Mosquitoes in Hong Kong

In Hong Kong, mosquitoes are pesky, but currently rarely carry diseases.

For most of my time in Hong Kong, mosquitoes have been pretty much just an irritant – especially from spring to autumn, in and near woodland, and worst around dusk. But lately, we've seen some increase in two mosquito-borne diseases: dengue fever, and Japanese encaphalitis. Especially as the latter has a high fatality rate, this makes mosquitoes here potentially a good deal worse than pesky little things that make you itch.

BUT – right now – neither are mosquitoes a great threat; we've had few cases of dengue, and only a handful of encephalitis (three in 2004).

That's a vastly different situation to Hong Kong's early years as a British colony, when malaria was a major killer – and there were plenty of other nasty diseases, including plague, typhoid and cholera (see A History of Hong Kong by Frank Welsh). Happily, all these are now rare or absent; mosquito control programmes, especially on Hong Kong Island streams, helped eradicate malaria.

Hong Kong mosquitoes as a darn nuisance

Though you might encounter mosquitoes pretty much year-round in Hong Kong, they're worst from aroudn the time it starts warming up and raining more in March/April, till the cooler and dryer weather sets in around late October. My first March here, a friend remarked, "The day mozzies will be out soon," – and, sure enough, the little black-and-white striped daytime active mosquitoes shortly appeared.

Most mosquitoes here are active from around sunset. That's not so bad if you're out in the country in the daytime, and home by dusk. But if you're sitting having a drink or a meal in a place with mosquitoes it's not so great. And if, like me, you live in an area with mosquitoes, it's not wonderful either: I've found it best to close windows towards dusk.

I've found Hong Kong's mosquitoes are worst in areas with and close to trees. I've only once found them bad at Mai Po Marshes – where normally I'm not bitten at all, though I normally only visit during daytime.

I've also noticed the dusk and night mosquitoes are larger, and are or seem slower than the day ones – presumably as they operate undercover of darkness. The day ones, again perhaps in response to predators, seem to like staying low, maybe biting my ankles.

Attracting mosquitoes

Mosquitoes have evolved to home in on a range of signals that suggest a nice, warm-blooded animal is  close by: including carbon dioxide, sweaty smells, humidity, and warmth. Hmm, seems on a hike, I give off all the right messages.

Repelling mosquitoes

"Repelling mosquitoes" is perhaps something of a misnomer – since at least part of the trick seems to be to not give off or mask the signals that suggest you are around. Easy if you don't exhale CO2, don't sweat, and aren't warmer than your surroundings, but otherwise it partly means trying to minimise sweaty smells – take a shower, say.

Then, of course, you can put on repellent. N,N, diethyl-methyl toluamide – commonly known as DEET – is a chemical that's highly effective at repelling mosquitoes, but can reportedly cause some unpleasant effects on human skin. I heard it's not good for plastics, even – so if it can impact plastic, I figure I don't want it on my skin if possible, and favour alternatives. (Maybe DEET repellents – the high concentration, jungle formula kinds – would be advisable for places where, say, there's substantial malaria; and then is best applied mainly to clothing.)

Extra (Sept 08): research shows mosquitoes actually dislike the smell of DEET: DEET's Not Sweet To Mosquitoes, Groundbreaking Research Shows.

Avon Skin-So-Soft is an unlikely sounding repellent I was told of a few years ago. Figuring this wouldn't give me skin rot, but might even iron out a wrinkle or two, I got hold of some. I've found it works pretty well (against mosquitoes, that is; not for wrinkles). Reading around the Internet before writing this item, I came across a page mentioning it's not effective for long – maybe 30-40 mins, a tenth the time of DEET, and maybe I too have found it needs reappyling before overlong, but that's no big deal.

The June issue of Esquire  featured a homemade recipe for a repellent it reckoned would be very effective, with catnip as the main ingredient. I didn't know quite what catnip was; on net, learned it's a mint relative that's now a common weed in the US, has smell that can drive cats bonkers; and research at Iowa State University found it repelled mosquitoes ten times as effectively as DEET.

Soon after reading the Esquire item, I noticed a repellent called Mozzie Guard – with catnip as the main ingredient – in Mannings; bought it, but yet to use. (If and when I give it a go, I aim to report back here.)

