Genetic Analyses of an H3N8 Influenza Virus Isolate, Causative Strain of the Outbreak of Equine Influenza at the Kanazawa Racecourse in Japan in 2007.

This research paper presents the findings of a study on an outbreak of H3N8 influenza virus among vaccinated racehorses in Japan in 2007. It investigates the vaccine’s effectiveness and analyzes how the virus isolated during the outbreak compares genetically to the strains included in the vaccine.

Study Context and Purpose

• In 2007, an equine influenza (horse flu) outbreak happened among racehorses at Kanazawa Racecourse in Japan. What puzzled researchers was that all horses were previously vaccinated with a commercial Japanese equine influenza vaccine.
• The main goal of the study was to understand the reason behind the occurrence of the outbreak among vaccinated horses, and to analyze the isolated virus strain in comparison with the strains included in the vaccine.

Vaccine Used

• The vaccine used included two inactivated H3N8 influenza virus strains and one H7N7 strain. These strains are commonly found in horses and are included in flu vaccines to help horses develop immunity against them.
• The H3N8 strains in the vaccine were A/equine/La Plata/93 from the American lineage, and A/equine/Avesta/93 from the European lineage. The H7N7 strain was A/equine/Newmarket/1/77.

Flu Outbreak and Virus Isolation

• The symptoms observed in the infected horses included fever and nasal discharge, with or without coughing. Around half of the infected horses demonstrated no symptoms, indicating a subclinical infection.
• The researchers managed to isolate the influenza virus from one of the nasal swab samples gathered from the diseased horses. The discovered strain was A/equine/Kanazawa/1/2007 (H3N8), also known as Kanazawa/07.

Phylogenetic Analysis and Findings

• Phylogenetic analysis, a method used to study the evolutionary relationships between different species or strains, showed that the Kanazawa/07 was part of the American sublineage Florida.
• Detailed genetic analysis also revealed four amino acid substitutions in the antigenic sites B and E in the HA1 subunit protein of Kanazawa/07 compared to the La Plata/93 strain included in the vaccine.
• These differences may explain why the vaccine wasn’t completely effective in preventing the outbreak and who the identified strain was able to infect the vaccinated horses.

Effectiveness of Vaccination

• Despite the outbreak among vaccinated horses, the researchers noticed differences in titer levels between the isolated strain and the vaccine strains in post-infection samples from recovering horses.
• This suggests that the commercial vaccination, even though it didn’t prevent the infection, reduced the severity of the disease and the manifestation of clinical symptoms among the infected horses.
• This finding highlights the importance of vaccination in controlling disease outbreaks and minimizing their impacts, even when the vaccine doesn’t provide complete protection from infection.