Parasitic diseases you should know about (Proceedings)

Toxocara spp. ("Roundworms")

Roundworms of dogs and/or cats (Toxocara canis, T. cati) are large (up to 4.5 inches), stout worms that live as adults in the lumen of the small intestine. T. canis is the species commonly found in dogs, T. cati is restricted as adult worms to cats. Both T. canis and T. cati usually undergo hepatotrachael migration prior to establishment in the small intestine. In older dogs and cats, a greater number (but not all) of migrating larvae are diverted to extra-intestinal tissues. The period of time from exposure to the parasite until mature worms are present in the intestine varies based on the route of infection. Generally, this period is between 2 and 5 weeks.

T. canis and T. cati remain prevalent in pets, based on recent surveys. Once such survey indicated that T. canis eggs were present in the feces of almost 15% of 6,458 dogs sampled from shelters in the United States. Although prevalences were reduced somewhat in adult dogs, the survey results indicate that T. canis is not restricted to puppies.

T. canis is a common parasite of dogs, regardless of geographic location, because of the potential for infection by several routes. These include embryonated eggs, transplacental transmission, transcolostral transmission (less common for T. canis), and by ingestion of transport hosts such as rodents and rabbits. Transplacental transmission accounts largely for the prevalence of T. canis in neonatal and juvenile dogs. Although age-associated immunity aids in the expulsion of some T. canis adults as dogs age, it is not completely effective in elimination of the parasites.

T. canis is important not only because of its ability to produce primary disease in dogs, but also because of its ability to produce several extra-intestinal disease syndromes known as "larva migrans" in many hosts, including humans. When ingestion of embryonated eggs results in migration and damage to internal organs, the syndrome is referred to as visceral larva migrans (VLM). VLM is observed most often in children less than 3 years of age. Encysted larvae induce nodules in organs such as liver, lungs, kidney and brain. Such infections generally manifest as profound eosinophilia, pneumonitis and hepatomegaly. Serological surveys suggest that exposure rates to Toxocara larvae vary from 3% in one US study, to 23% in other studies, depending on the region, socioeconomic group that is tested, and whether the test population resides in urban or rural areas. In older children (generally 3-13 years) the larvae apparently have a more pronounced predilection for the posterior chamber of the eye. The resulting granulomatous retinitis is the hallmark of this second syndrome known as ocular larva migrans (OLM). OLM can result in severe ocular damage and subsequent retinal detachment, loss of vision, and even blindness. Interestingly, OLM can occur in the complete absence of eosinophilia and signs or evidence of VLM. One graphic example of the potential for Toxocara spp. to cause ocular disease was the report in which an Atlanta, GA ophthalmology practice indicated that 37% (41 cases) of retinal diseases in children seen in children during a 18 month period were caused by Toxocara spp. Although T. cati is thought to be associated less often with human infections, recent research indicates that it too can cause them.

Adult female roundworms are prolific egg producers. For example, females of T. canis are estimated to produce between 25,000 and 85,000 eggs per day. Females of T. cati are capable of producing between 19,000 and 24, 000 eggs per day. Given these rates of egg production, it is easy to see how environments can eventually harbor large numbers of eggs. Parasite reproductive rates, combined with the resistance of embryonated eggs to adverse environmental conditions, can increase the risk of exposure and infection to both pets and humans. Roundworms infections can be controlled either by strategic deworming with narrow or broad spectrum agents or by repeated monthly administration of available broad spectrum heartworm preventives.

Baylisascaris procyonis ("Raccoon Ascarid")

Baylisascaris procyonis, is a prevalent and important parasite of raccoons. It is similar in structure and behavior to Toxocara spp. dogs and cats. However, it is not its disease consequences in raccoons that are important, but its capability to cause larva migrans in many other hosts. The raccoon has adapted well to encroachments by humans into its habitats. Given the high prevalences of Baylisascaris infection in raccoons (68%-82% in surveys in the United States), the raccoon's broad geographic range, and prevalences in urban environments, it is easy to see why B. procyonis is the most common and widespread cause of larva migrans in animals. Although larvae of Baylisascaris can invade a variety of tissues in man and other animals, including the eye, it is particularly prone to invasion of the central nervous system. The resulting "neural larva migrans" is the most serious of the migrans syndromes in humans or animals. The seriousness of the condition is a factor of rate of growth and size achieved by larvae compared to other ascarids. Documented infections in humans have resulted in severe, sometimes fatal encephalitis.

