Avian cholera is an infectious disease of domestic and wild birds caused by the bacteria Pasteurella multocida. This disease has been recognized for over 200 years. There are many strains of P. multocida that infect
different species of birds and mammals and cause varying degrees of disease; however, this description will focus primarily on avian species.
Avian cholera is the most significant infectious disease of wild waterfowl in North America. Single outbreaks can kill thousands of birds, and outbreaks occur annually and almost annually in some parts of the continent.
P. multocida has been found in many species of birds and mammals. Avian cholera infections have been reported in over 190 species of birds, though most bird species are likely susceptible to this disease. Waterfowl and coots are the most commonly affected and they frequently experience major mortality events. Scavengers such as gulls, raptors, and crows are also
affected with relative frequency, while other water birds and upland species are less commonly affected. In addition, domestic poultry and other captive species are affected by avian cholera. In general, adult and older birds are more susceptible to avian cholera than younger birds.
The various strains of P. multocida are more infectious in some species than others. For example the avian strains cause fatal disease in birds, rabbits, and mice, but not in other mammals. Some mammalian strains cause illness in birds, while others do not. Certain predatory mammals such as raccoons, foxes, and cats carry P. multocida in their mouths and can infect prey via a bite wound, while the predator itself does not suffer from disease. Many farm animals have been found to carry strains of avian cholera, but while strains carried by cows and sheep do not cause disease in birds, stains from pigs have shown to be very deadly for birds. Avian strains of P. multocida typically do not infect humans, though mammalian strains can infect humans via animal bites, scratches, or wound contamination.
Avian cholera was first reported in wild waterfowl in North America in the winter of 1943-1944 in Texas and California. These outbreaks marked the beginning of the emergence of this disease in North American waterfowl, and by the early 1980’s outbreaks had also occurred in Nebraska, Maryland, and the Northwest Territories in Canada. North America now experiences epidemics of avian cholera in wild waterfowl annually and outbreaks have been reported from coast to coast. The regions in the United States that experience mortalities from avian cholera most frequently include the California Central Valley, the Tule Lake and Klamath Basins in northern California and southern Oregon, the Playa Lakes and Gulf Coast regions in Texas, the Rainwater Basin in Nebraska, and the Lower Missouri River Basin in Iowa and Missouri. Less frequent mortality events also occur in the Chesapeake Bay area in Maryland. In 2008, thousands of birds died of avian cholera on the Great Salt Lake in Utah. Outbreaks are much less frequent in other parts of the world even though the disease probably occurs worldwide in domestic birds. While most states in New England have not had significant outbreaks, Maine occasionally experiences outbreaks of avian cholera in free-ranging waterfowl that result in large-scale mortality of flocks.
P. multocida is released into the environment by dead and dying birds or asymptomatic carriers, and it can be transmitted to susceptible birds in a variety of ways. One way is through direct bird-to-bird contact, especially when birds are crowded together. The bacteria tend to collect on the surface of water and can become airborne when birds take flight or land. Once the bacteria are airborne, it can be inhaled. Avian cholera can also be transmitted by way of ingestion of contaminated food or water . Ingestion is likely the most common route of transmission. Predatory and scavenging birds may acquire avian cholera by feeding on infected birds and small mammals. The bacteria may also be transmitted via the bites of insects and predatory mammals, but this is more likely a source of sporadic cases, not outbreaks. P. multocida can also be introduced to new areas by humans through transportation of contaminated equipment, cages, and clothing.
Avian cholera is transmitted easily between birds when they flock together in high densities. Birds are more susceptible to disease at times of stress, and avian cholera outbreaks often occur during the winter when birds are overcrowded and the weather is cold and damp. Other stressors influencing disease outbreaks include migration, harsh environmental conditions, and competition for limited food resources.
The first sign of an outbreak is usually large numbers of dead birds with very few sick birds. Most wild birds infected with avian cholera are not observed when they are showing clinical signs. When sick birds are observed they exhibit lethargy, convulsions, green, yellow, or blood stained diarrhea, and difficulty breathing. Birds may also swim in circles and fly erratically. They may have nasal or oral discharge, matted and soiled feathers, and bloody feces. This disease kills birds so suddenly that they may fall out of the sky or die while eating. Because avian cholera kills so quickly, most dead birds are in good body condition. At necropsy, hemorrhages are often found on the heart muscle and gizzard, and small white-yellow spots may be found on the liver. It was once believed that almost all birds infected with avian cholera die suddenly of the disease. However, more recent research shows that some birds survive and become carriers of the bacteria. These birds are a likely source of infection for subsequent outbreaks.
P. multocida must be isolated to reach a definitive diagnosis of avian cholera. Avian cholera shares clinical signs with several other diseases of wild birds, so it is important to conduct laboratory tests to confirm the cause of illness.
Antibiotics may be used to treat individual cases of avian cholera, but it is not feasible to treat wild birds when outbreaks occur. While antibiotics decrease mortality, once treatment stops death may continue, meaning antibiotics do not eliminate the disease from a flock. A vaccine is also available but is not used on wild birds.
Management efforts for avian cholera in wild birds focus on limiting transmission during outbreaks. Wetland areas where migratory birds congregate must be surveyed frequently to ensure early detection of disease outbreaks. When outbreaks are detected, carcasses must be collected and disposed of rapidly in order to minimize environmental contamination. Management practices may also be implemented to decrease population densities because the disease is most readily transmitted when during times of overcrowding.