Michael R. Lappin, DVM, PhD, DACVIM
Director, Center for Companion Animal Studies, Colorado State University, Fort Collins, CO, USA
Cryptosporidium spp. are intestinal coccidian parasites that are associated with disease in some infected hosts. In the United States, Cryptosporidium spp. DNA was amplified from 29.4% of cats with diarrhea. In cats, C. felis is most common and is transmitted between cats by the ingestion of feces from mutual grooming, shared litterboxes, ingestion of contaminated food or water, and possibly, ingestion of infected prey species. Approximately 20% of the oocysts produced in the intestine are "thin-walled" oocysts that fail to form an oocyst wall. These oocysts rupture within the intestines and when the sporozoites are released, auto infection occurs allowing for magnification of infection. Thick-walled oocysts are passed in the feces, are environmentally resistant, and are infective when passed. Cats experimentally inoculated with C. parvum had oocysts detected by Day 7 and C. parvum DNA detected by Day 2 after inoculation.
When it occurs, Cryptosporidium spp. diarrhea is associated with impaired intestinal absorption and enhanced secretion and so is generally characterized as small bowel. Infected cats show loss of microvilli, degeneration of host epithelial cells, atrophy of villi, and can have lymphocytic-plasmacytic infiltrates. While infection of cats with C. felis is common, many infected cats are normal. Diarrhea is generally more common in young cats. Dual infection with Giardia or Tritrichomonas foetus occurs in some cats and when this occurs in association with clinical disease it can be difficult to manage.
Cryptosporidium spp. oocysts are frequently missed on fecal flotation because of the small size and low numbers in feline feces. Modified acid-fast staining of a thin fecal smear can be performed in the small animal practice to aid in the diagnosis of cryptosporidiosis. Cryptosporidium spp. are generally the only enteric organism of approximately 4 to 6 µ in diameter that will stain pink to red with acid-fast stain. However, acid-fast staining will only detect approximately 50% of Cryptosporidium spp. infected cats when a single sample is tested. Fecal antigen tests for Cryptosporidium spp. in humans feces do not work well with C. felis. A fluorescein-labeled monoclonal antibody system is available that contains monoclonal antibodies that react with Cryptosporidium spp. oocysts and Giardia spp. cysts. In limited studies, this assay appears to detect both Giardia spp. and Cryptosporidium spp. isolates from cats and is a preferred diagnostic procedure in my laboratory. Polymerase chain reaction (PCR) is currently available to detect Cryptosporidium spp. DNA in feline feces and the assay is more sensitive than IFA. The PCR products can be evaluated by genetic sequencing to further determine what Cryptosporidium spp. is associated with the infection.
Over 100 compounds have been used to attempt to treat Cryptosporidium spp. infections in mammals and no compound is consistently effective. In cats, Cryptosporidium spp. associated diarrhea sometimes resolves after administration of tylosin (10–15 mg/kg, PO, q12hr), azithromycin (10 mg/kg, PO, daily), or nitazoxanide (10–25 mg/kg, PO, q12–24 hr). Tylosin and nitazoxanide are GI irritants. If the cat is responding to the first 7 days of therapy and toxicity has not been noted, I continue treatment for 1 week past clinical resolution of diarrhea. Some cats with Cryptosporidium spp. infection with or without Giardia coinfection have required several weeks of treatment prior to resolution of diarrhea.
In some cats with diarrhea and Cryptosporidium spp. in feces, the addition of fiber or probiotics to the diet and retreating may result in resolution of illness. Lastly, some of the cats with apparently resistant cryptosporidiosis have underlying diseases (i.e., inflammatory bowel disease, Giardia spp., Tritrichomonas foetus, etc.) and so the diagnostic workup should be continued if therapeutic failures occur. No drug treatment has been shown to consistently eliminate Cryptosporidium spp. infections. Thus, the primary goal for the treatment of cats cryptosporidiosis is to eliminate diarrhea. Cryptosporidium felis is not considered a zoonotic risk for humans.
