Tritrichomonas foetus in young cats

Introduction

Diarrhea is a common reason for young cats to be presented to their primary care veterinarian. Practitioners are generally acutely aware of intestinal parasitic disease as a differential diagnosis for diarrhea, and many such patients will be treated empirically with anti-parasitic medications without further investigation. In many cases this appears to be effective, with relatively rapid improvement and cessation of clinical signs. Whilst the likes of Giardia and various helminths are likely to respond completely to such medications, the less-often considered Tritrichomonas foetus, a protozoan similar in appearance to Giardia species, cannot be cleared by such empirical treatments. This parasite can be a source of profound frustration for both cat owners and their veterinary surgeons, as clinical signs may initially appear to respond to therapy with standard anti-parasitic medications, but often return quickly after treatment is discontinued. Furthermore, the parasite can be challenging to diagnose unless sought specifically. It is therefore likely that increased awareness of this pathogen could lead to faster recognition and more effective treatment in young cats with apparently unresponsive or recurrent diarrhea.

Incidence, prevalence and predispositions

Tritrichomonas foetus (T. foetus) has a worldwide distribution, with an estimated prevalence of between 10-59% 1. Relatively limited data exists on specific geographic prevalences; however one study in a population of UK cats estimated that roughly 20% of healthy cats may be carriers of the protozoa, and a separate study looking at the prevalence of T. foetus in diarrheic fecal samples submitted to a UK lab identified the parasite in 14.4% of samples 2. In the USA, on the other hand, a population of show cats was found to have a higher prevalence of T. foetus, at 31% 3.

The highest prevalence of T. foetus appears to be in purebred cats and cats kept as part of colonies. It has been demonstrated that individuals housed with a smaller number of square feet of housing space per cat were also more likely to be carriers of the disease, suggesting that population density plays a significant part in infection risk. Similarly, an increased risk of infection has been associated with the use of shared litter trays and with social grooming 2 3.

Diarrhea caused by T. foetus is most commonly a disease of young cats, with a mean age of 8 months, and with greater than 75% of affected individuals being less than one year of age. However, cats of any age have the potential to carry the disease, and a proportion of these may develop clinical signs 1 4.

Etiology and pathogenesis 

T. foetus is a flagellate protozoal organism that exists only in the trophozoite form. As such, there is no cyst formation (unlike the situation with giardiasis), and reproduction is achieved by simple binary fission. Visually the organisms are approximately 10-26 μm long and roughly 3-5 μm wide, and are often described to be “pear-shaped” or “spindle-shaped” in form. Each organism has three anterior flagellae (as compared to Pentatrichomonas, which has five), which allow the protozoa to be motile (Figure 1) 1 5.

In cats, T. foetus is primarily a parasite of the gastrointestinal tract, and localizes predominantly to the large intestine and, to a lesser extent, the distal ileum 6. This causes a mucosal inflammation manifesting as lymphoplasmacytic or neutrophilic colitis 7. Other clinical signs that have been documented repeatedly include anal irritation and occasional fecal incontinence. When present within the large intestine, the protozoa exist in close proximity to the intestinal mucosal surface and can often be found in association with colonic epithelial cells and crypts. Once in contact with the colonic wall, the organisms bind by specific receptor-ligand interactions, for which the binding is both competitive and saturable. Binding is to various cell surface molecules, including sialic acid lectins, lipophosphoglycans, cysteine proteases and adhesion proteins. Once bound, the organisms appear capable of breaking down both immunoglobulins and lactoferrins, allowing evasion of the immune system 8.

Transmission of the parasite is almost exclusively by the fecal-oral route. Trophozoites are shed in the feces of colonized cats, and are infectious in this form if consumed by another cat. There are currently no proven intermediate hosts, however it has been demonstrated that T. foetus trophozoites can survive ingestion and transit by terrestrial gastropod mollusks such as slugs, remaining infectious if subsequently eaten by a susceptible cat (Figure 2) 9. In the environment, the trophozoites can survive for several days if they remain in moist feces, however desiccation and death occurs in dry and aerobic environments 10. 

