Indirect transmission of highly pathogenic avian influenza in chickens

Highly Pathogenic Avian Influenza (HPAI), also known bird flu, is a serious infectious disease of chickens causing high mortality in flocks and economic damage for farmers. The control strategy to control an outbreak of HPAI in the Netherlands will include culling of infected flocks and depopulation of neighbouring flocks. Because of the high costs and the killing of large numbers of, often healthy, poultry, this strategy needs improvement. Better understanding of the spread of virus such as transmission routes of the virus between farms is a first step in this direction. The research described in this thesis provides a contribution. One of the hypothesised routes of virus transmission in an epidemic is by air and dust particles. It was however unknown whether and if so to which extent this occurs. Experiments were carried out to investigate the airborne transmission of a HPAI virus strain. The results of this study showed that airborne transmission between chickens over short distances up to 1.1m was possible, but the risk of transmission was relatively low. In another trial, airborne transmission between hens housed in different stables was studied. Also in this set up, transmission occurred, but the risk of transmission even lower. These studies showed that transmission via air and dust particles could indeed be one of the transmission routes for virus. Determining the origin of a virus infection in the field is extremely difficult, as the number and variation of contacts between farms is large. Molecular epidemiology is a tool that might help to elucidate which flock infected which other flock. To investigate what the variation in genetic make up is between isolates collected from infected chickens, virus isolates from samples from the experiments were genetically analysed with next generation sequencing in a so-called phylogenetic study. The aim was to determine whether this technique could help to follow virus transmission between chickens. It was however not possible to reconstruct the transmission route, because the genetic variation between newly arising viruses was too large, not only between chickens but even within one particular chicken. Instead of preventive culling, vaccination is considered to be an alternative measure to control HPAI outbreaks. Studies from the field show, however, that even in vaccinated flocks, outbreaks may occur, possibly because chickens received a dose that is too low to be efficacious. This hypothesis was also investigated. Despite the finding that virus was detected in the trachea of vaccinated and infected birds, none of unvaccinated contact chickens became infected. This study suggested that, under experimental conditions, also lower vaccine doses than recommended provide sufficient immunity to prevent further spread of the virus The studies described in this thesis show that airborne transmission is a causal pathway of HPAI virus transmission between poultry flocks, and that dust may act as a mechanical vector of the virus. Finally, phylogenetic studies to reconstruct the indirect transmission routes during an outbreak are hampered by the large genetic variation of the virus that arises in (a flock of) chickens