Parasitic Diseases of Rabbits
Coccidiosis is a common and worldwide protozoal disease of rabbits. Rabbits that recover frequently become carriers. There are two anatomic forms: hepatic, caused by Eimeria stiedae, and intestinal, caused by E magna, E irresidua, E media, E perforans, E flavescens, E intestinalis, or other Eimeria spp. Transmission of both the hepatic and intestinal forms is by ingestion of the sporulated oocysts, usually in contaminated feed or water.
Severity of disease depends on the number of oocysts ingested. Young rabbits are most susceptible. Affected rabbits may be anorectic and have a rough coat. Hepatic coccidiosis is most often subclinical, but growing rabbits may not make normal gains. Infrequently, death may follow a short course. Rabbits usually succumb within 1 mo after a severe experimental exposure. At necropsy, small, yellowish white nodules are found throughout the hepatic parenchyma. In the early stages, they may be sharply demarcated, whereas in the later stages they coalesce. The early lesions have a milky content; older lesions may have a more cheese-like consistency. Microscopically, the nodules are composed of hypertrophied bile ducts or gallbladder. Diagnosis of this form of coccidiosis is based on the gross and microscopic changes, along with demonstration of the oocysts in the bile ducts. An impression smear of a lesion in the liver examined under light microscopy often reveals oocysts. The oocysts may also be demonstrated by fecal flotation. It is important not to confuse the oocysts with the normal yeast (Cyniclomyces guttulatus), which is commonly seen in fecal examinations.
Treatment is difficult, and control rather than cure is expected. Sulfaquinoxaline administered continuously in the drinking water (0.04% for 30 days) prevents clinical signs of hepatic coccidiosis in rabbits heavily exposed to E stiedae. However, it may not prevent the lesions. Sulfaquinoxaline may also be given in the feed at 0.025% for 20 days, or for 2 days out of every 8 days. A single oral dose of sulfadimethoxine at 50 mg/kg, followed by its inclusion in drinking water at 1 g/4 L for 9 days, was found to significantly reduce fecal oocyst count. Because feed-grade sulfaquinoxaline can be difficult to obtain, liquid sulfaquinoxaline is used more commonly. Withdrawal time is 10 days for rabbits used for food. Other coccidiostats that may prove to be effective include amprolium (9.6% in water or 0.5 mL/500 mL), salinomycin, diclazuril, and toltrazuril. A single oral dose of toltrazuril at 2.5 mg/kg or 5 mg/kg has also significantly reduced oocyst count. Treatment is best administered for a minimum of 5 days and repeated after 5 days. Rabbits treated successfully are immune to subsequent infections.
Treatment will not be successful unless a sanitation program is instituted simultaneously. Elimination of fecal-oral transmission of infective oocysts is achieved by preventing feed hoppers and water crocks from becoming contaminated with feces. Hutches should be kept dry and the accumulated feces removed frequently. Wire cage bottoms should be brushed daily with a wire brush to help break the life cycle of the protozoa. Ammonia (10%) solution is lethal to oocysts and is the best choice to disinfect cages or ancillary equipment exposed to fecal material.
This form of coccidiosis can occur in rabbits receiving the best of care, as well as in rabbits raised under unsanitary conditions. Typically, infections are mild, and often no clinical signs are seen. In early infections, there are few lesions; later, the intestine may be thickened and pale. Good sanitation programs that can eliminate hepatic coccidiosis do not seem to eliminate intestinal coccidiosis. Intestinal coccidiosis is generally diagnosed by fecal flotation and microscopic identification of the oocysts (species). It is important to distinguish coccidian oocysts from the nonpathogenic yeast Cyniclomyces guttulatus that can also be found in large numbers. Treatment is similar to that for hepatic coccidiosis except that sulfaquinoxaline is given for 7 days and repeated after a 7-day interval.
Although adult tapeworm infections are rare in domestic rabbits, the discovery of larval tapeworm cysts on the serosal peritoneum is common. Rabbits are intermediate hosts for two species of canine tapeworm, Taenia serialis and T pisiformis. Although T serialis is rare in domestic rabbits, it is somewhat more common in wild ones. The larval stage of T pisiformis, a cysticercus, is found attached to the mesenteries. Before forming these fluid-filled cysts, the young larvae migrate through the liver, where they leave white, tortuous subcapsular tracts. Generally, there are no clinical signs, and diagnosis occurs at necropsy. Treatment is usually not attempted, but control is accomplished by restricting access of dogs (the final host of the tapeworm) to the area in which food and nesting material are stored. Mebendazole at 1 g/kg of feed (50 mg/kg) for 14 days is reported to be an effective treatment.
Baylisascaris procyonis has been reported in rabbits. Signs are similar to those induced by Encephalitozoon cuniculi. No treatment is available.
