Concomitant infection of cattle with the vaccine strain Anaplasma marginale ss centrale and field strains of A. marginale

Abstract Bovine anaplasmosis, caused by Anaplasma marginale , the intraerythrocytic rickettsia, is controlled by vaccination with live Anaplasma marginale ss centrale ( A. centrale ), a subspecies of relatively low pathogenicity. We have experimentally demonstrated that an animal primarily infected with A. marginale , or with the related vaccine subspecies A. centrale can be infected with the heterologous subspecies, and carries both bacteria. The co-infection was detected in experimentally cross-infected calves for up to 3 months after the last inoculation with the heterologous subspecies. The occurrence of characteristic cyclic rickettsemia of A. centrale and A. marginale was observed by examination of Giemsa-stained blood smears, or by the presence of specific rickettsial DNA confirmed in PCR assays based on specific msp1a and msp4 for A. marginale , and on specifically designed msp3 and msp4 primers for A. centrale . Sequence analysis of msp4 -specific fragments for each subspecies revealed the presence of dual infection in both calves on days 30 and 60 after cross-inoculation with the heterologous Anaplasma subspecies. The experimental cross-infection of calves clearly demonstrated that the concept of “infection exclusion” does not apply to Anaplasma infection in cattle; as there was no infection exclusion of A. marginale in A. centrale -infected cattle, and vice versa . The present results confirmed our previous findings that cattle grazing in an anaplasmosis-endemic field were subject to concomitant infection with both the vaccine A. centrale and the field A. marginale strains.