FECAL EGG COUNT REDUCTION PERCENTAGE CALCULATIONS TO DETECT ANTHELMINTIC RESISTANCE IN OESOPHAGOSTOMUM SPP IN PIGS

The results of four alternative methods of mean faecal egg count reduc tion percentage (FECR%) calculations were evaluated and compared using data obtained for Oesophagostomum spp. from ten sow herds. The estima tes of FECR% and 95% confidence limits obtained using the four methods were different. However, there were few discrepancies in the final de cision as to whether a given herd carried drug resistant isolates or n ot. The methods that used geometric means were more appropriate than t hose that used the arithmetic mean as the measure of central tendency for eggs per gram of faeces (EPG) values. The use of geometric mean EP G values in calculations has been criticized from several viewpoints, one of which is that its use reduces the comparability of reports betw een laboratories. If the geometric mean is to be used as we suggest in FECR% calculations, the appropriate references, number of animals in each group, minimum and maximum EPG values and the factor added to zer o EPG counts should be reported in order to improve the comparability. The difficulty in obtaining groups with similar pre-treatment EPG val ues in field situations suggested the inclusion of pre-treatment EPG v alues in the calculations as an adjustment procedure. The importance o f including a non-treated control group in calculations was demonstrat ed during this study. Therefore, we suggest the use of geometric mean EPG values, to include pre-treatment EPG values and to include the egg counts from the control group in FECR% calculations. The interpretati on of the resulting FECR estimate may be different according to the pu rpose for which the testing procedure is carried out, e.g. survey in d etecting anthelmintic resistance, control field tests, etc.