Clinical presentation

Any age, breed or sex of cat can be affected. The commonly reported clinical presentation is one of chronic, waxing and waning large intestinal diarrhea. The diarrhea is often of a semi-formed consistency (“cow-pat”) as oppose to watery, and in some cases may contain frank blood and/or mucous. Tenesmus is not infrequently reported, and some individuals may become intermittently partially fecally incontinent. The feces can also be profoundly malodourous in many cases. The consequence of the diarrhea and secondary regular grooming (Figure 3) is often the development of significant erythema and edema of the perineal region and anus that can be seemingly painful for the cat; less frequently rectal prolapse may occur secondary to inflammation and tenesmus 1 5. 

Aside from the diarrhea and inflammation of the perineal region, most affected patients otherwise appear generally healthy. Some have been reported to be a little unthrifty, and occasionally patients are seen who have dropped below an ideal body condition score. Other described abnormalities include general non-specific findings such as poor coat quality, but no other specific clinical abnormalities are to be expected 1 5.

Diagnosis

A compatible signalment and history should raise suspicion for Tritrichomonas foetus. Differential diagnoses are presented in Table 1, and a thorough clinical work-up should be performed with a view to differentiating T. foetus from these other conditions. 

Findings on routine blood testing in cases of T. foetus are generally non-specific, and parameters are often all within the reference intervals. However, bearing in mind the young nature of the patient population most at risk for T. foetus, it is important not to overinterpret parameters that are altered in young animals (for example, increased alkaline phosphatase and phosphate) when abnormalities on blood testing are present. Some changes may also be seen secondary to chronic clinical signs. The most common examples of this are alterations in electrolyte levels as a result of ongoing diarrhea, especially hypokalemia, hyponatremia and hypochloremia. These are generally mild and subclinical in nature, however more profound changes can occur in more severe disease 1 5.

While there are no changes on blood samples specific to T. foetus, it still remains a valuable part of the diagnostic work-up to exclude differential diagnoses, identify co-morbidities and assess for electrolyte changes that need to be addressed as part of the treatment plan.

Similarly to blood testing, diagnostic imaging offers relatively little in the way of specific findings in cats with T. foetus 5. In severe cases, where there is profound colitis, there may be evidence of thickening of the colonic mucosa (either locally or diffusely) on abdominal ultrasound scanning, although the absence of such findings does not exclude the parasite as a cause. In cases where increased wall thickness is observed, no loss of intestinal wall layering would be expected — if present, this would increase suspicion of an infiltrative neoplastic disease rather than T. foetus.

As with blood testing, there is still good reason to perform diagnostic imaging in these cases. The absence of other clinical findings is indirectly supportive of T. foetus being the causal agent, and the imaging is a valuable tool for investigation of differential diagnoses.

Definitive diagnosis

Definitive diagnosis of T. foetus can be achieved either by direct visualization of the organism or by polymerase chain reaction (PCR) testing of the feces. Fecal culture can also be performed prior to smear preparation or PCR to increase the number of organisms present, and therefore increase the sensitivity of the tests.

Direct visualization is achieved using light microscopy to assess a fecal smear. This is a relatively simple examination and requires only a light microscope and slides, and is an ideal first-line test for T. foetus. It should therefore be performed in all cases of feline large-intestinal diarrhea at first presentation. It is important to note when performing a fecal smear assessment that both Giardia and Pentatrichomonas hominis can look very similar to T. foetus on light microscopy, and therefore care should be taken to accurately identify any parasites that are visualized 3 11. The importance of this relates mainly to treatment decisions, as P. hominis is a non-pathological protozoan requiring no treatment 1, whilst giardiasis is generally substantially more responsive than T. foetus to treatment and typically can be cleared using routine anti-parasitic drugs (e.g., fenbendazole), unlike T. foetus.

Smears can be prepared from multiple sample types. Collection of samples from voluntarily passed fecal material is the simplest method, however if the patient is refusing to defecate within the hospital environment then alternative methods are reported to be successful. It is important to note that samples acquired from normal, non-diarrheic stools will rarely produce positive results, and therefore testing of such samples is not recommended 1.

Use of a rectal loop to acquire fecal material from within the colon has been found to be an adequate means by which to retrieve a sample, as has colonic flushing 1. Colonic flushing is a technique similar to that of a tracheal wash, whereby fluid is instilled into the rectum and colon with the cat under sedation. After instillation, gentle trans-abdominal palpation of the large intestine can be performed to facilitate mixing, before withdrawal of the fluid. This mechanism can generate two sample types: the first being a suspension that can be dripped onto a slide to create a wet preparation for direct light microscopy; the second being a larger sample for centrifugation to create a pellet on which PCR can be later performed.

