One of the major causes of reduced production and increased lamb losses, and an important cause of welfare problems, is having too many thin sheep in a flock. Internal parasites, especially intestinal worms, are a major cause of thinness in growing lambs, but are less commonly implicated in adult sheep, except as a secondary problem in animals debilitated by another disease. These parasites will be dealt with in Chapter 7. This chapter deals with the important causes of weight loss in adults, which may result in debility.

Feeding and Teeth – Some Reminders

Although the initial thought is usually to suspect the presence of a chronic disease, as we pointed out in Chapter 5, the adequacy of the diet should always be considered and thoroughly checked first. The second most common reason is tooth problems, particularly if older sheep are involved, so these should also be checked – see Chapter 3, where there are reminders of some of the main points concerning feeding and tooth condition.

If the diet is adequate and there are no significant tooth problems, then there are a number of diseases that cause weight loss which can be considered. You will need the help of your vet to investigate, and this will involve clinical examination, sample taking, and perhaps sending one or more affected animals to a laboratory for post-mortem examination.

Fig. 217 Assess the quality and quantity of the grazing.

Chronic Diseases Causing Thinness in Adult Sheep

Once nutrition and tooth problems have been eliminated, it is time to consider other possibilities, and for this you will need the help and expertise of your vet. There are various causes of thinness in individual sheep, but many of them are uncommon and rarely get investigated, although interesting conditions can come to light if they are checked out. More significantly, there are several common diseases that cause thinness in a substantial number of animals and lead to noticeable loss of production and premature culling or death of affected animals. Such a problem should trigger a veterinary investigation to establish the cause and to see whether any preventive methods are available. For commercial lowland flocks a breeding life of five seasons should be the aim. Any less than this and replacement costs increase dramatically and impact on the profitability of the flock.

The most important diseases in this category are:

Liver Fluke (Fasciolosis)

This is a really important disease caused by the fluke Fasciola hepatica (see also Chapter 7) and occurs in acute, sub-acute and chronic forms, but it is the chronic form which causes thinness, usually affecting many animals in the group or flock. The disease has become much more common in recent years as a result of a series of wet summers, which has favoured the development of the mud snail in which the developing and multiplying flukes spend part of their life cycle.

The chronic disease results from a steady intake of immature flukes which do not immediately kill the animal but develop into adult flukes, about an inch long, which live in the bile ducts. These cause chronic liver damage, weight loss and anaemia because they live on the blood of the affected sheep. Chronic disease usually occurs during the winter and spring, and it is very important to realize that the whole group or flock is likely to be affected, so clinical disease in a few animalsmuch more common in recent years as a result of a series of wet summers, which has favoured the development of the mud snail in which the developing and multiplying flukes spend part of their life cycle. The chronic disease results from a steady intake of immature flukes which do not immediately kill the animal but develop into adult flukes, about an inch long, which live in the bile ducts. These cause chronic liver damage, weight loss and anaemia because they live on the blood of the affected sheep. Chronic disease usually occurs during the winter and spring, and it is very important to realize that the whole group or flock is likely to be affected, so clinical disease in a few animals should signal the need to treat the whole group. Diagnosis is made on history, clinical signs and by finding fluke eggs in dung samples; blood samples may also be helpful. (For treatment and control, see Chapter 7.)

Fig. 228 This sheep is showing ‘bottle jaw’, a collection of fluid in the tissues under the jaw, a typical sign of chronic fluke infection.

Fig. 229 Looking at her mouth shows that this is a relatively young sheep (a six-tooth, about 2½ years old), so her teeth should not be a problem. Her gums look pale and bluish because she is very anaemic.

Fig. 230 The conjunctiva of her eyes is pale and bluish, also indicating anaemia.

Fig. 231 A typical fluke habitat: a shallow, slow-flowing stream with boggy areas and rushes.

The following four infectious diseases are all generally introduced into flocks as a result of buying infected animals that may not be showing signs of disease at the time. This shows the importance of checking the health status of purchased animals and making use of any available health schemes. CLA can also be introduced on contaminated equipment such as shearing gear, and illustrates another route by which various diseases can be spread from flock to flock. It is always best to minimize buying in animals if at all possible, or to buy from a trusted supplier. The most risky animals are likely to be groups bought through auction markets from unknown backgrounds. A number of other important diseases spread by the same methods will be encountered in other sections of this book.

