Why Do Some Chickens Resist Marek’s Disease?

Marek’s disease is so common you can assume your chickens have it, even if they don’t show any signs. But while some chickens resist Marek’s disease, others succumb. The question is: Why?

What is Marek’s Disease?

Marek’s disease is the result of infection by a herpes virus and can take on many forms. Signs range from none to temporary paralysis, to blindness in one eye or both, to sudden death.

This disease can cause slow-growing tumors that produce no outward signs. That is, until a tumor becomes so massive in an aging bird that the chicken can no longer survive.

The Marek’s virus can remain dormant in a chicken’s body, only to become active as a result of stress. Stress triggers include such common things as overcrowding, poor ventilation, or a worm overload. Even the natural process of maturing and beginning to lay is stress inducing.

The Marek’s virus works by switching off a chicken’s tumor-blocking genes. It also attacks the cells that produce antibodies, impairing the chicken’s immune system. Marek’s therefore often occurs in combination with other diseases, especially coccidiosis and E. coli respiratory infections.

The virus may be transmitted directly from bird to bird. It also spreads in contaminated litter, dust, down, and feather dander. And it spreads more easily in dry, dusty conditions.

Natural Immunity

Before commercial vaccines became widely available, chicken keepers induced natural immunity by raising young chickens with older ones. The young chickens thereby were exposed to any diseases the older ones had been exposed to, including Marek’s disease.

Inducing disease by this method has the same result as vaccination. In both cases chickens develop antibodies to fight future infections. But natural vaccination lacks the control offered by vaccination and therefore can backfire.

Chicks may die, especially if they encounter massive amounts of virus before they become completely immunized. Even among those that survive, some may not thrive.

Turkey Immunity

An alternative method that provides at least partial immunity is to keep a few turkeys with the chickens. Turkeys carry a related, though harmless, virus that prevents the Marek’s virus from causing tumors. So chickens kept with turkeys develop some measure of immunity to Marek’s disease.

On the other hand, keeping turkeys together with chickens can create a problem for the turkeys. The chickens may transmit histomoniasis (blackhead), which doesn’t affect chickens but can be devastating to turkeys.

You can, however, expose chickens to the turkey virus without potentially endangering the turkeys. Keep the turkeys in a separate yard. When baby chicks are in a brooder, mix a little soiled turkey bedding into the brooder bedding. The chicks will acquire the turkey virus without exposing the turkeys to chickens.

The Trouble with Vaccines

Vaccine used against Marek’s disease comes from the turkey virus, usually combined with a weakened form of the Marek’s virus. But vaccination isn’t effective for chickens already exposed to Marek’s disease. Chicks should therefore be vaccinated as soon as possible after hatch, and then raised away from other chickens while their immunity develops.

Many hatcheries offer vaccination as an option. However, it is not 100% fool-proof. Some 5% of vaccinated chicks will succumb if they become infected with the Marek’s virus.

Among the other 95%, vaccination still does not prevent infection. It does prevent the development of tumors and paralysis in vaccinated birds that become infected. But it does not keep the virus from spreading. As a result, the virus keeps mutating and becoming progressively more virulent.

This increased virulence creates two problems. One problem is the constant need for ever stronger vaccines. The other problem is the increasingly greater risk that infected unvaccinated chickens will die.

Natural Resistance

Sebrights and Silkies are particularly susceptible to Marek’s disease. And so are some strains of Polish, among other fancy breeds.

On the other hand, some breeds, strains, and individual chickens are resistant. Among resistant breeds are Fayoumis, with their ultra-strong constitution. So why do some chickens resist Marek’s disease, while others suffer and die?

Researchers at The Pirbright Institute in Surrey, England, have discovered that the T cells of resistant chickens, compared to non-resistant chickens, produce more protective proteins. T cells are so called because they come from the thymus.

Cytokines are one such group of protective proteins. Cytokines regulate immune system cells, and the increased output in resistant chickens protects them from Marek’s disease. Their T cells also produce more proteins (granzyme B and perforin) that assist in killing infected cells.

So, going back to the method of natural immunity, you could eventually develop a strain of chickens that is genetically resistant to Marek’s disease. All you have to do is remove from your breeder flock any birds that fail to thrive, so they won’t reproduce more weaklings. And keep as breeders only the robust survivors.