On 14 January 2014, Lithuania’s National Food and Veterinary Risk Assessment Institute confirmed two ASF cases in wild boars in area bordering with Belarus (VIL, Fig. 1). Since then, the country underwent a rapid spread of ASF almost simultaneously with three other EU countries: Poland, Latvia and Estonia. To date, more than three thousand of ASF notifications in wild boars have been reported in Lithuania alone . The average prevalence of ASFV in wild boars during the 2014–2017 period was 4.69% which greatly outnumbers outbreaks among domestic pigs, where average viral prevalence was 0.86%. Despite the fact, that the number of outbreaks in pig farms was several times smaller compared to the incidence rate in wild boars, ASF is a major threat to local pig industry today. ASFV mostly affects backyard holdings that gradually ruin small-scale farming.
Although a correlation between the pig density and number of recorded pig ASF- positive cases in affected regions of Lithuania was found in 2017 (R = 0.78, p < 0.05) it did not extend to the earlier years of the study where correlation was no longer statistically significant. This can be explained by a small number of infected areas where ASF cases had only recently been detected. The analysis of combined data from Poland and the other Baltic States, conducted by a panel of European Food Safety Authority experts, found no correlation between wild boar density and ASF case notifications . However, this uncertainty probably occurs due to inaccurate data on the number of wild boars since European Union countries are lacking of standardized accounting method for wild animals. Overall, it might be concluded that population density in wild boars does not correlate with virus prevalence in a given region, but might influence the risk of virus introduction to a new wild boar group due to increase in adult male dispersal distance. In turn, maintaining a low wild boar population levels might prevent long range dispersals of adult males. In addition, low-density barrier strategy in the surrounding areas has been previously proposed  although the effectiveness of the said strategy is debatable since the lack of possible mates for migrating male boars might increase their dispersal range even more.
Our data shows that specific anti-ASFV antibodies in wild boars tested in the framework of active surveillance appear at a very low rate. Based on ELISA test results in Lithuanian wild boar serum samples, the average prevalence of ASF specific antibodies during 2014–2017 was only 0.45%. However, it indicates that some wild boars can survive early stages of ASF infection and transmit the virus. This finding is confirmed by experimental infection of pigs with Lithuanian wild boar isolate LT14/1490 that is responsible for the first ASF case in Lithuania. ASF specific antibodies were detected 17–18 days after inoculation in 33% of experimentally infected pigs, while 10% of them survived the infection showing weak and intermittent peaks of viraemia .
ASF viral DNA prevalence in hunted wild boars using PCR analysis gradually increased from 0.83 to 2.27% in 2014 to 2016 respectively. However, 2017 saw a dramatic jump in the number of ASF positive wild boar cases resulting in prevalence of 12.39%. More than fivefold increase in ASF positive wild boar cases in 2017 compared to 2016 could be explained by extensive spatial spread of ASF infection in Lithuanian wild boar population during this period, since before 2016, ASF virus in Lithuania was restricted to a limited geographical area. In addition, intense active surveillance program in 2017, emergence of ASF in naïve wild boars in previously ASF-free regions and high level of wild boar density could have contributed to elevated prevalence. Passive surveillance showed a similar increase of ASF positive cases in 2017. Process of carcass collection has been actively and successfully encouraged in Lithuania by monetary incentives for each wild boar carcass retrieved, explaining overall ASFV prevalence rate jump in 2017.
Study results show seasonal prevalence of ASFV infection in wild boar population. We found statistically significant difference (p < 0.05) between the number of ASFV cases in winter and summer or autumn with prevalence rates of 3.90, 5.59 and 7.45% respectively. Differences in ASFV prevalence in wild boars could be explained by a more effective retrieval of carcasses in winter as part of passive surveillance program due to easier visualization of the carcass against a snowy background and lack of vegetation that may obstruct the view in other seasons. Similar statistically significant (p < 0.05) seasonal distribution was obtained by testing specific anti-ASF antibodies.
ASFV infection in Lithuanian wild boar population spread gradually across most of the country resulting in different prevalence rates with eight out of ten Lithuanian counties being affected. Statistically significant (p < 0.05) spatial differences could be explained by wild boar concentration differences in affected regions and temporal distribution factors.
Results of passive surveillance and total ASF positive cases did not show significant seasonal difference. ASF infected wild boar carcasses were found at very similar 61.00–62.06% prevalence rates in spring, summer or autumn, however highest level of ASF positive carcasses were found in winter.
The average prevalence of ASF-specific antibodies in 2014–2017 in Lithuanian pigs was only 0.35%, while during 2015–2017 period ASF-specific antibodies were detected in only 0.02–0.03% of tested serum samples (n = 48,530). A relatively low antibody prevalence could be due to the fact that most of ASF infected pigs are diagnosed and sampled in early stages of the disease, while stable specific antibody count is usually achieved at least two weeks after the infection .
The average ASF virus prevalence using PCR method in 2014–2017 was 0.86%, however a jump in positive cases, similar to wild boars, was observed in 2017 with 2.74% of tested pigs being ASF positive. ASFV infection in Lithuanian pig population spread gradually across most of the country, mainly in backyard holdings with low levels of biosecurity. All counties with positive pig cases overlapped with wild boar positive counties, indicating that regional ASF prevalence in wild boars is a risk factor for domestic pig farms in that particular region, especially where direct contact between wild boar and pig populations might be possible (e.g. backyard holdings).
Seasonal distribution of ASF cases in pigs showed summer to be a statistically significant (p < 0.05) factor for ASF outbreaks. All outbreaks in back yard holdings and two commercial farms in Lithuania during 2014–2017 have been detected in summer - mainly end of July and August. ASF outbreak peaks in summer could be attributed to human or insect activity with capabilities of virus transfer which could play a role in epidemiology of ASFV in pig population. A reason for dramatic increase of ASF cases in Lithuanian pigs during July and August needs to be further investigated. It would be useful to test if such outbreak tendencies could be observed in Poland, Latvia and Estonian pig population.
The ASF virus is extremely resistant to environment factors. It can persist in relatively high temperatures and processes of putrefaction. These properties provide excellent conditions for mushroom hunters to mechanically transport the virus. Therefore, the main challenge seems to be not the infected wild boars that are prone to contact with domestic pigs in small holdings, but high tenacity of the virus and its ability to be transmitted by indirect contact. Recent experimental studies indicated that domestic pig contact with infected wild boars is sufficient to transmit the infection [25, 35, 36] and there are no reliable evidence supporting the assumption that the spread of the disease in Lithuania is associated with direct contact between two mentioned species.