A fecal smear for analysis is produced using the wet preparation. From colonic flush samples, a drop of the suspended material can be placed immediately on a slide and allowed to air dry. For samples collected from voided diarrhea or rectal loop, the sample should first be suspended in sterile saline before following the sample procedure. Once dry, the sample can be fixed and stained in-house using a Romanowsky stain to assist visualization. After application of a cover slip the sample can then be viewed under a light microscope using 20x or 40x magnification. Lowering the condenser can help improve identification.

Samples for fecal smear need to be examined fresh (within six hours of collection) 10, and therefore transportation to an external laboratory is suboptimal. After six hours the sensitivity of fecal smears as a diagnostic test for T. foetus begins to decrease, and therefore performance of the examination in-house, immediately after the smear is prepared, is recommended 12. Fecal smear assessment is a very simple and inexpensive method for diagnosis, and the test has a very high specificity (i.e., apparent identification of the parasite means it is very likely present); however, it suffers from a low sensitivity. A study to quantify this found that a single fecal smear can have a sensitivity of only 14%, and therefore it is strongly recommended that multiple smears from multiple samples are assessed to improve the chances of identifying the organism 3. Sensitivity is further worsened by the presence of sample contaminants such as cat litter, and therefore in cases where samples are being sent to external laboratories it is imperative that such contaminants are separated from the sample prior to postage 12. Sensitivity of testing is also reduced by antimicrobial treatment such as metronidazole, and therefore all antimicrobial therapy should be discontinued for at least several days prior to testing to mitigate the effects of this 1.

The alternative means of diagnosis of T. foetus is by fecal PCR. This can be performed as a first line test, but it is more expensive and takes longer than fecal smear assessment, and so is often performed after a negative smear result. In view of the low sensitivity of the fecal smear, all cases where there is a suspicion of T. foetus but where nothing is identified on microscopy should have fecal PCR performed. The test can also be used for confirmation of a diagnosis made on smear if there is doubt about the accurate identification of the parasite.

The PCR works by detection of one of a number of highly conserved portions of the T. foetus genome, and these most commonly utilize primers for the ITS1 and ITS2 genes 13. PCR is capable of detecting both living and dead parasites, and it has been reported that as few as 10 organisms are required within a sample for detection 14. The sensitivity of PCR has not been reported, however it is likely to be considerably higher than that of the fecal smear. It is also likely to be variable depending on the density of organisms within the sample, with more individual parasites leading to a greater sensitivity.

An important characteristic of the PCR test is its high specificity for Tritrichomonas. This means that where a cat is carrying Giardia or Pentatrichomonas that may be misinterpreted as T. foetus on smear, the PCR will not detect these other parasites and will be negative unless there is co-infection along with T. foetus 11. This makes PCR a valuable confirmatory test after identification on smear, and thus it is strongly recommended that PCR is performed alongside a fecal smear analysis in all cases 15.

Samples for PCR can be acquired similarly to those used for fecal smear. Again it is recommended that multiple samples from multiple days are pooled and submitted to increase sensitivity, as intermittent shedding has been reported 7. Samples of diarrhea can be submitted fresh, or colonic flush samples can be centrifuged to produce a pellet which can be submitted for analysis.

Although the sensitivity of PCR is thought to be greater than that of a fecal smear, this test also suffers from false negative results due to small numbers of organisms within samples. In order to improve the sensitivity of the test, a greater number of organisms is required, and one solution to this is to culture the organism prior to testing. 

A commercial kit (Figure 4) for culturing T. foetus from fecal samples is available¹. This kit is a closed unit which requires inoculation with a very small fecal pellet (roughly rice-grain sized). The culture takes around 72 hours to perform if the pouch is incubated at 37^oC, although at room temperature it can take as long as 12 days 16. The disadvantage of this test is therefore the potential to markedly increase the length of time before diagnosis, however there are also advantages. The first is that an increased number of organisms will improve the sensitivity of both a fecal smear and a PCR test. The second advantage is that the kit also contains a number of growth inhibitors that prevent co-culture of Giardia or Pentatrichomonas. These inhibitors prevent growth of both parasites, so any detected protozoa on fecal smear after culture has been performed are very likely to be T. foetus rather than an alternative parasite 16.