Baylisascaris is particularly prevalent in young raccoons, resulting in very high fecal egg shedding rates. The high shedding rates, combined with the raccoon's habit of using communal defecation sites ("latrines"), can result in environments with astonishingly high egg numbers. When these contaminated areas occur in close proximity to urban developments, the risk of human infections increases immensely. Veterinarians should discourage clients from feeding raccoons or adopting them as pets. Clients should also be advised of the potential for environmental contamination with eggs, particularly sites such as fallen trees, tree stumps, and woodpiles that might serve as communal latrines.

Ancylostoma spp. ("Hookworms")

Canine and feline hookworms are small (up to about 0.75 inches) whitish or reddish-brown worms with a hooked anterior end (hence the name). As adults, they reside in the small intestine of dog, cats and rarely humans. Ancylostoma spp. include A. caninum (the universally distributed canine hookworm), and A. braziliense (found in both dogs and cats in the subtropical US). In the survey mentioned above, A. caninum eggs were recovered from almost 20% of 6,458 fecal samples from shelter dogs.

Developmental cycles of hookworms include a free living phase in which larvae hatch from eggs and develop through 3 distinct stages. The 3rd environmental larval stage (infective stage) enters the host either by oral ingestion or by skin penetration. Most (but not all) orally ingested larvae establish in the intestine without extraintestinal migration. Those that penetrate the skin follow a vascular/pulmonary migration prior to their establishment in the small intestine. In addition, both prenatal (transplacental) and transmammary modes of transmission can occur. The reservoir for such larvae is somatic as is the case with T. canis.

Hookworms may cause dermal disease, pulmonary disease and intestinal disease. The latter is the most common syndrome in the dog. Hemorrhagic enteritis caused by A. caninum can be a peracute, life-threatening disease in young dogs. In these animals, the transmammary route of infection can lead to the establishment of very high worm burdens in neonatal dogs in a short period of time. The remaining species are less significant pathogens, but not always innocuous. Ancylostoma caninum, similar to T. canis, is a prolific egg producer. It is estimated that mature females of A. caninum can produce up to 20,000 eggs per day. This magnitude of fecundity can result in substantial environmental reservoirs of infective larvae in rather short periods of time.

Free-living infective larvae of some Ancylostoma spp. may penetrate the skin of humans and migrate subsequently for short periods of time. These dermal wonderings result in reddish, pruritic, serpentine lesions. This condition is referred to as cutaneous larva migrans (CLM) or "creeping eruption". Although less significant than the larva migrans syndromes caused by the roundworms, the cutaneous syndrome caused by the hookworms remains a concern for veterinarians and pet owners. Larvae of Ancylostomabraziliense appear to be the most common cause, although cases of CLM caused by A. caninum have been documented. Recent evidence suggests that adults of A. caninum may also inhabit the intestines of humans. More than 200 such cases of "eosinophilic enteritis" have now been reported in the medical literature. Human infections with adult A. caninum can result in both acute and chronic signs. Clinical signs included recurrent abdominal pain, small bowel thickening, eosinophilia, increased levels of IgE and also inflammation of the distal ileum and colon.

As discussed for roundworms, hookworm infections can be controlled either by strategic deworming with narrow spectrum or broad spectrum agents, or by repeated monthly administration of available broad spectrum heartworm preventives.

Trichuris vulpis

Whipworms are common inhabitants of the cecum of dogs. My national survey of 6,458 dogs indicated that whipworms was more common than roundworms in 4 of 5 geographic regions. Data from a retrospective study of dogs presented to the University of Pennsylvania College of Veterinary Medicine indicate that canine hookworm and whipworm appears in combination infections more than other common internal helminthes. Cats are host to whipworms only rarely. These parasites have thin lash-like anterior ends and larger rounded posterior ends. Their anterior ends are "entwined" in the mucosa, while their posterior ends remain free in the lumen of the cecum. Years ago, the anterior "whip lash" was thought to be the posterior end. Consequently, they were first given the name Trichocephalus. This name is no longer used. Whipworms can demonstrated in any dog old enough to harbor adult worms (generally > 3 months). The life cycle is direct: eggs are shed nonembryonated in feces and embryonate in about 2 weeks. Embryonated eggs are extremely resistant to adverse environmental conditions and may survive for years in semi-protected habitats. Ingested embryonated eggs develop through additional larval stages and mature to adult worms in 70 to 90 days (some say the developmental period may be protracted to as long as 16 weeks). Heavy infections result in bloody, loose, mucoid feces. Unless treated and given supportive therapy, serious canine trichurosis can be a life threatening disease. T. vulpis is more problematic in outdoor dogs restricted to kennels with a soil or similar substratum that provides protection for the long-lived eggs. Diagnosis, treatment and control of canine T. vulpis infections can be difficult because of the density of the eggs, their intermittent shedding in feces, and the long development period. Brownish, bipolar eggs must be differentiated from eggs of capillarids. Keep in mind that some dogs may practice predation or coprophagy. As such they may ingest the viscera or feces of other animals that are infected with whipworms for which the dog is not a host. Whipworm infection can be treated strategically with products such as fenbendazole, Drontal Plus, or animals can be placed on a broad spectrum agent with heartworm prevention capabilities such as Interceptor, Sentinel, or Advantage Multi. The latter is preferred in chronic, difficult to manage cases. Treatment of the environment is not practical. However, replacing the soil or gravel substratum by tillage or entombing under the prior substratum under asphalt or concrete can help reduce the rate of reinfection. Canine whipworms are not considered zoonotic parasites.

