Birds and infectious disease
Birds, both wild and domestic, are susceptible to a variety of infectious diseases that can affect their health and, in some cases, pose risks to humans. While diseases like avian influenza (bird flu) are often asymptomatic in wild birds, domestic birds can become seriously ill and transmit infections through their saliva or droppings. Human infections are rare, but certain groups, including newborns, young children, and immunocompromised individuals, are at a higher risk when exposure occurs. Other notable diseases include West Nile virus, which is transmitted from birds to humans via mosquitoes, and bacterial infections like psittacosis and salmonellosis, primarily seen in pet birds and chicks.
Fungal diseases such as cryptococcosis and histoplasmosis can also be contracted from exposure to infected birds or their droppings, leading to respiratory issues or more severe health complications. The impact of avian diseases extends beyond health concerns; historical events like the bird flu pandemic of 1997 highlight the potential economic ramifications, as seen in the mass culling of poultry in Hong Kong. Ongoing research and public education are essential for managing these risks while also emphasizing the importance of protecting both wild and domestic bird populations.
Birds and infectious disease
Definition
Wild and domestic birds are at risk for infections with pathogens that may lead to disease in other birds and in humans. Each type of bird may carry infections that lead to different clinical diseases. Wild birds have adapted to urban environmental settings, increasing the risk of domestic infections.
Even though infections may be spread from birds to humans, a resulting human illness is rare. Newborns, young children, and immunocompromised persons are at greatest risk for disease transmission.
Avian Flu
Avian flu, or bird flu, is typically found in asymptomatic wild birds. However, domestic birds such as farm chickens and ducks may become sick and spread the disease through saliva or droppings. The avian flu does not pass easily from bird to human or from human to human. Typically, the avian flu is spread to pigs and other animals that can contract both bird and human flu strains. The bird and human flu strains combine to form a new mutant strain to which humans are susceptible. The avian flu is caused by strain H5N1.
Mild symptoms resemble the seasonal flu: fever, sore throat, and muscle aches. More severe symptoms include eye infection, pneumonia, and other respiratory difficulties. The risk for mortality is high because humans have no immunity to the avian flu strains. More than one-half of all persons who have been diagnosed with the avian flu have died.
Certain antiviral medications are effective, but the seasonal influenza vaccine cannot protect against avian flu. Newer vaccine combinations appear promising.
According to an article in the New England Journal of Medicine in 2024, as of November 1, 2024, more than 900 human cases of H5N1 in 24 countries have been reported since November 2003. Of these, 50 percent were fatal. Prior to 2024, only one human case of avian flu had been reported; a poultry worker became infected in Colorado in 2022 and reported fatigue as the only symptom.
West Nile Virus
West Nile virus is a flavivirus that is spread by a mosquito that first bites an infected bird and then, newly infected, bites a human. West Nile virus was first documented in the United States in 1999, and it spread rapidly. Urban communities have a higher risk than rural populations. Symptoms may be mild, with a rash, muscle weakness, and flu-like symptoms, or they may be more severe, with meningitis, encephalitis, or a lack of cognitive clarity.
In 2004, 2,200 human cases were reported; 73 of these cases ended in death. In 2023, the CDC reported 2,566 cases. Although the virus is not as prevalent in the human population now as it was during the original outbreak, West Nile virus remains a subject of medical research. Scientists continue to work on identifying the types of birds that carry the disease and on what mechanism causes the disease to infect both humans and birds. Antibiotics are not effective, and antiviral medication trials are ongoing.
Bacterial Infections
Psittacosis. Parrots and parakeets often carry the bacterium Chlamydophila psittaci. The disease is rare in humans; only a few hundred cases are reported each year in the United States. Veterinarians, pet-store employees, and bird owners between the age of thirty and sixty years are at greatest risk; the disease is rarely reported in children. Symptoms are usually flu-like, with a cough, and are treated with antibiotics. Rarely, the presentation is more severe and includes pneumonia, infection of the heart, hepatitis, and death.
Salmonellosis. Baby chicks and ducklings often have the bacterium Salmonella in their intestines; hence, they shed it in their droppings. Salmonella is also found on the feathers and beaks of birds. Chicks will not typically display signs of infection. Humans may develop salmonellosis either by holding the bird or by touching a contaminated object. Children are at greatest risk because of their improper handwashing technique and because of their tendency to put their fingers in their mouths after petting birds. Persons who are immunocompromised, elderly, or pregnant should avoid touching birds, especially chicks and ducklings. Salmonellosis results in diarrhea, fever, stomach pain, and other flu-like symptoms within a few days of exposure. The symptoms typically resolve within one week.
Fungal Diseases
Cryptococcosis. Cryptococcosis is a disease transmitted to humans from pigeons and chickens. The fungi Cryptococcus neoformans, which causes the disease, is found in soil droppings or in roosts, such as in attics and on ledges. Inhaling the spores causes the disease. Infections are usually asymptomatic or mild and include flu-like symptoms, a cough, and a skin rash. In more severe forms, infection in the lungs may spread to the central nervous system and cause brain damage or become fatal. Cryptococcosis is treated with antifungal medication.
Histoplasmosis. Histoplasmosis, a disease transmitted to humans commonly from pigeons, starlings, blackbirds, and bats, is caused by the fungus Histoplasma capsulatum. Humans may contract the disease by inhaling air near affected soil, near roosts that have been maintained for several years, or near droppings (from bats). The spores may be airborne, too, and can travel great distances. Construction workers, gardeners, and those in other outdoor occupations are at highest risk because of the disruption of soil at work sites. Infections are typically mild with flu-like symptoms. Rarely, the infection may lead to fever, blindness, and death. Young children, the elderly, and persons with lung disease are at greatest risk for these more significant symptoms.
Impact
One of the greatest impacts on global public health and the world economy can be found in the experiences of the bird flu pandemic of 1997. For example, government officials in Hong Kong, who feared outbreaks and a significant number of deaths, had ordered the slaughter of all poultry in that region (about 1.5 million birds) within three days. This slaughter led to economic problems in Hong Kong and elsewhere. The virus spread rapidly to other Asian countries, and with bird migration, the disease spread to Europe.
Outbreaks of bird flu occurred in the United States in 2024, infecting poultry, dairy cows, and people. According to the CDC in 2025, sixty-seven people had contracted H5N1 in 2024. There was one reported death.
Continuing research into vaccines and proper education about bird handling and care are vital to reducing the number of human infections and preventing avian disease-related pandemics. However, a balance should exist between preserving the wild and domestic bird populations and protecting humans.
Bibliography
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