An epidemiological study of gastrointestinal nematodes in cattle with particular reference to weather factors in Tetu division of Nyeri district, Kenya
An epidemiological study of gastroinLestinal nematodes of cattle was conducted in Tetu Division of Nyeri District, Kenya. This area has a trcpical climQte, being within the tropics. The aim of the study was to assess the ~nfluence of weather factors, stocking rate and herbage cover on the larval pasture contamination as indicated by the populations of infective larvae recovered from pasture herbage over a period of one year. Herbage was sampled at two week intervals. The study was carried out in five farms selected by the method of stratified sampling to cover a representative crosssection of the farming community within the division. Three age groups of cattle (under 6 months, 6 - 12 months, and over 12 months) were used for this study. Their faecal egg counts were used as an index of their worm.burdens. At any given period of time, the corresponding pasture larval contamination was considered as the source of the worm burden in the herd .at the time. Weather parameters which included rainfall, temperature, evaporation and relative humidity were recorded in a meteorological station located in the area of study. All the farms were within 10 - kilometre radius from the recording station. A W-transect method of herbage sampling was adopted. About 500gm of herbage was collected from each paddock and thoroughly washed with water containing a small amount of detergent. Larvae were recoveY'2d by a combination of sedimentation, flotation with concentrated solution of magnes iWI;sulphate, and finally by centrifugation to concentrate the larvae in about 0.5ml of water. Herbage was then dried to constant weight at 70°C. Infective larvae recovered were counted on a simple ordinary glass slide with calculating columns in doses of 0.1ml after adding a drop of Lugol IS iodine to kill the larvae. They were recorded in numbers of infective larvae per kilogramme of dry herbage at 70°C. During the counting, larvae were also identified to genus level. For the deter~ination of herbage cover, foliage from three quadrants per paddock and selected at random was cut to ground level and dried separately in an oven· to constant weight at 700e. The quadrants were 30cm. by 30cm. in size. The final dry weights were recorded, their mean calculated and herb~ge cover of each paddock recorded as dry weight of folia.ge per 30cm2. Faecal samples were collected per rectum. The modified McMaster egg counting method was adopted for the analysis of the egg counts. Faecal and herbage sampling in all the farms were done the same day. Calves under 6 months of age had the highest egg counts and were the main source of pasture contamination followed by the 6-12 months age group. The egg counts of the over 12 months age group remained constantly low throughout the year. The highest egg counts ~ere recorded during the dry seasons. The output went down during the rains. Larval populations were lowest during the dry periods (to a minimum of 8 L3/kg dry herbage) but went up to a maximum of 2728 L3/kg dry herbage during the rain seasons. The highest pasture larval count was recorded in the month of February when there was a brief rainfall following a prolonged dry period. This observation was made immEdiately after the onset of the rains and the eggs on pasture had not had adequate time to hatch and develop to third stage larvae. It was therefore concluded that there was a ready source of infective larvae that caused the sudden rise immediately the rains came. With continued rainfall, there was a gradual decrease in the number of infective larvae recovered. This meant that either the infective larvae were carried down into the ground by percolating rain water or there was an apparent dilution effect due to increasing amount of herbage. Rainfall and temperature were found to bave .apos it ive influence on the level of larval popul a cion in fns"'CuY·e.A rise in rainfall was accompanied by a rise in herbage larval population while a fall was followed by a decrease in pasture larval count. A regression line of temperature drawn against pasture larval count showed a positive correlation. Similar results were obta ined for herbage cover, stock in9 rate t·nd evaporation. A negative correlation was obtained between relative humidity and pasture larval. count. In this study, a negative correlation was observed between rainfall and faecal egg counts in all the three age groups of cattle. During the rains the herbage became ;,, abundant and lush. Animals grazing bn this herbage were passing out copious soft faeces. This was thought to dilute eggs passed in faeces and cause a frtlse decrease in faecal egg counts. Although not investigated in this work, the possibility of self-cure taking place and causing the decrease in faecal egg-counts during the rein season was also consideredalikelyexplanation. Nematodes of the genus· Cooperia were the most predominant while those of the genus Strongyloides were the least dominant. Other genera found were Haemonchus, Trichostrongylus and Oesophagostomum, in that order of prevalence; this was observed throughout the year. It was therefore concluded that no genus was more favoured or adversely affected by any seasons than others. A wide discrepancy was always observed between the - percentages of infective larvae of the genus Strongyloides recovered from herbage and those recovered from coproculture. It was further noted that the infective larvae of this genus did not survive as long as other larvae under laboratory conditions. This could mean that they were more susceptible to certain environmental conditions, thus leading to their short lifespan. It was suggested that under field conditions this could reduce Strongyloides larval population \ in herbage, thus leading the discrepancy between herbage / cover larval rscovery and coprocu1turc recovery. It was concluded that to control the problem of gastrointestinal parasitism, cont~mination of pastures that occurs during the dry period shouid be avoided or reducFd by deworming animals curing this period. Deworming during the rainy period, as is the common practice in this area, would be of temporary relief to the animals as the pastures at this time are heavily infested with infective larvae.