My world (well, the poultry part) got turned upside down a few weeks ago when a chicken that developed a swelling on his throat tested “likely positive” for Marek’s disease. I am the kind of person who doesn’t do well with uncertainty, so I dove head-first into researching this dreaded poultry disease. And what I discovered definitely made me feel better. Curiosity might have killed the cat, but information definitely brought her back!
Mareks’ is a herpes virus that affects chickens. It’s reportedly been seen in game birds, turkeys, ducks and geese but it’s pretty much accepted as a disease of chickens. It is NOT considered a risk to humans. Birds can carry it and never show signs of illness, but when they do, the virus can show itself in several different ways. From what I’ve gleaned, it causes the development of tumors which in turn cause the symptoms we see, the most common being paralysis. In our case, one of the birds developed a bit of a limp that we attributed to an injury, but it never progressed. The primary symptom was the growth on his throat.
The herpes virus is spread through dander, which can be carried on the wind, on clothing and shoes, by wild birds and insects… it can go EVERYWHERE. The birds we had were in a pen by themselves and never in direct contact with any of our other birds, but the pathologist of the lab we sent the one bird to said that the disease spreads so readily that we should consider all our birds exposed.
Since one of the major protocols when Marek’s is diagnosed seems to be to not let any chickens leave the property (ie don’t sell them) this of course put me in a panic. I asked the pathologist about our turkeys and geese and she reported that although Marek’s has been reported in these birds, she has never seen any positive tests at the lab for either, and doesn’t consider geese especially to be at risk.
BIG sigh of relief there. We DO have a few chickens but they aren’t our major poultry.
BUT… I learned some rather surprising facts about this disease that made me REALLY unclench.
- Marek’s is INCREDIBLY common in back yard flocks (likely because it spreads so easily through vectors such as wild birds and wind.) In fact, it’s so common that several sources I ran across say if you have chickens that aren’t kept in a tightly controlled facility (think commercial production) then your flock likely either has it or has been exposed to it.
- Marek’s cannot be spread through the eggs; if eggs from infected flocks are hatched where the emerging chicks have no contact with infected birds, the chicks will not have the virus.
- Birds carrying the virus can be perfectly healthy looking and acting, but are still able to spread the virus. So, you can do everything right; isolate incoming birds in quarantine for a period of time and they never show ANY signs of illness, but they could be silent carriers that spread the disease.
- There are vaccines, but they don’t actually STOP the disease, just the development of symptoms. Vaccinated birds can still pass on the virus and will likely test positive for Marek’s as well. Vaccination is a hot-button topic as some believe it is actually causing the virus to evolve more virulent strains as this National Geographic article explains.
- Marek’s is usually diagnosed via necropsy (like what we did… sent in a deceased chicken to the lab for testing) but there are a few tests available for determining if live birds carry the virus. This thread on backyardchickens.com describes them (though the post was from 2017 and I’m sure prices are much higher.)
- Eggs and meat from birds carrying Marek’s are safe to eat as the virus does not infect humans.
Here are a few of the resources I used:
Marek’s Disease in Chickens: Vaccination, Signs, & Symptoms (grubblyfarms.com)
The Great Big Giant Marek’s Disease FAQ | BackYard Chickens – Learn How to Raise Chickens