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This article has been cited by other articles in PMC. Abstract Background Cryptosporidium spp. Prevalence rate of Cryptosporidium spp. This study aimed to determine Cryptosporidium spp. Infection in humans and some animals in rural areas of Shushtar district from Khuzestan Province, south- west of Iran.

Methods In this study, Stool specimens were randomly collected from 45 cattle, 8 buffalos, 35 calves, 22 turkeys, 3 sheep, 2 geese as well as 62 humans in different seasons selected from rural areas of Shushtar district located in Khuzestan in the south- west of Iran from August to April The collected stool samples were examined by modified Ziehl-Neelsen staining method.

Conclusion In this study we found the high frequency of Cryptosporidium spp.

Keywords: Cryptosporidium infection, Human, Domestic Animal, Iran Introduction Cryptosporidium is a protozoan parasite that infects the gastrointestinal tract of a wide rang of vertebrates including humans, livestocks, wild animals and birds 1.

Cause by covering the extensive host rang, Cryptosporidium has been considered to be a zoonotic protozoa 2 , 3. An organism that cannot survive in any other manner is called an obligate parasite.

A facultative parasite is an organism that may exist in a free-living state or as a commensal and that, if opportunity presents itself, may become parasitic. It is implicit in this term that the organism does not of necessity have to be a parasite at any stage of its existence.

Some animals are obligatory parasites at one or more stages of their life cycles but free living at others. Parasites living within the host may be described as endoparasites, whereas those that are found on the surface of the body are called ectoparasites. Small organisms, such as mosquitoes, which must periodically seek out other and larger forms on which to nourish themselves, have occasionally been called intermittent parasites.

This generalization is certainly true of most predators and parasites, or at least of the most obvious ones. However, the essence of the parasitic relationship, which separates it from predation, is the protracted and intimate association between parasite and host. The association between the mosquito and its victim is neither prolonged nor intimate. Those blood-sucking arthropods, which lead an independent existence except for occasional nutritional forays, may be referred to as micropredators.

Many organisms customarily considered to be parasites are actually commensals. Entamoeba coli lives in the lumen of the intestine, subsists there on the bacterial flora of the gut, and does its host no appreciable harm.

This is a symbiotic relationship in which no advantage or disadvantage accrues to the host, whereas the ameba is supplied with food and protected from harm. Other cases are less definite. Adaptations to Parasitism The parasitic relationship probably evolved early in the history of living organisms.

We know little about how such relationships arose, but we may hypothesize that we can see in the facultative parasite one possible initial step along the road to obligate parasitism. The possibility of the adaptation of a parasitic mode of existence may depend on what is known as preadaptation, or evolutionary changes that make possible existence in an environment that otherwise would be unsuitable.

Such preadaptive changes might be in the nature of increased resistance to the enzymatic activities of the host. Further physiologic adaptations to parasitism might involve the loss of enzymes or enzyme systems, which are then supplied by the host. Such losses may be expected to make a parasitic, or at least a symbiotic relationship, obligatory. Certain groups of parasites exhibit profound morphologic adaptations to their way of life. As might be expected, these modifications are more striking in those groups that are wholly parasitic than in those that contain both free-living and parasitic species.

Organs not necessary to a parasitic existence are frequently lost. The only groups of protozoans that contain nothing but parasitic forms are the phyla Apicomplexa and Microsporidia. Members of these phyla have no locomotor organelles, although the structures are present in one form or another in all other phyla of protozoa, even in their parasitic representatives. Most of the free-living turbellarian flatworms are provided with a ciliated epidermis in the adult stage.

A digestive tract, moderately complex in the turbellarians, is generally reduced in the trematodes and is absent in the cestodes. The reproductive system is highly developed in the two latter groups; this seems a reflection of the difficulties inherent in transfer of these organisms to new hosts.

