6 June 2023

6 minute read

By: Gary Hartley

In global outbreaks of highly pathogenic avian influenza (HPAI) to date, vaccination has never been a tool actively considered by the global poultry industry, which has feared that vaccination would mask the spread of disease and subsequent trade issues. Things may be about to change.

The severity of recent outbreaks, rising egg prices and fears of a spillover into humans, as well as improvements in vaccine technology, are prompting a shift in mindset.

“In the old days, vaccination was not a solution. We only had low pathogenicity strains that sometimes mutated into highly pathogenic, and then if you killed the infected birds, you also eradicated the virus,” explained Professor Sjaak de Wit, DVM, PhD, of Royal GD (former Dutch Animal Health Service) and Utrecht University, who has been involved in recent vaccine trials in the Netherlands.

“In veterinary education, we were always used to making a list of problems connected to vaccination, such as their perceived low efficacy, the effects on trade and complexity of diagnostics, instead of thinking of a solution. Now, it’s very hard for some people to flick the switch from that to accepting that we don’t have a better option anymore.”

Modern vaccines offer improved performance

Several modern vector vaccines offer broader protection than the “classic” inactivated vaccines, de Wit said, meaning that the HPAI virus needs to mutate to a greater degree to make the vaccine ineffective. Also, technologies such as reverse genetics allow for quicker matching of inactivated vaccines to outbreak strains, leading to better protection.

Most new vaccines also only contain the H5 gene or protein, not a complete virus, which means diagnostic testing can easily show that flocks are free of the field strain. Serology is much simpler than in previous years, and on top of that PCR testing is much cheaper and more widely applicable, he noted.

More-targeted vaccines and better testing are vital, but ultimately, trust remains a major barrier preventing vaccination becoming normalized in the industry. Producers are concerned that infected birds may still be transported around the world in spite of vaccination programs, and of the possibility of trade bans.

“In this sense, it’s a good thing that a lot of countries have a headache with avian influenza. If only a few countries have a headache, that’s a few competitors less. But if a lot of countries in the world have a headache, then we all need a solution,” he said.

“This helps a lot of countries or companies to realize that ‘okay, if I am not going to trust the other, the other ones will not trust me either, because we are in the same situation.’ That will help to build up trust, but it will take time.”

Lab testing shows promise

In the Netherlands, an experimental animal trial in late 2022-early 2023 using a herpesvirus of turkey (HVT) vector vaccine with an insert of H5, a classical vaccine with inactivated antigen and a DNA vaccine in young layers showed positive results. Both HVT vaccines used in the study provided full protection against symptoms of disease, while also preventing the spread of infection.

Next, a small field trial using these products with or without a boost at the end of the rearing period will begin in the summer, with a view to vaccinating around 50 to 100 flocks later in the year to gather further information on diagnostics and surveillance.

Tests are also ongoing with ducks on French farms, using mRNA and subunit vaccines, due to the fact that HVT vaccines don’t work in ducks. Work in Hungary is looking at vaccinating geese, while in Italy, trials are being carried out with turkeys.

“At a recent meeting at the head office of the World Organization for Animal Health (former OIE), there was a general feeling that we really need vaccination now that HPAI H5 has become endemic in wild birds in five continents. Vaccination is really a valuable additional tool to help us to control HPAI. We cannot control nature, but at least we can have a more sustainable poultry production all over the world,” de Wit continued.

Demanding surveillance plans

Although vaccination against HPAI has been approved by the EU, the surveillance regime proposed may yet prove to be a stumbling block. Current directions demand weekly sampling plus an official veterinarian visit every month, producing a full report.

“You can do a lot of testing. Laboratories can adapt. But there are no countries which have a sufficient number of veterinarians to be able to do the number of visits to these flocks every month. We need a solution to that,” he said.

“Every farm visit is a potential break of biosecurity anyway, and we need to limit the number of extra farm visits as much as possible.”

A risk-based system, or an agreed subset of total flocks, may provide the answer to this problem — but a change in sampling approach, such as using environmental sampling rather than the birds themselves, could also help.

Research has highlighted the possibility of sampling for HPAI from boot covers, the dust in poultry houses or drinking water.[1]

“In the first few years, we will have to use the official scheme. But maybe we will get more experience and trust in environmental sampling so we can lower the cost but still have a reliable system. It is essential there is no discussion about that if products are to be trusted. Without trust there is no trade,” he said.

Overcoming trust issues

The issue, he said, is that while other countries would be likely to follow should another start vaccinating as standard, one nation still needs to take the risk of being the first. An EU-wide vaccination strategy could be the example for the major trade partners, such as the US, to replicate.

The risks of the present H5N1 virus circulating in Europe infecting humans is relatively low compared to some other HPAI H5 strains, but it remains a possibility which is hard for the industry to ignore, he continued.

While it is never possible to control viral mutation in nature, he noted, the broad coverage of modern vaccines is their advantage. For example, one of the HVT vaccines that has been tested in the Netherlands has an insert of an H5 strain from 2006 and is still working well against the present field strain.

“With all the technology that we have, and all the diagnostics, it will not suddenly happen that the vaccines don’t work anymore. We will see it coming; we will have time to adapt,” he added.

[1] Hood G, Roche X, Brioudes A, von Dobschuetz S, Fasina FO, Kalpravidh W, Makonnen Y, Lubroth J, Sims L. A literature review of the use of environmental sampling in the surveillance of avian influenza viruses. Transbound Emerg Dis. 2021;68(1):110-26.