The dengue fever has spread in Kerala just after the monsoon onset, which is alarming. The main reason is the sudden increase in the population of aedes aegypti, the day-biting mosquito. The threats from mosquitoes are manifold. They reduce real estate values, affect tourism and related business interests, besides impacting livestock and poultry production. Mosquitoes transmit dengue, malaria, filariasis, chikungunya, encephalomyeletis, zika, yellow fever etc.
Mosquito behaviour
Mosquitoes are insects showing crepuscular behaviour and conducting mass migration called swarming at dusk and dawn in places where human beings are located. They have feeble flying ability so that they can fly only up to hundred metres or less and up to a height of around 30 metres. Only female mosquitoes bite human beings to get the blood proteins for the maturation of their eggs. Humidity, rain, climactic changes etc can suddenly increase the population of mosquitoes.
Both male and female mosquitoes rest in plants during day time and feed plant sap. But in the evening, female mosquitoes seek human beings for a blood meal. Mosquitoes have complex methods for detecting the hosts and different types of mosquitoes react to different stimuli. They have chemoreceptors in the head to sense the presence of human beings. Body temperature is one factor that attracts mosquitoes. Many mosquitoes are attracted to the slightly cooler temperature of the extremities of body like hands, foot, nose and ear. Excess carbon dioxide in the closed rooms attracts mosquitoes. Humidity is another factor. Mosquitoes are attracted by perspiration because it increases the humidity around the body. Indoor plants, aquarium etc inside the home attracts more mosquitoes due to humid conditions. Some perfumes, floral decorations, cosmetics, fragrant cloths, dark clothing etc can also attract mosquitoes.
The female mosquito feeds off our blood by piercing the skin with the proboscis, a modified mouth part. While sucking our blood, mosquito deposits some saliva into the skin. The saliva contains anticoagulant proteins that remain in our body and react with the body chemicals, resulting in the characteristic itching and bump. The proteins present in the human blood are essential for the maturation of egg yolk in mosquitoes. On the first day of emerging from the pupa, female mosquitoes do not bite and on the second day, it mates and starts feeding human blood. After two or three days, it lays thousands of eggs in water and dies off.
How does aedes feed?
The female aedes aegypti feeds almost exclusively on human blood; however, it can also feed on other hosts like bovine, chicken etc. Feeding on humans generally occur at one to two-hour intervals preferring to bite typically from below or behind, usually the feet and ankles. The feeding habits of aedes aegypti is anthropophilic, endophagic and endophilic which means that they are biting mosquito, prefer to stay indoors and resting indoors after blood feeding while the blood meal is digested and egg matures respectively. It is believed that the dengue originated in Africa and later spread throughout the world.
Dengue infection
The characteristics symptoms of dengue infection include sudden onset of fever, headache, muscle and joint pain, skin rashes etc. Because of the severe joint pain during dengue infection, the disease is also known as 'breakbone fever.' The course of dengue infection is divided into three stages. Febrile phase - During this initial stage, high fever develops and may rise above 104 degree fahrenheit for 2-7 days. This is associated with headache, vomiting, skin rashes, joint and muscle pain etc.
Critical phase - It comes after the febrile phase, if not treated properly. The critical phase includes low platelet count less than one lakh per micro litre of blood, increased porosity of blood vessels and plasma leakage, low blood pressure, chances of internal bleeding etc. Critical phase is most important and proper monitoring and care must be given to save life. The critical phase lasts 1-2 days. Recovery phase - It occurs with the reabsorption of the leaked fluid back into the blood over a period of 2-3 days.
How do we contract dengue?
When an aedes aegypti bites a dengue patient, along with the blood, the dengue virus enters the gut of mosquito. It first infects the gut epithelium and later moves to the hemolymph from where it reaches the salivary glands. The dengue virus comprises four antigenically distinct serotypes called DEN V -1, DEN V -2, DEN V -3, and DEN V -4. These four serotypes are almost same in physical appearance with almost the same kind of antigens on the outer surface. Aedes aegypti transmits all these four serotypes causing dengue fever. It is estimated that 7-14 days are required before the mosquitoes transmit the virus to a new host. But actually, 4-5 five days are required for the viruses to appear in the salivary glands of the infected mosquito. After the 10th day of post-infection, the amount of viral antigens decreases in the body of mosquito. Interestingly, the dengue virus causes no harm to the mosquito and it takes only a space for multiplication.