Dipylidium caninum ("flea tapeworm", cucumber seed tapeworm")

Dipylidium caninum is a common tapeworm of dogs and cats. Current survey data suggests that intestinal cestode infections such as D. caninum and Taenia pisiformis (see below) are underdiagnosed. Dipylidium caninum is transmitted (cysticercoid larval stage) by consumption of fleas usually during the pet's self-grooming. Proglottids are elongate and contain bilateral reproductive openings (pores). Eggs are contained in egg packets containing up to 30 eggs. Eggs are not enclosed in a demonstrable striated embryophore as is characteristic of Taenia spp. Human infections with D. caninum occur when fleas are inadvertently ingested, usually by small children. Although not usually of pathogenetic significance, infections in children are a cause of considerable alarm and distress among parents and care-givers when proglottids are passed in feces or are found in soiled diapers. Human infections are best prevented by controlling D. caninum in dog and cat hosts. Flea control is a must is complete elimination of D. caninum is to be expected.

Tritrichomonas foetus

There are increasing numbers of reports of large bowel diarrhea in cats concurrent with infection with Tritrichomonasfoetus. A flagellated protozoan bearing some similarity to Giardia. Infected cats present with explosive, fetid diarrhea. Feces are usually foamy, light in color and may contain blood. Many of the affected kittens were purebreds and also suffered from inflammatory bowel disease. Direct examination of feces often revealed uninucleate, flagellated organisms, occasionally in very high numbers. Treatment with enrofloxacin and metronidazole at various dosages and for extended periods of time resulted in improvement of the condition in some, but not all of the cases. Recent use of paromomycin also has achieved some success (Table 11). Although the role of the flagellate in the pathogenesis of the disease is not known with certainty, it should be considered a possible etiologic agent in cases of large bowel diarrhea in cats, particularly when it can be demonstrated by direct examination of feces.

Treatment of canine, feline and human giardiasis*

Taenia pisiformis ("dog-rabbit tapeworm")

Taenia pisiformis is a common enteric cestode parasite of dogs and cats. It is second only to Dipylidium caninum in its prevalence in North America. Taenia pisiformis is transmitted (cysticercus larval stage) by canine predation on small mammals. The most common intermediate host is the rabbit. Taenia pisiformis is a larger tapeworm (up to 6.6 ft.) than D. caninum. Its proglottids are rectangular in shape and contain single lateral reproductive openings (pores). If present in feces, eggs are singular and contain a hexacanth embryo surrounded by a distinctly striated embryophore. Eggs are seldom recovered from feces because of their density compared to traditional fecal flotation solutions. As mentioned above, because of the infrequent passage of proglottids or eggs, taeniid tapeworms are greatly underdiagnosed in dogs and cats.

Treatment of canine and feline coccidiosis.

Echinococcus spp. ("hydatid tapeworms")

