Parasites of Public Health Importance in Veterinary Parasitology
After studying this chapter, the reader should be able to do the following:
• Detail the types of zoonotic diseases that might be encountered in a veterinary clinical practice.
• Communicate symptoms, diagnostic procedures, and prevention of major zoonotic diseases that might be encountered in a veterinary clinical practice.
• Understand that veterinarians and veterinary technicians should never provide direct treatment for a human patient infected with a zoonotic disease.
• Understand that veterinarians and veterinary technicians should only act in an advisory capacity regarding zoonotic diseases.
Schistosome cercarial dermatitis
Measly beef/beef measles
Measly pork/pork measles
Unilocular hydatid disease
Multilocular hydatid disease
Toxocaral larva migrans
Visceral larva migrans (VLM)
Ocular larva migrans (OLM)
Neural larva migrans (NLM)
Cutaneous larva migrans (CLM)
Dermal larva migrans
Incidental nymphal pentastomiasis
Chapter 1 defines zoonosis as any disease or parasite that is transmissible from animals to humans. In veterinary clinical practice, it is important for veterinarians to inform their clients concerning zoonotic diseases or parasites that may be transmitted from their domestic animals. Such zoonotic parasites include Toxoplasma gondii, Trichinella spiralis, Ancylostoma caninum, and Toxocara canis. This chapter discusses the major parasitic diseases that are of public health importance in veterinary clinical practice.
It is often the veterinarian’s role (and also the role of the veterinary diagnostician) to diagnose a parasite in a domestic animal. The veterinary diagnostician must also communicate to the public and often to medical personnel, such as physicians, laboratory personnel, and other public health workers, the significance of many of the zoonotic parasites. It is important for veterinary personnel to understand that in suspected cases of zoonotic parasites or conditions, the client should always be referred to a physician, family practitioner, obstetrician, or public health worker, as appropriate, for diagnosis or treatment. In no instance should a veterinarian or veterinary technician attempt to diagnose or treat any of these zoonotic parasites in humans. The veterinarian’s role is to inform, not to treat. Attempting to diagnose or treat a human is a violation of state veterinary practice acts across the United States. Treatments are included in this chapter for veterinarians strictly for the purpose of advising human health care workers and not for the purpose of treating human patients.
For each zoonotic parasite discussed here, the following areas are addressed:
• The symptoms of parasitic infection in humans and the ways in which infection is diagnosed.
• Treatment of the parasitic condition in humans.
• Prevention of the transmission of zoonotic parasites, conditions, or syndromes from animals to humans.
Just as parasites are broken down in previous chapters into major groups, the important zoonotic parasites are divided into zoonotic protozoans, zoonotic trematodes, zoonotic cestodes, zoonotic nematodes, zoonotic arthropods, and zoonotic pentastomes. The acanthocephalans and the leeches have no zoonotic significance.
Protozoans of Public Health Importance
Toxoplasma gondii (Toxoplasmosis)
Toxoplasmosis in humans is an extremely rare protozoan disease with which humans (particularly a pregnant woman and her developing fetus) can become infected. It is an important zoonotic parasite because a very popular domestic pet, the cat, is the only definitive host for this parasite. The cat can serve as a source of infection for the pregnant woman and her unborn child. Toxoplasmosis can infect the fetus in the womb and produce serious birth defects. This parasite can also wreak havoc in individuals with acquired immunodeficiency syndrome (AIDS).
Although this is a very rare parasite, several precautions must be taken to prevent infection. To sell papers, the “sensational press” often exploits this parasite in its headlines, alarming the general public. As a result, the veterinary clinical practice may receive phone calls regarding this parasite. Also, uninformed clients may request that veterinarians find a new home for (or even euthanize) the family cat. It is important for the veterinarian to alleviate the client’s concerns and to work in conjunction with the obstetrician-gynecologist to answer questions, thwart this parasite, and prevent toxoplasmosis. It is important to note that almost every warm-blooded animal can become infected with Toxoplasma gondii.
Human Infection with Toxoplasmosis
Humans become infected with toxoplasmosis in one of two ways: congenitally (in the case of unborn babies) or through acquisition. Congenital toxoplasmosis occurs when a woman becomes infected during her pregnancy; the woman ingests sporulated oocysts of T. gondii found in the feces of the cat. The domestic house cat with access to the outdoors (and predation) serves as the definitive host for T. gondii and will shed unsporulated oocysts in its feces. These unsporulated oocysts will sporulate (or become infective) in 1 to 5 days. If a pregnant woman accidentally ingests the sporulated oocyst, the parasite’s tachyzoites (rapidly multiplying stages) can infect the developing fetus.
