Seasonal patterns of faecal egg counts and gastrointestinal nematode species composition in Scottish dairy calves

Gastrointestinal nematode (GIN) infections impact livestock production globally. In pasture-based systems, GIN infections are ubiquitous, typically comprising co-infections with several different species within a single host. Nematode species vary in their epidemiology, pathogenicity, and anthelmintic sensitivity, which in turn can be influenced by weather, host factors, and management practices. The epidemiology of parasitic gastroenteritis in young cattle in temperate regions has been thoroughly researched. However, many studies were conducted more than fifty years ago, before the advent of modern molecular techniques and the widespread use of macrocyclic lactone (ML) anthelmintics. This study's objective was to survey dairy farms with different management profiles, using faecal egg counts (FEC) and GIN L₃ identification, to determine if any changes had occurred since these original studies. The longitudinal study of 23 Scottish dairy farms included 131 monthly sampling points, from which 1967 individual FECs were conducted, and a minimum of 94 L₃ from pooled coprocultures identified by PCR (n = 13,297) per visit. Species composition and FEC followed expected patterns, yet varied considerably in relation to management and anthelmintic use; Cooperia oncophora was more abundant earlier in the grazing season, while Ostertagia ostertagi became more abundant as the season progressed. Other GIN observed included Trichostrongylus spp., Oesophagostomum spp., and Haemonchus contortus. The majority of farms relied entirely on ML products and had done so for many years. Farmer concerns regarding anthelmintic resistance were minimal, and few farms routinely employed FECs to aid management decisions. Regardless of treatment strategy, the groups exhibited no evidence of clinical disease, and FECs remained relatively low throughout, even on farms not using any anthelmintic treatment (0-480 eggs per gram).