Transmission of African swine fever virus from infected pigs by direct contact and aerosol routes

Abstract

In 2014, African swine fever virus (ASFV) was introduced into the Baltic states and Poland. Since then, the disease has continued to spread within these regions, and recently, cases were reported in the Czech Republic and Romania. Currently, there is an increasing risk of ASFV introduction into Western Europe. Hence, there is an urgent need to assess current contingency plans. For this purpose, knowledge of modes-of-transmission and clinical outcome in pigs infected with new European ASFV strains is needed. In the present study, two experiments were conducted in pigs using an isolate of ASFV from Poland (designated here POL/2015/Podlaskie/Lindholm). In both studies, pigs were inoculated intranasally with the virus and contact pigs were exposed to the experimentally infected pigs, either directly (contact within and between pens) or by air. Pigs exposed to the virus by intranasal inoculation, by direct contact to infected animals and by aerosol developed acute disease characterized by viremia, fever and depression. Infectious virus was first detected in blood obtained from the inoculated pigs and then sequentially among the within-pen, between-pen and air-contact pigs. ASFV DNA and occasionally infectious virus was found in nasal-, oral-, and rectal swabs obtained from the pigs, and ASFV DNA was detected in air samples. No anti-ASFV antibodies were detected in sera. In conclusion, the study shows that the currently circulating strain of ASFV can be efficiently transmitted via direct contact and by aerosols. Also, the results provide quantitative transmission parameters and knowledge of infection stages in pigs infected with this ASFV.

Infection of pigs with African swine fever virus via ingestion of stable flies (Stomoxys calcitrans )

“…For group 2, the flies were homogenized in Eagle's minimum essential medium supplemented with streptomycin (Sigma‐Aldrich), neomycin (Sigma‐Aldrich) and 5% foetal calf serum (FCS; 200 μl per fly) using a 3‐mm stainless steel bead (Dejay Distribution Ltd., Launceston, UK) in a TissueLyser II (Qiagen, Hilden, Germany) for 1 min at 25 Hz. Note, blood‐fed homogenized flies from an earlier experiment were shown to contain ASFV DNA by PCR performed as described previously (Olesen et al., ); the C q ‐values ranged from 23.5 to 25.5 per fly, indicating that the feeding procedure was effective.…”

“…On arrival, the pigs were divided into groups of four animals within three separated BSL3 containment stables. Following acclimatization, pigs 1–4 in group 1 were inoculated orally with EDTA‐stabilized blood spiked with blood obtained from pigs infected with an ASFV isolate from Poland (Olesen et al., ). The pigs were restrained but not sedated and swallowed the inoculum.…”

“…The flies used for inoculation of groups 2 and 3 had been wild‐caught in the vicinity of a cattle herd using insect nets. Just prior to their euthanasia (by freezing), the flies had been fed for 1 hr on ASFV‐spiked EDTA‐stabilized blood that had a titre of 5.8 log 10 TCID 50 /ml, which is realistic for an infected pig (Olesen et al., ) using a heated membrane feeder equipped with a parafilm membrane. EDTA‐stabilized blood was used to prevent coagulation during the spiking and feeding procedures.…”

“…Table shows the dose of virus used for each group of pigs. The titre of the blood (group 1) was obtained from back titration in porcine pulmonary alveolar macrophages (PPAM) at the time of inoculation (Olesen et al., ). Virus titration directly from flies has been found previously to be difficult in our laboratory.…”

“…During the study, clinical scores, including rectal temperatures, were recorded from pigs on each day, and a total clinical score (CS) was calculated. The presence of clinical signs was defined as a CS >3 (Olesen et al., ). EDTA‐stabilized blood and unstabilized blood were collected prior to inoculation at 0 days, postinoculation (dpi), at 2, 4, 6, 8, 10, 12, 14 and 16 dpi and at euthanasia.…”