Describe the epidemiology and control of the important vector-borne diseases.

Malaria and Dengue

• Malaria — caused by plasmodium vivax and P. Falciparum in India (with malariae and ovale rare), transmitted by the female anopheles mosquito
• The vector characteristics that determine control — Anopheles breeds in clean, still or slow-moving water, bites at night (dusk to dawn), and rests indoors after feeding in most Indian species
• Those three facts explain the entire control strategy — night biting justifies bed nets; indoor resting justifies indoor residual spraying; and clean water breeding directs source reduction to wells, tanks, overhead reservoirs and rice fields rather than to drains
• P. Falciparum causes severe and cerebral malaria and death; P. Vivax relapses from dormant hypnozoites in the liver, which is why primaquine for 14 days is required for radical cure and why G6PD deficiency must be considered before giving it
• Diagnosis — microscopy of thick and thin films remains the reference standard, with rapid diagnostic tests used widely in the field
• The cardinal rule is parasitological confirmation before treatment — presumptive treatment of every fever wastes drugs, drives resistance and misses the real diagnosis
• Treatment — artemisinin-based combination therapy (act) for falciparum, never artemisinin alone, since monotherapy selects resistance; chloroquine with primaquine for vivax; and parenteral artesunate for severe malaria
• The national framework for malaria elimination targets elimination by 2030, with states categorised and case-based surveillance and investigation of every case in low-burden areas
• Dengue — a flavivirus with four serotypes, transmitted by AEDES aegypti
• The Aedes characteristics are the opposite of Anopheles in every respect that matters, and that contrast is worth drawing explicitly:
• Aedes breeds in small artificial collections of clean water — tyres, coolers, flower pots, tanks, discarded containers; bites by day, particularly early morning and late afternoon; is a strongly urban, domestic mosquito; and has a short flight range of about 100 metres
• Hence dengue control is source reduction in and around the house, and cannot be achieved by night-time bed nets or by indoor residual spraying
• Its short flight range means the breeding site is almost always within the premises or next door, which makes household inspection effective
• Clinically — dengue fever, dengue with warning signs, and severe dengue, in which the danger is plasma leakage and shock
• The critical clinical point is that deterioration occurs as the fever settles (the defervescence or critical phase), which is precisely when families assume the child is recovering
• Management is fluid replacement and monitoring; avoid aspirin and NSAIDs; and second infection with a different serotype carries a higher risk of severe disease, through antibody-dependent enhancement
• Chikungunya and ZIKA share the same Aedes vector, and therefore the same control measures

Other Vector-borne Diseases and Vector Control

• Lymphatic filariasis — Wuchereria bancrofti transmitted by CULEX quinquefasciatus, which breeds in dirty, polluted water and drains — a third distinct breeding preference
• Control is by mass drug administration with diethylcarbamazine and albendazole, now with ivermectin as triple therapy, given annually to the whole population of endemic districts; together with morbidity management and disability prevention for existing lymphoedema and hydrocele, which drugs cannot reverse
• Kala-azar (visceral leishmaniasis) — Leishmania donovani transmitted by the sandfly (Phlebotomus argentipes), which breeds in cracks in mud walls and organic debris; endemic in BIHAR, Jharkhand, West Bengal and eastern Uttar Pradesh
• Control — indoor residual spraying, early diagnosis with the rK39 rapid test, and treatment with single-dose liposomal amphotericin B; with attention to post kala-azar dermal leishmaniasis, which acts as the human reservoir between epidemics
• Japanese encephalitis — transmitted by CULEX mosquitoes breeding in rice fields, with pigs as amplifying hosts and ardeid birds as the natural reservoir
• Man is a dead-end host, so human-to-human transmission does not occur and eradication is impossible; control is by vaccination of children in endemic districts, pig management and vector control
• Other vector-borne infections — plague (rat flea), scrub typhus (trombiculid mite, an increasingly recognised cause of acute febrile illness in India), and leptospirosis (rodent urine, water contact)
• The methods of vector control, which form the core of any answer:
• Environmental (source reduction) methods, which are the most sustainable and should be listed first — filling and drainage, water management, proper storage and covering of water, removal of discarded containers and tyres, weekly emptying of coolers and pots, and improved housing and sanitation
• Chemical methods — indoor residual spraying, effective only against endophilic (indoor-resting) vectors; space spraying and fogging, which kills adult mosquitoes only and gives brief relief during an outbreak, and is greatly over-used as a visible but ineffective gesture; larvicides, chemical and oil-based; and insecticide-treated and long-lasting insecticidal nets, which are among the most cost-effective interventions in public health
• Biological methods — larvivorous fish (Gambusia and Poecilia), Bacillus thuringiensis israelensis, and newer approaches using wolbachia-infected Aedes
• Personal protection — bed nets, repellents, protective clothing, screening of houses and avoiding exposure at biting times
• The principle of integrated vector management — combining several methods, chosen on the basis of local vector bionomics and insecticide resistance data, with community participation and intersectoral action
• Insecticide resistance is the central threat, and arises from indiscriminate and repeated use of a single compound; it is managed by rotation of insecticide classes and by routine resistance monitoring
• The unifying lesson — one must know where the vector breeds, when it bites and where it rests before choosing any control measure; and applying the malaria strategy to dengue, or the reverse, is guaranteed to fail
• Know where it breeds, bites and rests before choosing any control measure, or the measure will not work
• Anopheles breeds in clean water and bites at night, which is why bed nets and residual spraying are effective
• Aedes breeds in household containers and bites by day, which is why only source reduction controls dengue
• Aedes flies about a hundred metres, so the breeding site is almost always within the premises or next door
• Fogging kills adults for a day and is a visible gesture rather than a control measure
• Culex breeds in dirty polluted water, unlike both the others, which is the third distinct preference to know
• Confirm malaria parasitologically before treating, since presumptive treatment wastes drugs and misses the real diagnosis
• Never give artemisinin alone, since monotherapy selects resistance in the one drug class that still works
• Vivax relapses from hypnozoites, so radical cure needs fourteen days of primaquine
• Check G6PD before primaquine, since haemolysis is the predictable harm in deficient patients
• Dengue deteriorates as the fever settles, which is exactly when families assume recovery has begun
• Avoid aspirin and NSAIDs in dengue, given the bleeding risk, and manage with fluids and monitoring
• Second infection with another serotype is more severe, through antibody-dependent enhancement
• Man is a dead-end host in Japanese encephalitis, so eradication is impossible and vaccination is the mainstay
• Pigs amplify Japanese encephalitis virus while ardeid birds are the natural reservoir, which shapes control
• Post kala-azar dermal leishmaniasis is the reservoir between epidemics, and must be sought and treated
• Mass drug administration controls filariasis, but cannot reverse existing lymphoedema or hydrocele
• Morbidity management matters as much as drugs in filariasis, since the disability persists after transmission stops
• Insecticide resistance follows repeated single-compound use, and is managed by rotation and routine monitoring
• Larvivorous fish are a biological method, cheap and sustainable in wells and ornamental tanks
• Treated nets are among the most cost-effective interventions in all of public health, not merely in malaria
• Source reduction is the most sustainable method, and should be listed first rather than last
• Indoor spraying works only against endophilic vectors, and is wasted on mosquitoes that rest outdoors
• Scrub typhus is an increasingly recognised cause of acute febrile illness in India and is easily missed