Acquired toxoplasmosis occurs when a human ingests the sporulated oocyst containing tachyzoites or ingests infected meat or tissue stages containing bradyzoites. Many domesticated animals (e.g., cattle, pigs, sheep) serve as intermediate hosts for T. gondii. Within their muscle, these intermediate hosts harbor bradyzoites (slowly multiplying stages) that can infect various tissue sites, including lymph nodes, meninges, eyes, and the heart, of humans. The infected human ingests the bradyzoites, which then infect a variety of tissue sites.
Symptoms and Diagnosis
If congenital infection occurs early in the pregnancy, abortion is common. If this infection occurs late in the pregnancy, the central nervous system may become infected, and a variety of neurologic abnormalities may result. These include cerebral calcification, chorioretinitis, hydrocephaly, microcephaly, and psychomotor irregularities. The child may be born dead or alive. If alive, the child may be mentally retarded.
Infected humans with acquired toxoplasmosis exhibit lymphadenopathy, malaise, fever, lymphocytosis, and myocarditis. Most cases, however, are characterized by mild fever and slight enlargement of the lymph nodes.
Diagnosis of clinical toxoplasmosis in humans is quite difficult. Diagnosis relies on the demonstration of organisms or antibodies against it. The best diagnostic test is the inoculation of suspected material into mice and demonstration of the organism multiplying in the mice. Serologic tests are also available. The veterinary diagnostician must remember that these tests need to be delegated to a more sophisticated diagnostic laboratory.
There is no completely satisfactory treatment for toxoplasmosis in humans, although pyrimethamine has been found effective. This drug, in conjunction with triple sulfa drugs, has given good results in ocular toxoplasmosis. Remember that suspected cases of toxoplasmosis in humans should always be referred to a neonatologist, pediatrician, gynecologist, obstetrician, or family practitioner. The Centers for Disease Control and Prevention (CDC) has many specialists who are qualified to answer the most complex issues of toxoplasmosis in humans.
Prevention of Transmission to Humans
Transmission of T. gondii to pregnant women is prevented by having someone else empty the cat’s litter box for the duration of her pregnancy. This step eliminates the woman’s potential exposure to the sporulated oocysts of T. gondii. A person should always wash his or her hands after handling a cat and before eating. Litter boxes should never be placed in the kitchen or dining area. Gloves should be worn while gardening because feral cats often defecate in flower beds. If children’s sandboxes are present in the backyard, they should remain covered when not in use. After gardening, hands should be thoroughly washed. Cats should never be allowed to roam freely and hunt. Likewise, uncooked meat should never be fed to cats. Rodent prey or uncooked meat may contain the tissue stages of T. gondii, which will set the life cycle in motion in the cat. Likewise, to avoid acquired infection with T. gondii (and a variety of pathogenic bacteria), humans should be wary of undercooked infected meat.
Cryptosporidium parvum (Cryptosporidiosis)
Cryptosporidiosis is a rare protozoan disease that produces a prolific, painful, watery diarrhea in humans. This protozoan parasite has been reported in the news because it has contaminated the drinking water supplies of several major metropolitan areas throughout North America and the world. As with toxoplasmosis, cryptosporidiosis is especially harmful in individuals with AIDS. For this reason, it is also an important parasite. Cryptosporidiosis was first reported in farmworkers and individuals who worked around very young calves. This organism is spread by ingestion of infective oocysts in calf feces.
Human Infection with Cryptosporidiosis
Humans become infected with cryptosporidiosis by ingestion of oocysts from feces of young calves or from contaminated supplies of drinking water.
Symptoms and Diagnosis
Cryptosporidiosis produces a transient, painful, watery diarrhea in humans, and all ages are susceptible. The duration of clinical signs in affected individuals varies considerably. Acute cases last from 3 to 7 days, and chronic wasting syndromes can persist for weeks to a few months. The development of natural immunity against the parasite determines the duration of clinical signs. Most humans infected with Cryptosporidium species develop immunity and recover from the infection.
Infections with Cryptosporidium species may persist indefinitely in people with immunodeficiencies, particularly AIDS. The prognosis for immunologically compromised individuals with cryptosporidiosis is poor.
Fecal flotation and concentrating solutions, such as zinc sulfate and Sheather’s sugar solution, may be used to identify oocysts of Cryptosporidium species. These parasites are extremely tiny, less than 5 µm in diameter, and thus may be easily overlooked. Acid-fast stains such as Ziehl-Neelsen or Kinyoun can be used, as well as auramine-rhodamine, Giemsa, and methylene blue. All these stains may be used to improve the identification of the parasite in fecal flotation.