Specialized attachment organs in the form of suckers and hooks have been developed by the parasitic flatworms. Body size may be greatly affected by the parasitic state. Although we think of parasites as small organisms, many of them are much larger than their free-living relatives. The majority of free-living turbellarians are less than a half centimeter in length, and while some land planarians may reach a half meter, none approaches the length of 10 m or more seen in some tapeworms.

Most free-living nematodes barely attain naked-eye visibility as adults, but Ascaris can reach 35 cm and Dracunculus as much as 1 m.

On a more basic level, the parasitic mode of existence may result in profound biochemical changes. Profound differences between metabolic pathways in parasite and host characterize the Kinetoplastida Leishmania and Trypanosoma species in humans , Entamoeba histolytica, Giardia lamblia, and Trichomonas vaginalis, as well as most, if not all, of the helminth parasites.

These metabolic differences between parasite and host may afford opportunity for strategic chemotherapeutic efforts, as will be seen later. Specialized mechanisms for effecting entrance into the body or tissues are seen in some parasites.


No such enzyme has been found in the commensal E. The cercarial stage in the life cycle of the blood fluke is able to penetrate through the skin of humans to produce infection. It does this with the aid of penetration glands, which produce an enzyme capable of digesting the skin. The embryo of Hymenolepis nana, before developing into a cysticercoid larva, penetrates an intestinal villus with the help of the six hooklets it bears.

Continuation of a parasitic relationship depends on how successfully the immune response of the host is overcome. Many different defense mechanisms have evolved, and many of these will be discussed in consideration of the individual parasites.

Immune evasion may involve such factors as location of the parasite in relatively protected sites, changes in the parasite surface antigenic structure brought about in a variety of ways, and active modification of the host immune response by products of parasite metabolism.

Increased reproductive capacity has already been mentioned as characterizing two parasitic groups in contrast with their free-living relatives. Most metazoan parasites exhibit such an increase, which in some cases involves larval stages as well as adults. The chances that a particular egg will successfully infect a new host are usually very small, and if more than one host species is involved, the chance of successful completion of the cycle becomes still smaller. If a parasite is successful in infecting an intermediate host, it is obviously advantageous if the larval stage that develops there can multiply to produce many additional organisms capable of infecting the definitive or a second intermediate host.

Such a modification is seen in the trematodes and many of the cestodes, where in the intermediate host a single egg develops into a larva, which in turn produces many larvae of a more advanced kind. Effects of the Parasite on the Host A parasite, by definition, is an organism that lives at the expense of its host; however, we have already found that many organisms that are loosely termed parasites are in reality commensals.

In many instances it cannot be said with certainty whether an organism injures the host. Even if we can be fairly sure that some injury is produced, we may not be able to detect it. Thus, a distinction is made between hookworm disease and hookworm infection on the basis of the presence or absence of clinical symptoms.

Overt symptoms of infection with this parasite may depend on the number of worms present, the nutritional status of the host, or both. Injury to the host may be brought about in many ways. Some of these mechanisms are common to all parasites, even if this term is used in its broad sense to include bacteria, viruses, and fungi.

The most widespread type of injury is that brought about by interference with the vital processes of the host through the action of secretions, excretions, or other products of the parasite. Such interference is probably largely or exclusively on the level of the host enzyme systems.

Parasites producing such effects may be in the tissues or organs of the host, in the bloodstream, or within the gastrointestinal tract, or they may even be ectoparasitic. Invasion and destruction of host tissue may be distinguished from injury that does not involve gross physical damage, although both types of injury reflect biochemical changes brought about in the host tissue by the parasites.

When the giant intestinal fluke, Fasciolopsis buski, is present in large numbers, toxic symptoms are seen, but the precise cause is unknown. Malarial parasites invade and multiply in red blood cells, which are destroyed in the process and may also attach to the walls of smaller blood vessels in the brain, occluding them to produce localized ischemia.

The helminth parasites, by virtue of their size, may damage the host in other ways impossible for the smaller parasites.

