The Netherlands suffered from five outbreaks of highly pathogenic avian influenza (HPAI) serotype H5N8. This strain of the disease was originally identified in China as part of a monitoring programme, and spread rapidly in South Korea before being found in the Netherlands.

The Dutch outbreaks were discovered on five farms in the latter half of November 2014. The farms included three egg farms, one pullet facility, and one meat duck farm.

Farmers discovered the outbreaks through increasing mortality and decreasing egg production, as well as some respiratory symptoms. Several of the farms were situated close to land on which abundant populations of wild waterfowl tended to aggregate.

When tracing the virus transmission backwards and forwards between outbreaks, researchers failed to find any links such as workers, transportation, deliveries or veterinarians in common between the Dutch outbreaks and outbreaks in Asia, Germany and the UK.

Scientists therefore wondered how the virus had managed to spread between the different outbreaks, and sequenced the genome of the virus on samples collected from all five Dutch farms to find out.

The scientists then compared the virus genome sequences and looked at the number of differences in the sequences to see how related the viruses were.

This analysis suggested that four of the five outbreaks were each introduced separately into the flocks, and were not transmitted from farm to farm.

However, the third and fourth outbreaks had closely related virus isolates. These farms were only 550 metres apart, suggesting that between-farm transmission was likely there, although the virus could possibly have been introduced separately from the same source.

As viruses isolated in Japan around the same time seemed very similar, the scientists said that it seemed likely the precursor virus for these outbreaks arose during the migratory bird breeding season, on breeding grounds in Siberia.

The low pathogenicity of the virus in wild ducks may have given the virus considerable potential to spread to commercial chickens, in which it had high pathogenicity.

Overall, the scientists stress the need for continued enhanced biosecurity measures in the Netherlands, to prevent avian flu spreading from wild birds to indoor commercial poultry.