Scientific Opinion of the Panel on Animal Health and Welfare (AHAW) on a request from the Commission on porcine brucellosis (Brucella suis)

Following a request from the European Commission (DG Health and Consumer Protection), the Panel on Animal Health and Welfare (AHAW) was requested for an opinion on porcine brucellosis (Brucella suis). B. suis consists of five biovars, however infection in pigs is caused by the first three biovars (biovars 1, 2, and 3). Infection of animals caused by biovars 1 and 3 differs from that caused by biovar 2 in the host specificity and geographical distribution. In the context of public health, biovar 2 is very rarely pathogenic for humans, whereas biovars 1 and 3 are highly pathogenic causing severe disease in human beings. There is currently no requirement for monitoring and surveillance of B. suis in domestic pigs or in wild life and therefore a lack of systematic epidemiologic data on porcine brucellosis in most MS. The occurrence of the disease is mainly sporadic (with the exception of certain areas where the characteristics of the production systems allow B. suis to be endemic). Within the EU, the epidemiological situation is varied, with some countries free of the disease, others reporting sporadic outbreaks and yet others reporting this disease as an emergent problem. Available epidemiological evidence shows that B. suis biovar 2 is the most common agent, but biovars 1 and 3 can also occur. Available evidence also suggests that currently the wild boar seems to remain the main source of infection for domestic pigs because several outbreaks of B. suis occurred in outdoor rearing systems, even on fenced premises, with the source of infection traced to contacts with wild boars. Transmission from wild boars to pigs is thought to be through the venereal route, as crossed piglets (striped) have been reported, at least in France and Portugal. Other routes might also be possible. Hares have been considered as a possible source of B. suis outbreaks in domestic pigs via swill feeding with offal from hunted infected hares. Some reported outbreaks have also been traced to the introduction of infected live animals originating from holdings where the diseases had not been detected. Based on the data of a systematic literature review, meta-analytical estimates of diagnostic sensitivity (Se) and specificity (Sp) of diagnostic tests for B. suis infection in pigs were generated. Highly sensitive and reasonably specific testing systems with the potential to combine more than one test are required for a rigorous detection and slaughter policy. Currently, serological testing in pigs is mainly useful to monitor the status of a herd but not reliable enough for single animals. Evidence from the systematic review suggests that indirect Enzyme-Linked ImmunoSorbent Assay (iELISA) and competitive Enzyme-Linked ImmunoSorbent Assay (cELISA) could be suitable candidates because of their high Se and Sp. However, the ELISA tests have not been fully evaluated and standardised for use in pigs. Primary reference standards are currently being developed. Formal procedures such as those implemented by the OIE should be considered for accreditation of candidate tests (e.g. iELISA and cELISA) for the purpose of control of B. suis in pigs. Little is known about the causes of false positive serological reactions to B. suis testing in pigs (FPSR), but it is believed that Yersinia enterocolitica O:9 could be the main factor of this problem. To address the FPSR issue it is important to improve the specificity of current diagnostic tests. Specific studies should also be conducted with the aim to identify the mechanisms of FPSR and to elaborate specific testing protocols to reduce this phenomenon. Further development of Brucellin-based tests should be encouraged since, in addition to bacteriology and molecular tools, these tests are the only confirmatory tests suitable to fully discriminate between true brucellosis infections and the infections caused by Y. enterocolitica O:9 or other cross-reacting bacteria. The risk factors (RF) for B. suis introduction and spreading into domestic pigs (in particular through contact with wildlife, and subsequent spread within the EU by trade in pigs and pig semen) have been identified and qualitatively assessed. The presence of infected wild boars and hares and the potential for exposure of outdoor pig holdings remain the most important risk factors in the currently affected areas. Exposure to infected wild boar would be influenced by the level of biosecurity resulting in variable level of either direct or indirect contact. In addition to the level of biosecurity, direct contact would also be influenced by the type of pig housing (e.g. outdoor vs indoor). Should the infection become established in holdings participating to intra-Community trade (e.g. outdoor, indoor, semen collection centres), the most important risk factor for wider spread within the EU would the infection remains unrecognised. This would create the potential for further spread within the EU either by direct or indirect contact. Movement of live pigs (mainly breeding pigs) and semen would be the most important risk factor given the intensive level of intra-Community trade. Indirect contact would mainly depend on mechanical transmission by people and shared contaminated equipment. The role of other means of transmission (e.g. rodents, scavenging birds) remains hypothetical. Awareness should be raised in the pig industry for indicative clinical signs of porcine brucellosis and to the additional risk posed by illegal swill feeding including offal from hares and wild boars. Semen production is well controlled by legal requirements related to the introduction of boars in semen collection centres, continuous monitoring of disease freedom and semen preparation requirements. However, transmission through this route could constitute an important way of disease dissemination. Boars kept in semen collection centres should continue to be selected and introduced from holdings that are epidemiologically proven as free from B. suis. Donors should continue to be serologically tested on holding of origin and in quarantine before being placed in the centre as well as on a regularly basis afterwards. The results indicate that the iELISA and cELISA could have the potential of being used for testing of boars for admission to semen collection centres and for compulsory routine testing.