Viruses are experts at hijacking cells to replicate, manipulating the conditions in the cell to optimize viral processes. But they manipulate their hosts on a higher level too, sometimes manipulating host behaviour to increase the chances of transmission. Take rabies virus for example: the virus induces aggression, then replicates abundantly in the salivary glands and stimulates salivation. The aggressive host is driven to bite, spreading the virus to its next host.
The basis for these bahavioural changes is poorly understood, on both the viral and host ends. However an experiment published in Science recently identifies the genetic basis for host behaviour manipulation by a baculovirus that infects the gypsy moth. When a baculovirus infects its host, the host eventually dies in a gruesome death appropriately called “virus melt.” The insect is liquefied, and the gooey, virus laden liquid drips down from the remains of the host on to the leaves below. Unsuspecting insects will then eat the contaminated leaves, becoming infected themselves.
So how could a virus maximize the dissemination of said liquid? How about having the host climb to the top of the plant and stay there to die, dripping all the liquid on the leaves below? The normal behaviour of the gypsy moth is to climb up a tree and munch on leaves during the night, and hide in crevices or climb down to the soil during the day, thus avoiding predation by birds. This behaviour is regulated by a hormone, 20-hydroxyecdysone, which tells the gypsy moth when to stop feeding and move down the tree (it also regulates molting and pupation). Baculovirus infected gypsy moths, however, climb up but don’t climb back down, staying up in the tree to die.
Baculoviruses expresses a gene that deactivates 20-hydroxyecdysone and prevents the infected host from leaving its “feeding state” and descending the tree. When researchers deleted the gene from the virus, the infected gypsy moths displayed normal behaviour.
I’d love to see a transmission experiment to see if the presence of this gene really helps with transmission to the next host.
(Also discussed on TWiV)
