Samoa guide. 32. Mosquitoes, Filariasis & Dengue Fever

The female mosquito finds you by following signals given off by your body such as body heat, odors and carbon dioxide. Light and movement also help her zero-in on you. She then sinks her long and thin mouth parts into your skin and sucks out a drop of blood, engorging so much that she can barely fly away. During the biting process, she secretes saliva into the cut to prevent the blood from coagulating. Your body’s reaction to this saliva is what causes the bite to itch and swell.

Mosquitoes can carry two diseases in American Samoa: filariasis and dengue. Both can cause serious problems. These diseases are spread by specific mosquito species: dengue (principally Aedes polynesiensis and Ae. aegypti) and filariasis (mainly Aedes polynesiensis, and Oc. samoanus).

Filariasis, also known as elephantiasis (tutupa, mumu), is caused by parasitic nematode worms called filariae. When an infected mosquito bites you, these minute worms crawl from the mosquito’s mouthparts onto your body and into the cut. This is not a particularly efficient way to enter your body, so the number of them transmitted from a single mosquito bite is probably limited. Multiple mosquito bites by infected A. polynesiensis may be needed before a person develops the full-blown disease. The worms lodge in a person’s lymphatic system where they can live for 4-6 years and produce millions of minute larvae (microfilariae). These larvae circulate in your blood and are sucked up by other
mosquitoes that bite you and thus can be spread to other people. In severe cases, filariasis can cause a very large swelling of arms, legs or genitals.

The number of people infected in American Samoa is considerable. In two surveys, people in several villages here were examined for filariasis and 17% (in 1999) and 13% (in 2002) were found to be currently or recently infected. But not all these people have swollen limbs. In general, about half of the people who are infected show no symptoms of the disease, but they may develop them as they get older. Our local Public Health Department together with the Center for Disease Control has joined a
worldwide program to reduce filariasis by giving every person on island a medication that kills the microfilariae circulating in your blood, thus preventing their further spread by mosquitoes. The cycle of infection can be broken if the number of people carrying this disease is significantly reduced.

Dengue fever is also transmitted by mosquitoes. Dengue is a viral disease that is rapidly expanding in tropical and subtropical areas of the world. It’s not native to American Samoa but is periodically brought here by travelers from other infected areas. It then gets spread around primarily by the daytimebiting Aedes polynesiensis and Ae. aegypti mosquitoes. This virus is present in the mosquito’s saliva, so it is injected directly into you when an infected mosquito bites you. Symptoms range from mild fevers to severe and potentially life-threatening illness. No vaccine is available yet.

Will tourists to American Samoa pick up these diseases? Not too likely. First, dengue is usually not present here, only when an infected traveler brings the disease here. Second, filariasis probably requires multiple bites by specific mosquito species, and probably only a small percent of this species actually carries the disease at any given time.

Scientists predict that we may see an increase in such diseases due to global warming and climate change. Although exact mechanisms are not known, many pathogens are sensitive to temperature; for example, growth, reproduction, and biting rates of insects all increase with higher temperatures. Global warming appears to be triggering a number of disease epidemics worldwide, involving a diversity of pathogens (viruses, bacteria, fungi, parasites) and a wide range of hosts (humans, corals, oysters, terrestrial plants, birds).

There are several basic measures that can be taken to reduce mosquito-borne diseases:

1. Find their breeding sites. Mosquitoes require water to complete their life cycle, so removing pockets of standing water around your house will help reduce their numbers. Survey your yard and eliminate areas where water can collect, such as discarded tires, buckets, coconut shells, saimin bowls, aluminum cans, cemetery urns, clogged roof gutters, etc. Drill holes in the bottom of large containers so that the water does not accumulate in them, or cover large objects like rain barrels with screening.

2. Avoid getting bitten. When the mosquitoes are out, wear long-sleeved shirts and long-pants. Use repellants and insecticides safely. Repair window screens on your house, and if needed, sleep under mosquito net tents while resting during the day and night.

3. Take filariasis medicine. Remember, you can be infected with the filariasis worm and not have symptoms. The filariasis medicine is free, so take it every year to break the cycle. If the health workers miss coming to your house each year, go to the hospital and ask for the medicine.

4. Be thankful for the swiftlets (pe’ape’a). They eat mosquitoes (and other flying insects).

Mosquitoes may play a role in the grand scheme of things, but I’m not really keen on the idea that they want to suck my blood. Worse still, they can infect us with two diseases in American Samoa -- more about that shortly. At least 12 species of mosquitoes (namu) occur in American Samoa. Most are common across the western South Pacific region, but some are endemic to the Samoan islands (Aedes upolensis, Coquillettidia samoaensis, Ochlerotatus samoanus, Oc. tutuilae) and a few are newly introduced. Their life cycle involves several stages, one is the irritating adult mosquito, but their juveniles live in water. To reproduce, the adult female needs to obtain a blood meal (from you) to develop her eggs. She then lays her eggs in quiet puddles of water where they hatch and grow. The larvae breath air, so they generally hang around at the water surface but will wiggle quite actively when disturbed. After a week or two, the larvae pupate (a resting stage) and then emerge as flying adults. Females may live for 2-3 weeks and lay several clutches of eggs (each requires a blood meal). Only the female mosquito searches for a blood meal; the males (the ones with fuzzy
antennae) feed on plant nectar. Also, not all the mosquitoes are after you personally -- some species seek out birds as their preferred blood meal.