Regular use of anthelmintics results in the development of anthelmintic resistant nematodes. This is likely to be aggravated in regions of high rainfall where year round treatment is required. In times of extended drought, where the only worms that reproduce in the new wet season are those that have survived treatment in the host, resistance is also likely to be worse. Kenya thus has two climatic conditions likely to encourage the selection for anthelmintic resistant nematodes. Existing publications and conversations with veterinary surgeons suggest that resistance is already quite widespread.

Two major factors enhancing for the selection and spread of anthelmintic resistant nematodes are underdosing and movement of animals (Coles and Roush, 1992). Poor farmers trying to economise on anthelmintics and are breeding improved stock for sale to peasant farmers provide an excellent method of distribution of resistance in addition to normal movement of animals. The relatively high reliance on group 2 anthelmintics (levamisole/ morantel) suggests that few people are practising rotation of anthelmintic types, a management system that is believed to slow the development of resistance.

Kenya thus has the potential for large problems with anthelmintic resistant nematodes on both ranches and in peasant farming. Obviously the availability of simple, inexpensive but reliable tests for anthelmintic resistant nematodes would be very valuable in determining the extent of the problem. Such tests need validating may be the incentive to apply strategies that should limit the development and spread of resistant nematodes.