So, you entered your chicken house today to find that, unfortunately, your poultry is dying at a relatively unhealthy rate. Birds are lying all over the floor, some looking at you pitifully through misshapen eyes. Some look like they're preparing for a track competition. A necropsy reveals a body full of lumps, bumps, and assorted nastiness. What are you to do? What has gone wrong here? I'm sorry to tell you, friend-- It's beginning to look a lot like Marek's. What is Marek's, you might ask? I'm glad you did. I'm here to tell you.

What IS this foul monster?

Marek's disease, discovered by Dr. Josef Marek, is a neoplastic disease caused by a type B herpesvirus. There are three different serotypes of the virus, though it seems that only the first serotype actually causes disease. This bugger has an incubation period of approximately 4-12 weeks, with manifestations normally seen around 5-6 weeks of age or after the 12 week period. The mortality curve often follows production curves in layers and broiler-breeders-- that is to say, as birds become more stressed out, the disease peaks. It can form latent infections that reoccur during times of stress, causing problems that can last a lifetime. This is a host-specific pathogen, meaning that humans are not at risk of infection.

But where does it come from?

Basically, all chickens produced in the United States (and probably most other intensive production nations) have Marek's disease. This means that the disease is ubiquitous. Because of the devastation that the disease caused in the industry before it began to be controlled, vaccination is now the absolute norm. Vaccination utilizes an attenuated strain, but still results in infection. We already established that infected birds form latent infections-- they become carriers for the vaccine in the environment. They can shed the disease at intermittent times throughout their life and production cycles. Shedding occurs in the form of sloughed-off epithelial cells from feather follicles. These, along with respiratory secretions and other excretions can cause lateral transmission from the vaccine-infected birds to those that were not previously infected. This can lead to a phenomenon called a rolling infection- if any birds are immunosuppressed, have vaccine failures or other confounding factors, the increased viral load produced by vaccinated birds will laterally infect the unprotected birds in the house. This results in a delayed peak of more severe disease. Bad times. Further, these danderbits can contaminate the environment long after the birds have come and gone, meaning that litter used between bird cycles can contain infected fomites from earlier flocks. Obviously, the older the litter, the stronger the build-up of junk in the dust and dirt, and the stronger the challenge to the baby chickies placed on said litter at day of age. Darkling beetles, a little litter beast, can also be a vector of the disease because they feed on chicken dander.

What did chicken #44369 ever do to deserve this?

  • Unvaccinated birds are very susceptible to this disease due to its field prevalence.
  • This infection hits all types of chickens, from broilers to layers to broiler breeders, but mostly occurs in older birds.
  • Turkey can be affected to some extent, but the disease is much more prevalent in chicken.
  • Birds in heavy production systems are at a greater risk than those who are in a lower intensity rearing system.
  • Birds in very unhygienic houses are at risk.
  • Birds in a multi-age flock are particularly at risk because of the rolling infections- imagine putting babies under the challenge of not only the stress of the house and movement, vaccine reactions, and contaminated environment, but also the established hot strain of the virus that has come rolling off of birds who have been in the system for several months. Recipe = disaster.
  • Other predispositions include damage to the respiratory tract through wet litter, disease, or vaccines, temperature abuse, and immunosuppression from other diseases.

What should I be looking for?

Marek's has many manifestations, including neural, visceral, and cutaneous. Any or all of these signs can help point you to a guess of Marek's for your sick birds. Here are some general signs to be on the lookout for.

  • General:
    • Severe depression-- caused by generally feeling like all hell
    • Emaciation-- caused by a refusal to eat, due to feeling like all hell
  • Neurological:
    • Paralysis-- a characteristic “hurdling” position- one leg forward and one leg back. Paralysis is a result of the neurological component of this disease, and is one of the more characteristic symptoms and is very important for differential diagnosis. The idea behind the paralysis is that tumor cells invade the nerves and brain tissue, resulting in neurological symptoms. Paralysis is also seen in some of the viscera- some birds will have a distended, over-filled crop due to nerve inhibition. The nerves controlling the tone and stretch of the bird's crop are out of commission, and so the little storage baggy overfills and looks pretty darn grotesque.
    • Swollen nerves-- especially the sciatic/isciatic nerve. This can be one of the most acute symptoms indicating the disease and is another important differential
  • Visceral:
    • Diarrhea-- either a green or white diarrhea. Because normal bird droppings are fairly dry, diarrhea can be said to be anything that is particularly wet, and especially anything that pastes up the vent. Diarrhea can be a complicating factor to many other diseases because wet litter is generally just not good to have around.
    • Eye lesions-- grey iris and misshapen pupils, caused by the infiltration of tumor cells into the eyes.
    • Visceral tumors-- Arguably the most important manifestation of this disease. Tumors will appear on/in/around nearly every organ in the body. This is not necessarily a diagnostic lesion, but it IS a very significant one. Tumors can cause the kidneys to be several times larger than normal, the liver to appear necrotic, and the nerves to lose striation. Tumors may also occur on the thymus and bursa of fabricius, resulting in immunosuppresion. Tumors are nasty business.
  • Cutaneous:
    • Shriveled, pale comb
    • Lesions on shanks and/or comb-- red shanks are indicative of cutaneous infiltration of tumor cells.
    • Follicular tumors-- These are relatively unpronounced infiltrates of tumors on the skin. Basically, they will result in what appears to be exaggerated goosebumps on the skin. Goosebumps are normal; very swollen bumps accompanied by redness and congestion are not.

