The subjects of Marek’s disease and vaccination pop up often in the chicken world, especially when discussing shows and viral transmission. Marek’s is an extremely contagious viral disease that lowers the immune response and eventually can result in multiple tumors. Some variations affect particular parts of the bird’s body, while some variations affect the entire body. A few variations do not appear to affect birds with certain genetics very much, while others kill most, if not all, of the affected flock regardless of genetics. All of the different types of Marek’s spread via dander, saliva, and poop. Poultry extension offices from agricultural colleges across America agree that if you have outside chickens, you most likely have some form of Marek’s in your flock, since the virus easily spreads and can be carried by wild birds.
A vaccine for Marek’s does exist. Should you get that show chicken vaccinated? Will all of your unvaccinated chickens get Marek’s if you bring in vaccinated chickens? What if you want to breed for resistance to Marek’s? Will that keep your flocks safe? These types of questions can become important very quickly, especially for a flock of rare, expensive, susceptible, or beloved birds.
No simple answer works for every breeder, but we can narrow down the choices a bit. First, what type of chicken do you breed? How susceptible is that breed to Marek’s? Ask several long term breeders for their experience with the virus in their flocks. Some general trends: Leghorns and light breeds who lay lots of eggs tend to be more susceptible, as do Silkies and Belgian breeds (Watermaal, d’Anvers, d’Uccles). Meat breeds tend to be less susceptible. Certain lines of all breeds can be more or less susceptible. Another consideration is that different strains of Marek’s can also affect the outcome – a very virulent strain can kill less susceptible breeds very quickly, so weigh your risk. If you have a rare, susceptible breed, vaccination should be a strong consideration. If you breed an easily obtainable, more resistant breed, vaccination may not be worthwhile for your flock or your wallet. Just starting a flock? Resistance to Marek’s could play a part in your breed choice!
Now let’s move on to the vaccine itself. Marek’s is a live vaccine made from a turkey virus that doesn’t affect chickens or turkeys. Vaccinated birds and unvaccinated birds can be put together, but you need to be aware that although the virus used for the vaccination will not cause disease in unvaccinated birds, the vaccine is “leaky,” which means vaccinated birds can still get the chicken version of Marek’s from infected birds. The vaccine helps prevent the bird from developing symptoms and tumors, but a vaccinated bird who has gotten a chicken form of Marek’s can pass that along to an unvaccinated bird.
In the past, a particular study has often been quoted to discourage use of the vaccine by implying that the vaccinated birds may cause more mutations in the virus, resulting in vaccines that don’t work and a worse variant of Marek’s. Thankfully a more recent study can calm our fears about that worry. This study found that vaccinated birds actually help decrease both the amount and the severity of Marek’s in their unvaccinated coop mates. Vaccinated birds are protecting unvaccinated birds to some extent. We don’t know if vaccinated birds actually pass some of the harmless turkey virus on to other chickens, since there has not been a documented case of that. Or it could just decrease the viral load on the unvaccinated birds by keeping infection to minimum. But we do know that the leaky nature of the vaccine is not causing the mutations that earlier studies worried about.
Years ago, every bird was considered positive for “range paralysis,” the common term for Marek’s. There’s a saying that if your bird breathes, it has some form of Marek’s. Every bird at a show, vaccinated or unvaccinated, should be considered a carrier. But most carry mild forms that don’t cause much trouble. Marek’s can kill your flock at any age, young and old. It does need to be in a bird long enough to cause symptoms, so chicks typically don’t start dying until 6 weeks or so. Marek’s weakens the immune systems of birds, so you may not realize your birds are dying of Marek’s when they succumb to a respiratory illness – one that may not have killed them if their immune system was strong enough to fend it off. As a bird ages, it can lose some of its ability to fight off disease. Combine that with the effects of Marek’s, and you may have older birds dying earlier than they should because of Marek’s disease. Many of us deal with Marek’s on a daily basis without even realizing it.
Do we stop showing and build protective fences around our farms with fans to blow away unwanted contaminated dander? Absolutely not. Marek’s has been around as long as chickens have, and chickens are still going strong. Should we vaccinate? That is a personal choice, and should be made based on considerations like your breed, your situation, and your tolerance of risk. No one answer is right for everyone, and no guilt or shame should be attached to your choice. Marek’s disease vaccination will not endanger the birds of others, and vaccinated birds are protected from disease should an unvaccinated bird expose them.
Pathogen transmission from vaccinated hosts can cause dose-dependent reduction in virulence.
By Richard I. Bailey, Hans H. Cheng, Margo Chase-Topping, Jody K. Mays, Osvaldo Anacleto, John R. Dunn, Andrea Doeschl-Wilson, Published: March 5, 2020, https://doi.org/10.1371/journal.pbio.3000619