Tapeworms are common intestinal parasites of dogs and cats. Many tapeworms are of little disease importance and present little danger to either humans or domesticated animals. Echinococcus species (hydatid tapeworms) are important exceptions. Human and animal echinococcosis, acquired through contact with the feces of infected canids or felids, is a potentially serious disease, requiring constant surveillance by knowledgeable, trained personnel. Echinococcus granulosus uses canids as definitive hosts and many omnivores and herbivores as intermediate hosts. Echinococcus multilocularis uses canids and felids as definitive hosts and microtine rodents (voles, lemmings, muskrats, water rats) as principal intermediate hosts. Echinococcus granulosus and E. multilocularis undergo complex cycles of development involving the following morphologically distinct stages: (1) the adult tapeworm (2-11 mm) which inhabits the small intestine of the canine or feline definitive host; (2) the egg, which contains the larval tapeworm (oncosphere); and (3) the metacestode, which contains infective protoscolices in either unilocular or multilocular cysts within the intermediate host. The oncosphere is enclosed within a striated wall (embryophore), which renders the eggs similar in appearance to those of Taenia spp. The metacestode (hydatid) is the replicative stage in the life cycle during which the parasite increases its numbers. When fully developed, the hydatid of E. granulosus consists of a single (unilocular) fluid-filled cyst. The cyst wall is a multilaminar structure which gives rise to brood capsules, each containing numerous protoscolices. The hydatid of E. multilocularis is alveolar-like and grows by outward budding of the germinative layer. Invasive growth into surrounding tissues forms many adjacent chambers (hence alveolar or multilocular) containing protoscolices. Both E. granulosus and E. multilocularis are transmitted through predator-prey cycles. In each instance, the carnivore definitive host becomes infected by ingesting the larval metacestode (hydatid containing protoscolices) within the intermediate host. Eggs and disintegrating gravid proglottids, excreted in the definitive host's feces, are dispersed widely in the environment. Intermediate hosts (including humans) ingest eggs and become infected. Oncospheres are liberated in the intermediate host's intestinal tract and are distributed to many extraintestinal sites via the venous and lymphatic systems. Development leads to the formation of a fluid-filled unilocular hydatid (E. granulosus) or a multilocular hydatid (E.multilocularis) in many organs. The life cycle is completed when the definitive host ingests the hydatid stage within the viscera of the intermediate host. Disease in intermediate hosts is caused by either the unilocular or multilocular hydatid cyst. In intermediate hosts, cysts of E. granulosus are usually found within the liver and lungs. Other less common sites include kidneys, spleen, heart, bones, and CNS. Disease caused by E. multilocularis is more serious than that caused by E. granulosus. The infection is progressive and malignant. Most multilocular hydatid cysts locate in the liver, rarely in other organs.

Treatment of feline trichomoniasis

Fecal flotation is not a reliable means of demonstrating Echinococcus infections in definitive hosts. The eggs are similar to those of taeniid tapeworms, and they are excreted erratically in feces. Diagnosis of hydatidosis in intermediate hosts such as livestock or wildlife is best accomplished at necropsy of suspect animals. An array of advanced techniques including radiography, computerized tomography, ultrasonography and scintigraphy has been applied to antemortem diagnosis of human infections. Such procedures, augmented by immunologic assays have proved useful in the detection of human infections. Cestocides are available for treatment of both juvenile and adult Echinococcus tapeworms in definitive and intermediate hosts. Treatment of infected dogs or cats with effective cestocides is the best means of control in urban environments.

Giardia spp.

Giardia spp. are dimorphic enteric flagellates that infect the intestine a wide range of vertebrate hosts. The genus Giardia consists of several valid species that parasitize mammals, birds, and amphibians. Giardia duodenalis, the principal species, can be further divided into at least 7 different genotypes. Human infections are caused by two assemblages of G. duodenalis that infect a wide variety of domestic animals or non human primates.

Giardia stages consist of a flagellated, binucleate trophozoite, and a quadrinucleate cyst . The trophozoite attaches to the surface of epithelial cells in the small intestine; formation of cysts occurs in the ileum, cecum or colon. Like Cryptosporidium, cysts of Giardia are immediately infective when passed in feces. Infections result from ingestion of cysts in contaminated environments, food, and water.

Although the mechanism(s) of Giardia-induced disease remains unknown, evidence suggests that the disease is likely multifactorial involving inhibition of brush border enzymes or other factors such as altered immune responses, nutritional status of the hosts, presence of intercurrent disease agents, and the strain or genotype of Giardia involved in the infection. Most infected animals remain asymptomatic, The most common presenting sign in clinically affected animals and humans is small bowel diarrhea. Feces are usually semi-formed, but may be liquid. Blood usually is not present in animal infections. Feces have been described as pale (often gray or light brown), fetid and containing large amounts of fat. Dogs or cats with giardiasis may present with poor body condition, weight loss, and occasional vomiting. It is not unusual to find Giardia coincidentally with other gastrointestinal diseases such as inflammatory bowel disease.

