DO you have sheep on your farm which lose condition for no obvious reason?

Most farmers would admit to having a few sheep like this but rarely investigate the cause. There is a collection of infectious diseases of sheep which cause weight loss, reduced production and early death or culling of breeding sheep.

The main diseases in this category are Johne's disease, ovine pulmonary adenocarcinoma (OPA or Jaagsiekte), maedi visna (MV) and caseous lymphadenitis (CLA).

If you lose even a small percentage of your breeding flock due to unexplained wasting you may well have one or more of these diseases present.

What are the diseases and what effects do they have?

- Johne's disease is a bacterial infection. Young lambs are the most likely to become infected but generally appear well until they are over 18 months of age. Weight loss is the most common sign and unlike cattle many affected sheep never show signs of scour. Infection can be passed between sheep, cattle and goats.

- OPA is a viral infection which causes lung tumours. Animals with the disease show weight loss and if moving a group over a distance affected animals with fall behind the others and be short of breath.

In the later stages of the disease fluid may be seen dripping from the nostrils. Also, some animals may die suddenly as a result of secondary pasteurellosis and be in good body condition at the time of death.

A recent survey of sheep at a fallen stock centre in the North of England by AHVLA and Flock Health Ltd found that 6% of ewes had Johne's disease and 6% had OPA.

- MV is a viral condition that causes loss of condition, pneumonia, mastitis and arthritis. In some cases it can affect the nervous system causing ewes to drag a leg or become paralysed.

As with OPA the animals may die rapidly due to secondary pasteurellosis and this can complicate diagnosis. Affected sheep in thin body condition have reduced fertility.

As the disease takes a long time to develop, affected sheep tend to be older. Often by the time the disease becomes apparent over half the stock are infected.

Lambs from affected ewes are often born small or weak and fail to thrive as their dams have lower milk production.

A recent survey conducted by SAC Consulting Veterinary Services with AHVLA showed that 2.8% (1 in 35) of flocks have MV infection. This has doubled in the past 15 years.

- CLA is a bacterial infection. The most obvious sign is the development of abscesses around the head and neck.

In some cases, these abscesses form on the internal organs and weight loss is the only outward indicator of a problem.

Rams are more affected than ewes, it is thought this is largely to do with rams being kept close together and also contact by fighting. Infection can be passed between animals and farms on shearing equipment, so disinfection is essential.

What is in your flock?

One of the most cost-effective ways of finding out which diseases are causing losses is to have postmortems carried out on affected animals.

These are useful because it is possible to check for all of the diseases mentioned and also for fluke and worms.

It is useful to have a few sheep examined to give the best chance of finding out which problem (or problems) are there. To make this as cost effective as possible choose animals with limited cull value.

Remember that if submitting carcases to SAC Consulting veterinary services a group of up to four animals can be examined for a reduced rate compared to the price of a single animal.

Make sure the animals you send for examination are typical of what you see as the problem. If you are concerned, for example, that two to four-year-old ewes waste away and die, then these are the one to send for examination. Don't send the five-year-old ewe with no teeth, it's unlikely to give you the answer you are looking for!

For some of the diseases taking a blood sample can screen for disease. Blood tests are available for Johne's disease, MV and CLA. Unfortunately, there is no blood test for OPA.

Initially 12 older or thinner animals should be sampled. Initial testing should be for MV and Johne's disease. This will give you an indication of whether these diseases are significant in your flock.

Testing of 12 animals for MV is at a reduced rate compared to individual samples through SAC Consulting Vet Services.

If you wish to sample animals to check for CLA then the ram field is a better place to start than the ewes.

Pus from suspicious lumps can be checked for the bacteria which causes the disease or blood can be checked for antibodies.

Managing an infected flock

If you have established that one or more of these diseases is in your flock it's time to have a sit down with your vet and establish a health plan.

There are things you can do to reduce the spread of infection within your flock.

Culling thinner ewes and their offspring is usually the first step to disease control.

Avoid keeping the smallest of the ewe lambs, as these are more likely to have been bred from diseased mothers.

Other measures such as vaccination against Johne's disease, or splitting flocks by age may also be useful in MV infected flocks.

If MV is widespread in the flock, culling and restocking with MV accredited sheep may be the best option.

Keeping a clean flock clean

If testing and post-mortem examinations fail to find any evidence of these diseases, then you are in a good position.

The challenge is, then, to keep disease out of your flock. You can minimise the disease risk by buying in as few animals as possible, for instance, only rams.

Purchased animals should be quarantined and tested for these diseases where possible before being introduced to the rest of the flock Buy MV accredited stock wherever possible to prevent introduction of this disease.