Maedi Visna (MV)

MV is a chronic disease of the lungs and other organs caused by infection with a virus (a lentivirus, which shows its effects only slowly, as the name suggests) that damages the lungs causing difficulty breathing, gradual weight loss and emaciation. Affected animals usually die of a secondary infection such as pasteurellosis. The other organs that can be affected include the nervous system (resulting in visna, the neurological form of the disease), the joints (causing chronic arthritis, seen more commonly in CAE, the similar disease in goats) and the udder (resulting in reduced milk production). The disease was first introduced into the UK with imported continental breeds but has been seen in recent years in some native and crossbreeds.

The difficulty with this disease is that because it develops slowly, it might not be diagnosed until a large part of the flock is infected although not showing clinical signs of disease at the time. The main ways in which it spreads through a flock are from ewe to lamb, as the colostrum and milk contain the virus, and from sheep to sheep by droplet infection, particularly in housed sheep. It is therefore a disease that can have a major economic impact on a flock’s production as a result of early culling, reduced milk production and poor lamb growth. Many farmers do not realize that this disease has the potential to devastate their flocks, and do not treat the risk with the seriousness it deserves.

Diagnosis is by blood testing and post-mortem examination, with further testing of lung tissues. There is no treatment. There is a voluntary blood-testing scheme under the Premium Sheep and Goat Health Scheme administered by the Scottish Agricultural College (SAC). Details can be accessed at www.sac.ac.uk/consulting. Briefly, to become an accredited flock, sheep have to be isolated from untested sheep and to pass two blood tests, with further negative blood tests at intervals in order to remain within the scheme. There is also a monitoring scheme with less rigorous requirements.

Where a flock has been found to be heavily infected, culling all the animals has been an extreme, but perhaps the only, practical solution. For less heavily in fected flocks, particularly in pedigree flocks, one solution has been to remove lambs at birth and rear them artificially, completely away from the main flock. This is obviously a labour-intensive and expensive procedure but can be successful in saving valuable genetic lines.

Ovine Pulmonary Adenocarcinoma (OPA, Pulmonary Adenomatosis, SPA, Jaagsiekte)

OPA is another viral disease of the lungs, caused by a retrovirus which usually does not cause obvious disease until the affected sheep are over two years old, although it has been seen occasionally in lambs less than one year of age. The virus causes the development of lung tumours that replace the normal tissue in the lower parts of the lungs.

There are major differences from MV: OPA affects different cells in the lungs, producing a copious, frothy, watery secretion, whereas MV causes fibrosis of the lungs without any abnormal fluid production. Infection is spread in a similar way to MV by the infected respiratory discharges, and also probably through colostrum and milk. It cannot be diagnosed by sampling as there are no detectable antibodies in the blood, although experimentally it is possible to detect virus in the blood, and it may be that tests will be developed in the live animal in the future.

Fig. 235 This sheep has an obvious watery nasal discharge which could indicate that she has OPA.

Fig. 236 An ultrasound examination of her chest: you can see some of the frothy fluid from her lungs on the floor.

Fig. 237 This shows the large volume of fluid that can be present in the lungs of some infected sheep. The fluid contains virus, so is infectious for other sheep.

Like MV, it is introduced by already infected animals and causes weight loss, difficulty breathing and coughing, and affected animals lag behind the rest of the flock when being driven (‘Jaagsiekte’ is a South African name meaning ‘driving sickness’). Also, as with maedi, it is often pasteurellosis that actually kills the animal. The main way to diagnose the disease in the live animal currently is by means of the ‘wheelbarrow test’. This involves raising the back end of the sheep so it is higher than the chest, which allows the surplus fluid in the airways to run from the nose. Surprisingly large quantities of fluid can be produced, making you wonder how the sheep manages to keep breathing.