Giardiasis is best diagnosed by fecal flotation using zinc sulfate (specific gravity = 1.18-1.20). Centrifugation of the preparation increases the likelihood of recovering cysts. Also, the addition of a small amount of Lugol's iodine to the slide prior to placement of the coverslip will aid in visualizing the small (10-12 ?m) cysts. Use of barium sulfate, antidiarrheals or enemas prior to sampling feces may interfere with detection of cysts and should be avoided if possible. Other diagnostic techniques that can be used to recover trophozoites, cysts, or proteins produced by the parasite include direct examination of feces (wet-mount), immunofluorescent procedures, and ELISA. These techniques are either too insensitive (direct examination) or impractical for the practicing veterinarian because of cost, necessary equipment or because of the effort required to conduct the test.

Given that cysts of the different genotypes are not differentiable based on their structure, it is best to be conservative about the potential for human infections with animal genotypes of Giardia. Consequently, it is my view that all animals that are positive by fecal examination should be treated. Several options are available for treatment of Giardia infections in dogs or cats. It is good practice to treat all animals in a household or kennel that have had contact with infected animals. Bathing animals to remove adherent fecal debris can aid in the control of giardiasis. Provide all animals with clean water since contaminated water is a common source of infection for both humans and animals. A commercial vaccine is available to aid in the prevention and control of giardiasis in dogs and cats. Though normally not considered a first line vaccine, vaccination for Giardia should be considered for pets in multiple-pet households, kennels or catteries, or in situations in which giardiasis has been a problem. It has also been suggested that the vaccine might augment treatment of giardiasis in certain problem cases.

Coccidiosis (Isospora [Cystoisospora spp[)

Although canine or feline coccidiosis is not of zoonotic importance, they do represent important diseases of pet animals. Coccidial infections in dogs and cats are caused by Isospora spp. (also called Cystoisospora). Our recent surveys indicated that about 5% of shelter dogs sampled were passing coccidial oocysts in their feces. The principal agents in the dog are I. canis and I. ohioensis. The principal agents in the cat are I. felis and I. rivolta. These parasites reside in the posterior small intestine and in the large intestine for some species. Their life cycles are generally self-limiting, after which the infection is terminated. The parasites replicate first asexually by schizogony resulting in destruction of many host enterocytes in which they develop. Asexual development is followed by production of gametes that fuse to produce noninfective oocysts that are passed in feces. The developmental cycles in the canine or feline host require 5-9 days depending upon the species. Development to the infective stage (sporulation) requires 1-2 days in the animal's environment. Only sporulated oocysts are infective to susceptible hosts. Clinical signs of coccidiosis include hemorrhagic or mucoid diarrhea, abdominal pain, dehydration, anemia, weight loss and emesis, as well as respiratory and neurologic signs. Death can ensue in extreme cases, particularly in young puppies and kittens. Nursing animals, recently weaned animals, or those that are immunocompromised are more likely to develop clinically apparent infections. There is some indication that stress that results from shipping can lead to outbreaks of coccidiosis in young dogs. It is my belief that clinical coccidiosis in adult animals reflects either underlying concomitant infection and/or immunosuppression. I base this on the self-limiting nature of the life cycle and on research that indicates that experimental reproduction of coccidiosis in adult dogs is difficult following inoculation with sporulated oocysts. Diagnosis of coccidiosis is based on signalment (usually puppies and kittens), clinical signs, and recovery of oocysts in feces. Fecal flotation remains the most practical means of recovering oocysts. A point to remember is that recovery of oocysts alone in feces is not sufficient proof to implicate coccidia as the cause of clinical signs. I have observed coccidial oocysts in the feces of many animals without evidence of intestinal disease. Oocysts of the coccidia mentioned above are round to oblong and measure from 16-51 m long, depending on the species. Oocysts of I.canis and I. felis are large (34-51 m), while those of I. ohioensis and I. rivolta are smaller (16-28 m). Although sulfadimethoxine is the only approved anticoccidial medication for use in dogs or cats, several agents have been used with success. I suggest that the amprolium regimen be given priority based on our successes. I am particularly supportive of the drinking water regimen for control of coccidiosis in dogs. Recent research suggests that ponazuril (Marquis, Bayer Healthcare) also is effective against coccidia. Continuing research will likely provide more definitive data regarding its use in companion animals. Many inquiries about outbreaks of coccidiosis that I have responded to have involved young animals housed in groups such as in kennels or cages maintained by breeders or in pet stores. In most of these situations, the use of the drinking water regimen has resulted in a resolution of the problem. Little can be done to disinfect environments because of the ability of the oocysts to withstand chemicals and adverse environmental conditions. Good sanitation, including prompt removal of feces to prevent development of oocysts to the infective stage and treatment of dams and queens with anticoccidial agents prior to parturition, have been shown to reduce the occurrence of coccidiosis in young animals.

References Available On Request