H. Wesonga[25], L. Manso Silvan[26] and F. Thiaucourt[27]
Introduction
The occurrence of post-vaccinal reactions has always been a burden for veterinary services using the T1 44 vaccine strain. These reactions occur with an unpredictable frequency; none sometimes, but up to 11% in some instances (Lindley, 1971). The lesions observed in those cases were strictly similar to what was observed by Louis Willems when he inoculated CBPP pleural fluid subcutaneously. It is characterized by an oedema that appears 10 to 20 days after inoculation. This oedema may vary in size and in some cases it extends to the dewlap and causes the death of the animal. In the past, veterinary services have used antibiotics such as tylosin, in order to treat animals with post-vaccinal reactions.
The origin of these “Willems reactions” was never elucidated. Most authors advocated susceptibility variations in the vaccinated animals. It was unclear if this exacerbated susceptibility was of genetic origin or due to other causes. Another explanation could be a reversion to virulence of the T1 44 strain. An experiment was designed in order to test this hypothesis.
Methods
The protocol included the use of 3 Mmm SC strains; the T1 44 vaccine strain as a control for an attenuated strain, a local pathogenic isolate as a control for a virulent strain and T1 B, a strain that had been isolated previously from a “Willems reaction” following vaccination with T1 44 in a previous experiment (Wesonga et al., 2000). Each strain was inoculated subcutaneously to 5 animals and the reaction induced was followed. The size of the local reaction was recorded as well as the body temperature for each animal.
Results
The control groups displayed the expected results. In the T1 44 group, only very small local reactions were measured. None of them exceeded 10 cm in diameter. None of the animals displayed fever. In the pathogenic strain group all animals displayed local lesions that were much more pronounced although there were marked differences between individuals. In two cases the oedema extended to the dewlap and animals were treated with antibiotics to prevent death. The group inoculated with the T1 B strain reacted similarly to that inoculated with the pathogenic strain.
Discussion
This experiment clearly demonstrates that strain T1 B has reverted to virulence. The difference of response is significant in spite of the low number of animals. This result opens new perspectives for research on CBPP. It is an indication that few genes may be involved in virulence. If multiple genes were involved, it would have been very unlikely to observe a full reversion to virulence during a single animal passage. As a consequence, the comparison between T1 44 and T1 B may allow an identification of the most important virulence genes and an understanding of their mechanism. In a future prospect it will also allow the construction of attenuated strains by the specific deletion of these genes (the final proof of their involvement in virulence). From a practical point of view, these results certainly do not call for an abandonment of strain T1 44 that confers good protection rates. It simply calls for caution when using it for the first time in a cattle population. Experience has shown that in subsequent vaccination campaigns the frequency of post-vaccinal reactions was reduced sharply.
References
Lindley, E. P. (1971). Experiences with a lyophilised contagious bovine pleuropneumonia vaccine in the Ivory Coast. Tropical Animal Health and Producton 3, 32-42.
Wesonga, H. O. and Thiaucourt, F. (2000). Experimental studies on the efficacy of T1 SR and T1 44 vaccine strains of MmmSC against a field isolate causing contagious bovine pleuropneumonia in Kenya, Effect of a revaccination. Revue Elevage Medicine Veterinaire Pays Tropicaux 53, 313-318.
Willems, L. (1852). Mémoire sur la pleuro-pneumonie épizootique du gros bétail. Recueil de médecine vétérinaire pratique 9, 401-434.
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[25] KARI, Muguga,
Kenya [26] CIRAD-EMVT, Montpellier, France. [27] CIRAD-EMVT, Montpellier, France. |