The ear mite Psoroptes cuniculi is a common parasite of rabbits worldwide. Mites irritate the lining of the ear and cause serum and thick brown crusts to accumulate, creating an “ear canker.” Infested rabbits scratch at and shake their head and ears. They lose weight, fail to produce, and suffer secondary infections, which may damage the inner ear, reach the CNS, and result in torticollis (see Rabbits:Clinical Findings). The brown crumbly exudate should never be removed in a conscious rabbit, because this is very painful. The crusts will slowly slough off as the mites die and the tissue underneath heals. The incidence is much lower when rabbits are housed in wire cages instead of solid cages. The mite is readily transmitted by direct contact. Rabbits should be treated systemically with any of the miticides approved for use in dogs and cats. A variety of injectable ivermectin treatment regimens effective against both fur and ear mites have been reported, with the dosage of ivermectin 200–400 mcg/kg, SC, two or three treatments 10–21 days apart. Mites may also be treated with selamectin (20 mg topically every 7 days has been effective).
Fur mite infestations are common, and two genera, Cheyletiella and Listrophorus, are found worldwide. A number of different species of the genus Cheyletiella are found on rabbits. The most common in North America is C parasitovorax. The genus Listrophorus has but one species, L gibbus. These mites live on the surface of the skin and do not cause the intense pruritus seen with sarcoptic mange. Fur mite infestations usually are asymptomatic unless the rabbit becomes debilitated. Cheyletiella may be noticed as “dandruff.” Scraping the dandruff onto a dark paper or background will demonstrate the “walking dandruff,” as Cheyletiella is called. Transmission is by direct contact. Diagnosis is accomplished by skin scraping and light microscopy. Cheyletiella mites may cause a mild dermatitis in people, especially on the arms. Weekly dusting of animals and bedding with permethrin powder can control Cheyletiella mites.
Rabbits are rarely infested with either Sarcoptes scabiei or Notoedres cati. These mites burrow into the skin and lay eggs. The rabbits are extremely pruritic, and the parasites are difficult to eliminate on domestic rabbits. The condition is extremely contagious and can be transmitted to people.
Fleas of the Ctenocephalides felis, C canis, and Pulex irritans species can affect rabbits and many other animals. Imidacloprid is a flea adulticide; the feline dose should be divided in two or three spots to treat rabbits infested with fleas. Fipronil is contraindicated for use in rabbits because of potential toxicity. Flea collars are also not recommended. It is important to also treat every cat and dog in the house, because the original host is not usually the rabbit.
Encephalitozoon cuniculi is a widespread protozoal (microsporidian) infection of rabbits and occasionally of mice, guinea pigs, rats, and dogs. Usually, no clinical signs are seen, but a few rabbits develop mild, chronic renal disease. Some develop brain lesions that may result in convulsions, tremors, or head tilt. Head tilt is often caused by bacterial infection with Pasteurella multocida; this can be difficult to distinguish from head tilt associated with E cuniculi infection, because both infections are common and can occur together. The mode of transmission is not definitively known, but the organism is shed in the urine and potentially can infect the fetus in utero. It seems to be mildly contagious in a rabbitry. At necropsy, the most significant macroscopic lesion is pitting of the kidneys. Microscopic lesions consist of focal granulomas and pseudocysts in the brain and kidneys. Sometimes a severe, focal, interstitial nephritis is seen. In some cases, an ocular manifestation is a key clinical presentation, and lesions range from phacoclastic uveitis to mature cataracts. Diagnosis is made by histologically identifying the lesions (pseudocysts) and observing the organisms when stained with Giemsa, Gram, or Goodpasture carbol fuchsin stains. Several serologic and skin tests are helpful in screening rabbits for antibodies to the organism, but positive serology only indicates past infection and does not confirm a diagnosis. Effective treatment has not been established. Some evidence suggests that oxibendazole or albendazole (20–30 mg/kg/day, PO, for 7–14 days, then 15 mg/kg/day, PO, for 30–60 days) or fenbendazole (20 mg/kg/day, PO, for 28 days) may be effective. Prevention entails good sanitation and possibly serologic screening of breeding stock with elimination of positive reactors. A differential diagnosis in rabbits with outdoor access is an aberrant migration of Baylisascaris spp into the nervous system. Encephalitozoonosis is an emerging disease of immunodeficient people.
Passalurus ambiguus, the rabbit pinworm, usually is not clinically significant but often is upsetting to owners. It is common in many rabbitries and is distributed worldwide. Transmission is by ingestion of contaminated food or water. The adult worm lives in the cecum or anterior colon. Diagnosis is made by observing the adults at necropsy or by finding the eggs during examination of the feces. Single treatments are not very effective, because the life cycle is direct and reinfection is common. Piperazine citrates in the water (3 g/L) for alternating 2-wk periods or fenbendazole (50 ppm in feed for 5 days) are effective treatments. Rabbit pinworms are not transmissible to people.