Parasites of dogs and cats from wildlife (Proceedings)

There are a number of diseases that dogs and cats acquire from wildlife. Some are acquired through vectors while some are acquired by ingestion. Some of this diseases are without sequelae, and some are very serious. This type of disease, a disease of a domestic animal with a pathogen of a wild animal is a form of zoonosis called zootherionosis.

Dogs

Hepatozoon americanum– This protozoan parasite undergoes sexual development and sporogony in the tick Amblyomma maculatum and merogony and gametogony in the coyote. Infections have been described from the southern United States. Ticks ingest gametocytes contained in the coyote's peripheral blood. Within the tick's gut, gametocytes fuse and then ultimately sporocysts form within oocysts in the body cavity of the tick. Dogs acquire infection by ingesting a tick containing sporulated oocysts. In the dog, released sporozoites penetrate the intestinal epithelium; enter leukocytes in the lamina propria, regional lymph nodes, or liver; and are transported to body tissues where merogony occurs. Schizonts form merozoites, which are released to invade another cell. After multiple schizogonous cycles, some merozoites invade leukocytes and produce gametocytes.

Disease is debilitating and often fatal. Dogs will present with periodic or persistent fever, weakness, muscle atrophy, generalized pain, reluctance to move, mucopurulent ocular discharge, and gradual deterioration of body condition. CBC and chemistries will reveal neutrophilic leukocytosis (leukocyte counts can exceed 200,000/µL), mild to moderate nonregenerative anemia, mild elevation in serum alkaline phosphatase, and occasional hyperglobulinemia. Periosteal proliferation of various bones may be demonstrated on radiographs. Without treatment, death will follow a period of chronic wasting.

Until a reliable serologic test is developed, the true prevalence of infection will be difficult to ascertain. It is unlikely that genetic predisposition, breed, and age are important factors in transmission or manifestation of the disease.

Diagnosis is by finding meronts in muscle biopsy samples or gametocytes in peripheral blood smears. No treatment is effective in eliminating H. americanum in infected dogs, but it can increase survival time, improve quality of life, and decrease the number and severity of clinical relapses. Two treatments are trimethoprim-sulfadiazine (15 mg/kg PO q12h for 14 days) or ponazuril (10 mg/kg PO q12h for 14 days). After treating the acute infection, dogs should be given prolonged therapy with a quinolone anticoccidial agent to help prevent relapses.

Alaria – This is a trematode parasite of raccoons, coyotes, and wolves that cycles through snails and amphibian intermediate hosts. Dogs get infected when they eat the amphibian. This parasite can also live in mammalian paratenic hosts, so, dogs can also become infected when they eat animals such as rodents that may have preyed upon the amphibian hosts. Fortunatley, this parasite does not cause any significant disease in the dog.

Heartworm – Dirofilaria immitis - One reason heartworm may have spread through California and much of the rest of the United States is the burgeoning coyote population. The coyote, Canis latrans, is a canid that is indigenous to the Americas that will support the growth and development of Dirofilaria immitis. It has been shown that heartworms have increased in the coyote population in the northern California counties around San Francisco between an initial sampling in 1975 and 1985 and a more recent sampling in 2000 - 2002. In the survey performed in these counties from 2000 to 2002, some 24% to 57% of the coyotes were infected. A survey of coyotes from throughout the state of California has revealed that some 5% to 60% of the coyote population is infected and spread throughout the state; thus, there are now unprotected reservoir hosts throughout the state. Coyotes have also increased massively in numbers in the eastern United States in the last 50 years markedly increasing their importance as reservoirs of infection putting dogs (and cats) at risk.

Dioctophyma renale - The "giant kidney worm" of carnivorans is one of the largest species of nematodes. Mink are the principal definitive hosts. The female may reach 1 m in length and 1 cm in diameter; males are somewhat smaller (less than 400 mm) and have a terminal bell-shaped copulatory bursa and one spicule. The eggs are passed in the urine of the mink and larvated eggs are infective to oligochaete worms in which they develop to the infective third larval stage. The oligochaetes are eaten by fish or frogs, and the the larvae persist in the tissues of these paratenic hosts. If the infected oligochaete (or paratenic host) is ingested by a dog, the D. renale larvae mature and complete the cycle. In dogs, the worms may be found in the pelvis of the right kidney or free in the abdominal cavity.

Macracanthorhynchus ingens – This acanthocephalan is a parasite of the raccoon (Procyon lotor) and black bear (Ursus americanus). The intermediate hosts are millipedes of the genus Narceus. Dogs will sometimes become infected by ingesting a millipede. Cases in dogs have been treated with ivermectin

Cats

Leishmania braziliensis and Leishmania mexicana – These organisms are found in the southern United States, Mexico, and Central and South America. The vector is a sandfly of the genus Lutzomyia. Within the mammalian host the parasite grows and multiplies within macrophages of the skin at the site of the bite by the vector. Within the sandfly are flagellated promastigote forms of the parasite.

There have been a few reports of cutaneous leishmaniasis in cats in the Americas. Lesions have been reported in the ear of a four-year-old long-haired domestic cat in Texas; the cat had no signs of systemic disease and was returned to its owners after a radical pinnectomy. Three cats (1 male and two females) have been found to have lesions in Venezuela. On the male cat, the lesions were on the nose and ears; on the female cats lesions were only on the nose. Recently there have been several sporadic reports of Leishmaniasis in cats in Brazil. There are no reports of attempted treatment of infected cats.

