Characteristics of an attenuated Anaplasma marginale of deer origin as an anaplasmosis vaccine.

A fully virulent field Anaplasma marginale isolate, passaged to a splenectomised white tailed deer, lost virulence for cattle after 18 months in the non-bovine host. This attenuated A. marginale produced a mild response in four 18-month-old Holstein steers characterised by an average 2.4% parasitaemia, a 13% drop in PCV and a positive serological response. All vaccinated steers were immune to challenge with a fully virulent, heterologous A. marginale when compared with non-vaccinated controls.