Feline infectious diarrhea: the usual suspects, plus how to deal with them

When it comes to feline diarrhea, the causes range from minor to life-threatening. By understanding the primary and secondary causes of this frustrating condition, it’s easier to determine the best treatment approach.

Just because feline diarrhea is relatively common doesn’t mean it’s not a big deal. Diarrhea is uncomfortable for our patients, and a source of distress for pet owners, cautioned Michael Lappin, DVM, PhD, DACVIM, professor of small animal internal medicine at Colorado State University College of Veterinary Medicine and Biomedical Sciences. “We don’t want our patients to suffer, so getting them back to having normal stool is a big goal for me. Also, defecating outside the litterbox can be a reason for relinquishment, so the quicker the cat begins defecating in the litterbox again, the better for the owner.”

During his live session at the Fetch dvm360 virtual conference on Thursday, Dr Lappin updated attendees on his approach to managing these cases

The bigger picture

Feline diarrhea is generally divided into primary gastrointestinal causes and secondary causes. Primary causes include parasites and enteric pathogens, whereas secondary causes may include etiologies like liver disease or feline hyperthyroidism. Basic diagnostics, including a chemistry panel, CBC, and urinalysis can rule out some important secondary causes, Lappin said.

History and lifestyle can provide clues, but the possibility of parasites shouldn’t be dismissed simply because the cat doesn’t go outside. Lappin reminded attendees that even indoor cats catch and eat flies, beetles, rodents, or cockroaches, which can increase their risk for parasites. So, this potential connection shouldn’t be overlooked just because the cat lives indoors.

Lappin also advised that it may help narrow the list of differentials to distinguish small bowel diarrhea from large bowel diarrhea. For example, a kitten from a shelter that presents with small bowel diarrhea is a good candidate for Giardia because of the nature of the diarrhea (small bowel) and the history of being in a shelter. In contrast, a cat from a cattery that presents with large bowel diarrhea is more likely to have Tritrichomonas than Giardia. Weight loss, appetite loss, and other historical and physical exam findings can also help differentiate underlying causes and help with developing an initial diagnostic plan.

The usual suspects

Giardia is becoming increasingly recognized in cats. “Giardia can be in any cat, but particularly in crowded environments,” Lappin reported, adding that small bowel diarrhea is a helpful clue. Isospora (coccidiosis) is also relatively common in crowded environments, like pet stores or some catteries.

Tritrichomonas foetus is not diagnosed as commonly as Giardia, but should be on the list of differentials for some cats with diarrhea. “A cattery cat with hematochezia and large bowel diarrhea should increase your suspicion for tritrichomonas,” Lappin warned. Another clue is that the diarrhea might fail to respond to routine treatments (e.g., dewormings and Giardia treatments).

Bacterial diseases should be on the differential list. However, Lappin noted “Bacterial diseases are quite rare compared with the protozoal diseases we’ve discussed.” He added that although Salmonella, Clostridia, and Campylobacter can occur, they’re less likely to cause chronic diarrheal disease.

Diagnostic tips

Fecal centrifugation, wet mounts, and flotation can offer important diagnostic advantages when used properly, Lappin noted. Although fecal centrifugation increases sensitivity of fecal testing, zinc sulfate flotation still has a role. “Zinc sulfate fecal flotation with Giardia antigen testing is about 97% sensitive for diagnosing Giardia.” He suggested that, since this combination of tests is so sensitive for diagnosing Giardia, it’s unnecessary to perform repeat testing 3 or 4 times. If fecal flotation alone is used, then repeat testing can be helpful. But when used with an antigen test, only one combination test is needed.

Lappin warned that fecal wet mounts are unlikely to find Giardia. However, they can help identify Tritrichomonas trophozoites, or help distinguish them from Giardia. Lappin commented that these 2 organisms look similar morphologically, but noting their patterns of motility can help with identification. Giardia move in a “falling leaf’ or “drunken sailor” type of motion, whereas Tritrichomonas trophozoites have a “darting” type of motility. These differences can help distinguish them microscopically, he suggested.

