their primary food-source, which illustrates the complexity and fascination of host–parasite relationships. ❖ Goblet cells derive their name from their goblet, cup-like appearance. ❖ The primary function of goblet cells is to secrete mucin and create a protective mucus layer. ❖ Other possible functions of goblet cells include immunoregulation. DR. WALID DARWICHE 47 47  Many gastrointestinal helminths migrate through body tissues en route to their predilection site and may consequently elicit different sets of immune responses during their parasitic lifecycle.  They are likely to have reached the gut before acquired immunity to the tissue-stage becomes functional, but the activation of these adaptive responses will help protect the host against future invasion by the same species.  Thus, there is an important difference between immunity that protects against reinfection and immunity that eliminates or ameliorates an existing infection. DR. WALID DARWICHE 48 48 Dr. Walid Darwiche 24  11/5/2022 IV. THE PARASITE'S DEFENSES AGAINST HOST IMMUNE REACTIONS DR. WALID DARWICHE 49 49 The survival of parasitic species is dependent on being able to escape the immune responses of its host. Such evasion strategies are multifaceted and can be divided into several main groups: DR. WALID DARWICHE 50 50 Dr. Walid Darwiche 25  11/5/2022 A. SEQUESTRATION Making it as difficult as possible for immune processes to reach the parasite. There are two main ways of doing this:  by adopting a relatively inaccessible predilection site, e.g. within particular cell types or organs (such as the CNS or within the lumen of the gastrointestinal tract)  by generating a protective capsule, membrane or cyst wall. DR. WALID DARWICHE 51 51 B. MASKING OR CHANGING SURFACE ANTIGENS Examples include: 1. Incorporation of host molecules onto the surface of the parasite; 2. Synthesis of parasite antigens which mimic host molecules; 3. Antigen variance – periodic changes of surface antigens, thereby rendering previous host adaptive responses ineffective. Some parasites have stage specific antigens that serve the same purpose. DR. WALID DARWICHE 52 52 Dr. Walid Darwiche 26  11/5/2022 C. DISTURBANCE OF IMMUNOLOGICAL EFFECTOR MECHANISMS Examples include: 1. Surface shedding to remove adhering immune cells or specific antibodies bound to parasite antigen; 2. Enzymatic digestion of antibodies; 3. Inhibition of oxidative products synthesized by leukocytes; 4. Reducing MHC-expression on the surface of infected cells, thereby inhibiting antigen presentation to the immune system. MHC: Major Histocompatibility Complex. Molecules that carry parasite antigen to the surface of the host cell so that it can be recognized by antigen-processing cells. DR. WALID DARWICHE 53 53 D. MODULATION OF THE HOST IMMUNE RESPONSE This can be achieved in various ways, for example: 1. Induction of multiple clones of T- and B-cells that produce nonspecific antibodies (polyclonal activation), thereby disabling the host’s ability to manufacture in sufficient quantity the specific antibodies needed to combat the invading parasite 2. Induction of immune complexes in the blood and cleavage of antibody/ complement factors, both of which result in severe immune suppression. DR. WALID DARWICHE 54 54 Dr. Walid Darwiche 27  11/5/2022 E. INFLUENCING APOPTOSIS 1. Release of pro-apoptotic factors that shorten the life of leukocytes that might threaten the parasite 2. Synthesis of anti-apoptotic factors by an intracellular protozoan parasite to prolong the life-span of its host cell Apoptosis: controlled and purposeful cell death (as opposed to necrosis, which is cell death due to an acute insult or injury, and autophagy, which is related to recycling cell components). DR. WALID DARWICHE 55 55 F. ARRESTED DEVELOPMENT AND HYPOBIOSIS  Some parasites are able to pause their development at a strategic point in their parasitic life-cycle.  This waiting phase (termed ‘arrested development’) is used to synchronize parasitic development with host or environmental events (e.g. parturition or the onset of a favorable season of the year).  There are various biological advantages to be gained from this.  During this process, parasites often ‘hide’ from targeted host immune responses by slowing or shutting down vulnerable metabolic processes (‘hypobiosis’). DR. WALID DARWICHE 56 56 Dr. Walid Darwiche 28  11/5/2022 V. THE HOST–PARASITE BALANCE DR. WALID DARWICHE 57 57  In nature, the coevolution of host defense mechanisms and parasite evasion strategies has resulted in an uneasy equilibrium whereby there is no undue threat to the continued existence of either at a population level, although the well-being or survival of individuals (host or parasite) may be compromised.  The parasite needs to feed and reproduce, yet it faces extinction should infection threats the survival of the host population.  In a stable ecosystem, a well-adapted parasitic species is one that survives in the host long enough to replicate but provokes no more than tolerable damage to the host population. DR. WALID DARWICHE 58 58 Dr. Walid Darwiche 29  11/5/2022  Disease generally indicates a disturbance of this ecological balance.  