As we know, many viruses, including Dengue, West Nile, Yellow Fever, etc., are transmitted by insect vectors. While these insects carry the virus, they do not necessarily exhibit any symptoms of infection. Due to the importance of these insects in the transmission of viruses, researchers have been looking into how the insect vector responds to different viral infections. A recent study using fruit flies has shown that depending on how the insects are infected, they have different immune responses.
In order to infect an insect for viral transmission to other species, the virus has to be able to replicate. Just like humans, insects also have ways of defending themselves against infection. One way insects do this is by preventing the virus from translating its genome. The other way insects protect themselves is through bacteria that they host.
When studying infections in insects, typically to mimic bites from other insects, which is the usual route of transmission of the virus, the researchers infect their model organisms using injections. However, in nature, often the route of transmission is oral and occurs when the insect consumes food that contains the virus.
Recently, it has been discovered that the immune response that occurs when the virus is transmitted to insects orally is different from when it is injected. When oral consumption of the virus occurs, the immune response that occurs is mostly regulated by the Toll pathways. When injection of the virus occurs, the Toll pathways are not activated as much and play a less important role in the response against infection. One difference between the routes of transmission, however, is the amount of time it takes for the virus to spread through the organism. With injection, likely due to the ease at which the virus enters the circulatory system, the infection becomes systemic much quicker than when the virus is consumed. When consumption of the virus occurs, the infection takes a few days to take hold.
One lesson to be learned from this research is that often studies done in labs are not necessarily indicative of responses in the real world. The protocols themselves often bias the results researchers find, as in this case. This caution should be applied to all studies. Organisms are often more complicated than what can feasibly be replicated in the lab.