Moving Forward- Breeding for Resistance to Marek's Disease

[Michael Apple]

Breeding for resistance will not ensure Marek's doesn't enter an otherwise healthy flock. From what I've read about Marek's Disease, vaccination is the only means for controlling it in infected flocks because there are no treatments that cure an infected bird, at least right now. Even a resistant hen who lays an egg only provides antibodies for the chick through passive immunity which may last for several weeks. After that, since the immune system of the chick is not stimulated, there will be no antibodies produced by the chick and no memory cells. If chickens are vaccinated when maternal antibody titer levels are elevated, the vaccine may have a reduced response. Opposite of that, if vaccinations are delayed and maternal titer levels are low, a severe vaccine reaction may result. I wonder if many of those selling chicks, whether it is a hatchery, or a private breeder, ignore this reality, and we see chicks, supposedly vaccinated, bearing the symptoms and a short existence.

 

[chooks4life]

Breeding for resistance will not ensure Marek's doesn't enter an otherwise healthy flock.

Yes, we know; that would be better termed 'breeding for immunity'.

One of the driving concerns is the very fact that without it present in your flock, you have no way of knowing whether or not your otherwise healthy birds are completely susceptible to a disease so many others don't die from.

It only compounds the problem to have them vaccinated, which means you don't know if the only reason some of your bloodlines exist at all is because they were artificially protected, and can't survive without the vaccine.

With many diseases it's a matter of when, not if, the disease gets in, particularly with Marek's which is so very common. It would be the most terrible culmination of a life's work to watch them all die from something the common mongrel frequently has decent resistance to.

From what I've read about Marek's Disease, vaccination is the only means for controlling it in infected flocks because there are no treatments that cure an infected bird, at least right now.

That's true, but part of the reason this thread exists is because vaccination is not a true solution, it's a short term crutch which may well be helping the problem get worse. Many of the experts on the disease (vets, scientists, etc) believe the vaccinations against it have only helped increase its virulence and perpetuate susceptible genetics, and it's an 'arms race' they're losing. Historical precedent doesn't make this look like a good plan to rely on.

So while vaccinations help prevent the tumors, it's apparently at a price, the future total of which is yet to be revealed, and which is disproportionately loaded against rare breeds and those breeders who practiced the most stringent biosecurity.

I'd be as happy as the next person if they found a way to eradicate the virus from existence but in the meanwhile it does make sense to look into other options, such as breeding for resistance, which of necessity requires exposure.

Even a resistant hen who lays an egg only provides antibodies for the chick through passive immunity which may last for several weeks. After that, since the immune system of the chick is not stimulated, there will be no antibodies produced by the chick and no memory cells. If chickens are vaccinated when maternal antibody titer levels are elevated, the vaccine may have a reduced response. Opposite of that, if vaccinations are delayed and maternal titer levels are low, a severe vaccine reaction may result. I wonder if many of those selling chicks, whether it is a hatchery, or a private breeder, ignore this reality, and we see chicks, supposedly vaccinated, bearing the symptoms and a short existence.

It's a good point, worth looking into for sure.

Best wishes.

 

[tridentk9]

Breeding for resistance will not ensure Marek's doesn't enter an otherwise healthy flock. From what I've read about Marek's Disease, vaccination is the only means for controlling it in infected flocks because there are no treatments that cure an infected bird, at least right now. Even a resistant hen who lays an egg only provides antibodies for the chick through passive immunity which may last for several weeks. After that, since the immune system of the chick is not stimulated, there will be no antibodies produced by the chick and no memory cells. If chickens are vaccinated when maternal antibody titer levels are elevated, the vaccine may have a reduced response. Opposite of that, if vaccinations are delayed and maternal titer levels are low, a severe vaccine reaction may result. I wonder if many of those selling chicks, whether it is a hatchery, or a private breeder, ignore this reality, and we see chicks, supposedly vaccinated, bearing the symptoms and a short existence.

Breeding for resistance typically involves the innate immune system. Depending on the disease, antibodies might not even be involved as the animal should never "catch" the disease so never has a chance to develop antibodies against it. This type of resistance for the Marek's Virus involves specific haplotypes, I think in the MHC. Different haplotypes confer greater or lesser degrees of resistance.
http://archive.wired.com/medtech/health/news/2005/01/66198 This article discusses the CCR5 gene in humans, it's involved with genetic resistance to AIDS.

The acquired immune system involves antibodies and memory cells. I think that in Marek's virus most antibodies would be vaccine related.

