African swine fever in the Lithuanian wild boar population in 2018: a snapshot

Abstract

The first cases of African swine fever (ASF) were detected in the Lithuanian wild boar population in 2014. Since then, the disease spread slowly through the whole country, affecting both, wild boar and domestic pigs. In the other Baltic states, which both are also affected by ASF since 2014, the recent course of ASF prevalence suggests that the countries might be well under way of disease elimination. In contrast, in Lithuania the epidemic seems to be still in full progress. In the present study, we aimed to extend a previous prevalence study in Lithuania. Looking at ASF virus (ASFV) and seroprevalence estimates of wild boar in all months of 2018 and in all affected municipalities in Lithuania, the course of ASF was evaluated on a temporal and spatial scale. A non-spatial beta-binomial model was used to correct for under- or overestimation of the average prevalence estimates. Within 2018 no big differences between the prevalence estimates were seen over time. Despite of the lower sample size, highest ASFV prevalence estimates were found in dead wild boar, suggesting higher detection rates through passive surveillance than through active surveillance. Accordingly, with the maximum prevalence of 87.5% in May 2018, the ASFV prevalence estimates were very high in wild boar found dead. The number of samples originating from hunted animals (active surveillance) predominated clearly. However, the ASFV prevalence in those animals was lower with a maximum value of 2.1%, emphasizing the high value of passive surveillance. A slight increase of the seroprevalence in hunted wild boar could be seen over time. In the center of Lithuania, a cluster of municipalities with high ASFV and seroprevalence estimates was found. The results of the study indicate that ASFV is still circulating within the Lithuanian wild boar population, constituting a permanent risk of disease transmission into domestic pig holdings. However, additional, more recent data analyses are necessary to re-evaluate the course of ASF in Lithuania and thus, to be able to make a statement about the stage of the ASF epidemic in the country. This is of huge importance for Lithuania for evaluating control measures and their efficacy, but also for neighbouring countries to assess the risk of disease spread from Lithuania.

African Swine Fever and Its Epidemiological Course in Lithuanian Wild Boar

“…Several studies have described the epidemiological situation of ASF in Lithuania in certain periods [ 9 , 10 , 25 ]. In the present study, all available and evaluable ASF wild boar surveillance data were used to illustrate the course of the disease over time and to obtain an overview of the current epidemiological ASF situation in wild boar in Lithuania.…”

“…Only in the west of the country are small parts (less than 2000 km 2 ) still free from ASF. In addition, outbreaks have occurred in domestic pig holdings every year since 2014 [ 10 , 25 ]. Recent data analyses in Estonia and Latvia suggest that the efforts to fight ASF are rewarded, since the number of ASF virus (ASFV)-positive wild boar has decreased.…”

“…The circulation of an attenuated virus or the existence of ASFV carrier animals has also been discussed as possible causes for this epidemiological course of ASF [ 26 , 29 , 30 , 31 ]. In Lithuania, however, the analyses of ASF surveillance data performed until 2018 indicate an ongoing epidemic situation with regular occurrence of newly infected wild boar [ 9 , 25 , 26 ].…”

African Swine Fever in Wild Boar in Europe—A Review

“…In most of the countries that have been affected by ASF in wild boar for several years, a significant reduction in the population density has been observed [ 7 , 13 , 100 ]. However, the disease is still present in these countries, so they also suffer from the corresponding economic restrictions, even if the ASFV prevalence decreased and no or hardly any cases in domestic pigs occur [ 3 , 5 , 13 , 16 ]. In Latvia, for example, several control measures had been implemented (e.g., incentives, targeted hunting, usage of supporting tools), but none of them showed an immediate effect on ASF prevalence [ 7 ].…”

“…Based on previous experience on the Iberian Peninsula and Sardinia with genotype I of ASFV, wild boar had so far not been considered a major and long-term reservoir for ASFV [9,10], and self-sustaining infectious cycles in wild boar were not anticipated at the beginning of the epidemic [11]. However, disease dynamics are different under the present conditions, particularly in north-eastern Europe, and long-lasting endemic cycles without any major involvement of domestic pigs were established in affected countries such as the Baltic States or Poland [3][4][5]7,[12][13][14][15][16][17]. Despite the high virulence of ASFV and considerable case/fatality ratio among wild boar, these cycles remained self-sustaining in many affected countries over several years, while the Czech Republic and Belgium were successful in eliminating introductions of ASF in wild boar [18][19][20].…”