There are various other repellents. Years ago, I tried citronella oil, but wasn't too impressed; lately had a go with a spray-on citronella concoction, and seemed ok. Also some using extract from tomatoes: work well for me, not for my wife…

I haven't tried electronic buzzers that are supposed to repel mosquitoes, but I've read they don't work.

Mozzie myths?

I've lately heard that eating bananas can encourage mosquitoes, while boosting Vitamin B can repel them. But seems some killjoy scientists have debunked these and other myths, according to the Ontario Ministry of Health and Long-term Care.

Keeping mosquitoes at bay

Current HK Government adverts with advice about mosquitoes recommend wearing light-coloured, long trousers and long-sleeved shirts. Well, I wonder if the people behind these ads spend much time outdoors here in summer: I find wearing long trousers and long-sleeved shirts almost unbearable, especially for hiking. Plus some mosquitoes happily bite through even trousers, or socks. So, for me, it's generally shorts, probably t-shirt, and repellent.

Light coloured clothing may be more worth considering: seems mosquitoes favour dark colours. I've even halted at times, put down black camera bag and seen mosquitoes heading for that and not me – maybe as black gives off more infra-red?

 One way of keeping mosquitoes from biting (so much) whilst outdoors is: keep moving, especially through shady areas where there are fair numbers of mosquitoes. Can perhaps stop in more open areas, where mosquitoes should be scarcer in daytime.

At home: closing windows helps of course (from around sunset, where I live). Air-conditioning is a boon: dry, cool air is the opposite of what mosquitoes are seeking.

Then, if mosquitoes still get in, maybe need mosquito net around your bed; perhaps treated with a suitable insecticide.

Slow-burning mosquito coils, and tablets heated on special electric pads, give off chemicals mosquitoes  avoid – but I'm always leery they also give off chemicals I shouldn't be inhaling.

Killing mosquitoes

Though I've slapped and clapped many a mosquito to an early grave (yes, I know, and me a bit of a bunny hugger), it's easier to get em using one of the electric things that look like mini plastic tennis raquets – these sizzle them in an instant.

Dengue Fever

Though there had previously been imported cases of dengue fever – when people who caught it elsewhere came to Hong Kong, and were diagnosed here – the first locally transmitted cases were in 2002.

In Hong Kong, the vector is Aedes albopictus, also known as the Asian tiger mosquito – which I believe is the same species my friend referred to as the "day mozzie". This small mosquito has some whitish bands. Because this mosquito thrives in urban environments, dengue is an urban disease – according to the US Centers for Disease Control, it's more likely to be caught in urban than rural areas.

Happily, though, although dengue is increasing worldwide, it apparently hasn't (yet) become endemic in Hong Kong. 

According to the WHO, " Dengue fever is a severe, flu-like illness  that affects infants, young children and adults, but seldom causes  death." While initial infections are usually not lethal – though fevers can be so bad, dengue has been  dubbed "bone-break fever" for the pain it can cause – second or third infections can lead to the more serious, potentially fatal   Dengue haemorrhagic fever, which the same WHO page describes as, "A potentially deadly complication that is  characterized by high fever, haemorrhagic phenomena–often with  enlargement of the liver–and in severe cases, circulatory failure."

Japanese Encephalitis

Dubbed the "brain bug" by the South China Morning Post – the WHO notes it can cause inflammation of the membranes around the brain – Japanese encephalitis is a widespread, mosquito transmitted disease that surfaced in Hong Kong this summer, with three cases to early July; two more in November (all 2004).

It's transmitted by Culex mosquitoes, which breed particularly in rice fields (in Asia?) – though an Internet search reveals that in the US at least, Culex mosquitoes also breed in containers and other receptacles around buildings. These mosquitoes prefer feeding on animals to humans, and it appears the disease circulates in wild birds – but can become amplified in pig farms. (Making it odd that second of the November cases was a man living in Aberdeen, HK Island; not near pig farms, and without Culex mosquitoes found nearby.)

In Hong Kong, there have been suggestions wild birds are to blame; WWF Hong Kong has refuted this notion.
It's worth remembering that with bird flu, wild birds made convenient scapegoats, even though no evidence suggested they were responsible – I got so riled by this, I did a webpage, Dead Ducks Don't Fly.