In addition to its toxic effects, F. Ascaris may perforate the bowel wall, cause intestinal obstruction if present in large numbers, and invade the appendix, bile duct, or other organs. Some parasites exert their effects by depriving the host of essential substances.

Thus, hookworms suck blood and by so doing may deprive the host of more iron than is replaced by diet and so bring about an anemia. The broad fish tapeworm Diphyllobothrium latum selectively removes vitamin B12 from the alimentary tract, producing a megaloblastic anemia in some infected persons. Effects of the Host on the Parasite The effects of the parasite on the host are more obvious than those that operate in the opposite direction, but the latter are nonetheless important.

The genetic constitution of the host may profoundly influence the host-parasite relationship.

There are racial variations in resistance to Plasmodium vivax, which are related to the presence or absence of the Duffy blood group. There is also considerable evidence that possession of the sickle cell trait, an inherited characteristic, is also associated with increased resistance to infection with the malarial parasite Plasmodium falciparum. The diet or nutritional status of the host may be of major importance in determining the outcome of a parasitic infection.

A high-protein diet has been found to be unfavorable for the development of many intestinal protozoa, while a diet low in protein has been shown to favor the appearance of symptoms of amebiasis and the complications of this disease. It has been shown that a carbohydrate-rich diet favors the development of certain tapeworms, and the presence of carbohydrate in the diet is known to be essential for some of these worms.

The general nutritional status of the host may be of considerable importance both in determining whether a particular infection will be accompanied by symptoms and in influencing their severity if present. Major nutritional disturbances may influence resistance through their effects on the immune mechanisms of the host. Every species of animal is naturally resistant to infection by many organisms that parasitize different species.

As we have seen in the case of certain strains of malaria, resistance may also be a racial phenomenon. In some cases it has been possible to adapt parasites to hosts that they normally infect poorly or not at all. Acquired immunity can be demonstrated in many parasitic diseases, and it is generally found to be at a lower level than that produced by bacteria and viruses.

Absolute immunity to reinfection, as is generally seen following infection with smallpox, measles, whooping cough, and a number of other viral and bacterial diseases, occurs rarely following protozoal infections and probably never with helminth infections of humans. As yet, no useful vaccines have been developed against protozoal or helminthic infections. Although malaria is a likely candidate for a vaccine, recent field trials of potential malaria vaccines have failed to meet expectations.

Primary infection with Leishmania seems to confer a degree of immunity to reinfection. While many protozoal and helminthic infections confer no longlasting immunity to reinfection, they do seem to stimulate resistance while the parasites are still in the body. This resistance to hyperinfection, known as premunition, may be of great importance in endemic areas in limiting the extent of infection with plasmodia, hookworms, and other parasites.

Acquired immunity may be very important in modifying the severity of disease in endemic areas, particularly diseases such as malaria, schistosomiasis, and filariasis. Infants born in such areas to a semi-immune parent are at birth, and for some time thereafter, partially protected by maternal antibodies acquired transplacentally.

If infection with one such parasite takes place during the first few months of life, it is likely not to be as severe as it would otherwise have been, and repeated infections over the years keep the acquired immunity at a high level and symptoms correspondingly mild.

If, on the other hand, such a person leaves the endemic area for a protracted period, the acquired immunity wanes, and on returning to the endemic area that person may fare no better than someone becoming infected after entering the endemic area for the first time. Exciting new areas of research have dealt with the role of eosinophils in killing young schistosomes and microfilariae, the ability of older schistosomes to induce immunosuppression in the host, the discovery of hostlike antigens on the surface of some parasites, and the phenomenon of antigenic variation in trypanosomes.

The role of cytokines, and particularly of tumor necrosis factor TNF or cachectin, has been the subject of much research activity. Cachectin, a major secretory product of activated macrophages, in low doses is protective against experimental malaria in mice, stimulates the killing of schistosomules by eosinophils in vitro, but paradoxically is thought to bring about the state of cachexia seen in trypanosomiasis.

