Leaky Vaccines: Marek's Disease
1 Overview of Marek's Disease and Leaky Vaccine
"The Marek’s vaccine is “imperfect” or “leaky.” That is, it protects chickens from developing disease, but doesn’t stop them from becoming infected or from spreading the virus. Inadvertently, this made it easier for the most virulent strains to survive. Such strains would normally kill their hosts so quickly that they’d die out. But in an immunised flock, they can persist because their lethal nature has been neutered. That’s not a problem for vaccinated individuals. But unvaccinated birds are now in serious trouble."
"This problem, where vaccination fosters the evolution of more virulent disease, does not apply to most human vaccines. Those against mumps, measles, rubella, and smallpox are “perfect:” They protect against disease and stop people from transmitting the respective viruses. “You don’t get onward evolution,” says Read. “These vaccines are very successful, highly effective, and very safe. They have been a tremendous success story and will continue to be so.”"
"They found that when unvaccinated birds are infected with mild strains, they shed plenty of viruses into their surroundings. If they contract the most lethal strains, they die before this can happen, and their infections stop with them. In the vaccinated chicks, this pattern flips. The milder strains are suppressed but the lethal ones, which the birds can now withstand, flood into the environment at a thousand times their usual numbers."
"Birds must get the vaccine before they are exposed to the virus.
"The best way to vaccinate the chicks is to have the hatchery vaccinate them.
"Birds become infected with Marek's Disease by inhaling virus-laden dander.
"Since infected birds are shedding virus, they will spread the disease as long as they are alive.
"Not all infected birds, however, will get sick."
Source: PennState Extension, Marek's Disease in Chickens, August 2018
https://extension.psu.edu/mareks-disease-in-chickens
2 Studies Showing Leaky Marek's Vaccine Causes Virulent Variants
-Vaccination of chickens has made Marek's disease more virulent
-Vaccinated chickens shed the virus on the birds around them
-Healthy unvaccinated birds die rapidly in the same cage as vaccinated, diseased birds
-Vaccines allow more deadly strains to persist
-Immunocompromised birds cannot vaccinate; vaccine overuse will be lethal to them
-Same idea as antibiotic overuse
-"When vaccines prevent transmission, as is the case for nearly all vaccines used in humans, this type of evolution towards increased virulence is blocked. But when vaccines leak, allowing at least some pathogen transmission, they could create the ecological conditions that would allow hot strains to emerge and persist. This theory proved highly controversial when it was first proposed over a decade ago, but here we report experiments with Marek’s disease virus in poultry that show that modern commercial leaky vaccines can have precisely this effect: they allow the onward transmission of strains otherwise too lethal to persist. Thus, the use of leaky vaccines can facilitate the evolution of pathogen strains that put unvaccinated hosts at greater risk of severe disease. The future challenge is to identify whether there are other types of vaccines used in animals and humans that might also generate these evolutionary risks."
Source: Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens, July 2015
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002198
"Vaccination of newly-hatched chicks with live vaccines has been widely used to successfully control MD since the early 1970s. However, vaccinated chickens still become infected and shed MDV."
Source: Use of Marek's disease vaccines: could they be driving the virus to increasing virulence?, February 2005
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