Several years ago I had a Silkie cockerel with the classic symptoms of Marek's disease. I took him to UC Davis where
he was euthanized and the diagnosis of Marek's disease was confirmed. I panicked. I was sure all my chickens were
going to die, including the cockerel's full blooded sister that hatched with him. I posted on numerous lists and many
people responded. Many people advised I immediately vaccinate all chicks from this day forward. Another person that
replied was a man named Rod Haefs. Rod is a dairy farmer in Minnesota. He has raised chickens since 1960 and is a
poultry judge. Rod said that from his experience, vaccinating for Marek's doesn't stop the virus, it allows infected birds
to breed non-resistant offspring. He advised breeding for resistance. After considering all the advice I received, I
decided to take my chances and see what happened. None of my other birds came down with Marek's disease. The
cockerel's sister was not a show quality bird but she was purchased by a breeder because she was resistant to
Marek's. I have never vaccinated for Marek's disease and I have never lost another bird to this nasty virus.
A link to the article below was recently posted on a Yahoo chicken list. It reinforced for me that I did the right thing
I am not opposed to vaccines in general. I do vaccinate for fowl pox and I have seen the ravages of polio and other
viruses in third world countries. But vaccines are not a magic bullet. I believe that in some cases they are not an
effective method of disease control, they actually make things worse. Here is the article, for what it's worth:
This chicken vaccine makes its virus more dangerous - BY Nsikan Akpan
July 27, 2015 at 3:01 PM EDT, PBS NEWS HOUR
The deadliest strains of viruses often take care of themselves — they flare up and then die out. This is because they
are so good at destroying cells and causing illness that they ultimately kill their host before they have time to spread.
But a chicken virus that represents one of the deadliest germs in history breaks from this conventional wisdom, thanks
to an inadvertent effect from a vaccine. Chickens vaccinated against Marek’s disease rarely get sick. But the vaccine
does not prevent them from spreading Marek’s to unvaccinated birds. “With the hottest strains, every unvaccinated
bird dies within 10 days. There is no human virus that is that hot. Ebola, for example, doesn’t kill everything in 10 days.”
In fact, rather than stop fowl from spreading the virus, the vaccine allows the disease to spread faster and longer than
it normally would, a new study finds. The scientists now believe that this vaccine has helped this chicken virus become
uniquely virulent. (Note: it only harms fowl). The study was published on Monday in the journal PLOS Biology.
This is the first time that this virus-boosting phenomenon, known as the imperfect vaccine hypothesis, has been
The reason this is a problem for Marek’s disease is because the vaccine is “leaky.” A leaky vaccine is one that keeps
a microbe from doing serious harm to its host, but doesn’t stop the disease from replicating and spreading to another
individual. On the other hand, a “perfect” vaccine is one that sets up lifelong immunity that never wanes and blocks
both infection and transmission.
It’s important to note childhood vaccines for polio, measles, mumps, rubella and smallpox aren’t leaky; they are
considered “perfect” vaccines. As such, they are in no way in danger of falling prey to this phenomenon.
But the results do raise the questions for some human vaccines that are leaky – such as malaria, and other
agricultural vaccines, such as the one being used against avian influenza, or bird flu.
Marek’s disease has plagued the chicken industry, it causes $2 billion in losses annually for fowl farmers across the
globe. The virus attacks the brain, spawns tumors in the birds and comes in different varieties or “strains”, which are
classified as “hot” or “cold” based on their brutality.
Andrew Read, who co-led the study, had heard about the severe effects of the hottest Marek’s strains before his lab
started studying the disease about a decade ago, but even he was surprised when he finally saw the virus in action.
“With the hottest strains, every unvaccinated bird dies within 10 days. There is no human virus that is that hot. Ebola,
for example, doesn’t kill everything in 10 days,” said Read, who is an evolutionary biologist at Penn State University.
In recent years, experts have wondered if leaky vaccines were to blame for the emergence of these hot strains. The
1970s introduction of the Marek’s disease immunizations for baby chicks kept the poultry industry from collapse, but
people soon learned that vaccinated birds were catching “the bug” without subsequently dying. Then, over the last
half century, symptoms for Marek’s worsened. Paralysis was more permanent; brains more quickly turned to mush.
“People suspected the vaccine, but the problem was that it was never shown before experimentally,” said virologist
Klaus Osterrieder of the Free University of Berlin, who wasn’t involved in the study. “The field has talked about these
types of experiments for a very long time, and I’m really glad to see the work finally done.”