Side effects of administration of TNF to cancer patients are almost identical to the various signs and symptoms seen in severe falciparum malaria. In Trypanosoma lewisi infections in rats, the metabolic products of the parasites are more effective in producing immunity than are the dead trypanosomes themselves.

Various immunologic tests have been devised based on the ability of the serum of an infected host to precipitate the secretions or excretions of eggs, larvae, or adults of a number of different helminths. Some of these are discussed in Chapter Parasites and the Compromised Host This subject is covered in some detail in Chapter We have already alluded to the compromised host in reference to the relationship between nutritional status and the outcome of a parasitic infection.

The therapeutic armamentarium of the modern physician is also capable of compromising these defenses. Benefits to be derived from the use of corticosteroids and other immunosuppressive agents, and of the antimetabolites, must always be weighed against their effects on the defenses of the patient. Aggressive treatment of leukemia and other malignancies may pave the way for fatal Toxoplasma infection, and acute amebic colitis may follow the use of corticosteroids for presumed ulcerative colitis.

Markell and Voge's medical parasitology. Eighth edition

Parasitic infection of tissues compromised by malignant involvement is typified by the report of primary gastric amebiasis in a case of reticulum cell sarcoma in which the resistant normal gastric mucosa was largely supplanted by tumor cells. There is also good evidence to suggest that certain helminthic infections, notably strongyloidiasis and trichinosis, may flourish in immunologically compromised hosts.

Life Cycles of Protozoa and Helminths Many parasitic organisms have but a single host, being transferred from one individual to another of the same species either through direct physical contact or by means of resistant or semiresistant forms that are able to survive a period outside or away from the host.

Entamoeba gingivalis, a commensal organism that inhabits the mouth, has no cyst stage or other means of survival outside of the host, and it probably is transferred by direct contact. Trichomonas hominis likewise is unable to form cysts, but it probably can survive for short periods outside the body so that direct contact is not necessary.

Many protozoa and helminths have cyst stages or eggs that survive for some time away from the host and by means of which new hosts become infected. Parasitic infections may be carried from one host to another by arthropod vectors. A vector may also be a host if development of the parasite takes place within its body. If the arthropod is simply an instrument of passive transfer, we refer to it as mechanical vector.

If a fly, feeding on fecal matter containing cysts of E. When an anopheline mosquito sucks blood from a malaria patient, the parasites must develop in the mosquito before she is able to transmit the infection.

In this instance the mosquito is both host and biologic vector. Some protozoa and many helminths have complex life cycles, with not one but two, and sometimes more, hosts. When more than one host species is necessary to the development of the parasite, that host in which sexual reproduction occurs is called the definitive host.

The species in which larval or asexual if both sexual and asexual forms occur stages of the parasites develop are called intermediate hosts; they are usually designated first and second intermediate hosts if there is more than one.

Disconcerting as it may be to those with a strictly anthropocentric point of view, humans are but the intermediate host of the malarial parasite Plasmodium, which undergoes sexual reproduction in mosquitoes of the genus Anopheles. Many protozoa are asexual; if an arthropod host is required in the life cycle of an asexual parasite, one may refer to its vertebrate and invertebrate hosts.

Important Groups of Animal Parasites The animal parasites of humans and most vertebrates are contained in five or more major subdivisions or phyla. Those containing organisms that can parasitize man include the Sarcomastigophora, Ciliophora,Apicomplexa, and Microsporidia. Other phyla containing parasitic species include the Platyhelminthes or flatworms; the Nematoda, or roundworms; the Acanthocephala, or thorny-headed worms; and the Arthropoda, which includes the insects, spiders, mites, ticks, and so forth.