Another method that might help in diagnosis is for your vet to perform an ultrasound examination of the chest, though it requires experience to carry out and interpret this. The only definitive way of confirming the diagnosis is by a post-mortem examination. Because there is no blood test it is not possible to have any control or eradication scheme at the moment. Once a flock is infected the only practical thing to do is to cull animals at the first sign that they have the disease. As with MV, it is possible to rear uninfected lambs by removing them at birth and rearing them artificially, but this is a large undertaking.

Caseous Lymphadenitis (CLA)

CLA is another disease that was introduced into the UK with imported animals, actually with goats, which are equally susceptible. It soon spread to sheep, becoming seen most commonly in terminal sire breeds but now present in some commercial flocks. It is caused by a bacterium called Corynebacterium pseudotuberculosis.

The main feature of the disease is the development of abscesses, most commonly in the head and neck area, but they can occur in lymph nodes anywhere in the body including in internal organs such as the lungs, liver and kidneys. The abscesses contain thick pus and usually have very thick walls consisting of several layers, and are referred to as ‘onion ring’ abscesses. Sometimes these internal abscesses interfere with normal body function and can lead to otherwise unexplained abnormal thinness. It is unlikely to appear in a flock purely as a cause of thin sheep, as some of the other sheep would have noticeable lumps. Diagnosis is based on the type and location of abscesses and submitting pus samples to the laboratory for culturing; as a cause of thinness post-mortem examination will be required.

Treatment is problematic because although individual superficial abscesses can be lanced and treated, others will continue to appear, some internally. Antibiotic treatment is of no real use as the thick walls of the abscesses prevent penetration by the drugs. Blood testing is of help in both trying to rid a flock of infected animals and in screening rams before sale. SAC (www.sac.ac.uk/consulting) offer a monitoring scheme.

Other control measures include keeping different age groups separately, particularly when trough feeding, as discharges from lung abscesses can easily get into feeding troughs and thereby infect other sheep. There is a vaccine, which is used in countries such as Australia, but it is not available in the UK except in very special circumstances, and permission to use it has to be sought. Your vet will be able to get more details.

Johne’s Disease (Paratuberculosis)

This is a chronic disease of sheep and cattle caused by a bacterium called Mycobacterium avium paratuberculosis (Map). It causes progressive loss of weight, reduced production and premature culling. In cattle, scouring is a prominent sign but in sheep it is much less likely to cause this. It is not known to what extent UK flocks are infected, although increasing problems in hill ewe flocks have been reported recently.

There are several strains of Map, some of which are found more commonly in cattle and some in sheep, although the separation is not absolute and it is likely that cattle can become infected from sheep and vice versa, although the importance of this in the spread of the disease is debatable. It is also known that some wildlife, particularly rabbits, can be infected, but again the importance of this as a route of infection and spread is not really known.

Diagnosis in the live animal can be difficult – sometimes the bacteria can be shown to be present in faeces samples, but some infected animals shed few bacteria, or shed only intermittently. So a positive result is reliable but a negative one is not. There is also a blood test but this is of more use to establish overall flock status than for individual diagnosis. Loss of protein through the damaged small intestine leads to low blood albumin levels, so blood sampling to measure this could be a helpful pointer towards a diagnosis. Usually confirmation of diagnosis comes down to post-mortem examination, the characteristic appearance of the affected area of small intestine (some sheep strains helpfully cause yellow pigmentation of the gut lining), and laboratory examination of samples.

In most infected flocks only a small number of sheep are affected at a time, and these should be culled as soon as they are picked out as becoming thin. In flocks where the disease is a greater problem, vaccination can be considered although special permission is required to do this – your vet will advise. As it is known that young animals are the most susceptible it makes sense to try to reduce exposure of lambs to infection which is via the dung of infected animals. So, good hygiene in lambing yards and pens, getting ewes and lambs outside quickly and away from heavily contaminated pastures, and removal of suspect animals as early as possible should be considered. A link between Map and Crohn’s disease in man has long been suggested, but evidence remains equivocal.

Other Chronic Diseases Causing Thinness

Individual sheep may become abnormally thin for a variety of other reasons but they seldom get investigated unless they are valuable breeding animals or perhaps pet sheep. These are some of the possible causes:

Fig. 245 Chronic joint infections such as this can lead to thinness because of associated pain and disinclination to move about to graze.