One of the most important aspects of sheep management is the control of diseases, which is carried out by vaccination, and the control of parasites by mouth dosing. Vaccines for all the common diseases can be bought, and the broad spectrum anthelmintics used for dosing sheep can be bought from commercial suppliers of veterinary products.
- INTERNAL PARASITES
LARGE STOMACH WORM OR WIREWORM
This worm will kill both lambs and adult sheep unless preventative measures are taken. The worm occurs in the fourth stomach, or abomasum, and it is about 25mm long. The worms feed on the blood of the sheep which they suck from the wall of the stomach and the small intestines. The sheep dies from anaemia caused by loss of blood. One female wireworm can lay 10 000 eggs a day, and these pass out of the sheep in the dung and infect the land where the sheep are grazing. One ewe with a moderate infestation of worms can pass 500 000 eggs each day onto the land. The eggs hatch into young worms which are picked up by sheep while grazing and these mature inside the sheep and produce more eggs. Hot, humid conditions are the most favourable for the eggs to hatch into infective larvae, and under these conditions, the cycle of the egg to adult worm is completed in three weeks. Sheep must be dosed at intervals of three weeks or less in order to break the cycle in the summer months. In winter, they should be dosed every six weeks. Any of the modern broad spectrum anthelmintics will kill both the adult and the young worms, if the worms are not resistant to that anthelmitic. If dosing is not carried out many of the flock will die.
Other worms which occur in sheep and which can be controlled by dosing are:
These roundworms live in the small intestine of their host and suck blood. Most of the blood passes through them and passes out in the sheep’s dung. This results in blackish diarrhoea which is a typical symptom of hookworm. Due to the large amount of blood each worm sucks and the damage it does to the intestine relatively few hookworms can kill a sheep from loss of blood. This can happen even before poor condition of the sheep is visible. Hookworm larvae are picked by sheep in muddy conditions e.g. around a water trough where the larvae penetrate through the skin of the feet.
- Brown stomach worms
These are thin strands about 12mm long. They are not as deadly as wireworms but they do cause loss of condition and weight in sheep.
- Bankrupt worms
These are very small worms 5mm long and they live in the small intestine.
- Nodular worms
These are 20mm long with a hook at one end. They burrow into the wall of the large intestine and the surrounding tissues form a nodule around the worm. Infested animals have periodic bouts of diarrhoea.
All of the above worms have a similar life cycle with the eggs being passed out of the sheep in the dung. These eggs hatch into infective larvae which crawl up the blades of grass in the veld or pasture, where they are ingested along with the grass.
Once inside the sheep, the larvae grow into adult worms, start laying eggs and the cycle commences all over again.
Figure 1: Typical Life Cycle of Worms which Infect Sheep
These occur in lambs and can be as long as 5 metres, living inside the intestine of the lamb. Segments of the worm break off and pass out in the dung of the lamb. These segments break up on the grass releasing very small eggs which are eaten by mites. The lambs pick up the mites when grazing and become infected. The segments of the tapeworm can be seen in the dung of infected lambs, which look like small grains of white rice. They can be controlled by dosing the lambs with Lintex.
- LIVER FLUKE
If sheep are grazed on topland and have access to clean water, they will not become infected with flukes. However, if they are allowed to graze on a vlei or drink from dams or pools, they can pick up flukes. The disease usually appears in late winter or early spring, with sheep dying suddenly. Adult flukes in the liver lay eggs which pass into the intestines of the sheep and pass out in the dung. They hatch into larvae which swim to their next host which is a snail, where the larvae multiply to produce cercariae. These cerceriae which look like very small tadpoles, leave the snail and form cysts on the leaves of grass and plants around the edge of the water. These leaves are eaten by grazing animals which then become infested by liver flukes. When large numbers of small flukes migrate to the liver and begin feeding on blood, the animal dies from loss of blood. There are a number of drugs which can be used to deal with liver fluke infestation.
2. EXTERNAL PARASITES
The common external parasites of sheep are lice, ticks and scab mites, and they can be controlled by dipping the flock in a plunge dip designed specifically for sheep. Care should be taken to immerse the sheep completely. It is not necessary to dip sheep as often as cattle, because the dip stays in the wool for longer. Therefore the effect of a single dipping lasts longer than with cattle.
The type of grazing, the height of the grass and the tick population of the veld or pasture will affect the number of times the flock should be dipped, but in general, dipping once a month is sufficient to give protection against lice and ticks. If the sheep are grazing short pastures, they do not require dipping very often but their feet and legs should be inspected for ticks fairly frequently.
These are very small, about 1mm, and live on the skin of the sheep. They lay eggs which hatch in 2 weeks and develop into adults. They cause great irritation to the sheep, and damage to the wool, due to the sheep rubbing and biting. Dipping the flock after shearing and again 2 weeks later will kill the adult lice and those hatching from eggs.
The brown tick
Does not transmit any significant sheep disease.
The red-legged tick
Can cause tick paralysis in lambs and severe infection of the ear.
The bont tick
Transmits Heartwater disease.
The bont-legged tick
Causes severe irritation through biting the sheep. All ticks can be controlled by monthly dipping.
This is caused by infestation of scab mite. It is a notifiable disease and must be reported to the nearest veterinary officer.
Treatment is by dipping and again after 14 days. It is good practice to dip all newly-purchased animals brought onto the farm.
