I encountered a neutered male domestic shorthair cat who had been with the practice from when he was five years old. He was regularly vaccinated and treated for fleas and worms. At the cat’s booster vaccination appointment aged six, the owner reported that there were no current concerns but did say that the cat had had diarrhoea ever since she had owned him. In spite of this, his weight was remaining constant. Several courses of action were suggested at this consultation: adjusting his diet, trying prednisolone as a treatment for inflammatory bowel disease or having a faecal sample tested to determine the cause of the loose faeces. Almost two years later the cat was examined because the owner had noticed blood in the loose motion and he had defecated outside his litter tray. Treatment stopped the diarrhoea, but the cat was presented again a year or so later with the owner saying that she had still never seen him pass formed faeces. A faecal sample was sent to the lab for analysis, and this came back negative for parasites, Cryptosporidia, Giardia, Campylobacter and Salmonella. These results were communicated to the owner with the suggestion that Tritrichomonas testing should be carried out and that an intestinal biopsy could be helpful in reaching a diagnosis, but the cat was not seen again at the practice.
This history emphasises the difficulty of establishing a diagnosis for chronic diarrhoea. Infection with Tritrichomonas may consist of intermittent episodes of soft, smelly faeces. Several practices are reporting that they have seen what looks like it, but that owners have chosen to live with the condition without establishing the cause.
The organism can survive for three days in moist faeces so transmission seems to be by the faecal-oral route, facilitated by shared litter boxes. As in my presumptive case, the affected cat appears to be healthy and has a good appetite without weight loss. The diarrhoea may be intermittent rather than continuous and antibiotic treatment often gives temporary respite. Infected cats may not show symptoms or they may have long-term passage of soft smelly faeces. Prevalence is higher in cats which have come from a high-density population either in the breeding situation or in rescue. The prevalence in the UK cat population is around 10 percent and it appears to be most commonly seen in pedigree cats.
Diagnosis can be achieved by faecal testing. The patient should have had a week free from antibiotic or any other medication before sampling and the sample should be freshly passed and diarrhoeic. It can be sent to the lab for PCR testing or examined microscopically in the practice. Gold-standard technique is to flush the colon with about 10ml of sterile saline so that a drop of the sample can be examined promptly in the practice to look for motile trichomonads.
T. foetus is a single-celled, flagellated protozoan parasite, found as a venereal pathogen in cattle but in the colon and distal ileum of cats. The “tri” in the name comes from its three anterior flagellae which allow it to be motile. It is pear-shaped and has an undulating membrane which extends along the length of its body. Too much faecal matter inhibits motility of the trophozoites and the pitfall is to distinguish T. foetus from the similar Giardia as mixed infections do occur. Direct microscopy is the cheapest but least sensitive diagnostic test. Culture in a pouch that contains a selective growth medium is more sensitive than the direct smear and PCR is the most sensitive test available, but is also the most expensive. It gives results on both living and dead trichomonads and a positive test is specific.
Treatment of T. foetus is far from straightforward. Ronidazole is a nitroimidazole like the better-known antiprotozoal metronidazole, but the latter will not clear a T. foetus infection. The drawback with ronidazole is that it is not licensed and needs to be made to order for each individual confirmed case. The drug is embryotoxic and possibly teratogenic and carcinogenic so it should not be used in pregnant or nursing cats. If the cat is overdosed, neurological signs such as anorexia, lethargy or seizures may develop but these will subside once the drug is withdrawn. The recommended dose rate is 30mg/kg once in 24 hours for 14 days. The disadvantages of the drug need to be explained to the owner who needs to sign a form giving informed consent to its use. The owner must be advised to wear gloves when handling the drug. Immunocompromised people and pregnant women need to be especially careful.
The relationship between the cat host and T. foetus makes a challenging clinical situation. Many cats are asymptomatic; about 88 percent of cats become clinically normal by two years of age but the problem of persistent, soft, smelly faeces can be lifelong.