Are Canada Goose Populations a Public Health Concern?

Goose populations are growing

Historically, the Canada goose (Branta canadensis) is a migratory species, wintering in the United States and migrating to summer breeding grounds in Canada (Fig 1). Canada geese have increased dramatically in urban areas and their numbers continue to grow. As a grazer, this species can take up residence in any large, grassy area like open yards and fields. A ready food supply paired with non-freezing water sources means the Canada goose no longer needs to migrate nearly as far and many birds have become year-round residents in the United States.

Figure 1. The Canada goose (Branta canandensis) is the most widespread goose in North America. Click image to enlarge.

Unlike many waterfowl…

Unlike many waterfowl species, Canada geese tend to travel further from water to feed. In turn, this allows birds to spread their feces over large areas—as anyone that has ever visited a park can attest! While feeding, one Canada goose can produce approximately one dropping per minute. Total daily fecal production may reach approximately 2 pounds or more of waste per goose.

Waterfowl may be asymptomatic carriers of potential human pathogens

The Canada goose can harbor human pathogens and intermittently shed microorganisms into the environment including coliform bacteria, Cryptosporidium, and Giardia.

Giardia

Giardia spp. has been reported in many orders of birds, including anseriforms. Giardia cysts are ingested by the host and then enter the gastrointestinal tract where trophozoites inhabit the small intestine. Although many birds are asymptomatic, some individuals develop small intestinal villous atrophy. The resultant malabsorption/maldigestion diarrhea can be acute and severe.

There are several methods to identify the presence of Giardia:

• Zinc sulfate fecal flotation detects the cyst form
• Direct saline smear of fresh feces detects motile trophozoites
• ELISA testing for the Giardia antigen in feces is also available

Feces collected from Canada geese frequently contain Giardia, particularly Giardia cytosi (psittaci). Giardiasis in mammals, including human beings, is much more commonly caused by G. lamblia. Birds with clinical disease are typically managed with metronidazole (50 mg/kg PO q24h x 5-7 days).

Cryptosporidium

Cryptosporidium species are protozoan parasites that infect humans and a wide variety of animals. Cryptosporidiosis in humans is an acute gastrointestinal illness that can be life-threatening in immunocompromised individuals. Cryptosporidium spp. is transmitted through ingestion or inhalation of sporulated oocysts. The most common mode of infection is ingestion of contaminated food or water. Direct contact with infected animals, persons, or contaminated surfaces can also easily transmit disease.

Cryptosporidiosis is considered an under-suspected, under-diagnosed and under-reported disease in people. In 2001, 3785 cases of human cryptosporidiosis were reported in the United States, however it is estimated that there may be as many as 300,000 human cases each year. Most human cases are attributed to contaminated water or direct person-to-person transmission. Infected individuals typically excrete large numbers of oocysts and the infective dose is relatively low. A few as thirty oocysts can transmit disease in healthy adult humans. The infective dose is likely lower in immunocompromised individuals.

There is high prevalence of Cryptosporidium in feces collected from Canada geese (Kassa 2001). Cryptosporidium can infect the salivary glands, proventriculus, small intestine, cecum, colon, and cloacal epithelium of affected birds. Although most birds are asymptomatic, clinical signs can involve the gastrointestinal tract, respiratory tract, and/or urinary tract, especially in immunosuppressed birds. Surveys of goose feces have primarily identified host-adapted Cryptosporidium sp. (goose genotypes I and II) (Xiao 2002). These goose genotypes have not yet been implicated in human disease (Table 2).

Cryptosporidium parvum (human genotype) and C. hominis are the most important causes of cryptosporidiosis in humans (McLauchlin 2000, Leoni 2006) and are responsible for most Cryptosporidium outbreaks worldwide. Principle sources are humans and cattle. Cryptosporidium parvum (zoonotic genotype), C. meleagridis, and C. felis are also occasionally isolated from infected humans.

Approximately 10% of Cryptosporidium-positive specimens in Canada geese contain C. parvum and C. hominis (Zhou 2004, Graczyk 1998). Although direct transmission of C. parvum from goose droppings to human has not been documented, C. parvum oocysts have been shown to retain viability and infectivity following passage through the Canada goose (Graczyk 1997). It has been suggested that geese may disseminate infectious oocysts of C. parvum via their feces into public water sources (Graczyk 1998), however it has also been suggested that Canada geese probably play only a minor role in the animal-to-human transmission cycle of the pathogen.

Cryptosporidium is difficult to detect because of its small size and intermittent shedding rate, however a Sheather’s flotation test is considered the best antemortem diagnostic test. Acid-fast cytology of fecal smears can also be used to detect the organism. Mucosal scrapings of the intestinal tract are a good postmortem diagnostic sample.

Treatment for cryptosporidiosis in birds has not been described. Control or prevention is also challenging since the oocyst stage is extremely resistant to drying as well as common disinfectants like chlorine bleach. Oocysts are destroyed by boiling water for a minimum of 1 minute. Filters can also be used to prevent the organism’s entry into a water source.

High fecal coliform counts

Geese carry moderate levels of fecal coliform bacteria (1.3×10⁴) per gram of feces (Table 1). When multiplied by the substantial amount of feces produced, this is a very large potential source of coliform bacteria.

Summary

There is high prevalence of Giardia and Cryptosporidium in feces collected from Canada geese, and waterfowl may be asymptomatic carriers, intermittently shedding these organisms into the environment. Public health concerns exist that aquatic birds like Canada geese can serve as a source of protozoa entering surface water, drinking water, and recreational water resources.

Although more research is needed, surveys of goose feces and studies investigating human cases suggest that the role of waterfowl in disease transmission appears to be relatively small. Although Canada geese are known to shed large numbers of Cryptosporidium and Giardia in their feces, the species isolated from geese are generally not the organisms responsible for human disease (Table 2).

Nevertheless the minor role that Canada geese are theorized to play in disease transmission should not be considered an academic issue for individuals routinely exposed to waterfowl feces. Veterinary health professionals are continually exposed to animals that potentially carry infectious diseases including cryptosporidiosis and giardiasis. This risk is an acceptable one as long as the risk is known, understood, and preventive measures are taken.