African swine fever (ASF) is a hemorrhagic fever disease of domestic pigs and wild boar caused by the African swine fever virus (ASFV). ASFV was re-introduced in Europe in 2007 in Georgia and has further spread to Europe and Asia [1
]. This spread is partly due to direct contact between pigs and wild boar, but also to human activities such as importation/transportation of infected animals or contaminated feed and fomites. Another possible way of spread is a vectorial transmission. Although ASFV can be transmitted by some species of Ornithodoros
soft ticks in endemic African areas, Pereira de Oliveira (2019) experimentally showed that Palearctic soft ticks, O. erraticus
(from Portugal) and O. verrucosus
(from Ukraine), were not able to transmit the current circulating European ASFV strain Georgia2007/1 [2
]. However, this study demonstrated that the virus was still infectious up to eight months in O. erraticus
and two months in O. verrucosus
], which raises questions about the possibility of transmission through other pathways than tick bites.
Among the different routes for introducing ASFV in new areas, including direct or indirect contacts with pig feces and urines [3
], the importation of infected food has been described as an important pathway for virus spread in Russia [6
]. ASFV was shown to be still infectious in different pig tissues, for example, in muscles collected from abattoir during epizooties in Spain during 1960–1990 [7
]. Furthermore, the virus was still detected in the muscle tissue for about 98 to 112 days after industrial processing [7
]. ASFV was also isolated from dried salami and pepperoni sausages but not after the curing period [8
]. More recently, Italian dry-cured meat products from pigs experimentally infected with the current Sardinian ASFV strain were able to infect naïve pigs [9
]. In addition, feed and feed ingredients could stay contaminated for a few weeks with the current Eurasian ASFV strain Georgia2007/1 [10
]. This makes infection by ingestion one of the most important routes of virus transmission.
Recently, pig infection due to the ingestion of ASFV from stable flies fed on blood from ASFV-infected pigs and leeches’ blood-fed on ASFV-infected pigs was described [11
]. Furthermore, stable flies were able to keep infectious ASFV up to two days [13
], and ASFV was able to persist up to 160 days in leeches [12
]. Since ASF has been re-introduced in the European Union, a seasonal pattern of outbreaks has been reported with more outbreaks in domestic pig herds in summer, the activity period for many dipteran species. One of the hypotheses to explain this epidemiological situation could be related to arthropod vectors, and more widely hematophagous species that could serve in the spread and the persistence of ASFV in infected areas.
Taking into account the foraging habits of Suidae
] and the endophilic character of Ornithodoros
] ticks, our study assesses the possibility of pigs to become infected by ingestion of infected ticks and to evaluate the risks of ASF persistence in Europe.
3. Results and Discussion
Virus was isolated from 9/10 and 7/10 OeG-0 and OeG-2 ticks, respectively. Viral titers of positive ticks were higher for OeG-0 ticks (viral titer > 104.4
/mL) than OeG-2 ticks (viral titer < 104
/mL) (Figure 1
). Therefore, we estimated that pigs that had eaten OeG-0 and OeG-2 ticks received an average viral dose of 106
Both pigs from group 1 that had eaten OeG-0 ticks became hyperthermic three to four days after exposure and were positive to ASFV diagnosis (Table 1
). Only one out of the three pigs from group 2 that had eaten OeG-2 ticks became infected (Table 1
). Against all expectations, the three control pigs from group 3 that received virus spiked brioche
and the contact pig from group 1 did not develop any clinical signs and remained negative for ASFV. Negative controls pigs from group 4 never displayed clinical sign. No specific IgG antibodies to ASFV were detected in any of the pig sera tested.
These results confirm the possibility of infecting domestic pigs by the ingestion of ASFV-blood-fed soft ticks O. erraticus
. Ticks with a high viral titer displayed the highest ability to infect the pigs through oral ingestion than ticks with a lower viral titer. The absence of infection with the “virus spiked brioche
” was surprising as it was considered as a control of infection. This could be due to a low viral titer, too. Indeed, these results are in accordance with a previous study showing a better efficiency when viral titer is higher in contaminated food [21
]. Moreover, the integrity of the virus when spiked in brioche
might have been altered by salivary proteases, conversely to the virus present in the ticks.
The fact that ticks remained sufficiently infectious two months post-infection to allow the infection of pigs by their ingestion confirms the possibility of O. erraticus to be a reservoir of ASFV strains circulating in Eurasia and suggest new transmission routes of ASFV by soft ticks Ornithodoros and more widely for others hematophagous organisms. Further studies are needed to better understand the role of the ingestion pathway in current epizooties in Eurasia, especially about the possible interactions between hematophagous species and wild boar.