All about internal parasites in animals.

Taruvinga Magwiroto

Internal parasites are just as dangerous as any disease. However, their effects may escape you because they do not cause dramatic illness. However, they are of economic importance to the farmer because they affect productivity of animals.

Internal parasites are more dangerous to young animals than to older ones. However, in all age-groups, internal parasites are a stress, and can worsen or trigger other infections.

Internal parasites favour humid and hot conditions, and therefore most of them are very active in the rainy season. However, they are active throughout the whole year, but tend to be concentrated in the hot, wet season.

When parasites enter the body, the animal will not get sick right away. They first need time to multiply. Only when the parasites and the toxins they produce are present in sufficient quantity, the animal gets sick.

Internal parasites are divided into three groups i.e. Nematodes (roundworms) Trematodes (Flukes e.g. liver fluke) and Cestodes (Tapeworms). Grazing/foraging animals take them up during grazing. They multiply by laying large numbers of eggs, from which larvae emerge when the environment is humid and warm.   

Internal or endoparasites cause economic losses in livestock production in one of the following ways:

  1. They suck blood resulting in Anaemia in livestock, particularly the wireworms (Haemonchus spp.) which parasitize the small intestines.
  2. They feed on the already digested feed e.g. Tapeworms resulting in animals getting thin.
  3. They pressurize organs e.g. the coenurus cerebralis, the dog tapeworm (Taenia multiceps) cyst which may end up lodged in the brain of sheep causing Gid disease.
  4. They damage organs and impair function e.g. liver flukes cause liver cirrhosis, a hardening of the ducts of the organ.
  5. They cause carcass condemnation at slaughter e.g. the human tapeworm cyst -Cystcercus bovis and Cystcercus cellulosae  (Taenia saginata and Taenia solium)
  6. They cause sloughing of the intestinal mucosa e.g. the Bankrupt worm
  7. They block the intestines blocking the passage of feed to the lower GIT/GUT e.g. tapeworms.
  8. They produce toxins as they die predisposing goats to Clostridial infections e.g. Pulpy Kidney disease.


The signs which indicate whether an animal is infected heavily with intestinal parasites, are

  1. Decreased growth rate (ill-thrift).
  2. Starry coat, a dry, dull and open hair coat
  3. Anaemia (shown by pale colour of inside of mouth) and decrease bodyweight
  4. Decrease appetite
  5. Consistency of the faeces is sometimes changed (scours).
  6. Tapeworm segments/eggs with stools.
  7. In seriously affected animals, bottle jaw (swelling below the jaw) or swollen tummy (stomach). This is a result of low blood protein levels.


Microscopic examination of stools. 

Treatment and prevention.

  1. In heavy infestation, animals can be treated with a drench (dewormer e.g. every 6-8 weeks, to cure and prevent heavy infections.
  2. Calves must not be put in pastures that are heavily infected with parasites.
  3. Pastures that are always used to graze calves are normally heavily infected with parasites.  A way to clean those pastures from parasites is by mowing them and by removing the old grass every time the calves have grazed the pasture.
  4. Calves must never stay for more than 14 days in the same pasture. After a fortnight, the number of infective larvae is increasing. This is due to the fact that the eggs that the calves expelled when coming in the pasture are developing into infective larvae around this time.
  5. Keep the calves indoors and feed them grass from a pasture that is never grazed by cattle.
  6. Put calves out to pasture when they are older.
  7. Let the calves graze ahead of the cows. They get good grass this way and because they only eat the top parts, they will not get heavily infected with intestinal parasites. However this system is often difficult to realize in practice, because of the extra fencing required. 

In the control of liver flukes, (whose life cycle involve snails), snails have to be controlled by the use of moluscides (chemicals which kill snails). Also, fencing off marshy areas/ draining of marshes/ grazing animals in uplands. Use of piped water / avoid watering animals with stagnant water (if you

In the next article, we will discuss specific strategic deworming regimes.

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