Vaccines that typically manage disease symptoms but fail to prevent infection and spread are more effective than first thought in controlling the severity of a viral disease in chickens, according to recent research conducted at The Roslin Institute in the U.K.

So-called "leaky vaccines" reduce the likelihood of chickens developing the painful symptoms of Marek’s disease, a study showed. This benefit extends to unvaccinated chickens in the same flock as vaccinated birds, the researchers found.

Their findings offer valuable insight into Marek’s disease virus, which can lead chickens to develop tumors in various parts of the body, eye cancer and wing and leg paralysis, eventually leading to death, Roslin said.

Marek’s disease has implications for animal welfare and the food production industry, as it can lead to a reduction in egg laying and an increase in meat deemed unfit for human consumption.

The virus spreads through birds inhaling a mix of their feed, bedding material, bird droppings, feathers and dead skin, according to the announcement. It is estimated to cost the poultry industry $1 billion a year.

Leaky vaccines are the most commonly used method of controlling Marek’s disease, but until now, little was known about how these vaccines affect overall populations, especially in unvaccinated chickens, Roslin said.

Flock impact

The team at Roslin, in collaboration with researchers from the U.S. Department of Agriculture’s Avian Disease & Oncology Laboratory (ADOL), tested the impact of vaccination on Marek’s disease transmission.

One group of chickens received a leaky vaccine that contained a related live virus originating from turkeys that causes an immune response but no symptoms. A second group received a sham vaccine that contained no biological material. Both groups of birds were then infected with Marek’s disease virus.

Groups of these infected birds were placed with sets of unvaccinated chickens, and more than 97% of birds became infected, Roslin reported.

However, unvaccinated chickens that had contact with vaccinated birds were less likely to develop full-blown Marek’s disease, and there were fewer deaths, the researchers said. This was found to be because vaccinated birds transmitted fewer copies of the Marek’s disease virus.

"In our study, we found that leaky vaccines can provide benefit in terms of reducing the presence and severity of symptoms and mortality caused by Marek’s disease, even for unvaccinated chickens. We need further research to understand how this effect changes as the virus mutates and in other strains of chickens," said lead scientist Dr. Richard Bailey, a research fellow at  Roslin.

The study was published in the journal PLOS Biology.

Roslin professor Andrea Doeschl-Wilson, personal chair in animal disease genetics and modeling, added, "Our findings suggest that even leaky vaccines can play a key role in reducing disease transmission. However, this is only one component in tackling disease in farmed animals; others include improved animal husbandry and breeding for disease resistance."