African horse sickness in The Gambia: circulation of a live-attenuated vaccine-derived strain.

The research study discovered that the African horse sickness virus, a disease mostly not causing any clinical illness in local horses, has been spreading in The Gambia mainly due to a strain derived from a live-attenuated vaccine.

Introduction

The topic of this research revolves around a virus known as African horse sickness virus serotype 9 (AHSV-9). This virus has been observed to circulate among equids (which includes horses, donkeys, and zebras) in West Africa. It has been noted, however, that the virus typically does not cause any noticeable disease in these local horse populations. The research investigates two theories: either the local breeds of horses are resistant to the disease or the AHSV-9 strains that are on the move are not capable of causing disease (avirulent).

Observations and Findings

• On investigating the situation in The Gambia, the study found that most horses and donkeys (96%) tested positive for African Horse Sickness (AHS). This finding was surprising given most of the animals hadn’t been vaccinated and hadn’t shown any previous signs of AHS.
• Younger horses, those less than 3 years old, were found to have neutralizing antibodies specific to AHSV-9, indicating they had been exposed to the virus.
• Eight of these young equids were also found to be positive for AHSV-9 via serotype-specific RT-PCR, a technique used to amplify and detect specific genetic materials. From two of these horses, live AHSV-9 was successfully isolated.

Conclusion: Vaccine-Derived Strain

The most significant finding from the study came from sequence analysis. The researchers discovered that the AHSV-9 strain present in The Gambia showed 100% identity to Seg-2 of the AHSV-9 reference strain. This discovery strongly indicates that the virus circulating in The Gambia likely originated from a live-attenuated AHSV-9 vaccine strain. Thus, rather than indicating disease resistance among local horse breeds, the widespread seropositivity appears to be due to the circulation of a less harmful, vaccine-derived strain of the virus.