World Health Day on 7 April focused on vector-borne disease – a fancy way of describing infections in which an insect carries the bug to humans. The best known vector is the mosquito which transmits the malaria parasite.
Now that summer is almost here it may be useful to look at what our unfriendly neighbourhood mosquitos are up to, especially in spreading dengue fever which has seen a huge surge in tropical South Asia in recent times. In recent years dengue has spread across the Tarai in summer, and even Kathmandu has seen the occasional patient.
This spread is not surprising because there is a migrant population with the disease and an abundant supply of the carrier mosquitoes. Because of the porous border with India, many migrant labourers (both Nepalis and Indians) with the disease enter Nepal. The vector mosquito enjoys a blood meal from these infected patients and transmits the disease to a healthy person after a bite.
More importantly, different species of mosquitoes transmit different diseases. So, the mosquito that transmits malaria will not transmit dengue. Mosquitos that transmit dengue are called Aedes while it is the female Anopheles that transmits malaria, and the Culex carries Japanese encephalitis.
The diagnosis of dengue should be strongly considered in any patient presenting with fever that has developed within 14 days of even a brief trip to dengue- endemic places like Bangkok and Delhi. The dengue mosquito is generally a city dweller unlike the malaria mosquito, which prefers a rural environment. In most people, dengue illness, which comprises of fever, headache, eye pain and joint aches, subsides in about a week’s time. Doctors will treat only the symptoms in dengue as there is no specific antidote.
For prevention, use mosquito repellents and wear full length clothing even during the day since the dengue mosquito is a day time mosquito unlike the malaria mosquito which is nocturnal. Turn over watering cans and pails after use so that the dengue mosquito does not breed in them. Although typhoid fever, commonly seen in Nepal, is not a vector-born disease as it is transmitted by eating or drinking contaminated food or water, typhus (which almost sounds like typhoid and has lead to confusion) is vector-borne.
Definitive studies done at Patan Hospital in collaboration with Oxford University have established that murine typhus fever is a very common cause of fever in Nepal. Murine typhus organism (called rikcettsiae) is carried by fleas which live on rats. These fleas transmit the disease to humans when the flea faecal material contaminate the area where the flea bite takes place. So even the vector (flea) is being carried by another animal (rat) before it finally reaches humans.
Clearly, diseases like murine typhus thrive in places where housing is poor and the environment contaminated. Hence the disease exacts its heaviest toll where the poor live. Indeed, on World Health Day, WHO’s Margaret Chan said: “The control of vector-borne diseases can make a major contribution to poverty reduction, as it precisely targets the poor.”
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