[The problem of recurrence of enzootic bovine leukosis in previously cured herds]

Within nine months, enzootic bovine leukosis (EBL) occurred in 23 well documented herds. Eight of them (= 35%) had previously conducted the eradication programme as laid down by law. This proportion is tenfold higher than anticipated from the average incidence rate since 1978. The conclusion is drawn that a higher risk for reinfection exists for herds previously infected and cleaned than for those that never had leukosis before. For such cases hypotheses are presented. In one case clear evidence for one of the hypotheses was obtained. In case of re-occurrence of EBL in a previously cleaned herd it is proposed to examine the white blood picture of the sero-positive animals. If hematologically positive cattle are detected, they should be removed from the herd including their offspring.