Aboubacar Yaya[28] and Francois Thiaucourt[29]
Introduction
When CBPP was reintroduced in Botswana in 1995 after more than 50 years of absence (Amanfu, 1998), a logical strategy was put in place to contain the spread of the disease by forbidding cattle movement from the infected zone towards free areas and to perform a massive vaccination campaign ahead of the disease front. The vaccine used was the T1 SR strain and it was administered to thousands of cattle in an emergency vaccination campaign. However logical the strategy and correct the implementation plans were, they failed to contain the disease. CBPP soon spread to regions where the vaccination had been performed. Subsequently, an ultimate massive slaughter strategy was put in place and all the cattle population in the infected zone were destroyed. This enabled Botswana to regain its CBPP-free status by the most rapid way although at a high initial cost. One of the tentative first explanations for the failure of the vaccine strategy was to advocate an error that could have been made when handling the seed strain of vaccine, a less potent strain might have replaced the original one. This proved not to be the case, as a subsequent polymerase chain reaction (PCR) technique (Lorenzon, 2002) clearly identified the vaccine used in Botswana as T1. The quality of the vaccine itself was not considered as a possible cause for the failure, given that it had passed all the internal controls by the producer and also from an independent laboratory (PANVAC) in Ethiopia.
Therefore, an international study was put in place to investigate the protection afforded by the two most well known CBPP vaccine strains T1 SR as a control for what happened in Botswana and T1 44, which was considered a more potent strain. When these strains were injected at the minimum dose recommended by the OIE (107 live mycoplasmas per dose) the protection afforded three months later was quite disappointing: from 30 to 60%. This would obviously not be sufficient to prevent the contamination of many animals, even if the vaccine coverage was high. A possible way to increase the protection afforded by these types of vaccines may be to increase the number of mycoplasmas per dose. However, there are very few data concerning the relationship between dose of vaccine and protection (Gilbert and Windsor, 1971). This is the reason why a new trial was put in place.
Materials and Methods
Two groups of 45 animals were vaccinated with strain T1 44 and T1 SR respectively. Within each group, 3 subgroups of 15 animals were injected with 107, 108 and 109 live mycoplasmas. Three months later, CBPP was artificially reproduced in a group of 22 animals inoculated by intubation with a culture of a local MmmSC pathogenic strain (Touroua) used in a previous challenge study. These intubated animals were put in contact with the vaccinated animals and a control group of 39 naive animals as soon as clinical signs of CBPP were evident. All the animals were slaughtered two months after the initial signs of disease in the control group. Lesions were monitored and a scoring system, modification from that by Hudson and Turner, was established.
Results and Discussion
CBPP transmission to the control group was successful and the mortality rate was 32%. In addition, most of the surviving animals displayed typical lesions of CBPP with or without seroconversion and presence of MmmSC.
The mortality rate in the vaccinated animals was significantly reduced (6%) and the protection was also evident from the shift in the lesion score distributions. These lesion score distributions seem to indicate a better protection afforded by strain T1 44 (70%) as compared to T1 SR (60%), although differences are not statistically significant. There is also no obvious pattern of correlation between dose and protection. The most likely explanation is that variations in protection are linked to the presence of some highly susceptible animals in one or the other group. The relatively limited size of each group and the degree of variations and individual susceptibility did not allow us to observe any significant trend. However, our results show that if there is a correlation between dose and protection within the range that was tested in this experiment, this correlation is not very marked. Accordingly, it is unlikely that injecting much higher dosages (109 mycoplasmas for example) will result in a significant increase in protection level. Increasing the viable content may be very helpful as it may increase the shelf life of the product. It has to be remembered that, although the minimum advocated dose is 107 organisms (Lefèvre, 2000), previous authors have advocated the production of vaccine vials containing 108 mycoplasmas per dose (Provost, 1987) in order to take into account the very probable losses of titre from the production site to the time and place when the vaccine is actually injected to cattle. Alternative ways to increase the protection rate with the present vaccines may be to perform a booster injection shortly after the first one. Such a trial is going on right now and results should be obtained by next year.
References
Amanfu, W., Masupu, K. V., Adom, E. K., Raborokgwe, M. V. and Bashiruddin, J. B. (1998). An outbreak of contagious bovine pleuropneumonia in Ngamiland district of north-western Botswana. Veterinary Record 143, 46-48.
Gilbert, F. R. and Windsor, R. S. (1971). The immunizing dose of T1 strain Mycoplasma mycoides against contagious bovine pleuropneumonia. Tropical Animal Health and Production 3, 71-76.
Hudson, J. R. and Turner A. W. (1963). Contagious bovine pleuropneumonia: a comparison of the efficacy of two types of vaccine. Australian Veterinary Journal 39, 373-385.
Lefèvre, P. C. (2000). Contagious bovine pleuropneumonia in "OIE manual of standards for diagnostic tests and vaccines. 12 rue de Prony 75017 Paris France" pp 957.
Lorenzon, S., David, A., Nadew, M., Wesonga, H. and Thiaucourt, F. (2002). Specific PCR identification of the T1 vaccine strains for contagious bovine pleuropneumonia. Molecular and Cellular Probes 14, 205-10.
Provost, A., Perreau, P., Breard, A., Le Goff, C., Martel, J. L. and Cottew, G. S. (1987). Contagious bovine pleuropneumonia. Revue Scitifique et Technique Office International des Epizooties 6, 625-679.
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[28] LANAVET,
Garoua,Cameroon [29] CIRAD, EMVT, Montpellier, France |
