The possibility of organizing systematic control and eradication of hog cholera in Europe: A survey of the epidemiology of HC in Europe reveals a generally low prevalence and large areas completely free of HC virus infections. These variations have resulted from differing risks of reinfection, the structure of the pig industries concerned, and the intensity with which control and eradication measures could be applied. Countries which have achieved freedom from HC virus infection must resort to protective barriers which impede free trade. Other countries have had to depend on nationwide or intensive zonal vaccination to minimize HC spread but almost all have supplemented vaccination with “stamping out” methods whenever the disease has appeared. The cost of these programmes, and the losses that occurred in epidemics in the recent past, clearly point to the economic, trade and welfare advantages of eradicating HC completely from Europe. To achieve this goal a number of technical, economic and administrative problems must be solved and a system of coordinating eradication activities is needed.
Diagnostic methods: It has been agreed that an adequate range of techniques is available for the quick and effective diagnosis of HC. Tissue section can be undertaken in field conditions at local diagnostic or mobile laboratories. A central laboratory is needed to prepare standard reagents for use throughout each country, and to conduct confirmatory examinations by cell culture, animal inoculation and serological methods.
The presence in some countries of BVD infection in a large proportion of pig herds presents problems of differential diagnosis but these can be overcome by the use of appropriate virus strains in comparative tests. The final conclusion on the infectious status of a herd will depend on the examination of a series of samples, and on a detailed epidemiological investigation. Procedures for handling samples should be standardized from the point of collection to the completion of analysis, and training of staff must be organized to assure continuing reliability of results.
Control procedures: Training of field staff, and education of the farming community and local authorities are prerequisites for securing rapid reporting of suspect cases, tracing of contacts, and early containment of outbreaks. Systems for the registration and control of animal movements, including identification of individual breeding animals and herd identification for others, should be developed where they do not already exist. It is necessary to have full legislative authority to take appropriate action where infection is suspected to have occurred, as well as where clinical disease has been observed. The principle underlying such legislation should be uniform for all countries. Since waste foods (swill) are regarded as a dangerous means of spreading HC virus, legislation should ensure that such material is effectively sterilized. In countries or areas where the pig population is free from HC infection, the rules which are followed in the emergency strategy should include the following: as soon as the presence of HC is confirmed, all infected and exposed animals must be destroyed, full compensation being paid to the owner to encourage cooperation. Rapid tracing of contacts, “standstill” restrictions, and intensive disinfection should be undertaken to minimize further spread.
With the aim of searching out and eliminating infection in areas and particularly breeding herds where the presistence of HC virus is suspected, serological screening procedures should be undertaken. The immune status of the young pig in relation to maternal immunity and immunotolerance is an important factor in diagnosis, which should be studied in more detail.
Neutralization test, immuno-electro-osmophoresis (IEOP) technique, and the modified CF test are capable of dealing with large numbers of samples. Positive and doubtful results require confirmation by neutralization tests using specified HC and BVD virus strains. The advantage of an initial IEOP disappears where the incidence of BVD infection is high.
HC diagnosis should be considered for herds experiencing reproductive failure, abortion, abnormal litters, and high piglet mortality. Countries in which HC is present should be encouraged to undertake serological screening of breeding herds.
Epidemiology: Epidemiological data should be routinely collected on the detailed characteristic of every herd in which HC is encountered. This should be complemented by background data on the pig population, trends of production, movement patterns and productivity. Regular analysis of this information will contribute to decision-making to improve the efficiency of programmes, indicate stages at which programme changes are desirable, and provide a basis for the economic planning of HC eradication.
National collection systems should be linked together with others. To make this possible, harmonization of the collection mechanisms should be organized. This would facilitate close inter-country collaboration and create the basis for common HC eradication policies.
Extensive serological surveys and investigation of problem herds for HC infection as well as constant surveillance are prerequisites for a declaration of freedom from HC infection. A period of at least twelve months must elapse after the last confirmation of HC infection before a country, or an area, can be declared free.
Security: When a country has reached a state of freedom from HC, all laboratories handling HC virus must be subject to strict virus security control measures to prevent the escape of HC virus.
Vaccines and vaccination: It was generally agreed that the application of “C” strain vaccine had made a great contribution to the reduction of HC prevalence in Europe. Since many different vaccines are now in use it was proposed that a collaborative investigation be undertaken to identify the safest and most effective products, and to organize a system of quality control. It was recognized, however, that when prevalence of infection has been sufficiently reduced, and the risk of reinfection removed by protective measures, vaccination should be replaced by measures to eliminate residual infection. There are two reasons for the eventual cessation of vaccination in Europe:
Cost:- a stage is reached when stamping out alone is cheaper than continued herd vaccination.
Free trade:- considering the existence of many European areas free from HC, unrestricted trade in pigs and fresh pig products will only be possible between all European countries when they have reached the same health status.
Epidemiology: The situation in Spain had improved in the last 2–3 years, in so far as the number of confirmed outbreaks was concerned but the emergence of strains of virus with gradually decreasing pathogenicity for pigs had created problems with differential diagnosis, and serological testing was necessary to determine the true extent of infection. The Spanish authorities planned to carry out surveys using the newly developed technique of immuno-electro-osmophoresis and the results are awaited with great interest.
In the enzootic area of the south and south-west of the country, control now relied particularly on encouraging changes in the structure of the pig industry, with moves towards more intensive units in which it was possible to ensure the absence of tick reservoirs.
So far as the tick Ornithodoros erraticus was concerned, it was considered that this reservoir/vector now accounted for about 5 percent of the total outbreaks of disease in Spain, compared with 50–55 percent in the 1960's. However, no surveys of the tick population had been carried out in the last 3 years and the frequency of virus in ticks was not known. It was pointed out that, although the disease would hopefully be eliminated in the future, virus-infected ticks could well remain in the affected areas and pose a continuing threat either locally or at greater distances if transferred on host animals other than the pig.
Diagnosis and control: It was agreed that existing techniques for the laboratory diagnosis of ASF were very satisfactory and it was noted that confirmation of ASF diagnosis could usually be obtained in Madrid within 2 hours of receipt of samples.
The usefulness of the IIF technique for demonstration of antibody in tissue extracts was noted. This has enabled a confirmation of infection to be obtained in 82 percent of positive cases. About 30 percent of all suspicious samples were now confirmed as ASF in Spain.
Research: The following areas were agreed to represent the most promising for continuing investigation:-
Studies of the pathogenesis of ASF caused by strains of a wide range of virulence.
The immunological reactions involved in ASF, whether humoral or cellular, need continuing intensive investigation.
It was considered that any research work directed towards the production of a safe and effective vaccine should have a very low priority.
Further studies should be carried out on the survival of ASF virus in meat and meat products, with a view to decreasing dissemination of virus in the food chain.
The necessity for additional work on reservoir/biological vectors has already been noted.
Editors note: There was no specific reference to ASF in Portugal but the disease remains a serious problem in the country.
