Contagious bovine pleuropneumonia vaccines and control strategies: recent data

Contagious bovine pleuropneumonia is one of the most threatening transboundary cattle disease in Africa. However, with the exception of Botswana, very few African countries were able to implement eradication strategies for this disease, after it had recently re-infected a number of countries. Previous experimental studies have shown that emergency vaccination campaigns, based on a single injection, were not inducing a sufficient protection level to prevent further spread of the disease. In addition, post-vaccinal reactions were sometimes reported in the field when using vaccine strain T1/44, leading cattle owners to refuse the vaccination. On the contrary, antibiotics are used quite often in the field but there are insufficient data to assess their efficacy properly. Therefore experimental studies were implemented: (i) to check if higher dosages of the vaccine would be able to induce higher protection rates and (ii) to elucidate the origin of the post-vaccinal reactions observed with T1/44 and (iii) to gain preliminary results on the efficacy of long-acting tetracycline. The first experiment included the use of three doses of vaccine strains T1/44 and T1sr: 10(7), 10(8) and 10(9) mycoplasmas per dose. T1/44 seemed to induce a higher protection (70%) than T1sr (60%). However, there was no observable dose effect for these vaccine strains. The second experiment was performed by injecting various MmmSC strains subcutaneously into susceptible cattle. One of these strains was an isolate obtained from a "Willems" reaction following a vaccination with T1/44. This isolate, called T1B, induced typical invading oedema at the injection site in a similar way to the pathogenic strain, whereas the original T1/44 vaccine strain did not. These findings indicate that the strain has reverted to virulence. Finally the antibiotic trials showed that long-acting tetracycline was able to reduce the losses due to the disease but could not prevent the persistence of viable MmmSC in treated animals. The consequences of these findings are discussed. They reinforce the need for additional research on new vaccines able to elicit longer lasting protection. However, once continuing additional field research is obtained, it should allow better defined strategies to be put in place. Meanwhile, immediate action should be taken to prevent the further spread of CBPP in the Southern part of Africa.