There are several commercially available enzyme-linked immunosorbent assay (ELISA) tests. These tests have a higher degree of sensitivity and specificity in diagnosing infections with Cryptosporidium species than can be obtained from stained fecal flotation smears. A commercially available indirect fluorescent antibody (IFA) test has been shown to be quite effective in diagnosing infection with Cryptosporidium species.
Although cryptosporidiosis is caused by a protozoan parasite with a life cycle similar to that of coccidia, coccidiostats have shown minimal to no efficacy when used to treat infected cattle. An effective chemotherapeutic means of eliminating the parasite is not available.
Human cryptosporidiosis is treated symptomatically. Individuals who become dehydrated should be given appropriate fluids either intravenously or orally. Clinical improvement has been demonstrated in patients treated with dialyzable leukocyte extract from calves immunized with Cryptosporidium species.
Prevention of Transmission to Humans
Infection with Cryptosporidium species can be prevented by good sanitation and hygiene practices when handling young animals, particularly calves. Infants, young children, or immunologically compromised individuals should not handle animals with diarrhea. Immunocompromised individuals should be advised to wash their hands after handling pets, especially before eating, and should avoid contact with their pet’s feces. Many of the agents and processes used to sanitize public drinking water have little effect on Cryptosporidium species.
Trematodes of Public Health Importance
Schistosomes of Wild Migratory Birds (Schistosome Cercarial Dermatitis, Swimmer’s Itch)
Swimmer’s itch is a highly pruritic skin condition in humans caused by repeated penetration of the cercariae of the schistosomes (blood flukes) of wild migratory aquatic birds and small mammals native to the water’s edge. On first exposure of the cercariae, there is mild redness and edema in the skin; with repeated exposure, however, there is severe pruritus (itching) and a papular or pustular dermatitis (a pus-filled pimple). This dermatitis may persist for several days or even weeks and can become secondarily infected.
Human Infection with Swimmer’s Itch
Migratory waterfowl frequently harbor schistosomes (blood flukes) in their blood vasculature. These schistosomes produce eggs that pass in the bird’s feces to the watery environment. The eggs hatch, producing miracidia, which in turn penetrate aquatic snails. Within the snail, the miracidia undergo asexual reproduction and produce thousands of cercariae. These cercariae exit the snail to penetrate the definitive host, the migratory waterfowl.
Humans serve as incidental hosts for these avian schistosomes. During summer, people swim or wade in the lakes, ponds, rivers, and even ocean waters frequented by wild birds. These waters are home to aquatic snails. The cercariae produced within the snails penetrate the skin of humans instead of the skin of migratory birds. The cercariae cannot complete the migration in the human host, and the host’s immune system kills the cercariae. At the same time, the cercariae release allergenic substances that cause severe dermatitis. Repeated exposure produces the highly pruritic, papular or pustular dermatitis, or schistosome cercarial dermatitis (“swimmer’s itch”). This condition may have many other regional or colloquial descriptive names.
Symptoms and Diagnosis
After the cercaria penetrates the skin, a reddened spot appears at the point of entry. The diameter of this spot increases, and the itching commences. If the area becomes raised, it is called a papule and will reach its maximum size in about 24 hours. In severe cases the affected individual may develop a fever, become nauseated, and spend several sleepless nights. The papule itches for several days before subsiding, but in a week or so the symptoms disappear.
Swimmer’s itch can be diagnosed by the observation of typical lesions in skin that has come in contact with pond, lake, stream, or ocean water containing infective cercariae from the snail intermediate host. A history of swimming in infested waters also aids in diagnosis. Laboratory findings usually have no role in establishing a diagnosis of swimmer’s itch.
Suspected cases of swimmer’s itch in humans should always be referred to a dermatologist or family practitioner. Antihistamines are prescribed to relieve the itch and topical steroid creams to reduce the swelling. Remember that these are prescription drugs and must be prescribed by a physician or dermatologist.
Prevention of Transmission to Humans
During the seasonal occurrence of swimmer’s itch, many public health agencies post warnings about swimmer’s itch on beaches adjacent to ponds, lakes, streams, or the ocean. The public should heed these warnings and comply with the ban on swimming in infested waters. Swimming in water away from the shore will reduce the chance of contact with the cercariae; cercariae tend to congregate close to the shoreline. If contact is suspected, the swimmer should towel off immediately after leaving the water.
For those who must work in such waters, protective waterproof clothing is available. Repellents, such as benzyl benzoate and dibutylphthalate, are available. Molluscicides (snail-killing compounds) are available; however, they may have adverse effects on plants and other animals in the environment.