Dengue virus is primarily transmitted by aedes mosquitoes, particularly aedes aegypti. These are day-biting mosquitoes which bite human beings during the early hours of the day and then in the evening. Dengue can also be transmitted through infected blood products, during organ transplantation, and sometimes vertical transmission is possible during pregnancy. Researches revealed that unlike other mosquitoes, aedes exhibits transovarian transmission of dengue virus. That is, the infected aedes transmits the virus to the progeny through eggs. This maintains a virus reservoir in the environment.
Impacts on body
If an infected mosquito bites a healthy man, it introduces the virus in his blood. The dengue virus primarily attacks the white blood corpuscles for multiplication. It also invades other cells of the body through the blood. The virus injects its RNA into the cell where it takes the control of the cell functions. Then the cell starts to produce viral proteins and other materials for the virus progeny. The nucleic acid then replicates to form the new viruses. During the infection, the white blood cells secrete chemicals like cytokines and interferon that initiates the symptoms of dengue fever. The symptoms first appearing are fever, skin rashes and headache which are collectively known as dengue triad.
In severe conditions of dengue infection, the virus infects the bone marrow cells called megakaryocytes which are the mother cells of platelets. Usually 1.5 - 4.5 lakh platelets are present per micro litre of blood. Due to the dengue infection in the megakaryocytes, they fail to generate platelets and the platelet count decreases below one lakh. Decrease of platelet count to 20,000 is the most severe condition because if an internal bleeding occurs during this stage, it may become fatal. This kind of dengue infection is called haemorrhagic dengue fever.
Another condition that turns dengue fatal is the plasma leakage. Plasma is the fluid part of blood. Dengue infection may increase the porosity of the blood vessel wall leading to plasma leakage. This can drop the blood pressure severely and may lead to a fatal condition called dengue shock syndrome.
Is dengue reappearing?
Some studies say so. Dengue infection produces antibodies in human body against the virus. Even though the antibodies remain in the body for long period, sometimes secondary infection occurs. This is due to the 'triking 'of the immune system by the dengue virus. There are four serotypes of dengue like DEN V-1, DEN V-2, DEN V-3 and DEN V-4. Suppose a DEN V-1 infection occurs in a person, his/her body will produce antibodies against DEN V -1. After some period, the same person gets a DEN V- 2 infection.
Antibodies already produced for DEN V- 1 will start to bind with the DEN V-2 virus since the outer antigens of all the four serotypes are almost same. But the antibodies produced for DEN V-1 fail to inactivate the DEN V-2 virus due a difference in antigenicity. So dengue infection starts. But if the same DENV- 1 is infecting the same person again, the person will get immunity against the infection. But it is a chance only since all the four serotypes are present in mosquito population. Moreover, subtypes of DEN V are also present as mutant varieties.
How to fight dengue?
Dengue spreads when the population of infected aedes aegypti increases abnormally. Unlike other mosquitoes, aedes prefers day biting. It likes to sit indoor during day time so that the chance of mosquito bite is high. Aedes usually comes through the back side of body and bites the feet and ankle since there are large veins there just beneath the skin. During the bite, the anticoagulant present in the saliva of mosquito prevents the clotting of feeding blood. So we will not feel the bite. When the mosquito completes the feeding, blood starts to clot at the site of bite and we feel pain and itching. Some allergic responses like itching and swelling also develops at the site of bite. The dengue fever is almost under control due to public awareness and adequate control measures. Let us hope for a dengue-free environment in the coming days.
No mosquito bite, no dengue
We can take many precautionary measures to avoid contracting dengue.
Clean indoor and outdoor: Aedes prefers indoor life and usually hides in dark places of home, bathrooms, dressing rooms, indoor plants, curtains etc. So cleaning such places can expel the mosquitoes from the home. Adequate sunlight and air flow in the rooms can expel the mosquitoes to outside.
Keep containers dry: Aedes is a "container breeder". So small quantity of stagnant water present in bottle caps, flower vases etc is the good choice for egg laying.
Empty water filled coconut shells, bottle caps, discarded plastic pieces, rubber tyres and anything that carry an amount of just a teaspoon of water.
Monitor house continuously: Aedes eggs can survive desiccation and it can survive for some days even without water. They will develop when they get water again. So monitoring the premises of home should be continuous for the effective control.
Carry out mosquitoes fogging: Fogging is the method in which, an insecticide is mixed with a solvent like diesel and pressure pumped as droplets to the air. This can kill most of the mosquitoes in an area.
Screened windows and doors can reduce the exposure to mosquitoes. Remove water from coolers and other water containers at least once in a week.
Use mosquito nets or mosquito repellents while sleeping during day time.
(The writer was associate professor and HoD of zoology (retd), Government College for Women, Thiruvananthapuram)
