@Sonya9...if by "number of adolescents that are more susceptible" you mean the number of chicks that are between 8 - 20 weeks old (the age at which the incidence of contracting
Marek's is highest), then yes it does.
Since Marek's often targets adolescents, and there are more adolescents in the summer then that could account for the increase in cases.
On the other hand sunlight has been linked to causing herpes simplex outbreaks in humans, it appears to be triggered by skin exposed to sunlight. Course chickens are covered in feathers so one would not expect their skin to absorb as much UV.
Also plenty of chicks are born in the summer months, and if they live in colder climates they would be stressed during winter (the cold and being possibly stuck in a coop) which one would think could trigger Mareks, perhaps it is related to the heat.
The scientists believe that the reason the sun triggers herpes outbreaks is due to the negative effect of ultraviolet radiation on the human immune system.
Another hypothesis is that solar ultraviolet radiation may stimulate the herpes simplex virus in nerve endings, called nerve ganglions.
Several other clinical studies confirm the fact that it is the ultraviolet radiation in sunlight that triggers herpes outbreaks in both oral and genital herpes.