3 Yr Old Hen Walking on Hocks, Thin

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UC Davis is my go to lab here in Cali. I just finished reading their recent newsletter - Marek's ranks above parasites, laying issues or anything else you can think of (although no mention of predators :) ) as the #1 cause of death here. It's really common.
 
Sorry to be ghoulish, but one thing we have learned is that on the Davis submission form where you can fill out the history section, put ANYTHING you can think of. The lab really uses that section. If they pick up on something on that section, they will run additional tests. For instance, they may not pick up that the hen (just an example) had a runny nose on gross necropsy, but if you mention she had a runny nose (again, just an example), they may run a test for Infectious Bronchitis. For instance you might mention her pupil to ensure they look at it. Or that she had lost weight first. They see SO many hens, you sometimes have to point things out you are concerned about and additional clues for them.
I've seen many people get the report back, then call the lab and say "what about this," as the symptom didnt show up on gross necropsy, so the lab doesnt know to run an additional say blood test on something. They really count on you to run down the history and symptoms.
 
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You guys are awesome to weigh in. Once Michelle's hen was diagnosed with Mareks via necropsy (also a 3 year old hen) in the thread MFE shared (Michelle was floored, so was I) I did some research and I was floored that a) vaccination does not prevent it b) vaccinated hens shed the virus like 10 times more c) there can be vaccine failure and d) the disease is incredibly common in CA.
 
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orrpeople said:
I've never found this in my research. Article?
Click to expand...
Google Mareks leaky vaccine. That’s what a leaky vaccine is, and they think it’s contributing to there being so much Mareks. I’ll find the couple of articles I read.

https://www.pbs.org/newshour/science/tthis-chicken-vaccine-makes-virus-dangerous

https://www.nationalgeographic.com/...s-enhance-spread-of-deadlier-chicken-viruses/

https://www.google.com/amp/s/amp.livescience.com/51682-vaccines-evolve-deadlier-viruses.html

https://blogs.plos.org/thestudentblog/2016/01/08/
 
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I would like to invite @casportpony into this discussion, if she is available. I can't remember the particular discussion, but I'm pretty sure the Marek's vaccine is not considered leaky (the propensity towards giving a disease to other birds) but that because it does not eliminate the virus, it has greater potential for creating resistant forms of the virus.
 
In response to questions about how vaccination impacts shedding of Marek’s disease virus, this study linked below is probably the most famous study on this topic, because this is the study that provided data supporting the hypothesis that leaky vaccination increases transmission of the most virulent strains of virus.
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002198
If you look at Figure 1, you’ll see Rhode Island Red chicks that are not vaccinated (dashed line, open boxes) or vaccinated (solid line, solid boxes) at 1 day old with the cell free HVT vaccine (the one that you can order online). The chicks were then inoculated one week later with different Marek’s strains, least virulent to most virulent from left to right. You can see that vaccination reduced mortality or death (over only a two month period!) with all viral strains. What is contentious is that for the very virulent or “hot” strains, the surviving chicks continued to shed virus because the vaccine reduces disease and death but doesn’t prevent viral replication and shedding. In contrast, all of the unvaccinated chicks given the hottest Marek’s strains are soon DEAD, so they don’t shed much virus; the hottest strains kill the chicks before it can even start shedding and get the virus all over the environment! Note that the less virulent strains still replicate and are shed in the environment by unvaccinated chicks. So, you can look at this study in a couple of different ways: 1) IF you have a very virulent Marek’s strain and vaccinate, then you increase viral shedding into the environment (via feather dust). But... IF you have a less virulent strain, then vaccination decreases shedding into the environment. 2) IF you have a very virulent strain of virus, you get more viral shedding because most of your birds live long enough to have the chance to shed; vaccination saves the birds lives (for a while) but now they carry and shed the very virulent strain that they were inoculated with.

So, commercial broiler and layer flocks all vaccinate, because they don’t want the birds to die. Some backyard flocks also vaccinate, because they don’t want the birds to die. But some backyard flocks don’t vaccinate because they want to know whether they have disease or they are trying to breed for resistance. The breeder who sold us our infected chicks didn’t vaccinate but “bred for resistance”. Her chicks were not actually resistant as most died of Marek’s by 2 months of age, plus it infected the rest of our flock. Therefore, I’m a little skeptical of the “breeding for resistance” route, especially if birds will be leaving the breeder’s farm...
 

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