Volume 23 • Issue 02 • 2019
Classification and importance of
Chlamydia species of ruminants
Based on the latest taxonomic classification
Chlamydia species of ruminants are associated
with two distinct syndromes namely:
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Enzootic abortion caused by Chlamydia
abortus. Abortions or birth of weak lambs
frequently occur in the absence of other
so-called Chlamydia “syndromes” viz.
conjunctivitis, polyarthritis, gastroenteritis,
mastitis and meningoencephalitis.
Sporadic abortion caused by Chlamydia
pecorum. This syndrome in ruminants is
characterised by outbreaks of conjunctivitis,
polyarthritis, gastroenteritis, mastitis and
meningoencephalitis and only sporadic
abortions.
Another separate Chlamydia specie, Chlamydia
psittaci, is not associated with abortions in
ruminants or with the other so-called Chlamydia
“syndromes” and is not considered a primary
lung pathogen of sheep or goats.
Epidemiology
Chlamydial abortion also known as enzootic
abortion of ewes or ovine enzootic abortion,
is a major cause of lamb mortality across the
world and is currently one of the most common
infectious causes of abortion in sheep and goats in
South Africa. It is caused by the bacteria C. abortus
which invades the placenta during pregnancy,
and which can lead to abortion or the birth of
weak lambs. Infection with C. abortus has both a
direct and indirect effect on the developing lamb.
Growth of the organism directly damages the
placenta and negatively affects the development
of the foetus resulting in late abortions or the
birth of weak infected lambs
The clinical consequences of infection are
determined by, amongst other causes, the timing
of infection, the immune status of the animal
and the number of organisms (infection dose).
When non- pregnant sheep, including young
lambs, become infected, they develop latency
in the uterus. Purchasing latently infected
sheep is probably the most common source of
initial infection in a flock. The stage at which
an infection is picked up during pregnancy will
determine the outcome as there is a lag period
between infection and onset of clinical signs. At
some point in mid pregnancy of infected ewes
an unknown trigger causes the organism to
become active resulting in a placentitis which
often results in abortions or the birth of weak
lambs. Organisms derived from infected birth
fluids and placentas from recently or latently
infected sheep are the main source of infection.
A small number of abortions usually occur in the
year after introduction of latently infected sheep
with a storm (due to infection spread to other
naive sheep within the flock) in the subsequent
year. Once an ewe has been infected, she becomes
immune, though will be a carrier of the disease in
subsequent years.
Understanding latent infection therefore is key
to disease control as it is important to realize
that infection in one year most often results in
abortion in the following year.
Clinical signs
In an infected flock first and second lamb ewes
are most susceptible to the disease. The ewe
shows no signs of illness. Flocks infected for the
first time may have an incidence of up to 70%
of abortions but drops to 1-5% once the disease
becomes endemic. Reproductive failure including
abortion, mummification, stillbirth (occasionally
one live lamb born along with a dead lamb) or
the birth of weak lambs (rarely survive) are often
experienced. It is common to have one or all these
syndromes on a farm at the same time.
Pathogenesis
A high level of environmental contamination
by infected ewes in lambing pens or fields is an
important source of infection for other animals
through ingestion and less commonly inhalation.
Following infection, C. abortus colonize the
intestine where they become established within
the intestinal mucosa. In pregnant animals
there is systemic spread from the intestinal
mucosal to the uterus and placenta. Initially
there is a localized placentitis at the level of the
placentomes at roundabout 100 days of gestation
Fig 2. Placental pathology is a hallmark of C. abortus infection.
Note the numerous chlamydia bacteria intermingled with
necrotic cells
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