Jody L. Gookin, DVM, PhD, DACVIM
Trichomonads are flagellated protozoans characterized by an undulating membrane that courses along the length of the body. They are similar in size to Giardia spp., and obligate parasites of warm, moist and anaerobic areas within the gastrointestinal or genitourinary tract of their hosts. Trichomonads do not form cysts, reproduce by binary fission and are transmitted directly from host to host in the form of trophozoites. Pathogenic and non-pathogenic species of trichomonads can be found in animals. Pentatrichomonas hominis inhabits the large intestine of a number of mammalian hosts and is considered to be a commensal. P. hominis infection has been described in both dogs1 and cats.2 In contrast, Tritrichomonas foetus has been long recognized as an important venereal pathogen of naturally bred cattle. T. foetus was first identified as a cause of chronic large bowel diarrhea in cats in 20033 and the duration of its existence in cats prior to that time is unknown.4,5 In cats, T. foetus colonizes the distal ileum and colon, resulting in lymphoplasmacytic and neutrophilic colitis and chronic foul-smelling diarrhea. Infection of the feline uterus with T.foetus has been reported in a single case9, although it is unclear whether T. foetus was a primary or opportunistic pathogen. The prevalence of T. foetus infection is high among densely housed cats: 31% of 117 cats from 89 catteries sampled at an international cat show in the U.S.6 and 14.4% of 111 cats with diarrhea in the U.K.7
Signalment and Clinical Findings
Cats with diarrhea and T. foetus infection are generally young but asymptomatic infection in older cats may be common. Cats originating from a cattery (e.g., purebred) or shelter appear to be at increased risk for infection due to history of dense housing.6,7 A true breed predisposition was not shown in the U.S.6 although Siamese and Bengal breeds were overrepresented in the U.K.7 Clinical signs are characterized by large bowel diarrhea occasionally containing fresh blood and mucus.8 Diarrhea is semi-formed to cow-pie in consistency and malodorous. In very young cats and with poor housing conditions, the anus may appear inflamed and painful; involuntary dribbling of feces or rectal prolapse may be present. In general, cats maintain good health, and normal appetite and body condition. Consistency of the diarrhea will wax and wane and typically improves during antimicrobial treatment, but quickly resumes when treatment is discontinued.8 T. foetus infected cats are often misdiagnosed as having Giardia spp.. Cats diagnosed with Giardia spp. on the basis of a fecal smear and that fail to respond to appropriate antimicrobial therapy should be closely re-evaluated for the possibility that the observed trophozoites were T. foetus.
Feline T. foetus infection may be diagnosed by identifying the organism in feces by direct smear examination, selective protozoal culture, and PCR using species-specific primers or by observation of trichomonads in colonic mucosal biopsy specimens.
Direct Fecal Smear Examination
Fecal samples should be diarrheic and freshly voided or obtained directly from the rectum using a fecal loop. Areas in which feces are particularly moist or contain mucus and blood are ideal locations to sample. A scant amount of feces should be mixed in a drop of saline and examined under a cover slip using a 40x objective. Lowering the microscope condenser to increase contrast will enhance visualization. Trichomonads are approximately the size of Giardia spp. and highly motile. The key microscopic differences between trichomonads and Giardia spp. are a "falling leaf" motility characteristic of Giardia spp. and presence of an undulating membrane on trichomonads. A video of these organisms can be viewed on the author's website (http://www.cvm.ncsu.edu/docs/jody_gookin.html). Multiple smears should be examined for organisms. If necessary, survival of T. foetus in feces can be extended from 0 to 4 days by removing adherent litter and diluting the sample with normal saline to avoid desiccation (˜3 ml 0.9% saline per 2 g of feces).10 Trichomonads will not survive refrigeration and are not typically observed using fecal flotation or sedimentation techniques. Concurrent antibiotic therapy will decrease the number T. foetus present in feces. It is often prudent to discontinue antibiotics for a few days in such cases and repeat the examination if negative results were initially obtained. The sensitivity of direct fecal smear examination for diagnosis of T. foetus is low (2% in cats with experimentally induced infection11 and 14% in cats with spontaneous disease6). T. foetus can be difficult to reliably distinguish from non-pathogenic trichomonads such as P. hominis on the basis of light microscopic appearance, making this means of diagnosis presumptive. However, P.hominis infection is rare in cats and all cats found thus far to be infected with P. hominis were also infected with T. foetus.2
