The epidemiology and control of gastrointestinal nematodes of small ruminants in a semi-arid area of Kenya with emphasis on hypobiosis of haemonchus contortus

Abstract

A study on the epidemiological dynamics of gastrointestinal nematodes of small ruminants was conducted in a semi-arid area of Kenya over a period of two years. Three major trichostrongylid species were identified; Haemonchus, Trichostrongylus and Oesophagostomum. Trichuris and Strongyloides species were occasionally encountered. Of the major trichostrongylids, Haemonchus gm. was the most prevalent (90%) and accounted for about 80% of the total worm burden. This species was found to undergo hypobiosis at levels that varied with seasons: nil levels of hypobiosis were recorded during the wet months and as much as 80% was recorded during the dry months. Hypobiosis was not investigated in the other nematode species. Evaluation of the relationship between the faecal egg count and the worm burden showed that the two parameters were more highly correlated during the wet months than during the dry months. This was a desirable situation because it is during the wet season that livestock owners in this area need to closely monitor the worm burdens in their animals. Treatment with ivermectin before the onset of the rains not only delayed the onset of egg shedding but also controlled clinical helminthiasis. In addition, a temporal change in the pattern of the appearance of infective larvae on pasture was observed; the appearance was delayed for about a month after the onset of the rains. The effect of treatment administered during the rains was a temporary and a short-Jived relief of infection as evidenced by a brief decline in egg output: it had no detectable impact on pasture infectivity. These results suggested that removal of hypobiotic larvae before they resumed development had the combined benefit of reducing both the severity of clinical helminthiasis and the level of pasture contamination. This impact was expressed in improved flock performance and particularly in the improvement of birth weights that subsequently enhanced kid and lamb survival rates. Probably due to the high selective pressure exerted on the parasite population in this treatment regime, evidence of loss of susceptibility to ivermectin was detected at the end of the study.