With the exception of the Apicomplexa, Microsporidia, and Acanthocephala, all these phyla contain both parasitic and free-living forms. Within each phylum only those groups that include species of medical importance are discussed here. Animal phyla may be subdivided into classes and the latter into orders. Each order may again be divided into families containing one or more genera and species. Assignment to these categories is made largely on the basis of morphologic characters; identification of any animal parasite requires some knowledge of its structure.

The ameboflagellates partake of the characters of both groups. The Mastigophora move by means of specialized structures known as flagella. A flagellum is a long, threadlike extension of cytoplasm that functions as a means of propelling the organism. Flagella always arise from small intracytoplasmic granules known as blepharoplasts.

The number and position of flagella vary a great deal in different species. In addition to the flagella, and often associated with them, one may observe a variety of structures that serve supportive and other functions and give a characteristic appearance to each species. A number of flagellates are blood parasites or inhabit the tissues, whereas others are found in the alimentary canal.

Most of the latter forms are commensals, but two species, Giardia lamblia and Dientamoeba fragilis, are pathogenic. Sarcodina contains those forms that move by means of cytoplasmic protrusions called pseudopodia. This group includes all free-living amebae, as well as those that are symbiotic in the intestinal tract and elsewhere in the body.

Most of the amebae of humans are commensals; one species, Entamoeba histolytica, is an important pathogen.

Markell and Voge's Medical Parasitology E-Book

While reproduction in the Mastigophora and Sarcodina is usually asexual, Apicomplexa have a complex life cycle with alternating sexual and asexual generations. Four species of Plasmodium are found primarily as blood parasites and cause malaria; species of Isospora, Cyclospora, Cryptosporidium, and Sarcocystis are parasitic in the mucosa of the intestinal tract, and Toxoplasma and Sarcocystis are found in various organs and tissues.

Microsporidia rarely cause disease in immunocompetent persons, but may do so with greater frequency in immunosuppressed persons.

A growing body of evidence suggests that the microsporidia may be fungi-related organisms Metenier and Vivares, Locomotion is accomplished by means of cilia, relatively short threads of cytoplasm arising from small basal granules. Some ciliates are multinucleate, while others contain but two nuclei, a large macronucleus and a small micronucleus. The only ciliate parasite of humans is Balantidium coli, found in the intestinal tract.

Although rare, it is important, as it may produce severe intestinal symptoms. Most flatworms are hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes.

Adults may be less than 1 mm long or they may reach a length of many meters. Most members of the phylum are symbionts, living on or in the body of their hosts.

Free-living species belong to the class Turbellaria, which also contains forms that are parasitic in lower animals. The classes Trematoda and Cestoda contain parasitic forms only. The Trematoda, or flukes, are leaf-shaped or elongate, slender organisms that possess attachment organs in the form of hooks or cup-shaped muscular depressions called suckers. A simple digestive tract is present.

Of the three orders of the Trematoda, the order Digenea contains all the species that are parasitic in humans. Members of this order have complex life histories, with at least one intermediate molluscan host. Included in the digenetic trematodes of humans are forms that parasitize the intestinal tract, the liver, the blood vessels, and the lungs.

Members of the class Cestoda typically have an elongate, ribbonlike, segmented body that bears a specialized attachment organ, the scolex, anteriorly.

A digestive tract is absent. Adult cestodes or tapeworms inhabit the small intestine. With the exception of Hymenolepis nana, cestode larvae require an intermediate host for development. Humans may be host to either adult or larval stages, depending on the species of cestode.

They possess a stiff cuticle, which may be smooth or may be extended to form a variety of structures, particularly at the anterior and posterior ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free living, a large number of species parasitize humans, animals, and plants.

Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species. Sexes are separate, and males are usually smaller than females. The life cycle requires an intermediate host. While thorny-headed worms are widely distributed among wild and domestic animals, only three genera have been reported in human beings.

The digestive system is well developed. Sexes are separate. The phylum is subdivided into a number of classes, many of which are of medical importance. The class Crustacea contains primarily aquatic forms, which breathe by means of gills.