Neither is foolproof, but I'd guess it would be better to never get a disease (not thinking just MDV).

FYI- there are genes involved in "tumor growth". Not just in MDV and not a sure thing but some individuals are less likely to grow tumors.

 

[ChicKat]

It is important not to forget the fact that the Merek's vaccine is non-sterilizing. It is not a cure or truly a prevention of Merek's it is only a prevention of death from the tumors.

On his site Peter Brown (the Chicken Doctor) First State Vet supply -- used to say that there is a race in the chicks body between the vaccine and the Merek's virus - usually the vaccine will win. Perhaps in the 10% of vaccinated chicks that die from Merek's despite vaccination - the virus won the race.

Just had an awful thought, what if the 10% of vaccinated chicks that die - had their own resistance (Let's say maternal resistance for example) squelched by the introduction of the vaccine.

I haven't got the med. knowledge that a lot on this thread display - but I agree with chooks4life that there could be a real downside to the vaccinations - such as increasing virulence of the Merek's virus strains.... And wasn't the statement put out a couple times in this thread that the home vaccine is pretty ineffective, only the liquid-nitrogen stored types that hatcheries use are truly effective? Just like a person who gets antibiotics from a doctor - and is instructed to take the full course even if they feel fine and the source of need for antibiotics is cured...the entire dose needs to be taken -- an ineffective does of Merek's vaccine could be doing more harm that good in the long run. JMO

 

[speckledhen]

It is important not to forget the fact that the Merek's vaccine is non-sterilizing. It is not a cure or truly a prevention of Merek's it is only a prevention of death from the tumors.

On his site Peter Brown (the Chicken Doctor) First State Vet supply -- used to say that there is a race in the chicks body between the vaccine and the Merek's virus - usually the vaccine will win. Perhaps in the 10% of vaccinated chicks that die from Merek's despite vaccination - the virus won the race.

Just had an awful thought, what if the 10% of vaccinated chicks that die - had their own resistance (Let's say maternal resistance for example) squelched by the introduction of the vaccine.

I haven't got the med. knowledge that a lot on this thread display - but I agree with chooks4life that there could be a real downside to the vaccinations - such as increasing virulence of the Merek's virus strains.... And wasn't the statement put out a couple times in this thread that the home vaccine is pretty ineffective, only the liquid-nitrogen stored types that hatcheries use are truly effective? Just like a person who gets antibiotics from a doctor - and is instructed to take the full course even if they feel fine and the source of need for antibiotics is cured...the entire dose needs to be taken -- an ineffective does of Merek's vaccine could be doing more harm that good in the long run. JMO

Some good and interesting points.


It's been 25 days since the second "preliminary" report from the state. I have an email to Dr. Davis asking when I might expect a final report all set to send and can you believe it? I'm scared to hit "SEND".

I want to ask a couple of questions that may have no answers. I have some 10 week old free range chicks who have been all over this property with their broody mama. If there is Marek's here, when would I expect them to show symptoms of it? If they don't, how could they even be carriers? Is that 12 week incubation period what I should rely on?

I wonder, since Karen has had several MD-negative results in necropsies, if it's wrong to assume they have it and that it just did not show up due to their ages. Can it simply be that some birds just do not contract the virus at all, even in a flock that has suffered deaths/symptoms from MD? Can some birds simply be not only resistant, but immune?

 

[seminolewind]

Cynthia, it's very nerve wracking to be waiting for that final diagnosis. I know with 3 lab-negatives and 2 necropsies done by me containing no tumors, and Nambroth had a negative rooster, It appears that if the virus is actively causing tumors and paralysis, it's easy to test positive correctly. In an older chicken who's been exposed or vaccinated and not symptomatic, it may be quite hard. And how do the opportunistic bacterial ailments secondary to Marek-suppressed immune system figure into this?

I've had numerous chicks hatch and be fine and adults die of common chicken ailments. I have only had 1 roo, 10 -six week old chicks, and recently 2 cockerels 8 months old who got paralysis. That's 13 out of hundreds of chicks that have been hatched here while I didn't know I had Marek's. Everyone else has died mostly of wasting away (supposed Marek's related deaths)

And it seems that broody hatched chicks have those antibodies for a few weeks. But if vaccinated during or after that, it supposedly reduces the results from the vaccine.

Trident and Chickat bring up some really good points about resistance and the vaccine. Yes, we have many vaccines out there that give a body a "taste" of an illness so it grows antibodies that will be ready to combat a true exposure. Both are exposures. I don't think that will ever change. Antibodies are the result of an exposure. Vaccines are a safe exposure vs the real thing.

