Risk assessment of bovine tuberculosis introduction and establishment in Norway. Scientific Opinion of the Panel on Animal Health and Welfare of the Norwegian Scientific Committee for Food and Environment

Summary

Following the verification of bovine tuberculosis (bTB) after an outbreak in 2022, concerns were raised about the true epidemiological situation of bTB in Norway. Consequently, the Norwegian Food Safety Authority commissioned VKM to assess the risk of introducing Mycobacterium bovis to Norway, and the risk of its spread and establishment in Norwegian livestock and wild fauna. VKM was also tasked with assessing the risk of infection to humans and identifying risk-reducing measures and diagnostic options for detecting infection in Norway. Background: bTB is a bacterial disease affecting animals and humans, caused by M. bovis. The prevalence varies greatly across European countries. Norway has held an official free status since 1963, with only a few cases reported in the 1980s. The 2022 outbreak was identified through routine meat inspection, revealing several infected animals in a specific herd. The source of this outbreak remains unidentified, and no infected animals have been detected since early 2023. Contact network tracing linked many farms to the index (outbreak) herd through cattle trade. The identified contact herds are still monitored for infection, and the possibility of a spread to other farm animals or wildlife cannot be excluded. Norway maintains strict regulations on live animal imports and monitors the presence of bTB through mandatory reporting, meat inspections, and breeding station testing. M. bovis can infect a wide variety of domestic and wildlife species. Furthermore, there is a significant public health concern due to its zoonotic potential. bTB is a chronic disease, and the incubation period can span from months to years. The bacterium can survive for months in the environment. Diagnosing bTB in live animals is challenging and time-consuming, implying that detection and eradication of the infection is difficult. Key Findings: Norway has had a very low number of imported cattle during the past 10 years. However, some imports of small ruminants and camelids (llamas, alpacas, camels) have occurred. Import of cattle and camelids from countries with bTB in the animal population is assessed as a risk of introducing the bacterium to Norwegian cattle. This risk assessment concludes that introduction of bTB to Norway from imported cattle is unlikely based on the current situation with low number of imports. However, introduction by camelids is regarded as more likely. There is significant domestic trade and transport of beef and dairy cattle within Norway, sometimes without proper registration. If bTB is established in the country, cattle movements are likely to spread the infection between herds. Furthermore, direct and indirect transmission to other domestic species or free-ranging animals (semi-domesticated reindeer and wildlife) may occur, which may complicate the control of bTB in outbreak regions. Indirect transmission can occur via contaminated feeds, pastures, and salt licks that are shared with free-ranging animals. Many species of free-ranging animals are susceptible to M. bovis. Depending on population density and other ecological factors, these species may play the role as hosts and a source of infection for cattle, other livestock and humans. Based on experience from Europe, M. bovis is considered as extremely difficult to eradicate in a country if established in free-ranging species. Badgers, cervid species (i.e. red deer, reindeer, roe deer, and moose), and a growing population of wild boars are of special concern. A contingency plan that takes into account the risk of spread to wild fauna may thus be crucial for successful control of an outbreak with bTB. In periods of severe drought, import of roughage to Norway may be necessary. It is uncertain how well different feed materials and ensiling methods will enable survival of M. bovis. Therefore, restricting import of roughage to Norway to countries and regions certified as officially tuberculosis free (OTF), will reduce the risk of introduction to Norwegian cattle. In the event of introduction and establishment of M. bovis to Norwegian cattle, slurry may pose a risk of spread to domestic and wild animals due to survival of the bacterium in liquid manure. Survival in slurry is uncertain; however, a minimum of six months storage before spreading or alternatively disinfection of slurry will reduce the risk. Zoonotic transmission of bTB remains a relatively rare event, also in countries where the infection is present in animal populations. However, M. bovis can be transmitted by direct contact between animals and humans, through handling (farmers, veterinarians, and slaughterhouse workers) carcasses, and indirectly by consumption of unpasteurised milk and dairy products, but rarely through consumption of meat and meat products. Meat inspection is the key measure for surveillance of bTB in cattle and other domestic animals. Diagnosing bTB is challenging due to the nature of the disease and the lack of a gold standard test. Test-positive animals may not show visible lesions postmortem, and sensitive methods like cultivation and PCR depend on the presence of bacteria in sampled tissues. Any test-strategy aiming to increase the possibility to detect latent infected animals will result in a higher number of culled animals where the infection cannot be confirmed. Too extensive testing in low-risk herds can lead to false positives and must be balanced against the financial costs of restrictions and culling. Combining different tests (skin test, IFN-γ test, and boosted antibody tests) improve sensitivity, and this strategy is particularly advised for imported animals and during outbreak investigations. To achieve the best sensitivity, one should apply the tuberculin test at the same time as the IFN-γ test, followed by serology 10-30 days after the tuberculin test. Culling testpositive animals, and retest after at least 60 days of animals with an inconclusive test will reduce the risk of introducing M. bovis to Norway. While tuberculin tests are labor-intensive and costly, they are regarded as the methods of choice for surveillance in endemic regions. Serological assays like Enferplex show promise for general surveillance, however, the sensitivity is relatively low without prior skin-test. Ongoing studies are evaluating the test performance in bulk-milk screening. In culled animals with suspected lesions, real-time PCR, alongside culture, is recommended for quicker diagnosis. Whole-genome sequencing is the preferred tool for molecular surveillance and outbreak investigations.