 bovine dermatophilosis is a severe skin infection of tropical ruminants inducing a severe loss in productivity and a 15% mortality rate . 
 this disease is caused by the actinomycete bacterium dermatophilus congolensis associated with the tick amblyomma variegatum . 
 currently there are no prospects for a vaccine , and acaricide or antibiotic control is hampered by the development of chemoresistance . 
 animal breeders have observed that dermatophilosis susceptibility seems to be determined genetically , and we previously identified a bola - drb3-dqb class ii haplotype marker for high ( r2 = 0.96 ) susceptibility to the disease . with this marker 
 , we developed a successful eugenic selection procedure for zebu brahman cattle in martinique ( fwi ) . over a period of five years 
 , a marked reduction in disease prevalence , from 0.76 to 0.02 was achieved , and this low level has been maintained over the last two years . 
 the selection procedure , based on a genetic marker system targeting the highly polymorphic bola locus , eliminates only those individuals which are at the highest risk of contracting the disease . in the present work , 
 we discuss the properties of this system , including the " heterozygote advantage " and the " frequency dependence " theories , and examine their involvement in the biological mechanisms at the host / pathogen interface . 
 we speculate on the exact role of the mhc molecules in the control of the disease , how the natural selection pressure imposed by the pathogens selectively maintains mhc diversity , and how our results can be practically applied for integrated control of dermatophilosis in developing countries . 