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Caseous Lymphadenitis in
Sheep
Michael
D. Piontkowski, DVM
Senior Staff Veterinarian
Technical Service
INTRODUCTION
Caseous Lymphadenitis (CLA), a disease caused by the bacterium
Corynebacterium pseudotuberculosis, is a
widespread chronic infection causing significant financial
losses for the sheep industry. The bacteria can enter a flock
either from the environment or more frequently from the
introduction of infected sheep. Once present in a herd, this
infection can be presented in an external form involving
superficial lymph nodes, an internal form involving internal
lymph nodes or organs (especially the lungs), or a combination
of both.
Losses caused by CLA are two-fold. The
first loss occurs when lambs are shipped to market with
lesions resulting in carcass condemnation. In a recent audit
by the sheep industry, CLA accounted for over 23% of the total
condemnation at slaughter. The second loss is "unthriftiness
in ewes", sometimes referred to as "thin ewe
syndrome". Although not typically associated with death,
CLA in this situation leads to decreased milk production and
quality, decreased wool production, as well as eventually
leading to condemnation at slaughter.
METHODS OF INFECTION
Wounds to the skin, either from shearing or the environment,
will allow bacteria to enter the body and establish in
external and internal lymph nodes or organs. Recent research
infers the bacteria may enter the body in an aerosol form,
thus allowing the bacteria to localize in lung parenchymal
tissue. Once established in the body, the infection results in
the formation of an abscess containing a cheesy
greenish-yellow to off-white material surrounded by a thick
capsule. This thick capsule prevents the body's own defense
system from effectively eliminating the disease and prevents
effective penetration by antibiotics. This makes any method of
treatment difficult and usually unproductive. These abscesses
eventually rupture, allowing the spread of the disease to
other parts of the body, as well as seeding the environment
and allowing the bacteria to spread to susceptible sheep.
CONTROL AND PREVENTION
The current recommendation for sheep with CLA lesions would be
cull those sheep to lessen the exposure of susceptible sheep.
Laboratory testing is available to diagnose CLA-positive sheep
with internal lesions to assist in eliminating infected sheep
from a flock.
Colorado Serum Company has developed two
effective vaccines to help protect susceptible sheep from CLA.
With culling sheep with CLA and instituting either of Colorado
Serum Company's CLA vaccines, CASE-BAC or CASEOUS D-T , into a
flock-health program, the incidence of CLA can be
significantly reduced. In a study by Colorado Serum Company,
sheep were challenged at 8 months post-vaccination. Ninety
percent of vaccinated sheep were protected from the internal
form of CLA and 58% were protected against the external form
of CLA. For comparison, non-vaccinated control sheep showed
100% infection of both forms of CLA. In animals having CLA
lesions after challenge, the vaccinated group averaged about
one lesion per animal compared with an average of 36 lesions
per sheep in the non-vaccinated control group, demonstrating a
significant reduction in disease severity (P<0.001).*
Although the vaccine does not help sheep
already infected with CLA abscesses, the protection offered to
susceptible sheep from exposure to a contaminated environment
and undetected infected sheep can amount to a considerable
financial gain to the sheep industry.
By incorporating CASE-BAC into an existing
health program or using CASEOUS D-T to form the basis of a
flock health program, Colorado Serum Company is helping the
sheep industry reduce the incidence of this disease.
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