| Description:
|
Mycobacterium is a
genus comprising morphologically similar, aerobic, non-spore-forming
and non-motile bacteria with wide variations in host
affinity and pathogenic potential. Mycobacteria are
more resistant to heat, pH changes, and routine disinfection
than are other pathogenic non-spore-forming bacteria.
In total, thirty-two species have been known to cause
infection in humans and animals. Avian tuberculosis
infection is commonly associated with two strains of
mycobacteria, M. avium and M. genavense. M. avium has
a wide spectrum of hosts including poultry, pigeons,
raptors, ratites, wild birds, psittacines, and passerines.
M. genavense commonly infects psittacines, and is occasionally
responsible for infection in passerines.
M. avium infections are considered to be "open",
meaning infected birds consistently shed large amounts
of organism into the environment. M. avium is transmitted
by ingestion and inhalation of aerosolized infectious
organisms from feces. Incubation in birds can range
from weeks to years. Oral ingestion of food and water
contaminated with feces is the most common method of
infection. Once ingested, the organism spreads throughout
the bird's body and is shed in large numbers in the
feces. In most cases, a bird infected with tuberculosis
will develop symptoms such as progressive weight loss
in spite of a good appetite, depression, diarrhea, increased
thirst, and respiratory difficulty. Decreased egg production
is often observed in birds that were laying eggs
Typically, three categories of infection are observed
in birds infected by mycobacteria. The classic form
is characterized by the presence of tubercles or granulomas
throughout many organs in the body (i.e. liver, spleen
lungs). A second observable form, known as paratuberculosis,
is associated with lesions in the intestional tract
and is often seen in amazons, pionus, and Brotogeris
parrots. With this form, high numbers of organisms are
shed in the feces. The third form known as nontuberculous,
or atypical form, is very difficult to recognize. This
form commonly occurs in finches, canaries, and small
pisttacines. The liver is usually very enlarged and
large numbers of mycobacteria are found in cytology
and histopathology. |
| Diagnosis: |
Diagnosis of tuberculosis
in live birds is often very difficult. Serological testing
and skin tests used to diagnose tuberculosis in humans
do not work well in birds. It is sometimes possible
to find acid-fast bacteria in the feces. However, this
method is not specific because other acid-fast bacteria
may also be present. Physical findings, such as a very
elevated white blood cell count and low red blood cell
count, as well as tests such as radiology (x-rays) and
endoscopy can lead to a preliminary diagnosis. Nevertheless,
definitive diagnosis is usually based on culturing the
organism from the feces or from an organ. This process
is labourious and time consuming and frequently requires
specialized testing that is beyond the capacity of clinical
laboratories. A PCR assay is now available that specifically
detects the DNA of M. avium and M. genavense species
in a sample. A positive result confirms M. avium or
M. genavense infection. |
