High Doses of Inactivated African Swine Fever Virus Are Safe, but Do Not Confer Protection against a Virulent Challenge

Abstract

African swine fever (ASF) is currently the major concern of the global swine industry, as a consequence of which a reconsideration of the containment and prevention measures taken to date is urgently required. A great interest in developing an effective and safe vaccine against ASF virus (ASFV) infection has, therefore, recently appeared. The objective of the present study is to test an inactivated ASFV preparation under a vaccination strategy that has not previously been tested in order to improve its protective effect. The following have been considered: (i) virus inactivation by using a low binary ethyleneimine (BEI) concentration at a low temperature, (ii) the use of new and strong adjuvants; (iii) the use of very high doses (6 × 10⁹ haemadsorption in 50% of infected cultures (HAD₅₀)), and (iv) simultaneous double inoculation by two different routes of administration: intradermal and intramuscular. Five groups of pigs were, therefore, inoculated with BEI- Pol16/DP/OUT21 in different adjuvant formulations, twice with a 4-week interval. Six weeks later, all groups were intramuscularly challenged with 10 HAD₅₀ of the virulent Pol16/DP/OUT21 ASFV isolate. All the animals had clinical signs and pathological findings consistent with ASF. This lack of effectiveness supports the claim that an inactivated virus strategy may not be a viable vaccine option with which to fight ASF.

Keywords: African swine fever; domestic pigs; inactivated virus; vaccine trial.

Figure 1

Individual post-challenge clinical scores of domestic pigs from the different vaccinated groups: Group 1 (Pol16-inactivated, MF59), Group 2 (Pol16-inactivated, Silica oil), Group 3 (Pol16-inactivated, mGNE), Group 4 (Pol16-inactivated, Montanide ISA201), group 5 (Pol16-inactivated), and the control group.

Figure 2

Individual post-challenge temperatures of domestic pigs from the different vaccinated groups: Group 1 (Pol16-inactivated, MF59), Group 2 (Pol16-inactivated, Silica oil), Group 3 (Pol16-inactivated, mGNE), Group 4 (Pol16-inactivated, Montanide ISA201), Group 5 (Pol16-inactivated), and control group.

Figure 3

Individual post-challenge viremia results expressed in qPCR cycles of quantification values (C_q) of domestic pigs from the different vaccinated groups: Group 1 (Pol16-inactivated, MF59), Group 2 (Pol16-inactivated, Silica oil), Group 3 (Pol16-inactivated, mGNE), Group 4 (Pol16-inactivated, Montanide ISA201), Group 5 (Pol16-inactivated), and the control group.

Figure 4

Kaplan–Meier survival curve of vaccinated and control domestic pigs subsequently challenged intramuscularly with the virulent Pol16/DP/OUT21 ASFV. DPC: days post-challenge.

Figure 5

View of thoracic and abdominal cavities of a vaccinated pig (group 2, ID 3) subsequently challenged intramuscularly with the virulent Pol16/DP/OUT21 ASFV. Hepatomegaly, splenomegaly and ascites are evident.

Figure 6

Mean of the cycles of quantification values (Cq) obtained by qPCR from the spleen, kidney, liver, brain, bone marrow, peripheral lymph nodes (submandibular, retropharyngeal, prescapular, and inguinal), abdominal cavity organs (urinary bladder, renal lymph node, gastrohepatic lymph node, and mesenteric lymph node) and thoracic cavity organs (heart, lung, and mediastinal lymph node) of the different study groups.