I’ve read a lot of posts about Marek’s disease on Facebook farm groups, but have never experienced it first-hand. I took a Small Flock Health Management workshop with two Avian Pathologists earlier this month and looked back on my notes for reference.

When I’m not blogging, I’m employed as a Health Promotion Educator in the field of HIV and Hepatitis c. When I read about how Marek’s is transmitted, replicated and works in the body I felt right at home. HIV and Marek’s are both viruses that affect the immune system and leave the host vulnerable to other infections.

HIV spreads through particular bodily fluids, while Marek’s is a Herpes virus spread by infected feather-follicle dander. The route of transmission is usually respiratory and the disease is highly contagious because it can survive at room temperature in feather dust for at least a year. It’s difficult to eradicate in the environment and is resistant to some disinfectants (quaternary ammonium and phenol), but is killed when frozen.

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 them as  foreign invaders and spark a complicated process of destroying them. 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.

Like HIV, 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 which 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.

The control of the disease is difficult and you need to have a good understanding of the basic principles of transmission in order to reduce infection.

Up to 50% of exposed birds will become infected and depending on the strain, death rates can be high. So what about the vaccine? The good news is that Marek’s is one of the few avian viral infections that you can vaccinate for. The bad news is that it’s problematic

I feel like I’ll be opening a can of worms discussing the Marek’s vaccines. I am not an anti-vaxxer, but I am cautious about the side-effects and inadvertent outcomes of mass vaccination. Like antibiotics, vaccines have saved millions through curtailing the spread of pathogens, but are not without legitimate controversy. I think that when looking at vaccines it’s important to view them on a case-by-case basis: How do they work? What are the rates of efficacy? What are the risks? Is it accessible?

Poultry Vaccines
Poultry Vaccines

Marek’s is an old virus and, like all viruses, has continued to evolve and develop various strains.

The different varieties are classified as ‘hot’ or ‘cold’ based on their virulence. Cold strains may infect, but not kill, birds, while the hottest strains have a 100% mortality rate within 10 days. Although hot strains have high losses, the upside is they flare up and die out quickly because they kill their host before the virus has time to spread.

Marek’s was a serious issue for commercial poultry producers by the 1960s due to its highly contagious transmission and the close proximity large numbers of chickens are housed in. A vaccine was developed and has made great strides in cutting the losses of large-scale producers.

The vaccine also produced an inadvertent result: vaccinated birds rarely get sick, but can still be infected and spread the virus to unvaccinated birds. Rather than stopping the virus from spreading, the vaccine has allowed it to spread faster and longer than it normally would. It’s the first time that this virus-boosting phenomenon, known as the imperfect vaccine hypothesis, has been seen.

The vaccine is considered ‘leaky’ or ‘imperfect’: the microbe is prevented from doing serious harm to its host, but doesn’t stop the disease from replicating and spreading to another host. A ‘perfect’ vaccine is one that creates lifelong immunity that blocks both infection and transmission.

The reason why the Marek’s vaccine works well for large-scale hatcheries and commercial growers is they vaccinate day-old chicks before they are exposed to the environment, have an all-in/all-out policy and strict biosecurity practices that most small flock growers can’t adhere to.

The other issue which prevents small flock keepers from vaccinating their birds is the problem of scale: the vaccine is sold in large quantities (i.e. 1000 doses), is stored in liquid nitrogen and once opened, must be used within one hour. That makes using it totally impractical for anyone but commercial operations.

If you’re like me, you started with a few birds, then got bitten by the chicken bug and added some more. Then maybe hatched some of your own chicks. It is this mixing of birds of different ages (some possibly vaccinated, others not) that makes disease prevention difficult. You might bring home vaccinated chicks, but expose them to unvaccinated birds.

Adapting to a new home and establishing a new pecking order is a particular stressful time for a flock. Stress is a trigger for infected birds to shed the virus into the environment. Since Marek’s is spread through dander it’s impossible to keep it from spreading.

So what do you do?

  • Buy chickens from breeders with no known history of Marek’s.
  • If possible, buy vaccinated birds.
  • Recognize that there are different vaccines and best one HVT (triple strain) is @95% effective, but vaccinated birds that are infected with the virus are still able to a) infect non-vaccinated birds and b) die.
  • If you want to add more birds, hatch them. The virus spreads between live birds, not from hen to egg.
  • Use recommended disinfectants and follow strict biosecurity protocols to try to prevent spread of the disease (including foot dips and cleaning vehicle tires and equipment that might carry pathogens).
  • Recognize symptoms of chicken illnesses: infected chickens are often vulnerable to other viral and bacterial diseases, respiratory issues and diarrhea because of their compromised immune systems.
  • Quarantine new and ill birds.
  • The only way to verify if your bird has been infected with Marek’s is through a post-mortem exam. When you experience a loss make the investment to have a necropsy done by a qualified vet. In British Columbia, necropsies are done at the Animal Health Centre in Abbotsford. Click here for complete details.

Update May 2020: When I first wrote this piece just over a year ago I had no first hand experience with Marek’s. A lot has changed since then: I have had three confirmed cases and another three suspected cases. I’ve seen different manifestations of Marek’s: wing and leg paralysis in young birds (9 -32 weeks) and tumours of the feather follicles and internal organs in adults. I was quite disheartened and struggled to pinpoint how and when Marek’s entered my flock.

I had a long conversation with Dr Bowes, the Avian Veterinarian/Pathologist at the Animal Health Centre who conducted the necropsies on my birds. When I asked her opinion of the timeline for the pathogen’s entrance into my flock she replied that it has always been there, that Marek’s is widespread and ubiquitous in all small flocks. Just because it is there, doesn’t mean birds are symptomatic, or for that matter, ever will be.

Two-thirds of the global human population have been exposed to the Herpes Simplex 1 virus, otherwise known as cold sores. Some people have the virus and don’t know it because they’ve never had an outbreak; some rarely have any evidence of it, while others are plagued with a few flare ups every year. Most folks, including me, picked the virus up in childhood and still carry it. We know that cold sores are often triggered by stress, but that doesn’t mean every stressful life event culminates with an outbreak.

We don’t always know why some pathogens get expressed in particular people, but not others. The same is true for chickens, but stress if often the catalyst or trigger that sets that pathogen in motion. And just like coldsores Marek’s is also a Herpes virus. Some birds will get infected, others not. Some will become symptomatic and others will live their whole lives without ever having a sign of carrying the virus. Dr Bowes assured me that a diagnosis of Marek’s is not life-changing and that I should just carry on as I have, doing what I can to keep my birds healthy and stress-free.

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