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Avian Pathogens and Zoonotic Health Threats

Avian Pathogens and Zoonotic Health Threats

By Rebecca Himebaugh and Lekshmi Gopalakrishnan, EMLab P&K Analysts

Birds are found throughout the world with around 10,000 species in existence. They inhabit ecosystems across the globe from the Arctic to the Antarctic1. In many areas they have cultural and economic significance. They are raised for meat or eggs or can be kept as pets. Since avian species are so common and wide-spread, they share many environments with humans. Therefore, humans need to be aware of the possible health risks that their presence poses. Birds and bats can act as disease spreading hosts; carriers of pathogenic organisms can cause diseases in humans.

Avian Pathogens Defined
Avian pathogens herein, are disease-causing organisms carried by birds or bats, which can be transmitted to humans. Exposure to contaminated avian fecal material, or soil contaminated by droppings, can lead to human infection. Individuals who care for or work with birds (e.g. bird-keepers, pet bird owners and poultry producers) should be aware of the routes and risks of exposure2. Additionally, those working in areas of the country where the pathogens have become endemic to the soil, need to take proper precautions to avoid infection. For most healthy individuals, infectious diseases caused by avian pathogens do not pose a serious threat. Many do not even show symptoms of having been infected. However, infectious risk increases for the young, elderly or immunocompromised.

Avian species can be carriers of infectious organisms. Shown here is the feral pigeon, Columba livia domesticaBats can be carriers of infectious organisms. Shown here is the Townsend's big-eared bat, Corynorhinus townsendii

Figure 1: Avian species and bats can be carriers of infectious organisms. Shown here are the feral pigeon, Columba livia domestica, and Townsend's big-eared bat, Corynorhinus townsendii.
Source: Wikipedia.

Histoplasma capsulatum, Cryptococcus neoformans and Chlamydophila psittaci are three primary types of pathogens that can cause zoonotic diseasess of avian origin. Histoplasmosis and cryptococcosis are infrequent or potential zoonotic diseases indirectly associated with birds. Psittacosis, also known as chlamydiosis, is a zoonotic disease of reasonable significance.

Histoplasma capsulatum and histoplasmosis

Methenamine silver stained Histoplasma capsulatum showing histopathologic changes in histoplasmosis

Figure 2: Methenamine silver stained Histoplasma capsulatum showing histopathologic changes in histoplasmosis.
Source: Wikipedia.

Histoplasma capsulatum is found throughout the world and was first described in 1905 by Samuel Darling9, but was not determined to be a widespread cause of infection until the 1930's. Before then, many cases of histoplasmosis were mistakenly attributed to tuberculosis. It prefers warm, humid soil and is most commonly found in central and eastern areas of the United States, particularly in states bordering the Ohio River Valley and the lower Mississippi River3.

Histoplasma capsulatum is a dimorphic fungus which grows in soil and material contaminated with bird or bat droppings (guano). It grows as mycelium in soil at ambient temperatures (roughly 10°C-37°C), and upon animal or human infection, converts to a yeast phase where spores undergo genetic, biochemical, and physical changes6.

The fungus has been found in poultry houses, bird roosts, caves and areas harboring bats. Transmission occurs when individuals working with infected soil or droppings inhale the spores. It is not contagious and cannot be transmitted from an infected person or animal to another6. This disease associated with infection from inhalation of the spores, is referred to as histoplasmosis10. It primarily affects a person's lungs and the symptoms can vary greatly6. For protection, workers should wear a face mask or respirator when working in suspect surroundings, or with animals known to carry the disease-causing organism7.

The acute phase of histoplasmosis is characterized by non-specific respiratory symptoms, often cough-like or flu-like3. Disseminated histoplasmosis is where the disease has spread to other organs from the lungs through the bloodstream. It can affect any organ, as well as multiple organ systems, and can be fatal if left untreated4. Antifungal medications are available to control infection7.

Cryptococcus neoformans and cryptococcosis

This photomicrograph depicts Cryptococcus neoformans using a light India ink staining preparation

Figure 3: This photomicrograph depicts Cryptococcus neoformans using a light India ink staining preparation.
Source: Wikipedia.

Cryptococcus neoformans is an encapsulated yeast that replicates by budding. Avian species are carriers and the spores are transmitted via bird droppings, especially pigeon droppings. It can also be found in soil that has been contaminated with bird droppings. Humans and a wide range of animals can become infected from these sources11.

Infection by C. neoformans is usually by inhalation of the fungal spores, although it can occasionally occur by ingestion. Humans can pick up C. neoformans from exposure to old pigeon nests or droppings, or by working in areas where the soil has become contaminated. The disease caused by this fungus is known as cryptococcosis. So far, it is not known to be spread from person to person, animal to animal, or from animals to humans11.

