In general, poultry diseases are highly contagious because they do not require direct contact between birds, or even between locations, to spread. With Marek’s disease, a tumor-forming virus, the disease can spread ﬁve miles in the air without a carrier. That radius increases once you account for the activities of native backyard birds, or your own shoes if you run to Tractor Supply after you clean the coop. It’s no surprise how widespread this disease is - especially when many chickens never show a symptom.
A chicken that is infected will be infected for life, and even if healthy, it will carry and transmit Marek’s to other chickens. There are many diﬀerent strains of Marek’s, and so there is a huge range of symptoms. These include chronic weight loss, “lazy” or “antisocial” behavior, strange wing carriage, sour crop, ﬂaccid crop, limping, respiratory disease, severe curling of the neck or complete paralysis. The unusual ones are changes in iris color, blindness, and strange feather growth. You may see just one symptom, many symptoms, or none at all. The death rate of Marek’s depends on the strain, but can range from 10-80% of the ﬂock.
Birds between six and nine months will be the most likely to become symptomatic and succumb, but birds over one year old handle the disease much better.
Marek’s disease can be tentatively diagnosed by a veterinarian trained in poultry medicine, however, to deﬁnitively diagnose Marek’s you need a postmortem exam. Therefore, if you or your veterinarian suspects Marek’s disease, and you desire a concrete answer, the best thing you can do is contact your veterinarian ASAP if there is a death so that the body can be tested. While waiting for testing, never freeze the body, but DO keep it refrigerated. There is no cure for Marek’s, so unfortunately it is something that a ﬂock must live with. Because of this, it is not advisable to welcome new chickens into a ﬂock that has dealt with Marek’s disease, because the new chickens will be burdened with this disease. On the ﬂip side, because the disease is so good at hiding, bringing new chickens into a ﬂock may also bring Marek’s disease into your ﬂock.