This may be caused by naturally occurring factors, such as unusual weather conditions, but is often due to human intervention.  Compare, for example, zebra roaming the African savannah carrying large worm burdens seemingly without ill-effect, with the vulnerability of horses confined to small enclosures.  The host–parasite relationship can be perturbed in two ways: DR. WALID DARWICHE 59 59 A. INCREASED HOST SUSCEPTIBILITY 1. Stressed, debilitated or immunocompromised 2. Exposed to parasites with which they have not coevolved (e.g. European cattle placed in a tropical environment) 3. Not allowed to express natural behavior (e.g. restrained so they cannot groom to remove ectoparasites) 4. Selectively bred for production traits at the expense of natural ability to resist infection (innate or acquired) 5. Inbred (e.g. some canine blood lines are particularly vulnerable to demodectic mange) DR. WALID DARWICHE 60 60 Dr. Walid Darwiche 30  11/5/2022 B. INCREASED PARASITE NUMBERS Exposure to host-seeking (infective) life-cycle stages may increase, for example, if: 1. Host stocking density is increased, thereby increasing the output of parasite eggs/larvae etc. per unit area (or per kg forage) 2. Parasitized animals are introduced into a previously clean area (e.g. through livestock movements, global trade etc.), thereby infecting susceptible local livestock, potential wild-life reservoirs or vectors 3. Short-term weather patterns or longer-term trends such as global warming produceconditions more favorable for the development of preparasitic life- cycle stages 4. There is a surge in the population of intermediate hosts or vectors, or an increase in the number infected or their accessibility 5. The parasite population becomes resistant to anti-parasitic medication DR. WALID DARWICHE 61 61 VI. MAJOR SYNDROMES RESULTING FROM PATHOLOGICAL PROCESSES DR. WALID DARWICHE 62 62 Dr. Walid Darwiche 31  11/5/2022 A. DIARRHEA Easy to diagnose, but responding to very complex and diverse processes. 1. Caused by primary parasites Lesions of the mucosal epithelium by intestinal cell destruction (by the coccids). The growth of Nematodes in the submucosa or mucous glands will lead to destruction. Production of intracellular toxins in the intestine. Portal hypertension. Presence of nodules that disrupt the structure. DR. WALID DARWICHE 63 63 2. Mechanisms Motor disorders (smooth fibers) Secretory disorders. Disorders of the permeability of the digestive wall, resulting in water leakage and the absorption of toxins. 3. Consequences Decreased food digestibility Loss of water and nutrients, which can lead to death from dehydration especially in infants. Resorption of toxins and germs. DR. WALID DARWICHE 64 64 Dr. Walid Darwiche 32  11/5/2022 B. ANEMIA Blood-sucking parasites are the most common cause of anemia 1. Causes Blood loss: hemorrhage (often small) or spoliation in the digestive tract or on the surface of the skin (ectoparasites and internal parasites) by blood-sucking parasites Hemolysis: destruction of red blood cells. Example: Piroplasma grows in red blood cells and its release causes bursting or an immunological reaction (the host reacts against the parasite and induces destruction of its cells at the same time as those of the parasite ). Disturbance of hematopoiesis Example: Some Cestodes will take vitamin B12 and cause anemia. DR. WALID DARWICHE 65 65 2. Clinical signs  Skin and mucous membrane problems are observed  Shortness of breath during exercise  Tachycardia.  A decrease in hemoglobin in the blood DR. WALID DARWICHE 66 66 Dr. Walid Darwiche 33  11/5/2022 C. WEIGHT LOSS AND CACHEXIA (GENERALIZED STATE OF FATIGUE) 1. Causes Direct, global or specific lack of certain nutrients, rarely very serious (i.e. the parasite can take up to 20% of the animal's diet , which can be enormous especially if the animal is already weakened ) Decreased intestinal absorption Decreased digestibility of food (by lesions of the mucous membranes) Loss of blood proteins Loss of nutrients (hence the association between diarrhea and weight loss) Decreased appetite because the animal is sick Mechanical obstruction, for example due to a ball of Ascaris (Tenia) Specific lack of a nutrient: a vitamin, a mineral salt which induces an imbalance. DR. WALID DARWICHE 67 67 2. Mechanisms  The general condition deteriorates, often leading to secondary diarrhea followed by anemia due to hematopoiesis deficit.  We can find these three major symptoms at once. DR. WALID DARWICHE 68 68 Dr. Walid Darwiche 34  11/5/2022 D. OTHERS  Bone dystrophy then rickets  Neurological syndromes: accommodation or passage of parasites  Endocrine disorders  And many others…. All this has consequences:  Medical. In sick animals, sometimes with polyparasitism, this can lead to death.  Particularly in the production sectors. Limiting the economic losses due to the drop in production is also part of the veterinarian's role. But also for human consumption and not only to treat sick animals. DR. WALID DARWICHE 69 69 DR. WALID DARWICHE 70 70 Dr. Walid Darwiche 35