New variant bird flu virus in Renswoude

Published on
November 16, 2023

The bird flu virus that struck the laying farm in Renswoude (The Netherlands) is a new variant of the bird flu virus. The virus is related to the highly pathogenic H5N1 virus that has caused many outbreaks in poultry since 2021, but contains a new piece of genome - the so-called PB1 segment - derived from a low-pathogenic virus. This is shown by genetic analysis of the virus by Wageningen Bioveterinary Research (WBVR, part of Wageningen University & Research).

On 11 November, an outbreak of highly pathogenic avian influenza was detected in Renswoude. This was the first outbreak since July this year. Since then two other outbreaks occurred, one in poultry at a care farm in Middelie on 14 November, and the other at a petting farm in Zaandijk on 15 November.

New variant of the virus

WBVR investigated the virus detected in Renswoude and showed that it is again a highly pathogenic H5N1 virus. Further research into the genetic composition of the virus shows that this is a new variant. The virus acquired a new PB1 segment by reassortment with a low pathogenic virus. “The virus in Renswoude has a new genetic composition, but bears a strong resemblance to the virus that has caused many outbreaks since 2021,” said WBVR researcher Nancy Beerens, head of the National Reference Laboratory for Avian Influenza.

Gull virus in summer

This summer, the HPAI H5N1 genotype BB caused high mortality among black-headed gulls and other gull species. This virus variant contains three pieces of genome of a H13 gull virus. The gull variant caused the outbreak at the laying farm in Biddinghuizen (24 July), but did not cause any outbreaks in poultry thereafter despite the fact that the virus circulated heavily among gulls. “Our analysis shows that the virus in Renswoude does not contain any of the three pieces of gull virus genome,” Beerens said.

Infections among wild ducks and geese

An increase in mortality of ducks and geese has been seen since November. Genetic analyses by WBVR show that these dead wild birds are also infected with the new HPAI H5N1 virus variant. A list of sites of infected dead wild birds is displayed on the WBVR website.


The compulsory caging law has been in force nationwide again since 14 November. The fact that the virus is genetically similar to the one that caused many outbreaks in 2021-2023, and found in wild ducks and geese is a concern. It is currently unknown how pathogenic or infectious the new virus variant is to poultry. For proper prevention and control of the avian flu virus, biosecurity measures on poultry farms remain key.

Update 24-11-2023: type Middelie and Zaandijk

Genetic analysis of the bird flu viruses in Middelie and Zaandijk shows that these are the old variant of the virus. The genetic composition is similar to the virus that has caused many outbreaks since 2021. This old variant was also demonstrated in a greylag goose that was found dead. This indicates that there are currently two variants of the HPAI H5N1 bird flu virus circulating among ducklings.