Cytauxzoon felis – This parasite is transmitted to cats from bobcats, Lynx rufus, by the bite of the American Dog Tick, Dermacentor variabilis. This parasite first appeared in domestic cats in Missouri in 1973, and the concern at that time was that it was an introduced species of importance to agriculture, perhaps related to Theileria parva. It was also at this time described as a new species. Cytauxzoon felis.

Two life cycle stages, schizonts and merozoites, occur in the feline host. Schizonts in histiocytes and macrophages of the bone marrow, veins, and venules of various organs, including the lungs, liver, spleen, lymph nodes, brain, and kidneys. Merozoites occur in circulating red blood cells later in the infection, and therefore, often will not be found in cats dying of acute disease.

The bobcat is the natural definitive host of C. felis. In domestic cats that are infected through the bite of the tick vector, schizonts develop within macrophages, which become markedly enlarged. Cats with acute disease typically develop anemia, depression, fever, dehydration, and icterus. The majority of cats die within nine to 15 days of infection. The cause of death is occlusion of veins and venules with schizont-laden macrophages. Hematologic changes may be severe, and result from displacement of hematopoietic tissue within the bone marrow. If the cat survives for more than six days, erythrocytes become infected and the merozoite stage develops, typically with no more than 1% to 4% of red blood cells infected. In the case of the bobcat, the schizogonous stage is shortened, and they become prolonged carriers of the erythrocytic stage.

In a recent survey of cases from the mid-Atlantic states of the US, of 34 cats infected with C. felis, 32 succumbed to the infection. The most common signs are pancytopenia and icterus. During the acute disease, organisms can be identified in smears of bone marrow or in biopsy specimens. Chronic disease is diagnosed by finding the merozoites in red blood cells. No treatments are consistently efficacious during the acute stage of the disease. Antiprotozoals with supportive care and antibiotics are often administered, but the prognosis remains poor. This is a disease that is currently best prevented by keeping cats indoors in areas where the ticks are active or using products that will kill or repel ticks.

Natural cases of cytauxzoonosis in cats have typically been described from the southeastern and south central United States, cases being reported from Kansas, Oklahoma, Missouri, Arkansas, Texas, Louisiana, Mississippi, Georgia, and Florida. More recently, C. felis has been diagnosed in Kentucky, Indiana, Tennessee (70 cases), coastal North Carolina and South Carolina. Thus, it is becoming obvious that cytauxzoonosis is spreading beyond its typical confined range in the south-central United Stated up into the lower Midwest.

As more and more people move into housing developments on the fringes of tick areas, it can be expected that more and more cases of this and other tick-transmitted diseases will be seen. This is due in part to the continued expanding range of the American bobcat, particularly in the Midwestern and several mid-Atlantic States. The range is increasing and the numbers are increasing. It is estimated that there are now at least some 1.5 million bobcats in the United States, but this may be an underestimate due to poor animal sighting abilities by census takers. The annual harvest for fur was about 35,000 per year in the mid 1990s. In recent years, there has been an increase in the harvest due to the demand of the fashion industry, but this does not seem to be decreasing populations in any areas. The expanding range of the host of the parasite and the vector would be expected to expand the range of both agents. Thus, it should be expected that cats will be under greater and greater risk of disease due to this pathogen throughout the United States.

Eurytrema procyonis - This trematode of the raccoon, Procyon lotor, is sometimes found in cats. The final host is probably infected via ingestion of an arthropod. The flukes commonly live within the pancreatic ducts, but they have also been reported in the gall bladder and bile ducts. In a number of feline reports, diagnosis has been made postmortem. One cat with a sub-clinical infection, diagnosed by detection of E. procyonis eggs in its feces, had normal serum amylase, lipase, and fecal trypsin activities. In one infected cat, serum pancreatic lipase immunoreactivity was increased. These findings supported the possibility of fluke-associated pancreatitis. Another cat that was euthanatized and necropsied after two years of persistent weight loss, vomiting, and ultimately incoordination had a fibrotic pancreas containing numerous E. procyonis in its ducts that were lined with hyperplastic epithelium infiltrated with eosinophils and mononuclear cells. Abdominal sonography may reveal an enlarged hypoechoic pancreas with a hyperechoic rim, and a distended and thickened pancreatic duct. Treatment with praziquantel, pyrantel, and febantel has produce clearance of the flukes and associated with resolution of sonographic abnormalities and normalization of pancreatic lipase immunoreactivity.

Trichinella spiralis – Cats acquire infections of this nematode parasite like other hosts by eating infected meat. Cats experimentally fed infected meat showed lethargy, weakness, weight loss, and hypersalivation, with other signs including diarrhea. Other experimentally infected cats have shown very little in the way of signs of infection, even though they have ultimately supported rather high loads of muscle larvae (about 300 larvae per gram of muscle). Cats that have been experimentally infected have not developed significantly increased numbers of circulating eosinophils. In a naturally-infected cat, the highest eosinophil count reported during a 5 month period the cat was followed was 22%. In this naturally infected cats, the initial presentation was puddles of fresh gelatinous, bloody feces in the litter box. About 3 weeks later, the cat underwent an episode of disorientation based on the observed unusual behavior.