Pathogenicity of an African swine fever virus strain isolated in Vietnam and alternative diagnostic specimens for early detection of viral infection

Abstract

Background: African swine fever (ASF), caused by the ASF virus (ASFV), was first reported in Vietnam in 2019 and spread rapidly thereafter. Better insights into ASFV characteristics and early detection by surveillance could help control its spread. However, the pathogenicity and methods for early detection of ASFV isolates from Vietnam have not been established. Therefore, we investigated the pathogenicity of ASFV and explored alternative sampling methods for early detection.

Results: Ten pigs were intramuscularly inoculated with an ASFV strain from Vietnam (titer, 10^3.5 HAD₅₀/mL), and their temperature, clinical signs, and virus excretion patterns were recorded. In addition, herd and environmental samples were collected daily. The pigs died 5-8 days-post-inoculation (dpi), and the incubation period was 3.7 ± 0.5 dpi. ASFV genome was first detected in the blood (2.2 ± 0.8) and then in rectal (3.1 ± 0.7), nasal (3.2 ± 0.4), and oral (3.6 ± 0.7 dpi) swab samples. ASFV was detected in oral fluid samples collected using a chewed rope from 3 dpi. The liver showed the highest viral loads, and ear tissue also exhibited high viral loads among 11 tissues obtained from dead pigs. Overall, ASFV from Vietnam was classified as peracute to acute form. The rope-based oral fluid collection method could be useful for early ASFV detection and allows successful ASF surveillance in large pig farms. Furthermore, ear tissue samples might be a simple alternative specimen for diagnosing ASF infection in dead pigs.

Conclusions: Our data provide valuable insights into the characteristics of a typical ASFV strain isolated in Vietnam and suggest an alternative, non-invasive specimen collection strategy for early detection.

Keywords: African swine fever; Alternative diagnostic specimen; Clinical signs; Incubation period; Pathogenicity; Vietnam; Virus excretion pattern.

Fig. 1

Pig survival rates in the ASFV infection (red line) and negative control (blue line) groups

Fig. 2

Individual rectal temperatures in pigs from the ASFV infection and negative control groups. Red and blue circles and horizontal lines indicate individual and mean temperatures of the ASFV and negative control groups, respectively. Red and blue short lines represent the average temperature of ASFV infection and negative control group, respectively, at each time point. * P < 0.05

Fig. 3

Clinical scores of pigs from the ASFV (red lines) and negative control (blue lines) groups. The scores were calculated based on the protocol published by Galindo-Cardiel et al. [18] with minor modifications (Table 2). * P < 0.05

Fig. 4

Patterns of ASFV excretion observed in experimentally infected pigs. Mean viral copy numbers per 1 µL in (a) individual ASFV-infected pig samples (blood, oral swab, nasal swab, and rectal swab), (b) ASFV-infected herd samples (oral fluids collected using the rope-based method and pooled fecal samples), and (c) environmental samples of the ASFV-infected herd (aerosol, feed, and water)