Uptake and Survival of African Swine Fever Virus in Mealworm (Tenebrio molitor) and Black Soldier Fly (Hermetia illucens) Larvae

Abstract

Insect production offers a sustainable source of nutrients for livestock. This comes with a risk for transmission of pathogens from the insects into the livestock sector, including viruses causing serious diseases, such as African swine fever virus (ASFV), classical swine fever virus and foot-and-mouth disease virus. ASFV is known to survive for a long time within animal meat and byproducts. Therefore, we conducted experimental exposure studies of insects to ASFV using larvae of two key insect species produced for food and feed, the mealworm; Tenebrio molitor, and the black soldier fly, Hermetia illucens. The larvae were exposed to ASFV POL/2015/Podlaskie, via oral uptake of serum or spleen material from ASFV-infected pigs. Using qPCR, the amounts of viral DNA present immediately after exposure varied from ~10^4.7 to 10^7.2 genome copies per insect. ASFV DNA was detectable in the larvae of H. illucens for up to 3 days post exposure and in T. molitor larvae for up to 9 days post exposure. To assess the presence of infectious virus within the larvae and with this, the risk of virus transmission via oral consumption, pigs were fed cakes containing larvae exposed to ASFV. Pigs that consumed 50 T. molitor or 50 H. illucens virus-exposed larvae did not become infected with ASFV. Thus, it appears, that in our experimental setting, the risk of ASFV transmission via consumption of unprocessed insect larvae, used as feed, is low.

Keywords: African swine fever virus; black soldier fly; feed safety; insect rearing; mealworm; virus survival; virus transmission.

Figure 1

The presence of ASFV DNA in the T. molitor larvae, in study T1 at 0 days post exposure (dpe) and subsequently at 1, 2, 3, 4, 6 and 9 dpe (A) and in study T2 at 0, 1, 2, 3, 4 and 7 dpe (B), was quantified by qPCR and values converted to log₁₀ genome copy numbers/larvae homogenate (mL) using a standard curve. Levels below 10^4.3 ASFV genomes/mL were below the detection limit (indicated by dashed line) of the assay.

Figure 2

The presence of ASFV DNA and infectious virus in sampled serum controls. The dashed line indicates the detection limit for the qPCR assay (log₁₀ 3.6 copies/mL). The solid line indicates the detection limit for the virus isolation assay (log₁₀ 2 TCID₅₀/mL).

Figure 3

The presence of ASFV DNA in the rinsed H. illucens larvae, wash fluids and spiked feed samples in study H1 (A) after 2 h and 24 h exposure and at 1, 2, 3 and 5 dpe and in study H2 (B) after 5 h and 24 h exposure and at 1, 2, 3 and 6 dpe was quantified by qPCR and values converted to log₁₀ genome copy numbers/sample (mL) by reference to a standard curve. Levels below 10^4.3 ASFV genomes/mL were below the detection limit (indicated by dashed line) of the assay. BSF = black soldier fly.

Figure 4

The presence of ASFV DNA and infectious virus in sampled spleen controls. The dashed line indicates the detection limit for the qPCR assay (log₁₀ 3.6 copies/mL). The solid line indicates the detection limit for the virus isolation assay (log₁₀ 2 TCID₅₀/mL).