Sweet Jesus on a stick! Are you SURE it's Marek's? Maybe it's something else!

One of the easiest methods of differentiating one disease from others is to pick out important-looking lesions/signs/symptoms/gross things and see if they can be related to any other disease.

Because this disease occurs in older birds and has tumors and swollen nerves and neurological symptoms and skin lesions, it is most likely Marek's. This is the art of differential diagnosis. Excluding other possibilities makes it possible to take a stab at what on earth is causing your problem. You can learn a lot by examining the epidemiology. Further tests for serotyping can be done by submitting pathology samples to the lab. Submitting samples of brain tissue, nerves, and fresh viscera and tumor tissues will provide the best results.

Determining the source of infection is much more difficult. Because birds are supposed to be vaccinated, it's important to look at the hatchery. They will tell you it is not their fault. Growers are supposed to keep their houses hygienic and keep up with vaccines. They will tell you it is not their fault. Finding the real culprit is going to be a party. Have fun.

Well... Hell. Maybe this isn't so bad. What's the worst that could happen?

Unvaccinated birds can reach high levels of mortality, either directly or due to the repercussions of their horrific pathologies. Many birds will be culled due to paralysis and stunted growth. Those that do make it to the processing plant are likely to be condemned due to the tumors and skin lesions. In a business where the loss of pennies is a catastrophic economic blow, this is not an acceptable scenario. So what are you going to do about it?

What CAN I do about it?

There is no real treatment for this disease other than the preventative treatment. Vaccine is the best means to this end. Vaccination is, in this day and age, normally performed in ovo at day 18 of incubation. This is simple, because at this age, the eggs are being moved from the setter to the hatcher. While they are suctioned up, a needle pops a tiny hole in the egg and injects vaccine into the fluids surrounding the embryo. When the yolk is taken up, the vaccine will be as well. Combined with maternal antibodies formed from vaccination of parent flocks, the chickies will be protected for some time. The other option is for hand-vaccination of day-old chicks. Chicks are pressed against an injection machine that pops the vaccine subcutaneously into their fuzzy yellow little chicken necks.

Unfortunately, the Marek's vaccine is fairly easy to screw up. Maternal immunity may be screwed up by only using a weak strain vaccine and never graduating to a more stringent vaccine that will induce high immunity. The vaccine must be stored in liquid nitrogen until it's used, and reconstituting can then be easily seen to get screwed up, rendering the vaccine ineffective. Some people are just bad at hand-vaccinating. They get tired when they get to the end of the 6 hour vaccination shift, and those little birdies don't get the protection they need. The diluent or vaccine itself may become contaminated and cause disease or simply fail to work.

Hygiene also becomes extremely important in prevention. Allowing litter to accumulate without reducing the disease load is a bad idea. Increasing air flow through the houses can help to prevent dander from building up and recirculating into the respiratory tracts of the birds. Adjusting temperature and humidity can help prevent respiratory distress that would aid Marek's ability to enter the body. Killing off little beetly-bugs can also help prevent the spread of disease. Biosecurity is a huge issue on most poultry farms, and practising good sanitation and biosecure practices can decrease risk between houses and between farms.

There are also attempts to genetically select birds that are resistant to the disease. This can help ameliorate some of the problems with vaccine failure and hygiene, but it should instead be viewed as a way to aid those other preventative practices, not supplant them.

So there you have it


Join us later for other horrific infectious diseases that could kill off thousands of chickens and lose you your Christmas bonus, won't you?


I'm ever thankful for the lectures and WebCT notes of Dr. Stephen Collett, professor of AVMD 3730 at UGA. Cheers!

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