Tuberculosis is a chronic, debilitating disease caused by the organism
Mycobacterium. It afflicts many species, including cattle and humans.
Though it was once widespread in dairy herds, effective control
programs have curtailed its incidence. In several countries, TB has
been eliminated. Except for periodic outbreaks, Canada is considered
free of Mycobacterium bovis.
TB typically lays dormant in cattle for years. During this time,
infected animals can shed the organism into the environment, posing a
threat to herd members and people.
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In the disease’s advanced stages, cattle will exhibit signs of chronic
debilitation and gradual loss of condition despite adequate nutrition.
Some cattle suffer from a deep cough, have difficulty breathing and
develop enlarged lymph nodes.
In most cases, TB is diagnosed at the time of slaughter. The most
characteristic lesion is enlarged lymph nodes, especially those in the
chest.
Spred through herd
Mycobacterium is spread through contaminated feed and water. Infected
cattle can shed the bacterium in feces and in secretions expelled with
coughing.
Milk can also be contaminated when the organism invades the udder.
Decades ago, when TB was common, Mycobacterium-laden milk was the
source of many human infections. TB in people dropped dramatically once
pasteurization of milk was accepted as an effective way to kill the
organism.
Calves can still be infected with Mycobacterium if they consume
contaminated milk.
Canada’s TB-free status was threatened when the disease erupted in an
Ontario dairy herd last April. Just recently, Mycobacterium was also
detected in Manitoba beef cattle.
Today, all provinces are classified as TB-free except Manitoba, which
has been deemed TB-
accredited. Ontario will keep its TB-free status unless the disease is
reported in another herd within the next five years.
TB was diagnosed in cattle in the vicinity of Riding Mountain National
Park in 1997 and 2001. Wild elk in this Manitoba park are known to
carry the disease and are believed to be the source of TB in the
neighbouring cattle.
TB is also endemic in the bison herd of Wood Buffalo National Park in
Alberta.
The dilemma faced by today’s agricultural industry is whether Canada
can maintain a TB-free status without eliminating or
controlling these sources of
Mycobacterium.
The United States has recently changed the border-crossing requirements
for cattle originating from TB-accredited areas. It now insists that
all breeding cattle and farmed bison be tested before entering the U.S.
All of Manitoba must be given a TB-accredited status. This means that
although the TB source is a specific area around the national park,
cattle from the entire province must be tested before being exported to
the U.S.
TB regulations are being reassessed to reduce the number of producers
affected by the new U.S. policy. It is hoped that the Manitoba region
deemed to be TB-
accredited can be redefined as just the small area around the park. The
rest of the province would then be eligible for TB-free status.
Jeff Grognet is a veterinarian and writer practising in Qualicum Beach,
B.C.
