An epidemic of itchy, burning rashes, irritated eyes, and sore throats struck Belgium in the spring of 2007. Victims’ reactions were severe enough to rally firefighters and dozens of troops—armed with gas-flame torches—to scour the northern countryside in search of the culprits. That summer a similar outbreak in west London set people to coughing and scratching, and the U.K. forestry commission launched its own extreme counterattack to blunt a recurrence this year.
What’s responsible? Fuzzy caterpillars. Oak processionary caterpillars (Thaumetopoea processionea), to be exact: they wreak havoc on anybody unlucky enough to cross their path. Considered tree pests as well, because they defoliate oaks, oak processionaries sport an armament of poisonous hairs. The fine, stiff, sharp-pointed bristles readily penetrate skin, releasing a toxin on contact from their hollow cores. In response to the toxin, thaumetopoein, a victim’s body releases histamine, which raises itching, red welts. Particularly sensitive people can suffer a much more serious, life-threatening reaction termed anaphylaxis.
Surprisingly, few of the victims in the outbreaks actually touched or saw the caterpillars. The hairs can be unavoidable on a windy day, floating invisibly in the air for more than a mile. High concentrations of the airborne hairs are common because the caterpillars live in large sibling groups—often of a hundred or more—and can achieve high population densities. The caterpillars move about in head-to-tail bodily contact, forming snakelike lines as long as twenty feet, a peculiar form of collective locomotion termed processioning. While this is an effective means of staying together, brazen marches render the insects conspicuous to predators, requiring strong defense against attack. Hence, the hairs—lots of them.
People all over the world have come into contact with the dozens of species of processionaries that have evolved around the world. In West Africa, for instance, inhabitants have eaten the Anaphe caterpillar for many generations. They deal with the hairs by singeing them as the caterpillars are roasted over a fire. Eating one or two of the tasty caterpillars is of no concern. But making regular meals of them, it turns out, often leads to serious symptoms: difficulty in speaking, impaired consciousness, rolling eyes, staggering, and tremors. Only recently has it been established that thiaminase, an enzyme in the caterpillar’s body, destroys the victim’s vitamin B1. The resultant vitamin deficiency, now known as seasonal ataxia, was responsible for about 70 percent of?hospital admissions in Ikare, Nigeria, in August 1993. Fortunately, the symptoms disappear quickly with vitamin supplements.
The most dangerous of the processionaries is the South American Lonomia. In Brazil, a seventy-year-old woman suddenly fell into a coma after she placed a slipper on her foot. Hidden within was a Lonomia caterpillar. Doctors found lesions on her left foot where hairs had penetrated her skin. The toxin had triggered intracerebral hemorrhages, from which she died seven days later. More and more people are being exposed to the hazard because of deforestation and a decline in the caterpillar’s natural enemies. An antilonomic serum, if injected in a timely manner, can save victims’ lives.
Central America has suffered in recent years from female moths in the processionary genus Hylesia. The moth, like that of some other species of processionaries, has poisonous spicules on its abdomen, allowing it to carry on the nasty business of its childhood. In 2005, Trinidad shut down offshore oil rigs when the moths fluttered about lights that burned through the night; the spicules broke from their abdomens, drifted invisibly through the air, and fell onto the exposed skin of the victims.