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PART ONE - The Helminth Diseases of Kenya

INTRODUCTION

Every animal is infected with worms. How these affect the animal and whether they are important depends on:

1. the type of worm
2. the number of worms present
3. the species and breed of animal
4. the age of the animal
5. the nutritional status of the animal
6. the amount of previous exposure the animal has experienced

THE PARASITES

Helminth parasites can be divided into three main groupings:

1. Roundworms (Nematodes)
2. Tapeworms (Cestodes)
3. Flukes (Trematodes)

Since the relative importance, the methods of control and the drugs for treatment vary between each of these groups, it is important to draw distinctions between them.

ROUNDWORMS

The most important of these worms live in the gut of the animal, in the stomach, small intestine or large intestine. There are a number of nematode species living in other sites (such as the lungs, eyes etc.) but in Kenya these other types are rarer and seldom of clinical or economic importance. While infection with nematodes may occasionally cause clinical disease or production loss in young cattle, they are rarely important in adult cattle. They are much more important as causes of losses amongst sheep, goats and camels.

Most of the nematodes of importance lay eggs in the gut which pass out with the animals dung and develop into infectious stages (larvae) in the pasture. Animals become infected during grazing.

ROUND WORM LIFE CYCLE

The larvae on pasture are quite vulnerable to drying and will not survive long In dry, hot conditions. In contrast, in cool damp conditions, the larvae can survive for longer periods and in some areas, grass can still be contaminated with larvae for up to six months after animals have last been grazing. As a very rough rule, if the grass is green and growing, larvae will be able to develop and survive long enough to infect the animals.

HAEMONCHUS. This worm lives in the stomach of sheep, goats, cattle and camels. It is large enough to be seen at slaughter without a magnifying glass. It sucks the blood of the animal and if present in large numbers, can cause a rapid depletion of the animal's blood reserves. In sheep, goats and camels it can be fatal if present in large numbers or if the animal is poorly fed. It is also a very prolific worm, laying many thousands of eggs, so build up of infection in a flock can be fast. In heavy infections, the animals become dull, thin, have very pale membranes around the eyes and inside the mouth and may develop "bottle jaw" - a pendulous swelling below the lower jaw. In camels this watery swelling is often seen above the eyes and each side of the sternal pad. When pasture is heavily infected, animals may die before showing clinical signs and before the onset of worm egg-laying. Haemonchus does not usually cause the animal to have diarrhoea.

Figure

TRICHOSTRONGYLUS. A number of species of worms in this family are found in sheep, goats, cattle and camels. Depending on the species, they live in the stomach or the small intestine. They are normally just too small to be easily visible on the gut surface at slaughter. In heavy infections, the surface of the small intestine may be reddened. While large worm burdens can cause an enteritis with a distinctive dark-coloured diarrhoea, Trichostrongylus is probably more often important in combination with Haemonchus and other nematodes in a general parasitic gastroenteritis.

OESOPHAGOSTOMUM. This worm lives in the large intestine where it causes formation of nodules. Both the nodules and the worms themselves are large enough to be seen at slaughter, but the occurrence of large numbers of hardened nodules is diagnostic. Apart from the loss of value in matumbo, large infections of Oesophagostomum can be a cause of scouring.

Figure

COOPERIA. This worm is found in the small intestine of all domestic ruminants. In sheep and goats in Kenya it is seldom of economic importance on its own but may contribute to parasitic gastroenteritis. Reports suggest that it may very occasionally be of significance in grazing calves.

BUNOSTOMUM. Infections with Bunostomum are acquired by larvae penetrating through the skin of the legs from contaminated pasture or through the wall of the gut while grazing. This is a more common infection of cattle than of sheep and goats. It may occasionally reach pathogenic levels. Since the infection is not dependant on pasture intake, even very young calves can become infected. It is more common where boma hygiene is poor.

DICTYOCAULUS VIVIPARUS. This is the cattle lungworm. While it is likely to survive well in the cooler damper parts of Kenya, it is seldom reported as a problem. Heavily infected calves have a distinctive drawn-out dry unproductive cough. Under normal conditions, animals become immune during their first grazing season and cases are unlikely to be seen in older animals. In the differential diagnosis of coughing cattle, other causes are more likely in the Kenya Highlands. Bacterial, viral or mycoplasma pneumonias should be ruled out before a diagnosis of lungworm is made. D.filaria, the large lungworm of sheep and goats, may occasionally be present and be a cause of a clinical pneumonia.

THELAZIA. Species of Thelazia are found in the eyes of cattle in the semi-arid areas of Kenya. The worms are transmitted by flies. Individual cases can be treated with levamisole applied into the eye or by using any of the injectable anthelmintics according to the manufacturers instructions. Where problems are recurrent, use of a persistent insecticide (such as cypermethrin) applied to the head of the animal may help.

TOXOCARA VITULORUM. This is a worm which infects young calves. Calves become infected through the milk from their mothers. Calves can become clinically sick and growth rates can be adversely affected. Larvae are only present in the milk during the first week or so after birth. For unknown reasons, this condition seems to be much more common amongst pastoralist owned cattle.

TAPEWORMS

All tapeworms have an indirect life-cycle. This means that there are two animals involved in the development of the infection. The animal harbouring the adult tapeworm is known as the final host, while that harbouring the juvenile stages is the intermediate host. While for domestic animal species the final host is always a mammal, the intermediate host is sometimes a mammal and sometimes an arthropod. Control is based on breaking the life-cycle or on drug treatment of the adult infection in the final host.