¹ “InPouch TF” (Biomed Diagnostics, White City, OR)

In view of the above information, the “gold standard” testing for Tritrichomonas foetus should be considered to be fecal culture followed by both direct fecal smear assessment and fecal PCR.

Treatment

Treatment and clearance of T. foetus can be a challenging and frustrating process. Patients often appear to respond clinically to courses of metronidazole, but this drug is not capable of clearing the infection and therefore, despite initial improvements in clinical signs, discontinuation of the drug will result in recurrence of the diarrhea 12. Treatment with metronidazole is actually thought to potentially increase the overall length of time an animal shows clinical signs, and it is not recommended for confirmed cases 1. Similarly, a treatment trial with metronidazole is not considered an appropriate method to differentiate Giardia (which may sometimes be cleared by metronidazole) from T. foetus (which is never cleared by metronidazole).

Unfortunately, T. foetus appears to be relatively resistant to conventional anti-microbial and anti-parasitic treatment. Currently only one drug has been reported to be effective at clearing infections. This is another drug from the nitroimidazole family, ronidazole 17. Ronidazole is not licensed or otherwise approved for use in cats in any country at the time of writing but is available from some veterinary compounding pharmacies for use off-license in the absence of an effective licensed treatment. This form of the drug is relatively expensive, but is recommended due to the reliable dose and formulation provided by a compounding pharmacy. If this option is not available, or if costs are prohibitive, the drug may also be acquired in powdered form as it is used to treat parasitic disease in pigeons. This formulation cannot be recommended as a first-choice drug due to the unknown quality and potentially variable formulation, but anecdotally it has been effective at clearing feline infections.

The reported dose for ronidazole is 30 mg/kg q24H PO for 14 days 18. The drug is well tolerated in many cats, however various side effects have been reported. These tend to be similar in nature to those seen with high doses of metronidazole, the majority of which relate to neurotoxicity. The most commonly seen side effects include lethargy, reduced appetite, ataxia and, if not discontinued, seizures 19. Most side effects will resolve quickly once medication is stopped, but substantial supportive care may be required if treatment is not discontinued as soon as side effects are noted 1. Side effects are considerably more common than those seen with metronidazole treatment, and this is thought to be related to the comparatively long half-life of ronidazole compared to metronidazole.

If the diarrhea recurs following treatment, in principle it would seem reasonable for the course to be repeated, particularly if a marked improvement is seen on medication followed by a sharp decline once finished. However, if there is a period of good health after the initial course followed by a deterioration weeks to months later, then it would be prudent to reconfirm the presence of T. foetus in case the clinical signs are caused by an alternative disease process.

Aside from ronidazole treatment, there is limited evidence for other management strategies. The author is not aware of any publication regarding dietary recommendation for this specific disease, but general rules for the management of enteropathies may be considered; any dietary alteration should be given careful consideration, as this can occasionally lead to large intestinal diarrhea, especially if a poor quality or unbalanced diet is employed. Supplementary antimicrobials are not appropriate. There is no evidence to support or refute the use of nutraceuticals and probiotics, therefore no recommendation can be made with respect to these.

Prognosis

Treatment with ronidazole usually results in relatively rapid improvement in both fecal consistency and fecal quality within the initial 14 day treatment period. However, in more severe cases, the time to improvement can take somewhat longer, and this is thought to reflect the level of colonic inflammation due to the higher parasite burden 1. In cats that are not treated for T. foetus, the diarrhea will eventually self-resolve in roughly 88% of cases, but this can take up to two years to occur 20. 55% of cats that self-resolve will remain infected and therefore may continue to shed trophozoites, making them a continuing infection risk for other cats. This management strategy is therefore not recommended.

Large intestinal diarrhea is a relatively common condition in young cats seen at primary care veterinary practices. Tritrichomonas foetus is an important differential diagnosis for this presentation and should therefore be tested for early in the diagnostic work-up. Gold standard testing involves fecal culture followed by both assessment of a fecal smear and fecal PCR. Treatment should be pursued once a diagnosis has been made, and ronidazole is currently the only known effective treatment.