What is the relationship between 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 and 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.

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 flulike 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.
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 flulike, 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 flulike 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 flulike 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 flulike 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

Perhaps the greatest impact to 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.

Continuing research into vaccines and proper education about bird handling and care are vital to reducing the amount 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

Clark, Larry, and Jeffrey Hall. “Avian Influenza in Wild Birds: Status as Reservoirs and Risks to Humans and Agriculture.” In Current Topics in Avian Disease Research: Understanding Endemic and Invasive Diseases, edited by Rosemary K. Barraclough. Washington, D.C.: American Ornithologists’ Union, 2006. A good outline of the problem. Also considers human health, agricultural concerns, and the potential effect on wild bird populations.

Krauss, Hartmut, et al. Zoonoses: Infectious Diseases Transmissible from Animals to Humans. 3d ed. Washington, D.C.: ASM Press, 2003. Explores the myriad infections introduced by human-animal contact.

Ligon, B. “Avian Influenza Virus H5N1: A Review of Its History and Information Regarding Its Potential to Cause the Next Pandemic.” Seminars in Pediatric Infectious Diseases 16 (2005): 326-335. Examines the history of the bird flu and its possible future effects on global health.

Marquardt, William C., ed. Biology of Disease Vectors. 2d ed. New York: Academic Press/Elsevier, 2005. A biology text examining disease vectors, including bats and wild and domestic birds. Written for graduate students and researchers, but accessible to general readers.

National Association of State Public Health Veterinarians. “Compendium of Measures to Control Chlamydophila psittaci Infection Among Humans (Psittacosis) and Pet Birds (Avian Chlamydiosis).” 2010. Available at http://www.nasphv.org/documents/psittacosis.pdf.

Thomas, Nancy J., D. Bruce Hunter, and Carter T. Atkinson, eds. Infectious Diseases of Wild Birds. Ames, Iowa: Blackwell, 2007. A detailed description of the health risks to birds, other animals, and humans from avian-related infectious diseases.