Can herd seroprevalence be used as an indicator of enzootic stability for bovine anaplasmosis? Insights from a case-control field study in Brazil.

Bovine anaplasmosis presents a significant challenge to livestock production in tropical, subtropical, and temperate regions. For many years, the concept of enzootic stability/instability (initially established for babesiosis) and herd seroprevalence as an indicator of outbreak risks have been applied to anaplasmosis. However, this model has never been definitively validated for Anaplasma marginale. The objective of this study was to examine the relationship between herd immunity (seroprevalence) and the occurrence of anaplasmosis outbreaks in Southern Brazil. A case-control study was conducted, categorizing farms into two groups: cases (farms with a history of clinical anaplasmosis) and controls (those without anaplasmosis). Thirteen farms were identified as "cases", while 23 were identified as "controls". A substantial difference in seroprevalence distribution between the two groups was observed. The majority of "control" farms exhibited over 75% of animals with antibodies to A. marginale in both calves and heifers, whereas the majority of "case" farms had a seropositive cattle percentage below 75%. Additionally, twelve months after cattle serology tests, we conducted a prospective follow-up survey to identify any clinical cases of anaplasmosis. Statistical associations (P < 0.05) were found between both retrospective and prospective anaplasmosis outbreaks and the hypothetical threshold of herd seroprevalence (75%). We hypothesize that herd seroprevalence may be an indicator of the risk of occurrence of clinical anaplasmosis. It appears that the epidemiology of cattle anaplasmosis, at least in our conditions, aligns with the well-known model of enzootic stability/instability originally applied to bovine babesiosis.