Infectious causes of gastrointestinal disease in the cat are important for two reasons. The first is clearly the impact on
the health of the cat itself. However, it must be noted that while unusual, our feline companions can be sources of zoonotic
disease as well. This is especially important for those households with immunocompromised people or young children. An excellent
resource with members of their household who are immunocompromised is the Centers for Disease Control website. A brochure
available for downloading there is located at http://www.cdc.gov/hiv/pubs/brochure/oi_pets.html.
Table 1: Clinical signs most commonly associated with small (SI diarrhea) or large intestine (LI diarrhea) diarrhea.
Zoonotic organisms associated with the gastrointestinal tract of the cat include Toxoplasma gondii, Giardia spp., Ancylostoma spp. (hookworm), Campylobacter spp., Cryptosporidium spp., Salmonella spp. and Toxacara cati (roundworm). In two studies regarding zoonotic gastrointestinal agents, prevalence ranged from 0.6% to 32.7% in feces from
cats with no clinical signs. The prevalence of any infectious zoonotic agent was 13.1% in adult cats and 40.7% in cats under
a year of age. Clearly, addressing the zoonotic potential of feline feces justifies the use of strategic deworming in any
cat, but especially in those households with young children or immunocompromised people. Kittens and their queens should be
dewormed at three, five, seven, and nine weeks of age. Finally, a convenient deworming regimen for adult cats is simply monthly
administration of heartworm preventatives. Two of the most significant and common gastrointestinal zoonotic agents in cats,
hookworms and roundworms, are readily controlled with both selamectin and milbemycin. Selamectin has the additional advantage
of controlling fleas, and important vector for Bartonella transmission as well as a risk factor for Rickettsia felis and Haemoplasmas. A broad spectrum dewormer, Drontal, is labeled for hookworms, roundworms, and cestodes. While not label
approved, clinical application of the drug for flukes has been reported as well.
Table 2: Bacterial causes of diarrhea in the cat. It should be noted that mixed bowel diarrhea can be present in most cases
that are commonly associated with small intestinal diarrhea only.
Addressing the cat with diarrhea, infectious causes include the above listed zoonotic agents in addition to heartworm disease
(another reason to consider preventative), Tritrichomonas, Cystoisospora, Taenia, and viral causes such as panleukopenia, coronoavirus, FIV, and FeLV, as well as Entamoeba, Histoplasma, and Clostridium. While some of these are associated with both large and small bowel diarrhea (FIV, FeLV, coronavirus), identifying the type
of diarrhea present may help guide the practitioner in choosing diagnostic assays. A table reviewing clinical signs associated
with both types is listed below.
Table 3: Nematodes associated with diarrhea in the cat.
Obviously, a significant amount of time, effort, and financial resources can be devoted to the detection of all the above
organisms. However, a stepwise approach to the workup will pursue the most commonly detected organisms as well as provide
an approach starting with the least invasive and less expensive assays. After the minimum database, all ill cats should have
a thorough physical exam and if not recently known, FeLV and FIV status checked via in house immunochromatographic assays.
Additionally, regardless of anatomic localization, at least a single, if not multiple, fecal centrifugation flotation should
be performed. Zinc sulfate centrifugation provides a 70% positive predictive value for detection of Giardia, allows detection
of eggs, cysts, and oocysts and can be performed in house and rapidly. In contrast, a simple flotation technique, without
centrifugation, decreases detection of Giardia to 50%. In both small and large bowel diarrhea, a fecal wet mount is a simple,
inexpensive assay and provides for the detection of trophozoites in fresh feces. To complete the first wave of diagnostics,
if small bowel signs are present, screening for cryptosporidium via acid fast thin smear of feces should be performed. If
signs of large bowel diarrhea predominate, rectal cytology for presence of white cells and fungal organisms should be examined,
as well as specific testing for Tritrichomonas foetus. If all diagnostics are negative, trial treatment with fenbendazole
(50 mg/kg PO q 24 hrs x 5 days) is warranted as organisms are only intermittently shed.
Table 4: Protozoal causes of diarrhea in the cat.
If response to therapy fails and first wave diagnostics are within normal limits, more expensive and targeted diagnostics
are recommended. Fecal culture and targeted assays for cryptosporidium (IFA, Meridian Laboratories) are warranted. If large
bowel signs predominate and in an appropriate geographic area or appropriate travel history, rectal scraping evaluating for
the presence of histoplasma organisms can be considered.