An overview of epidemiology of R. equi infection in foals is presented, emphasizing
the importance of the virulence-associated antigens and plasmids as epidemiological
markers. The monoclonal antibody-based colony blot test has been developed to identify
rapidly and accurately virulent R. equi. Epidemiological studies conducted during
the recent 5 years have revealed that: (1) avirulent R. equi are widespread in the
feces of horses and their environment on every farm; (2) the feces of horses and the
environment of the horse farms having endemic R. equi infections demonstrated heavy
contamination with virulent R. equi, but the farms without the problem did not, thus
suggesting that foals bred on a farm with endemic disease are exposed more frequently
to virulent R. equi in their environment than those of a farm without the problem;
(3) only virulent R. equi are isolated from lesions of naturally infected foals, showing
that natural infections in foals are principally by virulent R. equi, but not avirulent
organisms; (4) infected foals which constantly shed large quantities of virulent R.
equi in their feces are the major source of virulent R. equi, which this may be the
mechanism of progressive development of infection on farms with a history of the disease.
At present, farms with a potential for endemic infection can be distinguished on the
basis of the contamination with virulent R. equi, so regular examination of foals
and their environment by virulence markers might be the most practical approach to
control R. equi infection on endemic farms.