Protozoal Culture of Feces
If repeated direct microscopic examination results are negative for T. foetus, feces may be cultured in commercially available pouches originally marketed for diagnosis of T. foetus infection in cattle, but now reportedly optimized for use in cats (In PouchTM TF, Biomed Diagnostics, White City OR). Based on studies performed with the cattle product,12 pouches should be inoculated with 0.05 g (about the size of a rice grain) of freshly voided or loop-collected feces. The pouch can be incubated either at 37°C for 48 hrs or at room temperature (25°C) for up to 12 days. Bacterial overgrowth and gaseous distension of the pouch is a problem if over-inoculated with feces and more common when incubated at 37°C. Pouches should be cultured upright and in the dark and examined daily (37°C) or every other day (25°C) for the presence of trichomonads. Prior to examination, pouches are placed within a manufacturer-provided clamp that allows the pouch to be mounted onto the stage of a light microscope. Trichomonads are most easily found along the seams of the pouch and using a 20 or 40x objective. Although feces may be shipped to some laboratories for cultivation, given the fragile nature of the organisms it is strongly recommended that this user-friendly culture technique be performed in-house for best results. Culture of feline feces in the cattle pouches has a detection limit of > 1000 T. foetus organisms/0.05 g feces and is superior to direct fecal smear examination for diagnosis of T. foetus infection. Neither Giardia spp. nor P. hominis organisms survive in In PouchTMTF for longer than 24-hr and thus positive cultures are strongly suggestive of T. foetus infection.12 Strictly speaking however, the types of trichomonads potentially hosted by cats and the specificity of In PouchTM TF with regard to these other types of trichomonads is unknown. Positive In PouchTM TF cultures do not preclude the possibility of coinfection with P. hominis or Giardia.
Polymerase Chain Reaction
A sensitive and specific single-tube nested PCR based on amplification of a conserved portion of the T. foetus internal transcribed spacer region (ITS1 and ITS2) and 5.8S rRNA gene from feline feces is commercially available through the author's laboratory (http://www.cvm.ncsu.edu/docs/jody_gookin.html) and superior to fecal culture for diagnosis of infected cats6,13. PCR requires approximately 50 mg of feces devoid of litter and samples should be preserved in 3-5 ml of 70% isopropyl alcohol (rubbing alcohol) prior to shipment at room temperature. Sensitivity of the PCR is 10 trichomonads per 50 mg sample of feces.
Colonic mucosal biopsy is often appropriate in cases of large bowel diarrhea where less invasive diagnostics fail to identify a cause for diarrhea and appropriate empirical medical and dietary treatment trials have failed to result in a resolution of signs. Histopathology should not be relied upon to make a diagnosis of T. foetus infection, although a diagnosis can be attained if organisms are identified. The diagnostic feature is the presence of trichomonads, which can be found in close proximity to the surface of the mucosa or in the lumen of colonic crypts. Less commonly, subepithelial invasion of trichomonads may also be observed. Inflammatory infiltrates consist of plasma cells, lymphocytes and neutrophils. Lesions can be segmental in distribution therefore multiple sites should be biopsied. It is advisable to indicate that T. foetus is a differential diagnosis, as a minimum of six tissue sections should be examined to achieve >95% confidence that trichomonads will be identified.14
Coexisting Infectious Diseases
Any coexisting intestinal infection in a T. foetus infected cat should be identified and treated if possible. Studies of experimental feline infection suggest that coexisting intestinal infection (e.g., cryptosporidiosis) worsens clinical signs of diarrhea and increases the shedding of T. foetus organisms.11 Co-infection with Giardia spp. is common in T. foetus infected cats6,7 and if present, should be treated. If helminth parasites are not identified, empirical treatment with fenbendazole is recommended. In most cases, concurrent systemic disease or viral infection (FeLV/FIV) is not identified.