3. VIRAL DISEASES
These diseases are caused by virus infections, and can be prevented by vaccinating animals with the right vaccines.
This disease is found throughout Africa, and, as it is transmitted by midges, it occurs during the rainy season.
Infected animals have a high temperature of 41°C and the tongue, lips and gums become swollen and develop ulcers. The feet become hot and sore and the sheep will not walk. No treatment is effective, but the disease can be prevented by vaccinating the flock each year in the spring. Pregnant ewes should not be vaccinated.
RIFT VALLEY FEVER
This disease is not common in Southern Africa. It is transmitted by midges. It causes young lambs to die and pregnant ewes to abort. There is no treatment but it can be prevented by vaccinating the flock in the Spring. Pregnant ewes and lambs under 1 month should not be vaccinated.
ORF OR PUSTULAR DERMATITIS
Causes the formation of scabs around the mouth, especially in young, suckling lambs. Affected animals should be isolated and the sores bathed with mild antiseptic. Prevention is by vaccinating the flock.
Is transmitted by the bont tick which causes a high temperature in animals (42°C) together with nervous symptoms. Treatment with antibiotics can be effective in the early stages of the disease but the main control is by dipping the flock to kill off the ticks.
4. BACTERIAL DISEASES
PULPY KIDNEY OR ENTEROTOXAEMIA
This is a very common disease which kills sheep, especially lambs in a good condition. Infected animals are found dead. There is no treatment. Prevention is by vaccinating the flock carrying out the following programme:
- Ewes and Rams:
Vaccinate once a year. If the ewes are vaccinated 2 weeks before lambing it will protect the young lambs after they are born.
Vaccinate at 2 months and again 1 year later. Any sheep brought onto a farm should be vaccinated before reaching the farm.
QUARTER EVIL OR BLACKLEG
Bacteria live in the soil and will infect an animal through a cut or scratch on the skin causing death. Prevention is by vaccinating.
NAVEL ILL OR JOINT ILL
This disease is caused by bacteria entering the body, usually of a young lamb, through a cut or via the navel at birth. Infection causes an abscess to form on the navel and the joints of the leg often become stiff. Prevention is by washing the navel cord with a 20% iodine solution just after the lamb’s birth.
TETANUS OR LOCKJAW
The bacteria live in the soil and enter the body through a wound on the sheep, such as a cut caused by bad shearing, or by castration. Affected animals walk stiffly and the mouth is tightly closed. The disease can be prevented by cleaning and disinfecting wounds and a tetanus vaccine can be given to the whole flock. This vaccine is often carried out with other vaccines, such as Pulpy Kidney.
This is caused by bacteria entering the foot through a wound and is more prevalent during the rainy season. Treatment is by trimming the hoof with a knife and applying an antiseptic dressing such as copper sulphate and Stockholm tar or an aerosol spray made for dressing footrot and wounds. Affected sheep should be put into a separate paddock until they have recovered. The disease can be prevented by keeping the sheep’s feet trimmed and running them through a foot bath containing a 10 % solution of copper sulphate or a 5% solution of Formalin. Sheep should be run through the foot bath once every two weeks during the summer.
This is a very dangerous bacterial disease which can affect man. It is a specified disease and suspected cases must be reported to the nearest veterinary official. Animals are rarely seen ill and are usually found dead with a blood-stained discharge from the nose, mouth and anus. Such a carcass should be burned or buried in lime and must not be touched by anybody who is not wearing gloves and protective clothing. Prevention is by vaccinating. If the disease is confirmed on a property it is placed in quarantine and no animals may be sold or moved off the farm.
This is a bacterial infection of the udder that causes loss of milk in one or both teats. It is not a common disease with sheep, and ewes which suffer from it should be culled from the flock.
This is a bacterial disease which causes infertility in rams. Rams which have the disease can affect the ewes which they serve, and a clean ram will become infected by serving an ewe with the disease. Infected rams and ewes that have not produced a lamb should be culled from the flock. Ram lambs should be vaccinated against the disease at 4 months old. The disease is caused by bacteria similar to those which cause contagious abortion in cattle.
5. OTHER DISEASES
This occurs with sheep that are being fed on a high concentrate ration, especially those being fattened in pens. The activity of the rumen is upset; the animal refuses to eat and may suffer from diarrhoea. In acute cases, death may follow within 24 hours. Affected animals should be dosed with any of the following:
- Tablespoons of Milk of Magnesia (Magnesium Hydroxide); or 1 Tablespoon of washing soda (Sodium Carbonate); or
- Tablespoons of baking soda (Sodium Bicarbonate)
Whichever dose is chosen, it should be mixed with water in a small bottle and carefully poured into the sheep’s mouth. The treatment may have to be repeated. This condition can be prevented by making sure that any change to a high concentrate diet is carried out gradually.
TWIN LAMB DISEASE
This affects ewes that are close to lambing, and affected ewes are usually carrying twin lambs. The ewes lie down; stop eating and if not treated will die between 2 and 5 days of becoming ill. Treatment is to give the ewe sugar or molasses dissolved in water, by mouth. The disease is caused by poor nutrition before lambing and feeding molasses to in-lamb ewes is a preventive measure.
There are a number of conditions caused by minerals, trace elements and vitamin deficiencies, but these should not be a problem if the sheep are being fed a properly balanced ration.