A study of sheep helminthosis with special emphasis on patterns of infection and the development and survival of haemonchus contortus larvae in Muguga, Kenya

Abstract

Helminthiasis is recognised as a major production and health constraint of small ruminants in Africa, including Kenya. Among the helminthosis, haemonchosis has been recognised as a serious and persistent constraint in Kenya despite it's well known biology. This is mainly due to the significant influences of micro-climatic and management factors on it's epidemiology. The influences of these factors in specific regions have resulted in highly variable patterns of this disease in different agro-climatic zones of Kenya which in turn have made it difficult to formulate a common and effective management approach of combating it. In general, the level of haemonchosis has been determined by assessmg the faecal egg counts (fee) assuming that these correlate well to the total worm burden. However this is not always true since fee are influenced by various factors, for example fecundity of the worms, ingesta volume, age of the worms and host resistapce, yet many tactical and strategic management programmes are based on the total egg counts. The objective of this study was to establish the epidemiology of helminthiasis with special reference to Haemonchus contortus in Muguga, a cool temperate, semij humid to semi-arid area of Kenya with a view of recommending a cost-effective management approach based on the risks exposed to sheep. The study set out to asses the risk posed according to the level of third stage infective larvae (L3) available on pasture and how this level was influenced by climatic factors. A series of experimental plots were seeded with infected sheep faeces and monitored weekly for their ability to yield Haemonchus contortus L3 and their magnitudes and persistence over time. A parallel experiment monitored L3 recovered from a paddock naturally grazed by infected sheep. Faecal egg counts and total worm counts were monitored from a group of these sheep. Daily maximum, rmmmum and grass temperatures, rainfall, sunshine hours, evaporation and relative humidity were recorded for the duration of the study. Over the study period, absolute L3 recoveries ranged from 0.06-48.05%. Survival of L, on herbage varied between 5-22 weeks and persistence in faeces for 3-22 weeks. The eggs were viable for 10.5-19.0 weeks. A repeated measures analysis of variance showed that the magnitude of L3 recovered significantly differed (p=0.001) with the seasons, with the time (p=O.OOl) following deposition of faeces on pasture and was significantly affected (p = 0.035) by temperature. Optimal development of eggs to L) required at least 50 mm of rainfall, 11-21°C and a sunshine index (n/N) of less than 0.6. These conclusions were also supported by Life Table analysis. High rainfall increased survival of L) but decreased their magnitudes. Sunshine decreased both the magnitude and survival of larvae while the prevailing temperatures (11-21°C) were conducive for both development and survival of Lj. Faecal egg counts correlated with rainfall and patterns of infection on pasture. Packed cell volume (peV) remained within the physiological normal range. Total worm counts observed in this study were an indication of pasture infestation. This study found out that Haemonchus contortus was the most important helminth during the warm wet periods, while Trichostrongylus colubriformis was the common helminth during the dry cool periods. Larval inhibition (L4) was most significant (33%)during the cold season. In Muguga L3 could not survive in appreciable amounts over the cold and hot dry seasons. These seasons occurred in between the two rainy seasons. However, L3 developed from eggs deposited towards the end of the dry periods. These periods were therefore important in maintaining infection from one rainy season to the next. Whereas the measured weather parameters were related to development and survival of larvae, they did not account for all the differences observed between the seasons. Based on the results of this study. three anthelminthic treatments are recommended. A single strategic treatment could be given at the end of each rainy season to expel [he parasites at the beginning of the adverse hot-dry and cold-dry seasons. These would result in sanitation of the pastures. A third tactical treatment given in the middle of the long rainy season would reduce pasture contamination during this period when conditions are highly favourable for L3 development.