Immunology
The failure of the immune systems of both animals and humans to deal quickly and decisively with B. melitensis and B. abortus accounts for the persistence of infection and thus frequent cases of long duration.
Evans, who first drew attention to the similarities between the two organisms, contracted a laboratory infection with the disease in 1922 and suffered recurring episodes of debilitating undulant fever, alternative with periods of apparent recovery, for more than 20 years (in spite of this she lived, after final recovery, until the age of 94).B. abortus, in particular, can give rise to latent infection, leading to latent immunity, which occurs more frequently in persons exposed to milk infection over a period of time than in those suddenly exposed to heavy doses of infective material. This may help to explain the low rate of clinical disease in veterinarians, who as a group show immunologic evidence of exceptionally high levels of response to erstwhile infection.
Vaccination is possible against B. abortus in cattle and against B. melitensis in sheep and goats, but in human beings it is still at an experimental stage. Some knowledge of the immunogenic constituents of the bacillus has become available since the 1960s, but the animal vaccines developed thus far are too toxic for use in humans. In Britain a free calf vaccination service was introduced by the Ministry of Agriculture in 1962.