Isospora spp. are protozoan parasites in the coccidian group that have been recognized as potential pathogens in cats for years. The sexual phase of reproduction occurs in the gastrointestinal tract of cats which culminates in the passage of unsporulated oocysts in feces. Cats are the definitive hosts for I. felis and I. rivolta. Isospora spp. are host specific, have worldwide distribution, and infections are very common, particularly in young animals. In the United States, the Companion Animal Parasite Council reports that prevalence rates for Isospora spp. infection in dogs and cats can vary from 3% to > 30% (www.capcvet.org). In a study of 1,355 cats in the United Kingdom, Isospora felis oocysts were detected in 3%. Gender and breed does not usually influence the Isospora spp. shedding rates, but young animals are usually more likely to be shedding oocysts than adults. In the United Kingdom study, I. felis was found in the feces of 9% of cats less than six months of age.
Infection by Isospora spp. in cats is initiated by ingestion of sporulated oocysts in the environment or by ingesting tissues of other infected vertebrate intermediate hosts. Infection may also occur if the cat ingests sporulated oocysts carried by paratenic hosts like flies, cockroaches, or dung beetles. The enteroepithelial phase occurs in the small intestine of infected animals which culminates in the passage of unsporulated oocysts in feces. Depending on the environmental conditions, sporulation can occur in as little as 12 hours. Clinical disease is most common in young, debilitated, and immunocompromised animals. All the different Isospora spp. replicate in the small intestine but the regions with the heaviest infection varies by the species. Microscopic lesions observed in some infected animals include villous atrophy, dilation of lacteals, and hyperplasia of lymph nodes in Peyer's patches.
Isospora spp. infections are generally only associated with disease in kittens which can exhibit vomiting, abdominal discomfort, inappetence, and watery diarrhea that sometime contains blood. Depending on the age of the animal and the parasite burden, severe dehydration and death can occur. Kittens with subclinical infection can repeat shedding and clinical signs of disease during stressful periods.
The definitive diagnosis of coccidiosis is made by demonstrating oocysts in fecal samples from affected animals. Isospora spp. oocysts are large and often occur in large numbers and so are generally easy to identify on microscopic examination of feces after fecal flotation. However, normal animals also can pass Isospora spp. oocysts and so positive test results do not always prove a disease association. False negative fecal flotation results are uncommon in clinically infected animals but occasionally clinical signs precede oocyst shedding and so a second fecal flotation may be needed to prove infection in some cases.
Coccidiosis is generally self-limited and most otherwise healthy kittens will have diarrhea resolve clinically without therapy. However, administration of treatment can speed resolution of clinical disease and may lessen environmental contamination and the potential for infecting other in contact animals. The only approved treatment for coccidiosis in the United States is sulfadimethoxine administered at 50–60 mg/kg daily for 5–20 days (dogs and cats). Other sulfa drugs, amprolium, or clindamycin can be effective. The use of ponazuril and toltrazuril for the treatment of small animal coccidiosis has recently been studied. These drugs are coccidiocidal and so may be superior to other drugs for the treatment for coccidiosis. Ponazuril has been used most frequently in the United States. This equine drug can be administered off label at 20 mg/kg, PO, twice, 1–7 days apart or at 50 mg/kg, PO once. Many compounding pharmacies in the United States will appropriately formulate the drug by prescription. Most Isospora spp. infected puppies and kittens will survive infection and so the prognosis is considered good to excellent.
Isospora spp. oocysts are very resistant to environmental conditions and disinfectants. The key to control is to provide good sanitation including prompt removal of feces prior to oocyst sporulation. Steam cleaning can be used to destroy oocysts that contaminate surfaces. Treatment of queens with anticoccidial agents prior to parturition can lessen the occurrence of coccidiosis in young animals. In environments with heavy infections, treatment of all in contact animals, particularly kittens, could be considered. Ponazuril administered of to all at risk kittens on intake to shelters may aid in the control of coccidiosis.
Isospora spp. of dogs and cats do not infect people. However, some infected animals are co-infected with other parasites with potential for zoonotic transfer to people like Cryptosporidium spp. and Giardia spp. Thus, a complete diagnosis workup should be completed for animals with diarrhea.
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