Genomic changes of Lassa virus associated with mammalian host adaptation

Abstract

Background: Lassa virus (LASV) causes a severe haemorrhagic fever in humans, with estimates of 100,000 to 300,000 infections annually in endemic regions and accounting for around 5000 deaths. The natural reservoir is the Mastomys rat, but through zoonotic transmissions humans are accidental hosts. Regular outbreaks continue to exert pressures on public health systems, with its ability to cause nosocomial infections posing risks to healthcare workers. It is a concern that larger outbreaks and introduction of LASV to new territories will intensify, including risk of adaptation to new mammalian host reservoirs.

Results: To evaluate genetic changes in LASV during adaptation to a new host, a guinea pig model of infection was utilised. Initial infection with LASV stocks cultured from cell culture resulted in only mild or subclinical disease. To study the susceptibility in naïve animals, the virus was serially passaged which increased clinical signs during disease progression ultimately resulting in severe disease. An RNAseq and consensus mapping approach was undertaken to evaluate nucleotide changes in LASV genome from each animal at each passage.

Conclusions: During adaptation to guinea pigs, no significant new mutations occurred. Instead, a selection pressure on two genes of the L segment was observed resulting in their increased frequency in the genome population during passaging.

Keywords: Adaptation; Changes; Lassa virus; Mutation; Passage.

Fig. 1

Passaging of Lassa virus in vivo. Lassa virus was passaged through guinea pigs five times in order to increase virulence and increase lethality in a forced evolution model. There were 10 animals per group of which five were used at each passage to harvest LASV for the subsequent infection. The remaining five animals were observed for clinical manifestations of disease and to assess lethality of the virus

Fig. 2

Survival of guinea pigs after challenge with passaging of LASV. Kaplan-Meier survival plot showing animals that had reached humane clinical endpoint criteria with adaption of the virus to the guinea pig host

Fig. 3

Clinical details of guinea pigs post challenge with sequential passages of Lassa virus (LASV). Weight, temperature and clinical observations were recorded over the course of each study to determine the pathogenicity and virulence of subsequent passages of LASV. 10 guinea pigs were challenged in each study. Five guinea pigs were culled early in order to isolate passaged virus (indicated in red) and five continued until day 14 or 21, to assess clinical signs of disease and observe virological changes

Fig. 4

Variation frequencies in the L genome of LASV. LASV GA391 stock for initial inoculum (top); compared with guinea pig adapted stock after five passages (bottom)

Fig. 5

Differential expressed genes in LASV challenged guinea pigs using during passages two and five of the adaptation process