Evaluation and application of a molecular method to assess the composition of strongylid nematode populations in sheep with naturally acquired infections.

We evaluated the performance of a PCR method for the diagnosis of naturally acquired strongylid nematode infections in sheep (n = 470; in a temperate climatic zone of south-eastern Australia), using a panel of 100 'negative control' samples from sheep known not to harbour parasitic helminths. We compared the diagnostic sensitivity (98%) and specificity (100%) of this assay against a conventional faecal flotation method and also established a system to rank the contribution of particular strongylid nematodes to the faecal egg counts (FECs) from 'mixed infections' in individual sheep. The testing of faecal samples herein revealed that Teladorsagia circumcincta (80%) and Trichostrongylus spp. (66%) were most prevalent, followed by Chabertia ovina (33%), Oesophagostomum venulosum (28%) and Haemonchus contortus (1%). For the majority of sheep in this study, T. circumcincta and Trichostrongylus spp. represented the largest proportion of strongylid eggs in faecal samples from individual sheep. This is the first large-scale prevalence survey of gastrointestinal nematodes in live sheep using a molecular tool. The ability to rapidly rank strongylid nematodes according to their contribution to mixed infections represents a major advantage over routine coprological methods. This PCR tool has the potential to replace the conventional technique of larval culture. Future efforts will focus on enhancing and adapting this molecular method for high throughput application in routine, diagnostic settings.