Visceral Mareks

[pugspigchicks]

Does anyone have solid information on the incubation and development of visceral Mareks? I know there are several forms of Mareks and various presentations, but I was told visceral Mareks takes much longer to manifest. A vaccinated hen was purchased from a well known breeder and died 4 months later of visceral Mareks. I’ve never dealt with Mareks or ALV and all deaths in my many many years of ownership have been necropsy confirmed. The university and my vet said the hen was 100% infected at a young age before purchase and they suspect improper vaccination/improper quarantine before immunity. I’m still trying to find concrete information that supports this. I’m still assessing the damage…. Some of my newer ones weren’t vaccinated because the other breeder didn’t vaccinate. I only had new NPIP birds and newly hatched ones and I wrongly assumed NPIP tested for Mareks and AVL. I now know that is not correct.

 

[coach723]

There is not really a set amount of time for incubation, it's generally considered to be 4 - 12 weeks from exposure, but it varies a lot. Some birds don't show symptoms until they are older. It's common for symptoms to show up around point of lay, 20 weeks or so. The individual genetics and immune systems of each bird make a difference too. Some birds can be exposed/infected and never develop any symptoms (those are the ones to breed), some will show symptoms very quickly. The strain matters also, some strains are much more virulent than others. I've had birds with occular symptoms that lived a couple of years before developing visceral tumors. Some never did, or didn't have time as I culled them once their vision was too impaired for them to live in the flock. I've had some birds pass with no visible symptoms except a sudden illness and they pass in a few days and then show visceral tumors on necropsy. When chicks are vaccinated, they have to be quarantined away from any exposure to the virus, some sources say 4-7 days, some say up to 2 weeks, to allow the vaccine to take. Like all vaccines, it's not a 100% guarantee, and it's a leaky vaccine, it does not prevent the birds from being infected, it only stops them from developing the tumors. The vaccine does not give them Marek's, but they can still be infected by other infected birds, they just hopefully won't die from it. Whether to vaccinate or not is a personal decision, no one right answer. Many are choosing to try to breed for resistance.
Sadly, if you brought a Marek's infected bird into your flock, then they have all likely been exposed. It does not mean you will lose them all, time will tell. Often Marek's will have a terrible period as it goes through a flock, and then it will kind of settle down, it very much depends on the strain, and lots of other variables. Some breeds are more susceptible. I have fewer losses now, but still have them.
https://extension.psu.edu/mareks-disease-in-chickens
This is a very good article, worth the time to read:
https://www.backyardchickens.com/ar...-i-learned-to-deal-with-mareks-disease.76944/

 

[Eggcessive]

Did you ever get the necropsy results on your 2nd chicken that died with possible Mareks in 2018:
https://www.backyardchickens.com/threads/please-help-vet-has-no-idea.1265524/
Unfortunately, if Mareks is possible, it can remain in the environment for months or probably years. A lot of people are against vaccinating chicks for Mareks nowadays because the vaccine is leaky.

 

[pugspigchicks]

Did you ever get the necropsy results on your 2nd chicken that died with possible Mareks in 2018:
https://www.backyardchickens.com/threads/please-help-vet-has-no-idea.1265524/
Unfortunately, if Mareks is possible, it can remain in the environment for months or probably years. A lot of people are against vaccinating chicks for Mareks nowadays because the vaccine is leaky.

I did. A&M did the necropsy. And before death, my vet consulted with an international poultry board. Best guess pre necropsy was enterococcus of the spine. Necropsy confirmed spine injury/infection. Negative Mareks.

 

[pugspigchicks]

There is not really a set amount of time for incubation, it's generally considered to be 4 - 12 weeks from exposure, but it varies a lot. Some birds don't show symptoms until they are older. It's common for symptoms to show up around point of lay, 20 weeks or so. The individual genetics and immune systems of each bird make a difference too. Some birds can be exposed/infected and never develop any symptoms (those are the ones to breed), some will show symptoms very quickly. The strain matters also, some strains are much more virulent than others. I've had birds with occular symptoms that lived a couple of years before developing visceral tumors. Some never did, or didn't have time as I culled them once their vision was too impaired for them to live in the flock. I've had some birds pass with no visible symptoms except a sudden illness and they pass in a few days and then show visceral tumors on necropsy. When chicks are vaccinated, they have to be quarantined away from any exposure to the virus, some sources say 4-7 days, some say up to 2 weeks, to allow the vaccine to take. Like all vaccines, it's not a 100% guarantee, and it's a leaky vaccine, it does not prevent the birds from being infected, it only stops them from developing the tumors. The vaccine does not give them Marek's, but they can still be infected by other infected birds, they just hopefully won't die from it. Whether to vaccinate or not is a personal decision, no one right answer. Many are choosing to try to breed for resistance.
Sadly, if you brought a Marek's infected bird into your flock, then they have all likely been exposed. It does not mean you will lose them all, time will tell. Often Marek's will have a terrible period as it goes through a flock, and then it will kind of settle down, it very much depends on the strain, and lots of other variables. Some breeds are more susceptible. I have fewer losses now, but still have them.
https://extension.psu.edu/mareks-disease-in-chickens
This is a very good article, worth the time to read:
https://www.backyardchickens.com/ar...-i-learned-to-deal-with-mareks-disease.76944/

thanks for the reply. And the last article was great. That’s for providing the link. Yeah, I couldn’t find any definitive info on incubation period other than a range. I just found it interesting that two experts said visceral takes 9-12 months to develop and usually is seen in hens after 12 months. They also thought this form was highly unusual in a correctly vaccinated chick. But the vaccine is only 95% effective in producing clinical disease in a vaccinated hen with a moderately virulent strain. The neural form is usually seen earlier and around the time of lay. I hadn’t read anything on the internet that provided that specific information. And it doesn’t matter really because I’m not going to talk to the breeders. There’s no way to identify patient zero. Its Just unfortunate after buying really expensive breeder quality birds from high end NPIP flocks. I don’t think I suddenly developed this strain from my existing flock…This was a new yard post excavation of a patio, new soil, new xeriscape, new coop etc and all new birds in a separate area of the yard from the old existing flock-a bunch of 6-8 year old mixed background backyard misfits (separated by a house and trees). Per A&M essentially all flocks are assumed to be infected with a strain of Mareks.

 

[Eggcessive]

Interesting. I did read that whole thread. Neurologic symptoms are hard to diagnose without a necropsy. It’s unfortunate that you have a bird now diagnosed with visceral Mareks. Since becoming a member of BYC years ago, I usually will not buy started chicks or birds from others. One can have Mareks around since it could be spread from flock to flock by wild birds. It is a terrible disease. Many people breed their birds who do not get symptoms. It doesn’t help with those wanting special breeds or who want to sell or show birds. Unfortunately I think a lot of people have picked it up from shows or other breeders, but sales are the worst place. One would hope that vaccinations with a 2 week isolation from the feather dander would guarantee a healthy chick, but not always.