In an earlier Program for the Gifted Talented (PGT) course titled ‘Biochemistry and Infections‘, we came across infectious diseases caused by viruses, such as dengue fever (DF). The current local cases of dengue fever in Hong Kong present a good illustration of that disease.
The dengue virus is an example of enveloped RNA virus (with positive-sense single stranded RNA; positive-sense RNA means that the genetic materials of the RNA strand is similar to that of the messenger RNA). Dengue virus belongs to the genus of flaviviruses. Many of the viruses from the same genus are pathogenic; examples include Japanese encephalitis virus and Zika virus. From these well known examples, we may realize that the viruses are usually transmitted by insects, such as mosquitoes or ticks; therefore, the prevention of these diseases is best achieved by mosquito control, for instance.
In general, flaviviruses can cause symptoms, such as fever, bleeding, encephalitis, paralysis, jaundice, etc; yet, in DF, the real problem is often resulted from the secondary infection in adults; in other words, people with prior dengue virus infection are more susceptible. While the exact mechanism of the pathophysiology of DF is not fully understood, it is generally hypothesized to be related to the immune system. If a person is infected by one type of dengue virus, some antibodies will be induced; when the same person is infected by another type of dengue virus subsequently, the antibodies remained may not be the most specific to initiate the expected protective effect during the secondary infection triggered by another type dengue virus. In contrast, those antibodies might facilitate the viruses to enter a group of immune cells, called the antigen presenting cells (APC). This might enhance the viral replication inside the APC, which might eventually lead to some drastic immune responses resulting in severe illnesses, such as dengue haemorrhagic fever (DHF). DHF, for example, is characterized by the increase in the permeability of blood vessels, which is, in turn, caused by the inflammatory responses (cytokine storm) mediated by the vigourous activities of the immune cells.
In the laboratory, DF and DHF are confirmed by the diagnostic tests of antibodies present in the blood isolated from the suspected patients. While the diagnosis by antibodies reasonably suggests the role of the immune system in DF and DHF (but still, there is no direct demonstration of how the immune system leads to DF or DHF). In my humble opinion, the description above is still not a complete picture. Scientists will let us know more and more through the continual effect on their research in the future.