Control / Prevention

Unlike ASF, CSF can be well controlled by vaccination. The most widely used vaccines are live vaccines, most of which are based on the Chinese ‘C’ strain. They are highly effective and safe and have been successfully used in eradication programmes in many countries.

In countries where CSF has been eradicated, and particularly those which export pigs and/or pork, vaccination is generally prohibited. This is partly because absence without vaccination is considered the ideal situation, and partly because it is not possible to distinguish serologically between pigs vaccinated with the live vaccine and pigs that have been naturally infected. The occurrence of major outbreaks in countries that were free without vaccination (e.g. The Netherlands in 1997 and the United Kingdom in 2000), necessitating massive culling of pigs at high cost in order to regain their free status, as well as the fact that some countries where CSF is endemic nevertheless have the potential to export pork, makes the development of marker vaccines that will enable differentiation between infected and vaccinated animals (DIVA) attractive. To this end a number of subunit marker vaccines have been developed where for example an immunogenic protein is expressed in cell cultures, separated and combined with an adjuvant to form the vaccine. Some of the problems encountered have been lower protection than that afforded by the live vaccines, viral circulation in the presence of vaccination, failure to protect against vertical transmission of the virus, and the length of time required for immunity to develop, which precluded the use of this type of vaccine in emergency control of outbreaks. While the live vaccine apparently stimulates cellular as well as humoral immunity, since vaccinated pigs are protected before measurable antibodies develop, this has not been the case with marker vaccines. However, ongoing efforts to improve marker vaccines are reported to be yielding more promising results.

Like ASF, CSF can largely be prevented by biosecurity measures on farms. This would include limiting access to the pigs, supplying a change of footwear at least to people who have to approach the pigs, and not feeding swill that could contain raw or undercooked pork to pigs. This includes not feeding the entrails or other remains of wild boars that have been found dead or killed by hunters, as well as not permitting pigs to roam freely in areas where encounters with wild boars are likely. The 1997 outbreaks in The Netherlands suggested a possibility of airborne transmission over relatively short distances in areas with dense and contiguous pig farms in cool, moist conditions, and this is difficult to overcome by biosecurity measures unless pigs are permanently confined in barns with virus-proof air filters. Under these conditions vaccination would be a necessary adjunct to biosecurity measures. Because subclinical disease is not uncommon in herds with well developed immunity, quarantine of new pigs would be an important measure to prevent the introduction of CSF into a naïve herd.



Dead pigs going to burial

In countries where wild boars are known to be infected, oral vaccination using baits with live vaccine has been implemented. While the results were promising, it is recommended that this approach should be combined with other approaches such as controlling the size of wild boar populations through hunting and taking the normal precautions to prevent exposure of domestic pigs to wild boars or their remains. A particular problem was that, while the infection circulates in the younger members of the wild boar population, the larger boars tended to ‘hog’ the oral baits so that the younger pigs were not exposed to the vaccine.

During an outbreak, particularly in countries that have eradicated the disease, massive culling tends to be applied. During the 1997 outbreak in The Netherlands 11 million pigs were culled, of which only 0.7 million were from infected herds and 1.2 million from ‘dangerous contact’ herds; the remaining pigs had to be killed for welfare reasons due to overcrowding on farms when all movement and sales were banned. This kind of exercise is not only unpopular with farmers and public alike; it also creates massive challenges in terms of disposal of such a large number of carcasses. Modelling applied after the event suggested that some costs would have been saved by including vaccination as part of the control strategy. The vaccinated pigs could then have been removed from the population by commercial slaughter and the appalling waste of high quality protein that results from disposal avoided. When South Africa experienced CSF in the Western and Eastern Cape in 2005 the disease was eventually brought under control by a massive and logistically difficult culling operation that involved finding and culling an unknown number of free-ranging pigs in challenging terrain. The cost of compensating owners for close to half a million pigs slaughtered would likely make it difficult to justify another such operation should the same situation arise again. An alternative approach would be to vaccinate as many pigs as possible with the live vaccine, which requires only one vaccination, thus saving the costs of disposal and compensation. The saving would need to be balanced against factors like the value of South Africa’s pork export market. However, resistance to massive culling operations is growing and it is probable that in the future less drastic alternatives will become the norm rather than the exception.

In countries where CSF is endemic, efforts to eradicate it are supported by vaccination with the live vaccine to limit virus circulation as well as by attempts to improve pig husbandry and ensure permanent confinement of pigs. In a relatively recent development, the OIE has added biosecure compartments to the options that support export of livestock commodities (http://www.oie.int/fileadmin/Home/eng/Health_standards/tahm/2.08.03_CSF.pdf). The concept is particularly applicable to intensive production systems and offers the opportunity to commercial pig farms to enter the export market in spite of endemic CSF provided that they can prove that the measures in place are sufficient to ensure that the compartment is free of CSF. In response to the outbreak in the Eastern Cape, guidelines for CSF-free compartments were developed by the national authorities in collaboration with the private sector to at least permit some movement within the country for producers who were able to comply with the measures stipulated. Provided the measures applied to compartments are accepted by prospective trading partners, export from areas that are not geographically free of CSF becomes a reality.