| Description:
|
Johne's disease (paratuberculosis)
is a chronic, debilitating disease that affects the
intestines of all ruminant animals, including cattle,
sheep and goats. It is caused by Mycobacterium paratuberculosis
a small, gram-positive, facultative intracellular, acid-fast
bacillus. Although many animals in a herd may be infected,
usually less than five per cent of infected animals
will develop clinical signs of disease. Under circumstances
of stress, inadequate nutrition or parasitism, more
of the infected animals may develop clinical disease.
Once clinical disease develops, affected animals eventually
die. There is no treatment for Johne's disease currently
available, making this disease economically significant.
The most significant economic losses associated with
Johne's disease are decreased milk production and reduced
salvage value of clinically affected animals. Milk production
from dairy cows affected with Johne's disease has been
estimated to drop 7.8 percent in their last lactation
and the slaughter weight of these animals was 59 kg
less. Infection with M. paratuberculosis is
usually acquired by consuming feed and water contaminated
with manure from infected animals. Fecal shedding from
clinically affected animals has been estimated to reach
more than 500 billion organisms per day. Obviously,
the environment can become very heavily contaminated,
exposing most animals on these farms the infection.
Mycobacterium paratuberculosis is resistant
to environmental degradation, as well as many disinfectants.
This organism can survive in stagnant water, manure
or deep soil for up to a year. It also withstands freezing
at minus 14°C for up to a year. Calves under six
months of age are the most susceptible to infection.
Clinical signs of disease are seldom observed in animals
under two or three years of age. Consequently, disease
in adults is a result of infection acquired in the first
few months of life. M. paratuberculosis infects
cells that line the intestines, but does not damage
them. It is also able to resist being killed by the
body's immune cells. Signs of clinical disease are caused
by the body's immune response to the presence of Mycobacterium
paratuberculosis, not by direct damage of the intestinal
cells by the organism itself. Affected animals may develop
intermittent bouts of diarrhea that gradually become
more frequent. Other animals suddenly develop diarrhea,
which persists until death. Progressive weight loss
is typical of this disease and may begin before diarrhea
develops. Although affected animals appear unthrifty,
with a rough hair coat and declining milk production,
their appetite remains normal until the terminal stages
of the disease.
|
| Diagnosis: |
Previously, few practical
diagnostic tests could reliably detect infections of
Mycobacterium paratuberculosis in animals that
had not developed clinical disease. Culturing methods
are often employed for M. paratuberculosis
detection but are expensive because of the long time
required to grow the organism. Cultures must be incubated
for up to four months before they can be called negative.
Intermittent shedding also makes a negative fecal culture
difficult to interpret. Several blood tests are available,
but the number of false positives and negatives makes
these tests unreliable and carriers cannot be detected.
Fortunately, new generation DNA based diagnostic tests
enable accurate and timely detection of Mycobacterium paratuberculosis.
DNA based tests specifically target the genetic material
of M. paratuberculosis ensuring sensitive and
specific diagnosis. This test offers veterinarians and
livestock owners a definitive method of diagnosis for
Mycobacterium paratuberculosis and can be used to detect
asymptomatic carriers of the disease.
|
