Determination of anthelmintic resistance of Haemonchus contortus to three classes of anthelmintics in a Kenyan sheep flock based on results from faecal egg count reduction test

Haemonchus contortus is the most important bloodsucking gastrointestinal nematode in small ruminants worldwide. It is often controlled by anthelmintics. Even though other methods to control are available, the most efficient way to treat an animal with clinically manifested haemonchosis is to use one of the available broad spectrum anthelmintics: benzimidazoles (BZ), macrocyclic lactones (ML) or imidothiazoles-tetrahydropyromidines or the narrow spectrum drugs salisylanilides or nitrophenols. Anthelmintic resistance (AR) against all groups of anthelmintics in nematode parasites has been reported from many countries, and even multiple AR has been observed. In this study I have investigated the resistance levels to all major classes of broad spectrum anthelmintics in a naturally infected flock of sheep of Red Maasai sheep (RM), Dorper sheep (D) and their offsprings on a research farm in Kenya. Resistance to the BZ albendazole (ALB) andmost likely also to the imidothiazole levamisole (LEV) had previously been seen in this flock. Faecal Egg Count Reduction Test (FECRT) was now performed. The efficacy of ALB, IVM and levamisole (LEV) were tested. Faecal egg counts (FEC) and reductions after deworming were analysed according to the AHs used. Accordingly, a total of 88 sheep were divided into three treatment groups per breed or crosses (n=5-7) i.e. the number of sheep subjected to each AH were 27-31 animals. One untreated control group (n=15) was also included in the study. Prior to AH treatment animals were weighed and they were then treated orally by a veterinarian with either of the three substances. In addition, blood and faeces were collected and examined for the packed cell volume (PCV) and the number of parasite eggs per gram of faeces (EPG) both at the day of treatment plus at day 10 and day 22-24 post treatment. Furthermore, nematode larvae were cultured from pooled faeces per group at day 0 and day 10 and larval differential counts were performed. Results showed that the majority of the nematodes were H. contortus both at days 0 and 10. The FECRTs showed efficacy of all three tested substances, between 98 % and 99 % reduction of egg counts per gram faeces, on days 22-24. Thus, there was no resistance against any of the tested substances, even though resistance had been seen only about two years earlier in the same flock.