Incidence of Anaplasma phagocytophilum and Associated Co-Morbidities in Lambs Submitted for Postmortem Examination in Ireland

Anaplasma phagocytophilum causes tick-borne fever (TBF) in ruminants and is considered the most widespread tick-borne pathogen in sheep in Europe. This study aimed to determine the contribution of TBF to lamb mortality in Ireland and to identify factors associated with the risk of co-infection with A. phagocytophilum. Samples from dead lambs submitted to 3 Irish Regional Veterinary Laboratories (RVLs) in 2021 and 2022 were screened for the presence of A. phagocytophilum using real-time PCR. In total, 864 animals were sampled, of which 57 (6.6%) tested positive. The majority of the positive results originated in the northwest of the country; a region generally associated with high rainfall and a high prevalence of ticks and tick-borne infections in sheep. The most common causes of death reported in all lambs (including both TBF-positive and TBF-negative animals) were pneumonia, clostridial infection, and bacteraemia. Pneumonia accounted for 14.5% of deaths including 22.8 and 13.0% of TBF-positive and TBF-negative lambs, respectively. Bacteria from the family Pasteurellaceae were identified in 33.3 and 17.7% of TBF-positive and TBF-negative lambs, respectively. It was concluded that A. phagocytophilum is a possible concomitant infection and contributor to other infectious diseases in sheep, particularly those caused by bacteria in the Pasteurellaceae family. Understanding the prevalence and co-morbid associations of TBF is essential for improving disease surveillance and control strategies in endemic regions.