Cryptococcosis presents a wide-range of symptoms including pneumonia-like illness, fever and headache. Cryptococcus neoformans is an opportunistic fungus, in that it normally does not infect healthy individuals, but typically infects immunocompromised persons. Most people in the United States who develop this infection are HIV-infected or have other conditions affecting their immune system. However, occasionally persons with no apparent immune system problems develop cryptococcosis11.

Chlamydophila psittaci and psittacosis

Direct fluorescent antibody stain of a mouse brain impression smear showing Chlamydophila psittaci

Figure 4: Direct fluorescent antibody stain of a mouse brain impression smear showing Chlamydophila psittaci.
Source: Wikipedia.

Chlamydophila psittaci is an intracellular bacterial organism, which occurs worldwide and affects more than 100 avian species. Chlamydophila psittaci undergoes several transformations during its life cycle and can exist in a non-biologically active state which is resistant to environmental stresses and can even survive outside a host. Once inside a host, the organism becomes active and begins to replicate. Infection by C. psittaci causes a disease called psittacosis (or parrot fever) when it occurs in psittacine birds (parrots, parakeets, etc.), and called ornithosis, when diagnosed in all other birds, as well as in humans8.

Psittacosis was first reported in Europe in 1879. In 1929, a highly publicized outbreak of psittacosis hit the United States5. Infection is usually via the droppings of another infected bird, though it can also be transmitted via feathers and eggs, and is typically either inhaled or ingested. The bacterium, which is shed through both feces and nasal secretions, survives drying. This facilitates oral spread as well as transmission via contaminated clothing and equipment. Psittacosis can be transmitted bird to bird, feces to bird, and bird to human.

Psittacosis is an occupational hazard for persons working with psittacines and pigeons, or for people working in turkey slaughter plants and avian diagnostic laboratories2. Nationwide, 100-200 human cases are reported annually, although the actual numbers of cases are probably much higher.

Sampling and Testing: The Avian Pathogen Screen by PCR
Samples can be collected to determine environmental reservoirs and potential transmission routes for the organisms, although the mere presence of the organisms in an environment does not predict the potential for an outbreak of infection. A variety of matrices are suitable for sampling for avian pathogen testing. These include air, surface swab, water, avian feces, soil and dust. Timely determination of the presence of any of these organisms can alert individuals to take the proper precautions when handling affected animals or working in or near affected areas.

Testing of a sample for one or more of these avian pathogens can be performed quickly and accurately at EMLab P&K using the real-time polymerase chain reaction (also known as Q-PCR or KPCR). DNA is extracted from the sample and then as the DNA is amplified, it is simultaneously detected as the reaction progresses in real time. Detection of Cryptococcus neoformans and Chlamydophila psittaci, yield quantified results of the organisms in the sample. For Histoplasma capsulatum, the analysis yields a "Detected/Not Detected" result.

1. Wikipedia: Birds (accessed 30 April 2011).

2. Jacob, Jacqueline P., Jack M. Gaskin, Henry R. Wilson and F. Ben Mather. 1997. Avian diseases transmissible to humans. Document PS23, Animal Science Dept, FL Cooperative Extension Service, IFAS, University of Florida.

3. Silberberg, P. 2007. Radiology teaching files: Case 224856 (Histoplasmosis). (Retrieved 7/27/2007).

4. Kauffman, CA. 2007. Histoplasmosis: a clinical and laboratory update. Clinical Microbiology Reviews 20 (1): 115-132. doi:10.1128/CMR.00027-06. PMID 17223625. PMC 1797635.

5. Potter, Morris E., Arnold K. Kaufmann, Brian D. Plikaytis. 1983. CDC: Psittacosis in the United States, 1979. MMWR, Morb Mortal Wkly Rep 1983 Feb 01; 32(SS01):27-31.

6. CDC Report: Histoplasmosis: protecting workers at risk. Accessed 3/15/2011.

7. Wikipedia: Histoplasmosis (Accessed 3/16/2011).

8. Wikipedia: Chlamydophila (Accessed 6/21/2011).

9. Darling, ST. 1906. A protozoan general infection producing pseudo-tubercles in the lungs and focal necrosis in the liver, spleen and lymph nodes. J Am Med Assoc 46: 1283-1285.

10. Rapini, Ronald P., Jean L. Bolognia, Joseph L. Jorizzo. 2007. Dermatology: 2-Volume Set. St. Louis: Mosby. p. 1135. ISBN 1-4160-2999-0.

11. CDC Report: Cryptococcus. (Accessed 3/15/2011).


This article was originally published on May 2011.