Cestodes of Public Health Importance
Taenia saginata (Beef Tapeworm)
Taenia saginata, the beef tapeworm, parasitizes the small intestine of humans. Its larval or metacestode stage, Cysticercus bovis, is found in the musculature of beef cattle, the intermediate host.
Human Infection with Beef Tapeworm
Humans become infected with the beef tapeworm by ingesting the musculature of beef cattle that contains the cysticercus, or bladder worm, the larval (metacestode) stage for this tapeworm. This cysticercus stage has a scientific name, Cysticercus bovis, and may be found in a variety of sites in the bovine musculature: skeletal and heart muscle, masseters, diaphragm, and tongue. Humans are infected by ingesting raw or undercooked, cysticercus-infected beef. This meat is often referred to as measly beef or beef measles. The covering of the cysticercus is digested away, and the young tapeworm is released and attaches to the wall of the small intestine. This young tapeworm begins to grow its proglottids. The gravid proglottids are released to the outside environment. Each gravid proglottid contains about 80,000 eggs. In the outside environment the gravid proglottids rupture, and the released eggs may be ingested by a beef cow. The eggs hatch, the embryos penetrate the intestinal mucosa and enter into general circulation; they are then distributed throughout the musculature of the beef cow.
Symptoms and Diagnosis
Adult Taenia saginata in the small intestine of humans may cause a variety of nonspecific abdominal signs, such as diarrhea, constipation, and cramps. About 10 gravid proglottids are passed in the feces each day; this fact is quite evident to the infected individual. These proglottids are quite motile and will migrate a few centimeters over the human host’s body, clothes, or bedding.
The gravid proglottids from human feces are unusual in appearance. Each proglottid has a prominent uterus with 14 to 32 lateral branches. The uterus contains approximately 80,000 eggs. Perianal swabs may be used to detect these eggs. Diagnosis of cysticerci in cattle is usually made by meat inspection procedures.
Suspected cases of the beef tapeworm in humans should always be referred to a family practitioner or internist. The treatment of choice is praziquantel or niclosamide.
Prevention of Transmission to Humans
All infected humans should be treated by a physician. Feedlot employees should be educated concerning transmission of bovine cysticercosis and personal hygiene practices. Adequate and accessible toilet facilities must be provided for all workers. Meat inspection should be thorough. Infected carcasses may be condemned for human consumption or treated by freezing for 10 days to 2 weeks at −10° C or by cooking at 50° to 60° C. Likewise, humans should be wary of undercooked meat, to avoid acquired infection with T. saginata (and a variety of pathogenic bacteria).
Taenia solium (Pork Tapeworm)
Taenia solium, the pork tapeworm, parasitizes the small intestine of humans. The larval, or metacestode, stage of this tapeworm, Cysticercus cellulosae, is found in the musculature of pigs, the intermediate host. This is an unusual parasite in that humans can also harbor the larval stage, which may occur not only in the musculature, but also within the eye and brain. Therefore, humans can serve as both the definitive and the intermediate host for this parasite.
Human Infection with Pork Tapeworm
Humans become infected with the pork tapeworm by ingesting the musculature of pigs that contains the cysticercus, or bladder worm, the larval (metacestode) stage. This cysticercus stage has a scientific name, Cysticercus cellulosae, and may be found in a variety of muscle sites: skeletal and heart muscle, masseters, diaphragm, and tongue. Humans are infected by ingesting raw or undercooked, cysticercus-infected pork. This meat is often referred to as measly pork or pork measles. The covering of the cysticercus is digested away, and the young tapeworm is released and attaches to the wall of the small intestine. This young tapeworm begins to grow proglottids. The gravid (terminal) proglottids are released to the outside environment. Each gravid proglottid contains about 40,000 eggs. In the outside environment the gravid proglottid ruptures; the released eggs must be ingested by a pig. The eggs hatch, and the embryos penetrate the intestinal mucosa and reach general circulation, to be distributed throughout the musculature of the pig.
If a human ingests one of the eggs, the egg hatches in the intestine. This embryo penetrates the intestinal mucosa and reaches general circulation, to be distributed not only throughout the musculature of the human, but also in subcutaneous sites and within the brain and the eye. In such sites, tremendous damage can result.
When the metacestode stage develops in the brain, the condition is known as neurocysticercosis. The parasite usually develops in the ventricles and is proliferative.
