Do Mareks-vaccinated chickens spread the vaccination?

MaeIstrom

Chirping
Jul 10, 2019
45
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89
Perth, Western Australia
So according to https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/ a mareks vaccination is accomplished by injecting a chick with a harmless form of the virus (MDV-3) which allows the immune system to recognize and fight the deadly strain (MDV-1). If the bird becomes infected with the deadly variant post-vaccination and lives it now becomes a carrier leading to the myth that vaccinated birds are always deadly to non-vaccinated.

My question is what if the immunized bird does NOT encounter the deadly strain and only has MDV-3? Does it spread that with the same virulence and could chicks raised in a coop infected with MDV-3 be considered de-facto immunized?

Now I know the first answer from anyone who's read the faq will be no because it explicitly states that the immunization must be done inside 36 hours of hatch and that the chicks must not be exposed to the deadly strain for 3 weeks after that to give the immune system time to develop the immunity which makes sense to me but only the 3 weeks parts. I can think of no good reason why exposing a bird to the non-deadly strain via any means at any age would be less effective unless exposure to the deadly strain also occurs before the 3-week window is up.

Anyway I have a batch of chicks, 2 months old currently, that I'm running with vaccinated birds that I believe/hope are clean and will have more definitive answers in a few months I guess but I'd like to try selling/giving away the cockerels before killing them as I'm not able to keep them but not sure what the ethical way of describing them to potential buyers would be. Probably safe to run with vaccinated flocks but proceed with caution otherwise I think. Would appreciate other opinions but preferably ones backed by a medical/veterinary degree as opposed to what your neighbour or some guy at a swap-meet told you.
 

cavemanrich

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Apr 6, 2014
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I can think of no good reason why exposing a bird to the non-deadly strain via any means at any age would be less effective unless exposure to the deadly strain also occurs before the 3-week window is up.
You bring up a VERY INTERESTING POINT. I will fellow thread,, since I'm interested in this as well.:thumbsup
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, and :welcome
 

MaeIstrom

Chirping
Jul 10, 2019
45
119
89
Perth, Western Australia
Well if none of my chicks come down with it in the first year I would consider the theory proven. If they do it might still be correct and I was unlucky enough to have a live carrier in my pen. Fingers crossed I'll be able report good news in a few months.
 

Tesumph

Knotenolk
6 Years
Jul 10, 2015
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The short answer is no, the vaccine cannot be shared horizontally throughout a flock.
There is some passive immunity from maternal antibodies that will be passed from mother to child, but this is actually troublesome as this can neutralize the weak live vaccine in day old chicks. Maternal antibodies also increase viral shedding and contribute to hyper-virulent strains.
In any case, this is not a reliable method for preventing MDV in a flock.

The long answer is that MDV replicates in a chicken by multiplying inside lymphocytes (B and T cells). As the virus spreads, it kills these lymphocytes which causes the hallmark immunosuppression we see with MDV. This is also the biggest reason we are dealing with leaky vaccines: the virus “hides” inside T cells, which are the very cells that are meant to remember and destroy the virus for future infections! This results in “latent viral infection,” aka the virus can and will suddenly re-emerge even if the immune system has dealt with it before.
The reason these leaky vaccines are so dangerous is because very hot strains of MDV (or any virus) would normally wipe out a population before it could become an epidemic. But if you're a real nasty virus and you find a disease resistant host that you can replicate inside of until you’re nice and ready to be shed out into the world, you’re in luck. Your infectivity just went through the roof!

Back to the question..
There is still a lot of research needed to be done on the Marek’s vaccine itself and how the immune system responds to it. My understanding is that the vaccine boosts T cell count (as all vaccines do) to ward off the disease, but this is useless to prevent shedding if the virus is specialized at infiltrating and recruiting the T cells themselves.
When the virus is shed from the body, it’s already mature enough to start attacking another host’s lymphocytes. This is different from a controlled vaccine where the virus is altered to be weak enough to fight off before disease can set in.

Hope that wasn’t too long winded. :) Here’s some more in-depth reading, if you’re interested.
https://veterinaryresearch.biomedcentral.com/articles/10.1186/s13567-016-0404-3
 

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