Read’s group started their investigation by exposing vaccinated and unvaccinated Rhode Island Red chickens to one
of five Marek’s disease strains that ranged from hot to cold. The hottest strains killed every unvaccinated bird within
10 days, and the team noticed that barely any virus was shed from the feathers of the chickens during that time.
(The virus spreads via contaminated dust in chicken coops). In contrast, vaccination extended the lifespan of birds
exposed to the hottest strains, with 80 percent living longer than two months. But the vaccinated chickens were
transmitting the virus, shedding 10,000 times more virus than an unvaccinated bird.
“Previously, a hot strain was so nasty, it wiped itself out. Now, you keep its host alive with a vaccine, then it can
transmit and spread in the world,” Read said. “So it’s got an evolutionary future, which it didn’t have before.”
But does this evolutionary future breed more dangerous viruses?
This study argues yes. In a second experiment, unvaccinated and vaccinated chickens were infected with one of
the five Marek’s disease strains, and then put into a second arena with a second set of unimmunized birds, known
as sentinels. In particular, the team was interested in a middle-of-the-road strain called “595” and whether it would
It did. The virus spread to sentinel birds nine days faster if it came from a vaccinated chicken versus an unvaccinated
one. In addition, sentinels died faster when exposed to vaccinated chickens versus unvaccinated chickens.
“One way to look at that experiment is that shows vaccinating birds kills unvaccinated birds. The vaccination of one
group of birds leads to the transmission of a virus so hot that it kills the other birds, said Read said. “If you vaccinate
the mothers, the same thing happens. The offspring are protected by the maternal antibodies of the mother and that
allows the virus in the chicks to transmit before they kill the host. So they transmit and kill the other individuals.”
This trend persisted when the team tried the experiment in a setting meant to simulate a commercial chicken farm.
“At the moment, the vaccines are working well enough, and you can vaccinate every bird,” Read said. “There are
20 billion birds on the planet at any time; the vast majority are Marek’s vaccinated.”
However, both Read and Osterrieder worry about what might happen if Marek’s continues to change or if its vaccines
were to fail.
“If the virus continues to evolve, then it could be pretty devastating for the chicken industry, which is suffering quite a
bit right now in the U.S. with the influenza virus,” Osterrieder said.
Like Marek’s vaccines, vaccines for avian influenza are leaky. For this reason, they’re banned from agricultural use
in the U.S. and Europe. When bird flu breaks out in these western chicken populations, farmers must cull their herds.
However, Southeast Asia uses these leaky vaccines, raising the possibility for virus evolution akin to what’s happened
with Marek’s disease.
“In those situations, they’re creating the conditions where super hot avian influenza could emerge,” Read said.
“Then the issues become what does that mean when it spills over into other flocks, into wildlife or into humans.
Avian flu is the setting to watch for evolutionary problems down the line.”
Bird flu isn’t alone. The world’s first vaccine for malaria, which was recently approved by European Medicines
Agency, is also leaky. Vaccines for HPV and whooping cough can leak too; however it is unknown if this scenario
creates more dangerous viruses for each of these diseases.
“Our concern here, primarily and foremost, is whether this is going to happen with any of the vaccines that we give
to people,” said molecular biology James Bull of the University of Texas Austin, who specializes in the evolution of
viruses and bacteria. “But there is a lot we don’t know about how the scenario with Marek’s could apply to newer
To test the imperfect vaccine hypothesis in humans, you would need monitor the vaccine response for either a
large or isolated population for a long time. Doing this would allow a researcher to gauge how the vaccine interacts
with the virus and if that relationship is evolving. Does the vaccine merely reduce symptoms, or does it also keep
patients from getting infected and transmitting the virus?
Clinical trials for Ebola might be an arena for keeping an eye on this trend.
“It’s important that we pay close attention to the Ebola vaccine in the ongoing trials. We want to know if a person
who has been vaccinated and comes in contact with Ebola, whether there is any virus replication in that person and
whether that means there could be onward transmission,” Read said. “If those are leaky in humans, it would be
potentially very disadvantageous as it could help establish an endemic.”
However, in the end, Read said, leakiness isn’t a strike against these vaccines, but more motivation to conduct
surveillance of their effects after they exit clinical trials and enter the broader population. Take Marek’s disease for
“Even if this evolution happens, you don’t want to be an unvaccinated chicken,” Read said. “Food chain security and
everything rests on vaccines. They are the most successful and cheapest public health interventions that we’ve ever
had. We just need to consider the evolutionary consequences of these ones with leaky transmission.”