Giardiasis is caused by an intestinal infection of flagellate protozoa belonging to the Giardia spp. The parasite is distributed worldwide. The lifecycle of the organism includes trophozoites in the vegetative state and cysts in the transmission stage. The trophozoites live in the upper part of the small intestine, especially the duodenum, and are carried to the ileum where they secrete a heartening substance and become encysted. Cysts leave the infected host in the feces. They can survive more than two months in cool water, and about one month at room temperature, and are sensitive to desiccation, sunlight, and freezing. They are readily killed by quaternary ammonium compounds but not by normal concentration of chlorine in drinking water.
Infection in Humans
Endemic throughout the world, the prevalence of giardiasis ranges from 2-4 % in industrialized countries, to 15% or higher in children of developing countries. Regardless of location, both infection and disease are more commonly seen in children. Epidemics of giardiasis also can occur; the latest outbreaks in the United States were in 1993-1994 and 1995-1996 from drinking infected water or exposure to infected recreational waters. Contaminated drinking water supplies also caused Giardia spp. outbreaks in the United States. Children and travelers to other countries can be infected from contaminated drinking or other waters, whereas patients with AIDS are less likely to be infected, perhaps because HIV virus interferes with the parasite’s ability to multiply in the intestine.
The primary source or reservoir of infection in humans is other infected humans, who shed parasite cysts in their feces and often contaminate water supplies. The infection is usually mild and self-limiting. While as little as 10 cysts can be infective, infected individuals can excrete up to 900 million cysts daily in the feces! Hand-to-hand and hand-to-mouth transmission occurs from infected to susceptible people (usually children) in institutions that care for children and adults. Fortunately, the organism is susceptible to desiccation and sunlight and so does not survive on contaminated food or other fomites for very long.
Some animals may also serve as a reservoir for human infection. Known outbreaks have occurred from infected beavers contaminating mountain water, and from non-human primates infecting zoo personnel. Specific-pathogen- free puppies have been infected from Giardia cysts isolated from beavers.
Infection in Animals
A wide variety of domestic and wild mammal species are known to be hosts of Giardia spp. The infection is less common in adult animals, and prevalence surveys throughout the world have found the parasite in 20-35% of young dogs, 10-15% of young cats, 5-90% of calves, 6-80% of lambs, 17-32% of foals, and 7-44% of young pigs. In the United States, beavers and coyotes have been shown to harbor the organism, and an outbreak among non-human primates and zoo personnel also has been documented. Rats and other rodents can harbor a high rate of infection in both domestic and wild stock.
Disease in Humans
The majority of infections are asymptomatic even in children, although disease most often occurs in children with an impaired humoral immunity. The incubation period is 3-25 days and symptoms of disease are mainly diarrhea and bloating, with abdominal pain. Occasionally, nausea and vomiting occur, and the acute phase of disease typically lasts several days. In some patients, giardiasis may be a prolonged gastrointestinal illness associated with food intolerance, episodes of recurring diarrhea and flatulence, and urticaria. The presence of concurrent bacterial infections apparently does not influence the pathogenicity of infection.
Disease in Animals
As with the human disease, is usually asymptomatic in animals and more commonly seen in the young. Symptoms in dogs and cats are similar to those in people, whereas only mild diarrhea occurs in calves and lambs.
Diagnosis is confirmed by finding Giardia cysts in feces. Cysts are seen in formed stool, whereas trophozoites are usually seen in diarrheal stools. As cysts are illuminated intermittently, at least three samples taken every other day should be examined to rule out infection. Stool concentration methods are helpful in detecting infection. For trophozoites, fresh and stained stool preparations are examined for the presence of the flagellate form of the parasite. Tests using specific fluorescent antibodies or ELISA are also available and afford increased sensitivity in detecting past and present exposure. It is important to remember that the finding Giardia cysts in an individual with gastrointestinal illness may not be causally related, so that other etiologies need to be investigated as well.
Control and Treatment
Public water suppies should be protected from contamination with human and animal feces that could harbor Giardia spp. Removal and sanitation of human and animal waste in the environment is also important. Water should be boiled in situations of a potential exposure, and travelers should drink only bottled water. While there is no evidence that domestic animals are a significant source of infection for humans, dogs and cats infected with Giardia cysts should be treated because of the likelihood of contact with children.
A killed vaccine for canine giardiasis is available but is currently not recommended for routine use by the 2006 Canine Vaccine Guidelines of the American Animal Hospital Association. The vaccine may prevent oocyst shedding but does not prevent infection. There is no duration of immunity data for this vaccine, because it doesn’t prevent infection. Most puppies and kittens exposed to Giardia spp. develop sub-clinical infection, and are not considered a zoonotic risk to immunocompetent humans.
Metronidazole or fenbendazole are commonly recommended treatments for giardiasis. Bathing during treatment is also advised to prevent re-infection from ingestion of Giardia oocysts during grooming. Control measures should be employed to prevent reinfection.
References: Shedlock and Weiner, J Leuk Biol 68: 973-806, 2000; Giardiasis ICD-10 AO7.1. Pan American Health Organization, Zoonoses and Communicable Diseases, vol. 3, pp 52-58; Paul et al, AAHA Canine Vaccine Task Force, JAAHA March 2006; pp 8, 27.
Test Profiles: Giardia ELISA Antigen Test (Profile # T820 ) ; Fecal Combo Test (Profile # 405), includes ZnSO4 Fecal Centrifugal Flotation Test and Giardia ELISA Test.
W. Jean Dodds, DVM
Hemopet / NutriScan
11561 Salinaz Avenue
Garden Grove, CA 92843
Originally published: Antech Diagnostics News; August 2006.