Symptoms and Diagnosis
Adult Taenia solium in the small intestine of humans may cause a variety of nonspecific abdominal signs, such as diarrhea, constipation, and cramps. Chains of gravid proglottids do not leave the host spontaneously but are passed in the feces each day; this fact is quite evident to the infected individual. These proglottids are quite motile and will migrate a few centimeters over the human host’s body, clothes, or bedding.
Neurologic symptoms vary with the site of the offending cysticercus in the nervous tissue. Pain, paralysis, and epileptic seizures have been associated with neurocysticercosis. Ocular lesions may result in blindness.
The gravid proglottids from human feces are unusual in appearance. Each proglottid has a prominent uterus with fewer than 16 lateral branches. The uterus contains approximately 40,000 eggs. Diagnosis of cysticerci in pigs is usually made by meat inspection procedures.
Sophisticated radiographic imaging techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI), may reveal the presence of cysticerci within the brain and other sites in the central nervous system (CNS).
Suspected cases of pork tapeworm in humans should always be referred to a family practitioner, neurologist, surgeon, or internist. The treatment of choice is praziquantel. Because of the infectivity of the eggs for humans and the resulting CNS involvement, these cases must be handled with great care. Treatment of human cysticercosis is by surgical removal of the offending lesion.
Prevention of Transmission to Humans
All infected humans should be treated by a physician. Feedlot employees should be educated concerning transmission of porcine cysticercosis and personal hygiene practices. Household workers such as maids and cooks should be educated regarding proper hygiene, such as handwashing before preparing a meal. Adequate and convenient toilet facilities must be provided for all workers. Meat inspection should be thorough. Infected carcasses may be condemned for human consumption or treated by freezing for 10 days to 2 weeks at −10° C or by cooking at 50° to 60° C. Likewise, humans should be wary of undercooked meat to avoid acquired infection with T. solium (and a variety of pathogenic bacteria).
Echinococcus granulosus and Echinocuccus multilocularis (Unilocular and Multilocular Hydatid Disease)
Hydatid disease is a syndrome characterized by the development of the larval, or metacestode, stage of a genus of tapeworm, Echinococcus species, found in the small intestine of dogs and cats, the definitive hosts. Hydatid disease is characterized by the formation of large, fluid-filled cysts in the internal organs of the intermediate host. There are two species of importance in veterinary parasitology: E. granulosus, a tapeworm that produces a unilocular (large, singular, thick-walled, fluid-filled) hydatid cyst, and E. multilocularis, a tapeworm that produces a multilocular (multiple, extremely invasive, thin-walled, fluid-filled) hydatid cyst. These hydatid cysts may occur in a variety of internal organs in the human intermediate host: the liver, lungs, kidney, spleen, bone, and brain.
Human Infection with Hydatid Disease
Humans become infected with hydatid disease by ingesting the egg of Echinococcus species. Once ingested, the egg hatches in the intestine of the human intermediate host. The released oncosphere (“growth ball”) penetrates an intestinal venule or lymphatic lacteal and reaches the liver, lungs, or other internal organs. Once in these extraintestinal sites, the oncosphere develops into the hydatid cyst. E. granulosus produces a unilocular hydatid cyst. This cyst has a thick, multilayered cyst wall that keeps the developing cyst restricted to a single compartment. The hydatid cyst may grow up to 50 cm in diameter; however, it will not invade the surrounding tissues of the parasitized organ. E. multilocularis produces a multilocular hydatid cyst. In contrast to the unilocular hydatid cyst of E. granulosus, this hydatid cyst lacks the thick, multilayered cyst wall. Without this cyst wall, the developing cyst is capable of “budding off,” or producing additional compartments, which in turn, bud off other compartments. As a result, this type of hydatid cyst readily invades the surrounding tissues. This multilocular hydatid cyst takes on a “malignant,” invasive role.
Symptoms and Diagnosis
The symptoms of infection with hydatid disease depend on the site where the organism develops; these sites include the liver, lungs, kidney, spleen, bone, and brain. Neurologic symptoms vary with the site of the offending cysticercus in the nervous tissue. Pain, paralysis, and epileptic seizures have also been associated with echinococcosis.
In humans, examination of histopathologic sections of unilocular hydatid cysts reveals the unique structure of the germinal membrane supported by a thicker, acellular, laminated membrane. Protoscolices are contained in the saclike brood capsules. When viewed macroscopically, these brood capsules look like sand. Humans with E. multilocularis develop tumorlike masses, or nodules, in their livers. When sectioned, these masses reveal the alveolar-like microvesicles containing protoscolices.
Sophisticated radiographic imaging techniques, such as CT and MRI, may reveal the presence of hydatid cysts within the brain and other organ sites throughout the body.