Treatment failures are common in people and other animals with giardiasis. It is likely that no drug will be universally effective
for treating giardiasis. So in clinical practice, vary the drug and protocol you use according to each individual patient,
and consider all other options for treatment. In chronic cases, also consider the possibility of underlying disorders such
as inflammatory bowel disease, bacterial overgrowth, exocrine pancreatic insufficiency, and immunodeficiency. Infection with
Giardia species does not appear to cause permanent immunity, so reinfection can occur, hampering the assessment of treatment studies.
Multiple treatment regimens have been attempted in cats with T. foetus infections with generally poor responses. In some cases, temporary improvement was noted with a variety of drugs, but relapse
was invariable. Recently, ronidazole was shown to have in vitro and in vivo activity against one strain of T. foetus in cats and, for now, should be considered the drug of choice.14 Administer ronidazole at a dosage of 30 mg/kg orally every 12 hours for 14 days. The drug has to be formulated for use (Westlab
Pharmacy, Gainesville, Fla.).
Optimal follow-up testing recommendations for Cryptosporidium species, Giardia species, and T. foetus infections have been difficult to make because 1) even in cats with negative test results, it is unknown whether infection
still exists below the sensitivity limit of the assay used, and 2) none of the infections result in permanent immunity, so
infection can be quickly reacquired from the environment or other infected cats. Thus, it is our opinion that the primary
goal of therapy is to eliminate clinical signs of disease.
The AAFP Zoonoses Guidelines committee recommends at least one follow-up examination for animals with previous diagnosis of
an enteric zoonotic agent.41 Assuming the diarrhea has resolved, perform a fecal flotation for Giardia species cysts and Cryptosporidium species oocysts within nine days. If the results are negative, perform testing by fecal flotation at least once or twice
a year as part of a routine health check. Cats without diarrhea in general are not considered human health risks.
PREVENTING INFECTION IN PETS
It is extremely difficult to prevent cats from being exposed to Giardia species, Cryptosporidium species, or T. foetus because the organisms are transmitted by fecal-oral contact and reinfection can occur. Attempts can be made to avoid contact
with infected animals; however, shedding can be intermittent, and treatment does not eliminate infection in most animals.
Tritrichomonas foetus does not have an environmentally resistant stage. But Giardia species cysts and Cryptosporidium species oocysts can exist outside the host. While Giardia species cysts can be killed on surfaces after one minute of contact time with quaternary ammonium compounds, Cryptosporidium species oocysts are resistant to routine disinfectants.74 Only extreme temperatures affect the viability of the oocysts: Exposing the oocysts to 139.5 F (59.7 C) for five minutes,
freezing the oocysts to -94 F (-70 C), and desiccating the oocysts for four hours kills the oocysts.74
If giardiasis is recurring in a cattery, shelter, or multicat household, control measures could include
1. Cleaning and decontaminating the environment
2. Administering drugs with anti-Giardia species effects to all animals
3. Cleaning cysts from coats
4. Preventing reinfection.75
Animals can be bathed with regular pet shampoos and thoroughly rinsed. The use of Giardia species vaccines as preventives in cats is still controversial because it is unknown whether the vaccine induces protection
against feline-specific Giardia species isolates. In dogs, administering the vaccine in the field did not change the incidence of giardiasis in several studies.48,76
ZOONOTIC CONCERNS AND PREVENTION IN PEOPLE
Tritrichomonas foetus infection is not zoonotic. Most cases of cryptosporidiosis in people are caused by the C. parvum cattle genotype and C. hominis; most cats are infected with C. felis. However, C. felis DNA has been amplified from both immunocompetent and immunosuppressed people.1-3 An epidemiological study failed to find an association between pet ownership and cryptosporidiosis in people with HIV infection.77 Similarly, Giardia species isolates from people and cats often vary, and to our knowledge, transmission of Giardia species from a cat to a person has not been documented. However, all cats infected with either Giardia or Cryptosporidium species should be considered potentially zoonotic.
People can attempt to avoid Giardia and Cryptosporidium species infections by avoiding contaminated food or water and by disinfecting contaminated areas. Water collected in the
wilderness or rural areas should be filtered or boiled before drinking. The Centers for Disease Control and Prevention's online
publication recommends using filters labeled as reverse osmosis and others with a pore size of 1 micron or less.78 Chlorination of drinking water is not sufficient to kill Cryptosporidium species oocysts.
Andrea V. Scorza, MV, MS
Michaell R. Lappin, DVM, PhD, DACVIM
Department of Clinical Sciences
College of Veterinary Medicine and Biomedical Sciences
Colorado State University
Fort Collins, CO 80523