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Control / Prevention

Eradication of CBPP has been achieved in North America, Europe, Australia and apparently in Asia, with only sub-Saharan African continent remaining significantly infected. Methods used have generally involved movement control and stamping out of infected herds, although eradication in China, in which infection was widespread, was finally achieved by the use of an attenuated vaccine involving numerous passages in rabbits that was apparently effective, although it took more than 30 years to achieve eradication.

Efforts to control CBPP in Africa have a long history in especially West Africa, where cattle keepers are reported to have vaccinated cattle against CBPP by inserting pleural fluid or a piece of infected lung tissue under the skin either of the tail or on the bridge of the nose. Lack of the necessary resources as well as resistance by cattle-owning populations preclude the use of massive culling as a control measure for CBPP in most countries and vaccination is usually regarded as the only alternative. The use of antimicrobials is controversial because of fears that it could result in more carriers, but cattle owners nevertheless do use antimicrobials to cure clinical disease and save the lives of their cattle. It is widely agreed that vaccination will play a central role in reducing the impact of CBPP in Africa and that it may be necessary to combine vaccination with therapy using approved antimicrobials. For this approach to succeed, a more effective vaccine than those currently available would be a great advantage.

The only commercial vaccines available are live attenuated vaccines using the T1/44 and T1sr strains. The former is more widely used, as it provides coverage for a year, while the duration of immunity of the T1sr vaccine is shorter. The latter has the advantage of inducing fewer adverse reactions and being unlikely to cause clinical disease, as sometimes occurs with T1/44, where especially first time vaccination may induce a Willems reaction that is sufficiently severe to require treatment. Additionally, suspicion that T1/44 subcutaneous vaccination could cause outbreaks of clinical disease was supported by an experimental study. The level of protection provided by T1/44 has also not been determined as it appears not always to correlate well with the humoral immune response, and animals without antibodies may also be protected. Studies have suggested that a reduced ability to produce interferon-gamma in the early phase of the disease leads to the development of severe lesions. There are also practical problems of vaccine delivery in less developed areas. Both T1sr and T1/44 are freeze-dried vaccines that are thermostable until reconstituted, but after reconstitution they have to be used within a short period of time, approximately one hour. This is a serious disadvantage when undertaking vaccination in areas with poor infrastructure and relative small herds, as it may only be possible to vaccinate a small number of cattle during the recommended period after reconstitution. Farmer resistance on account of adverse reactions has also been reported from various countries including Kenya, Namibia and Zambia, leading to lower vaccine coverage. 

Investigation of the immune responses of cattle to infection with MmmSC as well as molecular studies of the pathogen to determine the genes coding for antigens have provided information that may lead to development of improved vaccines. It has been proposed that mucosal subunit vaccines would probably be the best option, since IgA appears to be important in mitigating the effects of the infection. However, it is apparent that new generation vaccines are still a long way from becoming available. Some candidate vaccines tested in cattle apparently enhanced the pathogenic effects of MmmSC. Even if an excellent new generation vaccine can be developed, questions have been raised about the practical implications of producing and delivering it. T1/44 is a relatively low tech vaccine that can be produced at a reasonable cost, while high tech vaccines are inevitably more expensive. Willingness on the part of cattle owners in Africa when it has been investigated has been variable, but given that most of them have limited resources it is unlikely that they would be able to buy a more expensive vaccine.

Modelling to determine the best approach to control of CBPP in pastoralist cattle in East Africa based on published data and participatory research to obtain information on the knowledge and perceptions of cattle owners indicated that vaccination alone would be insufficient to eradicate the disease as it could persist indefinitely even in small closed herds. In practice pastoralist herds are never closed and there is a high level of inter-herd exchange and interaction. An approach combining vaccination and prudent, effective antimicrobial treatment was therefore proposed. In spite of the fact that antimicrobial treatment is officially discouraged at international level and is prohibited in many countries, it is widely used to limit damage due to the disease and is regarded as effective by livestock owners and field veterinarians. Effectiveness and sustainability would be increased by the involvement of private and community-based service providers.

Oxytetracyclines (OTC) are the antimicrobials most widely used in Africa to treat CBPP. In spite of widespread and probably sub-optimal use over a long period of time resistance has not been detected, but a number of other products have been investigated. In vitro studies have indicated that relatively few drugs are efficacious against MmmSC, and some of these, for example Ciprofloxacin, are not recommended for use in food animals to prevent possible resistance developing in humans. The most effective drugs, according to the in vitro trials, were OTCs and tilimicosin, a macrolide. Further investigations included newer generation macrolides danofloxacin, which proved an effective contributor to eradicating an outbreak in northern Namibia, tulathromycin and gamithromycin. The studies concluded that OTC and all the macrolides studied showed good activity against MmmSC and that the macrolides would be good candidates for clinical trials. Both OTCs and macrolides are mycoplasmastatic and therefore do not necessarily sterilise the infection. It has been suggested that a mycoplasmcidal effect would be ideal but so far no suitable candidate medicine has been reported.

Although the use of plants to treat CBPP has not been documented but is to an extent preserved in oral tradition, with at least one specific example in Niger, a recent investigation demonstrated good potential for various Nigerian plants tested in vitro. In particular a plant known as giant milkweed, Calotropis procera (family Asclepiadaceae), showed antimicrobial activity against MmmSC that was comparable with that of the macrolide tylosin.