Included here are crabs, shrimps, crayfish, and copepods. Certain of these serve as intermediate hosts of human parasites. The first pair of appendages is modified as poison claws. The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites.

Scorpions and spiders produce venom, which in some species may be extremely toxic. Certain ticks and mites may transmit disease. From a medical or economic point of view, the class Insecta includes by far the most important of the arthropods.

Insects have three pairs of legs and a body divided into three distinct parts: head, thorax, and abdomen. Several orders of insects are worthy of special mention. The Anoplura, or sucking lice, are wingless, dorsoventrally compressed insects, among which are included human lice.

The order Hemiptera, or true bugs, includes the wingless bedbugs as well as the more characteristic forms with wings. Two pairs of wings are seen in this group, and the first pair has thickened membranous bases. The cone-nosed bugs, or reduviids, are important as vectors of American trypanosomiasis.

The coleoptera, or beetles, also have two pairs of wings, but the anterior pair is thickened throughout. Certain grain beetles are intermediate hosts of tapeworms. The Hymenoptera include ants, bees, wasps, and so forth. Bees, wasps, and fire ants are medically important because of the venom of their stings; other ants may serve as intermediate hosts for one of the human trematode parasites. The Siphonaptera, or fleas, are wingless and laterally compressed; in addition to their irritating bites, some fleas act as intermediate hosts of a species of tapeworm.

The Diptera are insects with only one pair of true wings. This order includes several groups of medical importance, notably mosquitoes, flies, and gnats. Some larval flies are parasitic in humans and animals, while mosquitoes and gnats transmit many different diseases. The name is derived from their body shape, which is elongate and in some species tonguelike.

Other species have a ringed or annulated body. Linguatulids lack external appendages and possess two pairs of hooks near the mouth. Adults live in the respiratory tract of vertebrates. Encysted larval stages may occur in the lungs and other internal organs of humans, and they are found principally in tropical areas.

Prevalence of Parasitic Infections Estimates of the prevalence of parasitic diseases are at best extremely rough, as reporting of morbidity is essentially nonexistent in many of the areas in which these diseases occur. Owing to its lengthy half-life and in case of therapeutic failure of mefloquine, it must be borne in mind that the substitution of quinine or quinidine may expose the patient to increased risk of cardiac conduction problems or convulsions.

Doxycycline The tetracyclines are blood schizonticides, and while they have some effect on the preerythrocytic stages of the malarial parasites, they cannot be relied on to achieve radical cure. As with other chemoprophylactic drugs, it should be continued for 4 weeks after the patient leaves the endemic area. Proguanil Paludrine Proguanil is a blood schizonticide that initially was effective against all four species of malaria. It is now generally useful against P.

Strains of P. It is well tolerated in suppressive doses but should not be used for treatment. Paludrine is not available commercially in the United States but may be obtained in Canada and many European and African countries. It is used extensively in East Africa.Qinghaosu appears to act more rapidly as a blood schizonticide than either quinine or mefloquine, but patients treated with it have a high rate of recrudescences, and it is possible that its chief value will be its use in combination with other antimalarials.

Clinical studies of atovaquone, alone or in combination with other antimalarial drugs, for treatment of acute uncomplicated malaria in Thailand.

As might be expected, these modifications are more striking in those groups that are wholly parasitic than in those that contain both free-living and parasitic species. One aspect of prevention chemoprophylaxis has already been discussed. The only ciliate parasite of humans is Balantidium coli, found in the intestinal tract. Strickland GT ed. Adults may be less than 1 mm long or they may reach a length of many meters.

This is a symbiotic relationship in which no advantage or disadvantage accrues to the host, whereas the ameba is supplied with food and protected from harm. Important Groups of Animal Parasites The animal parasites of humans and most vertebrates are contained in five or more major subdivisions or phyla. Further physiologic adaptations to parasitism might involve the loss of enzymes or enzyme systems, which are then supplied by the host.

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