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

Anaplasma phagocytophilum (AP) is a tick-transmitted bacterium, causative of tick-borne fever (TBF) in sheep. Symptoms of TBF vary in severity from asymptomatic to fatal, and the disease manifests generally with immunosuppression. Here, we describe the effects of a naturally occurring AP infection on lamb welfare (bodyweight, dag score and faecal egg counts (FEC) in a UK farming setting. In 2022 TBF was incidentally diagnosed in a group of lambs undergoing parasitological field studies. Lamb performance and strongyle FEC were collected fortnightly over 21 weeks, and blood was collected for AP quantitative polymerase chain reaction (qPCR) at study end. Of 101 lambs, 16 tested positive (4 with clinical symptoms and 12 subclinical), 1 equivocal and 84 negative. Linear models compared mean total weight gain between groups with birthweight and strongyle FEC included as continuous covariates. The sub-clinically infected group showed a 2.5 kg deficit in total weight gain across 21 weeks. There were no significant differences in strongyle FEC between infected/uninfected lambs, but increasing strongyle FEC was associated with lower weight in TBF-positive animals, but not in TBF-negative animals. Other clinical parameters were not correlated with AP infection. The 2.5 kg reduction parallels findings from Norway (3.8 kg loss) and this effect seems to be accentuated in animals with higher strongyle egg counts, suggesting an interplay between the two infections. Given the recognition of AP in UK tick and livestock populations, further research is warranted into the long-term effects of AP on lamb performance under UK-specific management systems.