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Mick Millar outlined the signs and treatments
for a group of ‘iceberg’ diseases, so called
because animals showing clinical signs
were just the tip of the iceberg, and others
would be infected at sub-clinical levels.
Mick highlighted three diseases with causative
agents that persist in the environment; Johne’s
disease, CLA (Caseous Lymphadenitis) and
OPA (Ovine Pulmonary Adenocarcinoma).
Mick explained: ‘These diseases are
probably under-diagnosed as they are
difficult to detect because the tests available
have poor sensitivity, or there is no test
available at all.
Johne’s disease
Johne’s is caused by a mycobacterium in
the gut which causes the lining to thicken.
Infected animals shed the bacterium in faeces
and colostrum – into the environment, where
it can persist for a long time. There are
different strains, and cattle strains can also
affect sheep and goats.
‘Johne’s is a major reason for culling in
goat herds,’ said Mick. ‘It has a long
incubation period, so disease may only
become apparent when animals are two or
three years old.
‘Symptoms are progressive weight loss and
often anaemia. Sometimes fluid swelling
under the jaw is seen. In cattle, diarrhoea
is seen, but in sheep and goats diarrhoea
is less common and the main clinical sign
is wasting, and a consequent drop in
milk yield.
‘Diagnosis is difficult in the early stages,’
explained Mick, ‘but blood and faecal
tests in the later stages can identify it.
Post-mortems are useful for diagnosis though
in sheep and goats typical changes in the
intestine can be variable.
‘There is no treatment for Johne’s disease so
the only option is to cull.’
To prevent the disease, he advised farmers
not to graze cattle with sheep and goats;
to ensure good hygiene in lambing/kidding
sheds to reduce exposure to faecal
contamination; and snatch kids/lambs at
birth and feed them pasteurised colostrum,
rearing them away from adults.
There is a vaccine for Johne’s disease which
needs to be given between four weeks and
six months of age. However, Mick warned
that this can interfere with TB testing, so
ideally a closed herd/flock should be
maintained.
Caseous Lymphadenitis (CLA)
CLA is caused by a bacterium which causes
lymph node and internal abscessation in lungs
and the abdominal cavity. It is associated
with wasting, and only occasionally results
in death.
‘It is highly infectious and will survive in the
environment for months,’ explained Mick.
‘There is no effective treatment for CLA, and
infected animals remain so for the rest of
their lives.’
The disease is spread by contamination
of feed troughs, shears and ear-tagging
equipment, by infected pus and nasal
discharge, faeces and skin wounds. A blood
test is available but it has poor sensitivity
so may not identify infected animals (13%
can be missed). Testing should be done
every six months on animals older than
six months if attempts are being made to
eradicate it.
‘The introduction of infected stock is the main
source of infection on farm,’ explained Mick.
‘So prevention is best achieved with strict
biosecurity measures; this includes disinfection
of clothing and equipment.’
Ovine Pulmonary
Adenocarcinoma (OPA)
There are several names for this disease;
sheep pulmonary adenomatosis (SPA),
Jaagsiekte disease, or ovine pulmonary
adenocarcinoma (OPA).
‘OPA is associated with 25% of the
pneumonia cases in sheep in Scotland,’
said Mick.
The causative agent is a virus that gives rise
to tumours in the lungs. It is spread by
respiratory secretions, and outward signs
include weight loss, coughing, difficulty
breathing and nasal froth/discharge.
Secondary infections can also follow.
‘OPA is difficult to control because there is no
blood test available, and it relies on waiting
for clinical signs to appear. However,
infected sheep often show no visible signs
so it is easily spread between flocks when
sheep are purchased,’ said Mick.
Disease control
Unlike clostridial diseases, control of these
three production-limiting diseases centres
on biosecurity measures, as there is only
a vaccine for Johne’s disease. Mick
recommends that farmers only buy
disease-free accredited stock; cull any
infected animals, segregating them
immediately; and reduce the level of
challenge e.g. through good hygiene,
and by not pooling colostrum.
SPRING 2016 ISSUE
LIVESTOCK MATTERS
14
SHEEP AND GOAT
CONFERENCE
Mick Millar
Production-limiting ‘iceberg’ diseases
Typical thickening of the intestine in a ewe with
Johne’s disease (the thickening seen here may not
be so obvious in other affected animals)
OPA is associated with 25% of pneumonia cases
in sheep in Scotland
Clostridial disease; the classic cause of sudden
death in sheep