Spontaneous Resolution of Diarrhea
When left untreated, 88% of cats with T. foetus infection had spontaneous resolution of diarrhea within 2-years (median 9 months; range 5-mos to 2-yrs).10 Time to resolution of diarrhea was significantly longer for cats from multiple-cat households, and those receiving a variety of different diets and antimicrobial drugs in attempt to treat the condition. Stressful events may be associated with a relapse in diarrhea (e.g., surgery, oral antimicrobial therapy, travel, or a change in diet). The wait-for-resolution approach to treatment of T. foetus infection is best suited for a stable, low-number-cat household where further spread of infection is contained and owners are willing to tolerate the long course of diarrhea. Importantly, spontaneous resolution of diarrhea does not imply recovery from infection, as 57% of these cats remain infected with T. foetus as determined by PCR when performed 2-5 yrs after diagnosis.10
Dietary trials, homeopathic remedies, and antimicrobial drugs have not been consistently effective in ameliorating signs of diarrhea in T. foetus infected cats and studies suggest that these interventions may prolong the duration of clinical signs, perhaps by perturbing the balance of colonic microflora on which the trichomonads depend. Despite the presence of inflammatory colitis, diarrhea does not appear to be improved by treatment with corticosteroids. In vitro studies suggest that T. foetus are inherently resistant to many drugs or rapidly acquire resistance in vivo.11,15-18 Trichomonads lack mitochondria and ferment pyruvate in reductive organelles called hydrogenosomes. Reduction of nitroimidazole antibiotics by hydrogenosomes results in formation of toxic anion radicals that serve as the basis for susceptibility of trichomonads to drugs such as metronidazole. Unfortunately, feline T. foetus infection is not responsive to metronidazole.
Tinidazole (TDZ) is a second-generation 5-nitroimidazole that is FDA approved for use in treating metronidazole-resistant T. vaginalis, Giardia spp., and Entamoeba histolytica infections in people. Susceptibility of feline T. foetus to tinidazole has been shown in vitro15,16 and pharmacokinetic studies in cats have demonstrated a long elimination half-life that would be consistent with once a day dosing.20 When administered to experimentally infected cats at 30 mg/kg PO q24hrs for 14 days, TDZ decreased fecal shedding of T. foetus but failed to eradicate the infection from 2/4 cats for which repeated PCR testing of feces was performed over a 33-week period.
Ronidazole (RDZ) is a 5-nitroimidazole relative of metronidazole that has been used for the treatment of Histomonas meleagridis (turkey blackhead), Treponema hyodysenteriae (swine dysentery) and Trichomonas gallinae infection (pigeon canker). RDZ has been demonstrated to be effective at killing feline isolates of T. foetus in vitro at concentrations > 0.1 µg/ml and eradicated T. foetus from experimentally infected cats (on the basis of PCR) when administered at 30 mg/kg q12hr, PO for 14-days.15 RDZ is not registered for human or veterinary use in the United States and is currently banned for use in food-producing animals due to human hazards. Due diligence is required for protection of humans from exposure to RDZ and veterinarians are advised to obtain informed consent prior to use of this drug in cats. Gloves should be worn when handling RDZ. Several pharmacies will compound chemical grade RDZ for veterinary use. Due to its foul taste, compounding into gelatin capsules rather than flavored liquids is highly recommended. Storing the capsules in the freezer may improve drug stability. RDZ is popular in Europe for the treatment of T. gallinae in racing pigeons. Several 10% active-drug formulations for dilution in the drinking water of birds can be obtained without prescription from pigeon supply warehouses. Due to their undetermined quality, composition, and low active-drug concentration, these products are not recommended.
Neurotoxicosis may be a common and serious side effect of RDZ.19 Accordingly, treatment with RDZ should only be considered in cases of confirmed T. foetus infection where informed consent has been obtained. Cats should be isolated during treatment to decrease the risk of re-infection. It is suspected that asymptomatic infections are common in multiple-cat colonies and isolation from only cats with clinical signs may not be sufficient to avoid re-infection. A prolonged follow-up period is necessary to determine if infection has been eradicated. Testing by PCR at 1-2 weeks and 20+ weeks after completion of treatment is recommended. Importantly, negative results should be interpreted with caution as PCR cannot prove the absence of infection and prolonged asymptomatic carriage of the organism after antimicrobial therapy may be common.
Studies undertaken at North Carolina State University have recently shown that RDZ is rapidly and completely absorbed from the proximal GI tract in cats and persists in plasma for over 48 hrs. These attributes may predispose cats to neurotoxicity with twice-daily administration while also delivering sub-optimal concentrations of drug to the lumen of the colon. The group has developed a delayed-release formulation of RDZ, the absorption of which corresponds to arrival of the drug in the colon. As such, delayed release tablets of RDZ may provide improved efficacy at lower doses or less frequent intervals that reduce risk of systemic toxicity.
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