Pets and People

Giardiasis is a gastrointestinal infection of many mammalian species.  The disease is caused by Giardia duodenalis, a flagellated protozoan parasite found worldwide.  Infection among dogs and cats is common, with prevalence rates of 5-15%.  Humans can also become infected with strains of Giardia spp.  At the Manchester Animal Hospital, we routinely test for Giardia in pets presented with diarrhea. 

While many infected dogs and cats are asymptomatic, acute small bowel diarrhea is common.  Stools may be liquid to semi-formed and normal in quantity, though the frequency of defecation is usually increased.  Acute or chronic large bowel and chronic small bowel diarrhea may also occur.  Acute or chronic vomiting and fever are rarely noted. In severely affected patients, dehydration, poor body condition, and lethargy may be noted, as is abdominal discomfort. 

Prevalence rates vary in dogs.  A study of 16,114 symptomatic dogs seen at veterinary clinics in the US showed a prevalence rate of 15.6%, as determined by an in-house SNAP ELISA test. Giardia spp. have been found in fecal samples of up to 30% of shelter dogs.  Many of these samples were from asymptomatic dogs.  Overcrowded and unsanitary conditions can increase the risk of infection.  Younger animals are more likely to be infected, and infection is often more severe in young animals.  No gender or breed predisposition has been reported.  

As in human cases, some animals are asymptomatic or have clinical signs that are self-limiting. Diarrhea is the most common clinical sign.  Many symptomatic patients continue to eat well and maintain a normal energy level, although abdominal discomfort may be notable.  For severely affected dogs, anorexia, weight loss, and lethargy may occur.  Severity is often worse in patients with concurrent gastrointestinal parasitism, other illnesses (e.g. inflammatory bowel disease), or debilitation. 

The primary goal of treatment is to abolish clinical signs, but the elimination of giardial infection can be difficult, so a cure is often considered a secondary goal.  Multiple drugs or multiple attempts at therapy may be necessary to affect a cure. In some cases, giardial infections are subclinical and treatment may not be necessary.  Otherwise healthy animals can shed the organisms, with little evidence to suggest that shedding predisposes them to disease.  

The overall prognosis for infected animals is generally good because they may be asymptomatic and clinical signs are not often severe.  However, some animals have infections refractory to treatment, and others may become re-infected. 
Human infection is associated with ingestion of Giardia spp. via contaminated water or food; person-to-person transmission; or animal-to-person transmission.  The most common method of infection in humans is drinking contaminated water.  Animal-to-person transmission is thought to represent a very small percentage of cases. 

The zoonotic potential of giardiasis is controversial. It is unclear how much dogs and cats contribute to human infections.  Technically, certain strains of Giardia spp. are considered potentially zoonotic; however, studies indicate that human infections are more frequently associated with strains that rarely infect dogs and cats. 

Because it is difficult to differentiate assemblages and cross-species transmission is possible, it is reasonable to consider any animal infected with Giardia spp. to potentially be infectious to people.  Basic hygiene and handwashing must be practiced when handling any infected feces. Children and immunocompromised individuals are at higher risk for infection and may require isolation from infected pets. 

Dr. Lamb is the Veteranarian at the Manchester Animal Hospital.