The vaccine does nothing for carrying and spreading the virus to many more. I think that it's more important to find some way of preventing a chicken from shedding the virus. Not really focusing on the tumors. In fact you can hypothetically have 50 chickens and only 10 die from the tumors. Even without a vaccine. But the rest that survive will be spewing virus all over the place anyway. To me, that's the much bigger problem. I would rather have 10 die of tumors than have 40 survivors that spew virus.

 

[seminolewind]

Quote:
Cynthia, I think that unless a chicken has active tumors or symptoms, it becomes more difficult to test for the virus. Since the virus is spread in dander, I can't see one chicken not getting it. No chicken is [error, edited out] immune to Marek's virus. They get it and become spewing factories of more Marek's. The chickens do most likely have varying amounts of resistance. Since Marek's also suppresses the immune system, I don't believe there can be an immunity. All the unvaccinated chickens I have right now are resistant. They have been exposed 4 years ago and have not died.

What does immunity mean to you?

 

[seminolewind]

It is important not to forget the fact that the Merek's vaccine is non-sterilizing. It is not a cure or truly a prevention of Merek's it is only a prevention of death from the tumors.

On his site Peter Brown (the Chicken Doctor) First State Vet supply -- used to say that there is a race in the chicks body between the vaccine and the Merek's virus - usually the vaccine will win. Perhaps in the 10% of vaccinated chicks that die from Merek's despite vaccination - the virus won the race.

Just had an awful thought, what if the 10% of vaccinated chicks that die - had their own resistance (Let's say maternal resistance for example) squelched by the introduction of the vaccine.

I wonder if this has something to do with B cells and T cells. A chick's resistance (theoretically mom's antibodies) may be created of B cells which are the first line of defense and sort of generalized. When they get used up, the T cells take over. They are more specific antibodies. Maybe somehow the T cells get delayed because of the sequence of who gets to fight first if the vaccine is given to a chick that has the temporary antibodies.

I haven't got the med. knowledge that a lot on this thread display - but I agree with chooks4life that there could be a real downside to the vaccinations - such as increasing virulence of the Merek's virus strains.... And wasn't the statement put out a couple times in this thread that the home vaccine is pretty ineffective, only the liquid-nitrogen stored types that hatcheries use are truly effective? Just like a person who gets antibiotics from a doctor - and is instructed to take the full course even if they feel fine and the source of need for antibiotics is cured...the entire dose needs to be taken -- an ineffective does of Merek's vaccine could be doing more harm that good in the long run. JMO

 

[speckledhen]

Cynthia, I think that unless a chicken has active tumors or symptoms, it becomes more difficult to test for the virus. Since the virus is spread in dander, I can't see one chicken not getting it. No chicken is resistant or immune to Marek's virus. They get it and become spewing factories of more Marek's. The chickens do most likely have varying amounts of resistance. Since Marek's also suppresses the immune system, I don't believe there can be an immunity. All the unvaccinated chickens I have right now are resistant. They have been exposed 4 years ago and have not died.

What does immunity mean to you?

"No chicken is resistant or immune to Marek's virus" I'm not sure we could know that for certain, even if it is true. How you'd know a chicken is absolutely immune, I'm not even sure. To me, immunity means the bird cannot "catch" the virus, therefore, cannot shed the disease nor become symptomatic because it has a "force field" immune system where that particular disease is concerned. Resistant, on the other hand, to me would mean a bird has a strong immune system and unless it it constantly bombarded by the virus, it can "bypass" it in small amounts most of the time without being infected or affected. Again, that's my own take on it, using plain English language type meanings.

I have a few birds I want to rehome. I don't need them, they are making my life too complicated to have to keep several males I didn't plan to keep, two of them are those 10 weeks old chicks I mentioned. I need to know what the real deal is with that hen (from actual tissue testing, not someone's off the cuff visual guess), so I can decide what to do going forward from this point. She died from massive ovarian infection, complicated by capillaria, but there were other odd findings that need to be put to bed once and for all, if it can be done. If I have to keep the extra birds, then I will, inconvenient though it may be, but if there is no reason to, I need to know that. I guess you can tell I'm frustrated and unsettled about the whole thing and it's been so darn long now. But, this state does free necropsies, and I'm not a commercial operation, so they may be very backlogged, which would make sense that it may take a full month to get a final report.

 

[seminolewind]

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