Polymerase chain reaction (PCR) testing has become more widely available through diagnostic laboratories. However, Dr Lappin generally doesn’t use PCR as a screening test for feline diarrhea. He warned that false negatives can occur with Giardia PCR testing, and since fecal floatation combined with antigen testing can effectively diagnose Giardia, PCR testing isn’t needed.

Lappin stated that he only uses Giardia PCR testing to determine strain. Human Giardia strains have been identified in cats (e.g., a cat can swallow human Giardia cysts), but because the human strain doesn’t replicate well in cats, it’s unlikely to be a cause of feline diarrhea. Nevertheless, since the discussion about zoonosis may require answers, it’s helpful to use PCR testing to provide them. Dr Lappin sometimes uses PCR to detect Cryptosporidium coinfection in cats with persistent small bowel diarrhea, and for detection of Tritrichomonas.

When to treat

Although it may seem counterintuitive, it isn’t always necessary to treat every infectious organism we diagnose. "Some cats have chronic Giardia, but they usually don’t have chronic Giardia diarrhea. Those are 2 different things,” Lappin advised, noting that it’s not always mandatory to treat these cats. A Giardia-positive cat with normal, formed stool probably doesn’t need to be treated. Most Giardia is not zoonotic, Lappin said, and the risk to humans is reduced if stools are normal. Also, treating the cat doesn’t prevent reinfection from the environment. Frequent retesting could lead to repeated, unnecessary treatments. Lappin added, “Most feline Giardias are specific to cats. So, even if you have a cat with Giardia, the odds are that it’s a cat Giardia, and it probably won’t infect the owner.” Cryptosporidium felis (the species of Cryptosporidium that infects cats) is also unlikely to infect a human.

Dr Lappin made a few recommendations for treatment, such as fenbendazole, metronidazole, or secnidazole (for Giardia); ronidazole (for Tritrichomonas); azithromycin or tylosin (for cryptosporidium); and sulfadimethoxine or ponazuril (for Isospora). However, when treating feline diarrhea, Lappin cautions, ”If you’ve tried 2 drugs and the cat still has diarrhea, don’t reach for that third drug. If a good diet, fiber supplementation, or probiotic fails, there’s likely something else wrong with the cat—it’s unlikely to just be Giardia.” Coinfections with other parasites, or even viral or other causes of the diarrhea should be explored.

Lappin further advised that the goal of therapy shouldn’t be to “sterilize” feces by eliminating everything potentially infectious, but instead clinicians should try to resolve the diarrhea. Normal, formed stool is less likely to contribute to disease transmission to humans. “So, from a One Health perspective, if you have a ‘healthy’ pet, meaning normal stools, there’s no coughing, sneezing, vomiting, or skin lesions, and you’re performing strategic deworming for appropriate parasites, there’s probably minimal danger to humans.” Lappin added that if our prevention strategies stop cats from becoming parasitized, then we don’t have to worry as much about treatment.

Don’t dismiss probiotics and diet

Dr Lappin reminded attendees that some of the antimicrobials we use for diarrhea can be hard on the gastrointestinal biome. It’s also notable that drug resistance to some medications can occur with Giardia, so it may be prudent to consider other therapies before reaching for antimicrobials.

Some cats with Giardia respond to psyllium dietary supplementation or probiotics. This can also be useful in cats that continue to have diarrhea despite appropriate drug therapy. Dr Lappin also suggested that some cats with stress diarrhea be considered candidates for probiotics or diet change, instead of reaching for antibiotics as the first therapeutic choice.

Karen Todd-Jenkins received her VMD degree from the University of Pennsylvania School of Veterinary Medicine. She is a medical writer and has remained in clinical practice for over 20 years. She is a member of the American Medical Writers Association and One Health Initiative.