SHEEP TAPEWORM CYCLE

MONIEZIA. This is the large tapeworm seen in the intestine of calves, sheep, goats and camels. The domestic animal is the final host with species of mites that live on pasture as the intermediate hosts. While these infections are highly visible (due to the tapeworm segments that pass out in the faeces) and quite dramatic (due to the size of the worms found at slaughter), they rarely cause the animal any ill effects. Animal health workers who can find nothing else to report at post-mortem often ascribe death to the presence of these tapeworms.

However, in trials where animals have been infected with many thousands of Moniezia, no production loss or clinical signs have been observed. Farmers often treat their animals with anthelmintic whenever they see tapeworm segments in the faeces, these infections are therefore a cause of over-frequent dosing. Also since few of the broad-spectrum anthelmintics are particularly effective against tapeworms, the failure of the treatment to totally eliminate the segments leads to reports of anthelmintic failure, anthelmintic resistance or fake products.

Figure

AVITELLINA. In many respects this worm is similar to Moniezia, however, it is smaller, less dramatic and causes the animal no harm.

CYSTICERCUS BOVIS. This worm is found as small, rice-grain-like cysts in the meat of cattle. Cattle are the intermediate hosts and when these cysts are eaten by man, they develop into an adult tapeworm in the human intestine. The cysts are thought to cause no pathogenic or production losses in the cattle. However, where cattle are sold through a reputable slaughterhouse, the carcase may be downgraded with subsequent loss of value. Control is by treatment of humans and prevention of contamination of pasture with human faeces.

Figure

HUMAN/CATTLE TAPEWORM LIFE CYCLE

Camels are occasionally infected with Cysticercus bovis, however a similar parasite Cysticercus dromedarii is more common - in this infection, hyenas rather than man are the final host for the adult tapeworm.

CYSTICERCUS TENUICOLLIS. This worm occurs as long, thin-necked, balloon-like fluid-filled cysts in the peritoneal cavity of domestic ruminants. Most commonly these are found attached to the surface of the liver, intestinal mesentery, other abdominal organs or the body wall. These cysts can be confused with hydatid cysts (see below). In this case the domestic ruminant is the intermediate host and the final host is the domestic dog (or perhaps wild canids such as jackals). The dog becomes infected by eating cysts in animal carcasses. The domestic animal becomes infected by eating eggs passed out by the dog. Since the dog may pass out many millions of eggs per day and these can be spread by flies, it is very difficult to control this infection. Treatment of dogs with praziquantel and preventing dogs having access to carcasses (or raw carcase trimmings) can reduce the incidence.

HYDATID. Hydatid cysts are the intermediate stage of the tapeworm Echinococcus granulosus. The final host is again the domestic dog or wild canids. Like Cysticercus tenuicollis (above) the dog passes out eggs which are infectious to domestic ruminants or camels. The cysts are more rounded than the tenuicollis cysts and are found on or in the liver, the lungs or more rarely in almost any other part of the carcase. While the cysts cause no adverse effects to the animal, they can be a cause of condemnation or downgrading if the animal is sold to a reputable slaughterhouse.

Unlike C.tenuicollis, the eggs passed by the dog are also infectious to man. The resulting infection in man is known as hydatid disease and is very hard to treat. Human cases are best treated with surgical removal of the cyst. However, since the cysts easily rupture these can spread throughout the body and some cases are untreatable and fatal. It is important where hydatid cysts are found in the domestic animals that dogs are kept away from carcasses and that household dogs are regularly treated with praziquantel. In addition, dog-human contact should be restricted and hands washed very carefully after handling dogs. Farm children are particularly vulnerable to infection.

HYDATID LIFE CYCLE

Figure

COENURIASIS. This is a condition where the intermediate stage of another dog tapeworm, Taenia multiceps, forms in the brain of a sheep, goat or camel. The pressure on the brain from the growing cyst causes behavioural changes in the sheep, often resulting in the animal circling and bumping into objects. Sometimes a softening of the skull can be felt over the cyst which is frequently just behind the horn-buds. While surgical removal of the cyst is often successful and has been practiced by shepherds in the past, it is perhaps best if the animal is simply slaughtered.

STILESIA. This small tapeworm has its adult stage in the liver of sheep, goats and camels. As far as is known, it causes no pathogenesis but is a cosmetic problem leading to loss of value in the sale of the liver. The intermediate stage is thought to be found in a pasture mite.

FLUKES

LIVER FLUKE CYCLE

All the flukes of importance to livestock have indirect life-cycles with adults in the domestic animal and intermediate stages in various species of water snails.

FASCIOLA. This worm is the liver fluke, which is found in areas of Kenya where there are marshy areas or slow-moving permanent water courses. The infectious stages passed out by the snail form cysts on the grass which are subsequently eaten by livestock. The young flukes develop and migrate through the animal's liver causing damage. When present in large numbers, over a long time or when nutrition is restricted, the animal can suffer a loss of condition and show signs of "bottle jaw" similar to those described for Haemonchus above. In very heavy infections, death can occur before these signs are apparent. A history of grazing along stream sides, near ponds or on areas that flood during the rains would indicate that liver fluke control should be included in routine farm management.

PARAMPHISTOMES. These flukes are often found in very large numbers in the rumen of cattle, sheep, goats and camels and are known as stomach flukes. Despite their large numbers, the adult worms are thought to cause little harm to the animal. However a sudden large intake of infectious stages can cause scouring, emaciation and death.

Figure