Genetic and antigenic analysis of the influenza virus responsible for the 1992 Hong Kong equine influenza epizootic.

The research studies the genetic makeup of the virus that caused an outbreak of equine influenza in Hong Kong in 1992, which affected 37% of the local thoroughbred racehorses. The findings suggest that the virus shares a high similarity with a particular type of equine influenza virus commonly found in Europe and the likely source of the outbreak were horses imported from England or Ireland.

Analysis of the Hong Kong Influenza Outbreak

The researchers investigated a significant influenza outbreak among racehorses in Hong Kong during November and December 1992. This outbreak resulted in illness in 37% of the local horse population. Even though all affected horses had been vaccinated against equine-1 and equine-2 influenza viruses, they were not immunized against the specific virus type that was responsible for an equine influenza outbreak in northern China in 1989.

• It was observed that horses had not been vaccinated against the influenza A/equine/Jilin/89 virus, subtype H3N8, which led to the 1989 outbreak.
• Due to this occurrence, the researchers took on the task of investigating the source and nature of the virus responsible for the Hong Kong outbreak.

Genetic Analysis of the Virus

The researchers isolated the virus from an infected horse and conducted a genetic analysis.

• It was observed that all the viral gene segments shared similarities with those of equine-2 (H3N8) influenza viruses.
• Moreover, these segments did not show any relation to the genes of the equine/Jilin/89 virus.
• When they examined the nucleotide sequence of the viral hemagglutinin gene, it showed a remarkable homology – 99.4% similarity – with that of the influenza A/equine/Suffolk/89 (H3N8) virus, which has widely circulated in Europe.

Antigenic Reactivity and Epizootiological Information

On further analysis, researchers found that despite the genetic similarity, the viruses showed different antigenic reactivity when exposed to a panel of monoclonal antibodies. Preliminary epizootiological information and the concordance of the amino acid sequence between the hemagglutinins of the Hong Kong isolate and a concurrent equine-2 influenza virus isolate from the UK, suggested the source of the Hong Kong outbreak.

• Finding these points of difference throws light on how closely related virus strains can significantly differ in their antigenic reactivity.
• The infection patterns (epizootiology), aligned with the high sequence match between the Hong Kong isolate and the UK’s equine-2 strain, pointed towards the possible source of this outbreak.
• The research concluded that the probable source of the Hong Kong equine influenza outbreak was horses that had recently been imported from England or Ireland.