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Bollith Bred Poultry

Specialising in quality, free range poultry, perfect for your backyard

Marek's Disease (MD)

The Facts, and Why We Choose To Vaccinate.

Lately, I've seen heard and seen some interesting comments on Marek's Disease (MD), from both hobbyists and breeders alike. It's quite concerning how much misinformation is out there, so I thought I'd put together a few facts about Marek's and why we've decided to vaccinate against this horrendous disease.

Firstly, we always appreciate people's views and opinions. If you choose not to vaccinate or buy unvaccinated poultry, ensure you're doing it for the right reasons, not because a breeder is lazy and chooses not to vaccinate, a common line is "We choose natural immunity" thereby telling you vaccinating is 'wrong'. Vaccinating poultry is no different to immunising your puppy, or even immunising your own children. Why would poultry any different?

It's also important to note I have not added any photos to this page. I considered it, but there is no such thing as a 'nice' or 'PG rated' photo when it comes to Marek's Disease.

Now, a few facts on Mareks Disease.:

  • The disease was first described by Josef Marek a Hungarian veterinarian in 1907 which was characterised by paralysis and lymphoctic infiltration of the peripheral nerves.
  • Marek's Disease Virus (MDV) is a member of the genus Mardivirus within the subfamily Alphaherpesvirinae.
  • MD is an airborne virus which hosts on dander on the feather follicle of chickens. As a member of the Herpes family, it riddles the affected chicken with internal lesions (tumours), causing paralysis and/or blindness and most often death.
  • There is no known cure or treatment for affected birds. Euthanasia is often considered the best option for most diagnosed cases.
  • Marek's disease is one of the most ubiquitous avian infections; it is identified in chicken flocks worldwide. Every flock, except for those maintained under strict pathogen-free conditions, is presumed to be infected. Clinical disease is not always apparent in infected flocks, so one may not know the virus is present.
  • Marek's is a common virus which kills more birds than any other disease except Coccidiosis. (By the way, Cocci is both preventable AND treatable, unlike MD).
  • The disease is highly contagious. Due to being airborne, the infected dander can be transmitted in many forms, such as in clothing and shoes, dogs or cats, rodents or birds, even equipment or cars. Even having your birds in extreme quarantine may not help as it's also been found to have been blown many kilometres by the wind, infecting neighbouring flocks some distance away.
  • Marek's cannot be passed on through the egg.


There are three main serotypes of MD.

  • Neural form. This results in tumours in the nerves and brain. Tumours of the nerves will cause weakness and paralysis of the wings and/or legs.
  • Visceral form. This causes tumours of the internal organs, such as liver, spleen, heart, kidney, gonads, feather follicles.
  • Ocular form. This changes the colour of the iris from normal orange/red/yellow to gray. It can also cause the pupil to be irregularly shaped.
  • A chicken affected with MD can show one or a combination of these forms.
  • Symptoms may include: Paralysis of leg/s and/or wing/s; staggering, loss of balance; difficulty eating; difficulty breathing; darkening comb; lymphomas (cancerous tumours) located throughout the chicken which can be seen on the skin; severe weight loss; depression; loose, watery and/or bright green or yellowish stool; discolouration of the iris turning grey or pale blue colour; deformed pupil; blindness; immunosuppression.
  • Mareks Disease may also look like a respiratory disease with symptoms such as gaping (gasping for breath). Some birds may gasp and show respiratory distress that makes us think we are looking at a respiratory disease such as Aspergillosis, ILT, Infections Bronchitis (IB), Gape worm, or even Wet Fowl Pox.
  • Paralysis is a well known symptom of MD, however birds can be severely affected with Marek's who show no such signs.
  • The only way to diagnose MD is to perform a necropsy (autopsy) on a dead bird.
  • Marek's usually affects chickens between 4 to 25 weeks old, however it CAN affect them at any age.

About the vaccine:

  • Like any vaccine or immunisation, it is not 100% guaranteed. It is believed to be around 80% - 90% effective, which is within the same range as all other immunisations across the board.
  • It is not a cure. It is not an immunity. It is exposing chicks to a virus while they're young and still developing an immune system, which allows the chicks to have exposure to it and build up a resistance. The aim is if the chicken is ever exposed to the virus later in life the immune system will react appropriately to fight off progression of the virus in to lymphomas, and therefore the chicken will not develop symptoms.
  • It cannot CAUSE the chicken to shed live Marek's disease, therefore vaccinated chicks are not a carrier any more than unvaccinated chicks.
  • The vaccine cannot cause a chicken to contract Marek's.

Now, we respect those breeders who don't vaccinate (although we may not agree with them), however we do not respect those breeders who say vaccinating is an unnecessary expense which is unnecessarily passed on to the customer. Typically, these same breeders also say they'd rather breed genetically resistant chickens or chickens with a natural immunity rather than simply vaccinating. The theory of breeding resistant birds sounds wonderful (why didn't I think of that?) although unfortunately it's not that easy. Why can't we breed humans resistant to Measles? Ok, ok, I know. But the truth is for a breeder to START to show resistance (ie not become fully disease proof) is a big headache. Marek's virus has a life cycle of 7 years. Think about it. That's a huge amount of culling birds for about 10 years, possibly dealing with immense loses (not including the birds you need to euthanise as a control), and conducting routine necropsies (autopsies) to monitor progress. Now THIS creates a huge unnecessary expense which is forced upon the customer, (not to mention countless unnecessary deaths). Now, don't get me wrong, someone who genuinely has a set program for breeding resistance has my upmost respect and admiration, however there are far too many breeders who just don't understand what "breeding for resistance" means.

At the end of the day, we did our own research and we found our customers to be more than happy to pay just $1 more per chick for a vaccinated bird, if even there is the slightest chance it can possibly save so much heartache, vet bills, and avoidable loss of life.

If, however, you would still rather purchase unvaccinated chicks, like I say we respect that, please contact us and we can arrange some unvaccinated chicks on a pre-order pre-paid basis.


Marek J. Multiple Nervenentzuendung (Polyneuritis) bei Huehnern. Dtsch Tierarztl Wochenschr 1907;15:417-421.,Poulvac%20CVI%20HVT%20Vaccine_R.pdf