Cross‐Protection Between Geographically Distinct A naplasma marginale Isolates Appears to be Constrained by Limited Antibody Responses

Summary The rickettsia A naplasma marginale causes the haemolytic disease bovine anaplasmosis, an economic problem in tropical and subtropical areas worldwide. The closely related but less pathogenic A naplasma centrale is commonly used as a live vaccine to prevent anaplasmosis, but it can only be produced from infected blood. UFMG 1 is a low pathogenic B razilian strain of A . marginale, which has been shown to protect cattle against a high pathogenic B razilian isolate. As UFMG 1 can be grown in tick cells, the strain was proposed as a possible cell culture‐derived vaccine. We have evaluated whether UFMG 1 could protect cattle against a geographically distant heterologous strain, using A . centrale vaccination as a standard for comparison. Trial calves were infected with UFMG 1, A . centrale or PBS . UFMG 1‐infected animals were more symptomatic than those infected with A . centrale , but none required treatment. All calves were then challenged with the I sraeli A . marginale Gonen strain (one of the most prevalent strain in I srael). The A . centrale group had the mildest symptoms, while UFMG 1 and control groups both had a more severe response. Nevertheless, the challenge did not cause life‐threatening disease in any group. Animals infected with A . centrale had a significantly higher I g G response than UFMG 1, when measured in an ELISA against initial bodies from their homologous strain or G onen. The level of cross‐reactivity of the response to initial infection correlated significantly with reduced symptoms after challenge. In conclusion, UFMG 1 had limited effect in preventing disease by the geographically distant heterologous G onen strain. While the low pathogenicity of the G onen strain in this trial makes it impossible to conclusively state that UFMG 1 would have given no protective effect against more serious disease, the comparatively low I g G response to UFMG 1 suggests it would not have been as effective as A . centrale .