An examination of the relative reliability of laboratory case submissions in determining the prevalence of anthelmintic resistance in sheep nematodes in New Zealand, and the possible influence of test analysis methodology on such data

Abstract

AIM: To evaluate the likely reliability of laboratory case submissions in assessing the prevalence of anthelmintic resistance in sheep nematodes in New Zealand, and to examine the possible influence of two alternative faecal nematode egg count reduction (FECR) analysis methodologies on such data.

METHODS: A comparison was made between the prevalence of anthelmintic resistance determined using faecal egg count reduction tests (FECRTs) conducted on randomly selected sheep farms in a national survey with those derived from similar case material submitted to a veterinary pathology laboratory on a more ad-hoc basis. A comparison was also made between two alternative FECR analysis methodologies using the latter data. One methodology involved a partially differentiated procedure in which FECRs for individual nematode genera were only undertaken in those instances where reductions in total strongylid faecal nematode egg counts (FECs) (excluding Nematodirus) of <95% were recorded. The other was a fully differentiated method where reductions in FECs for individual parasites were undertaken in all cases.

RESULTS: Although there were some differences between them the results showed that there were considerable similarities between the prevalence data obtained from both the national survey and laboratory case submissions. This was particularly evident in relation to the overall pattern of involvement of the various nematode genera and the types of anthelmintic concerned. A comparison between laboratory case submission data analysed using a partially differentiated FECR methodology with that of a fully differentiated procedure, however, suggested that the use of the former practice was likely to lead to the ‘true’ prevalence of resistance being underestimated.

CONCLUSIONS: The results of this study indicate that examination of FECRT case submissions to veterinary laboratories may offer a useful source of information regarding changes in the prevalence of anthelmintic-resistant sheep nematodes in New Zealand. They also lend support to suggestions that the recently completed national survey may have provided a conservative estimate of the prevalence of such resistance.

KEY WORDS:

• Sheep
• nematode
• anthelmintic resistance
• prevalence
• faecal egg count reduction test
• larval culture
• benzimidazole
• levamisole
• benzimidazole-levamisole combination
• ivermectin