Subclinical infection with Anaplasma phagocytophilum and gastrointestinal nematodes results in reduced weight gain in grazing weaned lambs.

Subclinical Anaplasma phagocytophilum infection causes a 2.5 kg weight gain deficit in lambs, worsened by concurrent strongyle infections.

This UK-based observational field study investigated the impact of naturally occurring Anaplasma phagocytophilum (AP) infection on production parameters in grazing weaned lambs. During a parasitological field study in 2022, tick-borne fever (TBF) was incidentally diagnosed in a cohort of 101 lambs. Blood samples collected at study end via quantitative PCR identified 16 AP-positive lambs (4 clinical, 12 subclinical), 1 equivocal, and 84 negative. Lamb bodyweight, dag scores, and strongyle faecal egg counts (FEC) were recorded fortnightly over 21 weeks. Linear models incorporating birthweight and strongyle FEC as covariates revealed that subclinically infected lambs experienced a statistically significant 2.5 kg deficit in total weight gain compared to uninfected animals over the study period. No significant differences in strongyle FEC were observed between infected and uninfected groups; however, increasing strongyle FEC was associated with reduced weight gain specifically in AP-positive lambs, suggesting a synergistic immunosuppressive interaction between AP infection and gastrointestinal nematodiasis. Dag scores and other clinical parameters showed no significant correlation with AP status. These findings are consistent with Norwegian data reporting a 3.8 kg weight loss in AP-infected lambs. The study highlights that subclinical TBF, often overlooked in field conditions, carries meaningful production consequences. Given increasing AP prevalence in UK tick and livestock populations, the authors emphasize the need for further longitudinal research examining the long-term economic and welfare implications of AP infection under UK management systems.

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