A critical evaluation is made of reports in the literature on the block method of vaccination and the relevant factors that play a role in the immunization against heartwater are also discussed. The most important of these is the fact that in heartwater immunogenicity parallels pathogenicity. It is shown that the more severe the reaction of the host to the immunizing infection, the stronger its immunity to subsequent challenge. The importance of this principle in the block method is emphasized. Other factors that play a role are the average incubation periods recorded in the different domestic ruminants after experimental infection and differences in age and breed susceptibility. The survival rate of experimentally infected Bonsmara cattle and Merino sheep that were treated on different days of the febrile reaction also serve as a guideline to determine the day after infection on which block treatment can be applied. The danger of a fatal recrudescent infection if block treatment is given too early, necessitating additional treatment and close observation, is indicated. In conclusion, recommendations on the day of block treatment are made for each domestic ruminant species. It is emphasized that the other methods of immunization of large groups of animals, such as treatment only after the commencement of the febrile reaction determined by the daily recording of early morning temperatures, or the prolonged prophylactic chemotherapy of susceptible stock exposed to heavy tick challenge, are preferable to the block method. The block method does, however, find application in certain instances where these procedures are impractical or inappropriate.
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