I hope you're doing well. I also hope that you'll find the following article
about avian pathogens by EMLab P&K analysts Rebecca Himebaugh
and Lekshmi Gopalakrishnan both interesting and useful.
With best wishes,
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.
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.
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
Histoplasma capsulatum and histoplasmosis
Figure 2: Methenamine
silver stained Histoplasma capsulatum showing histopathologic changes in histoplasmosis.
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
Figure 3: This
photomicrograph depicts Cryptococcus neoformans using a light India ink staining preparation.
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
Figure 4: Direct
fluorescent antibody stain of a mouse brain impression smear showing Chlamydophila psittaci.
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).