Marek’s Disease is a Herpes virus spread by infected feather follicle dander. Once in the body the virus initially circulates in the blood stream and within a week, it infects white blood cells and kills B Lymphocytes, which produce antibodies that are essential for a healthy immune system.
Antibodies attach to pathogens and stimulate other immune cells to recognize it as a foreign invader and spark a complicated process of destroying it. As the B Lymphocytes are destroyed by the Marek’s virus the bird’s immune system becomes compromised making it more vulnerable to other diseases such as respiratory diseases and coccidiosis.
Marek’s also infects the T cells, which are involved in controlling the bird’s response to infection and its effects on the immune system. T cells are not killed by Marek’s, but are changed in a way that will eventually kill the bird. The infected T cells stay dormant for about 10 weeks, then start to multiply forming tumours in various organs including the liver, kidney and spleen. Birds slowly lose weight but outwardly appear normal, until at some stage they become incapacitated.
The disease manifests in various ways:
- Young birds (10-12 weeks old) often present with neurological issues: nerve cells are affected resulting in nerve failure and paralysis of the leg, wing and neck. It typically affects the sciatic nerve causing the classic one legged paralysis, which makes the bird appear to be doing the splits.
- The second phase, seen as birds mature, is the development of tumours in the heart, ovaries, testes, muscles and lungs.
- Birds can have ocular abnormalities: greying of the iris; misshapen iris; blindness; eye lesions; and tumours in the feather follicles.
Between the fall of 2019 and the spring of 2020 I had six casualties die from Marek’s (3 suspected and 3 diagnosed by professional necropsy). Some of them presented with wing or leg paralysis, while others had internal tumours. I’m happy to say I haven’t had any symptomatic birds in almost a year and my flock has been healthy.
Some pathology labs provide photos with their necropsy reports – unfortunately not the one I’ve dealt with. I’m always curious to decipher what the terms in those reports actually mean so I was happy when I stumbled across Becca’s post of her DIY necropsy, complete with good photos and some explanation of what she found. I’m presenting them here as a learning experience for folks wanting to do their own necropsies.
Necropsy: Bantam Belgian D’Anvers Pullet
“I have opened up every one of my birds that I’ve lost, except for three which I took to the state lab for a professional necropsy. I experienced a number of losses and wanted to know why my birds were dying. The results came back that all three died of Marek’s Disease. Since then, I have done tons of research, and with the help of Dr. Reddy Bommineni from the state lab, have learned to identify what I am seeing with my own necropsies.
Uncommonly, almost all of my chickens that died of Marek’s had both internal tumors and leg paralysis. Only a very few didn’t have paralysis.
This is the case of a bird with the visceral form of Marek’s and a fairly extreme example at that. The classic form (i.e. leg paralysis) is caused by cells invading the nerves to the legs and may not have any tumors in the organs.
I thought it would be beneficial for those of you who do (or plan to do) your own DIY necropsies to share photos of what you might see in a bird who had Marek’s Disease.”
As soon as I opened her up, I knew it was Marek’s just by seeing her liver, which was riddled with tumors.
Intestines with tumors:
Gizzard with tumors:
Gizzard and intestine, both with tumors:
Cross-section of one of the tumours:
Normally this is when I would stop seeing tumors. All of my birds that died before this one had tumors on, or in, the intestines and liver. A few had them on/in the gizzard. Unfortunately that was not the case with this pullet. I continued to find tumors, some on/in her heart.
I also found tumors in her lungs:
“This was by far the worst case of Marek’s I have ever seen in my own birds. How she lived so long with all those tumors in her is beyond me.”
Many thanks to Becca Alley for sharing her notes and photos, used with permission. Featured Photo (top): Trendy Global Research.
For more info on doing your own DIY necropsy click here.
Most backyard chickens are exposed to Marek’s because it is carried and spread in feather dust. The vaccine stops the tumors form forming on the brainstem and prevents paralysis. There are still tumors to be seen in chickens with sudden unexplained death, but also chickens can live with these tumors for a long time. Marek’s is 100% fatal, especially when you see paralysis setting in. It’s important to know the difference between paralysis caused by Marek’s than vitamin deficiency or wryneck. I dose with B complex to see if they improve, but if they don’t or can’t sustain improvement, then its pretty certain that its Marek’s.
LikeLiked by 1 person
Sad, but very helpful.
LikeLiked by 1 person
I believe I have had several Marek’s tumor fatalities in my hens. I am a dog and cat veterinarian but am learning about chickens in hopes of helping mine and others. I have necropsied several of mine and have pics of my own of widespread tumors in the internal organs. Can I ask what lab you had the necropsies performed at and what testing (if any in addition to the necropsy) was done to verify Marek’s? I am in Hawaii so sending the entire bird for necropsy isn’t feasible but I would like to be able to send samples to verify. We have a large wild chicken population here so I believe Marek’s is pretty widespread. I have only seen the leg paralysis in one wild hen so far but I believe the abdominal tumor variety may be more common here. Excellent post, thank you.
LikeLiked by 1 person
I live in British Columbia (Canada) and am fortunate to have the services of our provincial Animal Health Centre that provides low cost, full necropsies (i.e. gross exam, blood work, lab tests, etc) on a variety of animals, including chickens. Marek’s and Avian Leukosis tumours can look similar to the naked eye, so they do histopathology to confirm the diagnosis.
If you type ‘necropsy’ into the search box on my home page you’ll find an article about what our lab provides, as well as a link to a list of labs that do necropsies in the the USA at the bottom of the post.
I’m glad to hear you’re widening your scope. Do you have a good Avian Veterinary text?