Elucidating variations in the nucleotide sequence of Ebola virus associated with increasing pathogenicity

Abstract

Background: Ebolaviruses causes a severe and often fatal hemorrhagic fever in humans, with some species such as Ebola virus having case fatality rates approaching 90%. Currently the worst Ebola virus outbreak since the disease was discovered is occurring in West Africa. Although thought to be a zoonotic infection, a concern is that with increasing numbers of humans being infected, Ebola virus variants could be selected which are better adapted for human-to-human transmission.

Results: To investigate whether genetic changes in Ebola virus become established in response to adaptation in a different host, a guinea pig model of infection was used. In this experimental system, guinea pigs were infected with Ebola virus (EBOV), which initially did not cause disease. To simulate transmission to uninfected individuals, the virus was serially passaged five times in naive animals. As the virus was passaged, virulence increased and clinical effects were observed in the guinea pig. An RNAseq and consensus mapping approach was then used to evaluate potential nucleotide changes in the Ebola virus genome at each passage.

Conclusions: Upon passage in the guinea pig model, EBOV become more virulent, RNA editing and also coding changes in key proteins become established. The data suggest that the initial evolutionary trajectory of EBOV in a new host can lead to a gain in virulence. Given the circumstances of the sustained transmission of EBOV in the current outbreak in West Africa, increases in virulence may be associated with prolonged and uncontrolled epidemics of EBOV.

Figure 1

Passaging of virus in vivo . In order to give a reproducible model of infection EBOV was passaged five times in guinea pigs in a forced evolution model. There were 10 animals per group, where four animals were used for harvesting spleens for virus preparation and the remaining six animals used to measure clinical parameters.

Figure 2

Clinical data in the form of weight gain/loss and departure difference from EBOV-infected guinea pigs using virus that had been passaged from spleens harvested 7 days post infection: (A) weight and (B) temperature changes compared to day of challenge, compared to control uninfected animals. Data points represent mean values from 10 animals up to day 7, and six animals up to day 14, with error bars denoting standard error.

Figure 3

Kaplan-Meier survival plot of EBOV-infected guinea pigs when different virus concentrations were used for challenge. Survival studies lasted for 14 days.