There are a number of well-known diseases we battle on an annual basis such as pneumonia, lameness and abortion. However, there can be other diseases lurking in our flocks. These diseases are often referred to as iceberg diseases, meaning we often only see some clinical symptoms and often they are missed or diagnosed as something else. My experience is most of these diseases aren’t widespread in our national flock, but most are there. I feel we underestimate these five iceberg diseases – border disease, caseous lymphadenitis, maedi visna, ovine partuberculosis (Johne’s) and ovine pulmonary adenomatosis.
1 Border disease
This virus can cause infertility, abortion and the birth of small and weak lambs. This pestivirus is worldwide and affects both sheep and cattle. It has been linked with BVD infections so our eradication of BVD will help the prevalence in our flocks. It is spread by nasal or oral secretions in the ewe. If the pregnant ewe is infected before 65 days it can cause abortions or barren ewes. If lambs are born they are persistently infected, being born normal or else what we call a “hairy shaker” lamb. These lambs have a hairy rough coat with tremors as well. Sometimes infected flocks will see poor growth rates in lambs.
We must investigate if we are getting these hairy weak lambs. This can be done by post-mortem on these suspect lambs or blood testing ewes that have aborted.
Long term, it is better to not retain replacements from within the flock if there is a high incidence of the disease and instead source replacements from disease-free flocks.
2 OPA ovine pulmonary adenocarcinoma
This disease, also called jaagsiekte, affects the lungs of sheep. It causes soft coughing, secondary infections and pneumonia. The virus causes cancer-like growths in the lungs. Affected sheep can slowly pine away and often have clear discharges from the nose and mouth.
It can often only be diagnosed through ultrasounds of the lungs and post-mortem findings. Sheep can show symptoms from a year of age onwards, with some dying from pneumonia.
The wheelbarrow test is a crude method – if you lift the back legs of affected sheep, a clear discharge may run from their noses.
Infected flocks need to carry out scanning of the lungs to take out carriers or infected sheep. At a minimum, once diagnosed, culling ewes with poor condition and respiratory signs must be considered.
3 CLA caseous lymphadenitis
Caused by a bacteria, the symptoms are abscesses around the head and neck. These can burst and start discharging a thick yellow pus. Clinical signs appear slowly. Infected sheep transfer to each other at times of close contact such as when they are being yarded for dipping or other treatments. Sheep can also develop abscesses internally as the disease spreads through the lymph nodes, often affecting glands around the lung. These sheep can spread the bacteria by aerosol.
Once infected, the sheep are infected for life. It can be difficult to diagnose but great care should be taken when handling sheep with abscesses around the head and neck. Once a diagnosis is made through blood tests (not a very sensitive test), carrier sheep need to be identified and handling protocols need to be reviewed to reduce spread.
It is advisable to cull sheep with symptoms described earlier to prevent the spread of the disease in the flock.
There is a vaccine available which can be got under special licence but I am not aware of anyone using this currently in Ireland.
4 Maedi visna
This virus can cause respiratory symptoms in sheep, it is unknown if it is affecting the Irish flock but it is present in the UK. It is spread by aerosol from infected lungs and also through milk, semen and saliva. Once a flock becomes infected, the number of sheep infected can increase over a period of years.
It typically affects older sheep, causing a swelling and hardening of lung tissue over time. These ewes often have poor fertility and weight loss. It can also cause mastitis in some older sheep.
Once a flock is infected, it is very hard to control the disease. Whole flocks need to be culled in severe cases. Again, like all these diseases, if you have a high turnover of chronic culls, ask the question why?
5 Johne’s disease
This is a disease similar to that in cattle but caused by the sheep species of mycobacterium avium. Again it is a chronic wasting disease of sheep affecting older sheep. The clinical signs in sheep are different to cattle as we don’t get chronic scouring or diarrhoea, but weight loss and low production are common.
The bacterium can survive a long time in the environment and water. Sheep of any age can be infected but the most susceptible are young lambs which can ingest it in the first months of life. The earlier the lambs are infected, the more likely they are to show clinical signs as adults.
Infected sheep can also shed the bacteria in their dung which keeps the cycle of the disease going.
This disease can be missed in our flocks, with these thin poor performers being culled without a diagnosis being made. However, even with culling, the incidence will continue to increase slowly in the flock.
When a diagnosis is made, it should involve blood testing over time to pick up positive sheep and better hygiene, especially in lambs.
Managing these diseases can be difficult due to the fact that a lot of the blood tests aren’t very reliable and we often only find the symptoms at the end stage of the disease. Post-mortems do have value as a way of at least identifying the diseases. Some flock owners, if they want to be proactive, can use screening blood and faecal tests on a number of sheep as an early way of identifying the diseases in our flocks. This does come at a financial cost and often is only carried out in pedigree flocks.
If you have none of these symptoms you still need to be aware of the risk and take steps to keep these diseases out. A closed flock will always offer huge advantages. Even when buying in rams, is it worth screening for these diseases?
SHARING OPTIONS