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Giant Snails, Once a Delicacy, Overrun Brazil

Sabrina Valle in Rio de Janeiro, Brazil
for National Geographic News
October 19, 2007
 
The giant African snail, originally brought to Brazil as a delicacy for gourmet restaurants, has instead become a major nuisance in the country.

The invasive mollusk—which can measure nearly 8 inches (about 20 centimeters) long and weigh more than 1 pound (500 grams)—is widespread in Brazil, thriving in nearly every state.

The snail is currently at the height of its invasion, experts say, and the its success makes eradication near-impossible.

Silvana Thiengo is a mollusk expert from Brazil's national health organization, the Oswaldo Cruz Institute.

"Raining season starts in November, and that's when they like to [lay] eggs," she said. "The snails will show up more, so we do expect the situation to get worse."

Escargot Substitute

The giant snail, native to eastern Africa, was brought into Brazil as a profitable substitute for the common garden snail, which is used for escargot.

There is no record of when the species was first imported, but an agribusiness fair in southern Brazil in 1988 was probably pivotal in sparking the invasion. (See photos of other invasive species.)

At the fair, people sold kits with snails and brochures detailing how to raise them.

At first the African snails seemed promising for food: They had more meat, grew faster, and were more resistant to disease than the garden snail. The African snail was also cheaper to keep.

Brazilians countrywide began growing the giant snail in their backyards, planning to sell the mollusks to fancy restaurants.

Yet eating escargot is unusual in Brazil, and the few diners who would pay to eat the delicacy were not willing to substitute it for a new species with different texture and taste—and suspicious origin. This resulted in thousands of frustrated people with unwanted snails slithering through their backyards.

Most of the snails were then released in the wild, where they rapidly grew in number.

Disease Vector

If bred in captivity, the snail is unlikely to carry any parasites, according to Fábio Faraco of the Brazilian Environmental Institute.

"But out of rigid control, they can get contaminated," Faraco said.

The snail is a potential vector for several pathogens and an intermediate host for worms that can cause parasitic diseases in humans.

The garden snail, found in some areas of the country, can also carry the same worms, he added.

One worm, Angiostrongylus costaricensis, causes strong abdominal pain and fever and may cause internal bleeding.

The second, Angiostrongylus cantonensis, can cause a type of meningitis in humans, which in extreme cases leads to blindness and paralysis.

Two cases of meningitis in the state of Espírito Santo were diagnosed this year, and both were caused by worms that came from African giant snails.

People can get infected from touching the traces left by the snails on the ground, or ingesting vegetables that the animal partially ate.

Environmental Damage

Thiengo of the Oswaldo Cruz Institute led the latest study about the plague, published in the August 2007 issue of the journal Biological Invasion.

The research highlighted the damage the pest has caused to the environment.

The study also showed that the snail has become part of the food chain. For example, there has been an increase in populations of rats and snakes that eat the snails, Thiengo said.

The African species has also begun to compete with many of Brazil's large native snail species, which may be vulnerable to losing habitat to the prodigious invader.

Native mollusks lay a few eggs at a time, but the African snails lay clutches of up to 400 eggs and produce up to 1,200 eggs a year. The species can live up to ten years.

Stopping the Spread

Although eradication is thought to be impossible, control is needed, experts say.

Management measures for the giant African snail are underway through a national plan created in 2004.

But the plan's suggested control method—collecting the snails by hand and destroying their shells—requires a serious commitment from Brazilians.

The snails can also hide themselves underground for long periods, making eradication even more difficult.

"Everyday I have to collect at least six in my back garden," said Luiz Roberto Bragança of Niterói, a town near Rio de Janeiro.

"They like to come out at night, especially when it rains," he said. "I can't water my plants at night anymore."

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