Sneaky little insect hitched a free ride

DIARMUID, our youngest son, returned from the jungles of Bolivia last month. He brought home a guest. Some odd characters stay with us from time to time, but the Bolivian visitor, who is still here, is the strangest to date.

The Rio Beni, which flows into the Amazon, has one of the finest intact ecosystems in South America. The jungle teams with life, including a bewildering profusion of insects, many of them as yet unknown to science. Some of the little beasties can deliver a painful sting. I visited the area myself some years ago but few of the insects stung me. Diarmuid was not so lucky. One particular sting, to the back of his head, was particularly irritating.

Some weeks later, Diarmuid noticed a swelling at the site of the sting. The doctor thought it was a harmless cyst but the lump continued to grow. There seemed to be something, about the size of grape, under the skin. A black liquid was discharged from time to time. There were occasional itches and aches, but Diarmuid, who has been roughing it around the world for the last year, is used to discomfort and he was not unduly perturbed by the bulging mini-volcano on his head.

Then, at 4am one night last week, he woke to a tickling sensation in the head. It seemed that the lump had started to bleed. Something, at any rate, was flowing from it. Turning on the light, he found that he was not shedding blood but a huge writhing maggot. The thing wormed its way out of his head and fell onto the pillow. Diarmuid put it in a jar where it continued to wriggle. Reddish-brown in colour the giant maggot resembled a large, very fat, caterpillar but without hair, legs or other appendages, apart from two tiny horn-like projections at one end. These turned out to be anal hooks, which the creature uses to cling to its victim. The maggot would stretch to about 4cm on occasion and then contract, accordion-style, to a fat roundish ball. It was time to consult an expert.

Dr Sam McCluskey is Ireland’s leading tropical disease specialist. He has lived in the Gambia where he encountered many exotic squatters. Telling such creatures apart when they are still at the larval stage is difficult but Dr McCluskey thinks that Diarmuid’s hitch-hiker is a young Dermatobia hominis, a notorious South American body parasite.

Dermatobia is a bot-fly, a member of the diptera order which also includes the familiar house-fly and bluebottle. Lifestock farmers will be familiar with bot-flies which are known here as warble-flies, although our ones target farm animals.

The adult bot-fly is bluish-grey, about the size of a bumble bee. One of its relatives, the deer bot-fly, was listed in the Guinness Book of Records as the world’s fastest flying insect until it lost the title to an Australian dragonfly.

Dermatobia has an unusual lifestyle. The adult insect needs to get its eggs onto warm animal flesh. Because it’s so large and would attract fatal attention to itself, it does not approach its victim directly. Instead, it enlists the help of a smaller creature, usually a mosquito. The bot-fly lays its eggs on foliage, where they become attached to the legs of mosquitoes, or it may deposit them directly on an insect. The mosquito must be a female as only females attack large animals; she needs a meal of blood to form eggs.

When the mosquito lands on flesh, the bot-fly egg drops onto the skin, where the person’s body heat causes the egg to hatch. The tiny larva scratches a hole in the skin and burrows into the flesh underneath. The hole is kept open to provide air for the larva as it grows. Its black faeces are expelled through the hole and these were the discharges which Diarmuid reported. The larva secretes antibiotics to control infection in the host and feeds on the rich serum, which the host’s body produces in response to the invader. It can take up to 12 weeks to reach full size. Then it burrows its way out through the hole in the skin and drops to the jungle floor.

According to Manson’s Textbook of Tropical Diseases, the larva, despite its large size and ghoulish appearance, does not usually harm its host, unless it lodges in the eye cavity.

People have lost eyes to bot-flies and small children have died when a larvae lodged inside the skull. Blocking the air-hole with Vaseline may force the larva to emerge, but if the parasite dies under the skin, a nasty infection may result. The beast can be removed surgically but it is important that no part of it is left behind. If the larva has not lodged in a difficult site, the safest strategy is probably to let it go to term and emerge of its own accord, which is what Diarmuid inadvertently did.

The stowaway has now developed into a chrysalis and is receiving tender loving care in our hot-press, in the hope that it will hatch into the adult insect. It will then be taken to Dr Jim O’Connor of the Natural History Museum for specific identification and addition to the growing list of alien insects which have been arriving in Ireland in recent years.