Welcome back to Microbial Mondays after the Easter long weekend! This week will kick us off on a three-part series on viruses, and the many roles they can play in human life. Today, we'll start with a virus that causes human disease, and has been called the most important contemporary arbovirus (which just means viruses transmitted by arthropods, such as ticks, fleas, and mosquitos): Dengue Virus.
Dengue virus has been estimated to infect 400 million people each year, and rates of infection have doubled each decade since the 1990s. The virus is also expected to travel northward in the future, partly due to climate change. This northward bound trend is not due to the virus intrinsically, but due to changing habitat of its 'animal vector', the arthropod that transmits it: the Aedes aegypti mosquito. This is the same mosquito that transmits the yellow fever virus, the Zika virus, and the chikungunya virus, meaning that these diseases could also begin to travel North.
Dengue virus is interesting in that it is only transmitted from human to mosquito and back again: not directly between humans. Infected mosquitos, however, do pass on the virus to their offspring, meaning that the virus can survive over the winter in mosquito eggs, after all adult mosquitos have died off and humans have recovered.
Mosquitos are infected when they bite humans carrying the virus. When Aedes aegypti mosquitos are infected, the virus replicates (makes more of itself) in the bug's salivary glands. This is at the root of the subsequent mosquito infection of humans. When mosquitos bite, they inject their saliva, which contains material that keeps human blood flowing smoothly, into the wound. Dengue virus travels with the saliva into the human skin when the feasting mosquito is infected.
When an infected mosquito bites a human, the virus is shot into the human bloodstream and into the space between tightly packed individual human skin cells. This is where the human immune system comes in. In human skin, there are immune cells that patrol the tissue looking for any unwanted invaders. Upon recognizing anything suspicious, i.e. non-human, these cells will eat it up, then travel through the body to lymph nodes. Lymph nodes are sort of like the fire stations of the immune system. They spring into action once they get a signal that something is wrong in the rest of the body, and then send out immune cells to fix the problem.
The dengue virus takes advantage of this normally beneficial (for the human, anyways) process. The virus cruises along inside the 'alert' cells which travel to the lymph nodes, and then attacks all the immune cells in the lymph node station once it arrives there! The lymph nodes throughout your body are also all connected, making things worse: the virus quickly spreads all across the poor infected human.
The symptoms people have after infection with Dengue Virus depend on a lot of different factors. If you are lucky, and have the right genetic background and are infected with a less virulent (causing less serious disease) virus, you might only get symptoms similar to those of a bad case of flu. If you're less lucky, however, you can end up with Dengue Haemorrhagic Fever.
Dengue Haemorrhagic Fever only occurs in a small percentage of cases, but it simply looks scary. In patients who develop Dengue Haemorrhagic Fever, the virus triggers an enormous over-activation of the immune system. Scientists call this a "cytokine storm". Cytokines are sort of like letters mailed out by your cells that tell other cells, "GO!". When Dengue virus sends all the letters out at once, it overwhelms the "postal system": your immune system. The result is not only a high fever, but also leakage of blood from its vessels. Patients with Dengue Haemorrhagic Fever may bleed internally, but also from orifices - from the gums, ears, nose, mouth.
As of now, only general treatment like aspirin for mild cases or oral rehydration therapy for Haemorrhagic fever is available for people infected with Dengue virus. There is no "specific treatment", for instance drugs that target the virus itself. For now, we can only try to treat its symptoms. Dengue virus is one of many microbes that have been causing serious disease for a long time, but for which there is no vaccine or specific treatment. Historically, Dengue only caused problems in South America and Southeast Asia. This shows how microbes are tied up with the economics of the human world: it is often only when disease starts to effect the richest humans that humans target the responsible microbe for destruction.
Until next week - swat away those mosquitos!
- Alex