Kanobana used an infection model in which, based on their genetic background, animals exhibited varying degrees of resistance to gastrointestinal worms. Broadly speaking there are three groups of animals. Two percent of the animals are naturally immune to a first infection. Another group reacts to the first infection with an effective immune response. In the event of a second infection these animals are protected. A third group is highly sensitive for infection and scarcely acquires any immunity even after repeated infections.
The researcher distinguished the three groups of animals by using two types of measurement. Three-month-old calves were infected with 100,000 larvae of a small-intestinal worm. After the infection an initial distinction was made on the basis of parasitological variables such as worm counts and the detection of eggs in the animals' excreta. Secondly, immunological parameters were used to confirm the three groups of animals.
In an animal that has no resistance to a worm infection, the worms occur in the first part of the small intestine. In animals that develop resistance, worms are translocated towards the end of the small intestine.
Interestingly the male worms disappear out of the intestine first, followed by the female worms. This conclusion is based on a study six different parts of the small intestine, which in calves has a length of between 25 to 40 metres. In addition to this, Kanobana also discovered a number of mechanisms that are responsible for ensuring the disappearance of the worms from the small intestine.
Cattle pick up larvae from the grass, which develop into adult worms in the cattle's gastrointestinal system. The worms reproduce sexually and lay eggs. The eggs pass out of the cattle with the excreta. In the dung, the eggs can once more develop into larvae. In this manner cattle can continually be reinfected by eating the grass.
Preventative anti-worm drugs are effective but are a potential risk to public health, as they are sometimes found in dairy and meat products. An understanding of how cattle acquire immunity might contribute to the development of a vaccine, which would be a good alternative for preventing gastrointestinal worm infections.
For further information please contact Kirezi Kanobana (Department of Parasitology and Tropical Veterinary Medicine, Utrecht University, now at the Department of Veterinary Science, Institute for Tropical Medicine in Antwerp), tel: 32-641-439-635. e-mail: firstname.lastname@example.org or email@example.com. The doctoral thesis was defended on 18 September 2003. Dr Kanobana's supervisor was Prof. A.W.C.A. Cornelissen.
The research was funded by the Technology Foundation STW.