As I talk to different backyard producers, and offer vaccines to keepers in my area, the number one issue that causes consternation and confusion is Mareks vaccination. Opinions range from “don’t vaccinate, it will make your birds sick”, to “vaccinated birds pose a risk to non-vaccinated birds”, to “vaccines result in hotter strains of Mareks Disease in the field”, to “don’t vaccinate, just breed for resistance”. When I try to discuss these opinions with the people who promote them, I get half-truths, misunderstandings and different philosophies on health provision. So, to decrease the number of times I explain my point of view on Mareks vaccine, here is a summary of what I know, and how I think the vaccine works. Keep in mind, I have looked after roughly 100 million vaccinated birds that have ALL been housed in places where large numbers of birds have been kept before, and have almost definitely all been exposed to Mareks Disease.
Vaccines work by exposing an animal’s immune system either to a weakened form of a disease causing agent, or a protein that mimics one that is produced by a disease causing agent, thereby tricking the immune system into attacking the agent the first time the body actually sees it. Every foreign protein that enters the body is separated and presented to the immune system by cells called macrophages. It is important to note first off that Mareks Vaccine CANNOT cause Mareks Disease, and CANNOT shed the vaccine to cause disease in other birds in the flock. Unfortunately, not sharing the virus with flockmates means that only the chicks that have been injected will have any immunity to Mareks.
Antibodies that are effective against a virus must be very specific. The common analogy is a lock and key relationship. Creating a key to fit a lock takes a long time, and a bunch of energy, but cutting identical keys is quick and easy. The proteins presented by macrophages are the templates for key production. Once the first key is produced, it is stored in the body for reference (called immunoglobulin M or IgM). If that same protein enters the body, IgM is used as a template to make tons of immunoglobulin G, which meet up with the protein, and cause it to be killed. (I hope you appreciate that entire text books are written on Immune function, and immune stimulation. This is a Cole’s notes version of a Cole’s notes version of immunology).
So, since this process is so INCREDIBLY simple, why the confusion and disagreement on Mareks vaccines? Well, unfortunately, with MD immunology, there is a wrinkle. The IgG that is produced by the body does not kill ALL the Mareks virus that enters the body. Much of the virus dies, and the amount that is available to be shed is very much decreased, but the IgG merely surrounds some of the virus particles, and keeps them from entering the cells of the vaccinated chicken, saving it from disease, but not preventing the bird from carrying the virus, and potentially shedding it. This is why the vaccine is described as “leaky”. Even vaccinated birds can harbour Mareks Disease Virus, if they are exposed to it. Most people have read this somewhere, and form strong opinions around it. Here is what many people DON’T know. The vaccine reduces the amount of virus shed to about 1/1000 to 1/10000 the amount that is shed in non-vaccinated birds. A study by Nair, et al. in 2010 found the following. 9/9 birds who got challenged with MD with no vaccine died. 0/9 birds with one vaccine, and 1/9 birds with another vaccine died. But the authors also measured the shedding rate of vaccinated and non-vaccinated birds:
What I want to point out is the scale. The non-vaccinated chickens shed around to 1 million virus particles per thousand cells while vaccinated birds shed less than 10% as much virus. So the leakiness of the vaccine is less than 10% that of unvaccinated birds. Even though Mareks can be carried and shed by vaccinated birds, the risk is reduced to less than 10% of the non-vaccinated birds….assuming the non-vaccinated birds survive.
In a way, that is probably the greatest risk posed by vaccinated chickens with respect to shedding Mareks Disease….they don’t die as young, so they survive long enough to be a source of virus longer. The problem is, most backyarders will replace the birds that die, and if the replacements are non-vaccinated as well, they harbour and shed vast amounts of the virus they pick up when they join the infected flock.
As for vaccines increasing the rate of mutation and the increase in virulence of Mareks disease viruses present, that is probably true. Anything that makes it harder for any biological being to survive will add pressure to evolve to be more successful. It’s why cheetahs run so fast….the slow ones died out during hard times. (Interestingly, the 3rd and 4th fastest animals on earth are the springbok and wildebeest, 2 of the cheetah’s main prey). But, when you consider the risk of a virulent virus developing in backyard flocks when compared to the evolutionary pressure of literally billions of vaccinated commercial chickens yearly, I think the risk is put into perspective. The odds of having to protect your flock from a virulent strain from somewhere else is astronomically higher than the risk of developing a highly pathogenic strain in your flock.
Finally, the arguement for selecting robust breeds of chickens that are naturally resistant to Mareks, rather than vaccinating. As a vet, I see that as an arguement to let any weak ones die. There is no way to predict what birds will succumb to Mareks. Often it takes well over a year for the tumors to cause symptoms in a chicken. So, is the answer to keep your chickens until they are 2-3 years old before hatching any chicks? By then, you may only have 10% survivability. You need 90% of your chickens to die a preventable death before you can even begin to select for resistance. If that was it, it might be doable. The problem is that the next generation is only slightly more resistant to Mareks than the current one. You have to sacrifices birds for generations to get a noticeable increase in resistance. And then hope the virus doesn’t mutate, and wipe out all your gains. Believe me, all the genetic companies are very interested in increasing genetic resistance to Mareks Disease, but they haven’t made significant progress.
So….there are my views on Mareks Vaccines. There are 2 types of chickens….those that have been vaccinated for Mareks Disease, or those that should have been vaccinated for Mareks Disease. I expect some of you will disagree, and I welcome any questions….I will answer them as clearly as I can.
Oh, and my experience with Mareks Disease in the 100+ Million vaccinated birds I have dealt with? I carry my phone with me, and take pictures whenever I find a bird with tumors….it’s rare enough that it is an event for me to find a bird full of tumors. Make of that what you will.
Mike the Chicken Vet