LATIN AMERICA: Radioactive Attack on Flesh-Eating Screw-Worm

  • by Emilio Godoy and Raúl Pierri (mexico city/montevideo)
  • Monday, October 18, 2010
  • Inter Press Service

The screw-worm is the larva of the Cochliomyia hominivorax fly and feeds on the living flesh of warm blooded animals, including humans. The insect is a native of the tropical zone of the Americas, but it has been spread by international trade to other regions, like North Africa.

The female of the fly copulates just once and lays its eggs in any open wound on animals. The larvae literally drill through the tissues, sometimes reaching as far as the bone.

In Uruguay, a livestock-raising country, it is known as the 'bichera' worm and it affects 5.7 percent of sheep and 3.4 percent of cattle, according to an official study involving 530 producers. Losses are estimated at 210 million dollars a year, a heavy cost for a small country of 3.4 million people.

'It's a big job to keep it at bay. In a rainy year, the hooves of sheep soften and the fly lays its eggs there; the worm starts eating away as soon as they are hatched,' cattle rancher Gustavo Rianni, who has 6,000 hectares of land in the northern department (province) of Artigas, on the border with Brazil, told IPS.

'Infected animals stop eating and prefer the shade. They are kept in isolation to avoid spreading the infection, and if they cannot be cured they have to be slaughtered,' he added.

Extermination of this parasite, which can be transmitted from animals to humans, is the goal of the Mexican American Commission for the Eradication of Screw-Worm (COMEXA), established by the United States and Mexico in 1972. The commission produces sterile male flies in its laboratory in Tuxtla Gutiérrez, the capital of the southern Mexican state of Chiapas.

The flies are bred in the laboratory and made sterile by irradiation with caesium 137, a radioactive isotope. Then they are released into the field where they mate with female flies without fertilising them, so they cannot procreate and the species gradually dies out in the area.

Screw-worm was eradicated in the United States in 1966, and in Mexico in 1991. Since then, COMEXA has managed to eliminate local outbreaks of the infection, and to extend eradication to nearly all of Central America, the Caribbean, and also Libya in North Africa, COMEXA director Alejandro Parra told IPS.

The sterile insect technique was developed by U.S. entomologists Edward Knipling (1909-2000) and Raymond Bushland (1910-1995) who began research into the problem in 1937. Only in the 1950s did they hit on the idea of applying short bursts of irradiation that knocked out the reproductive capacity of male flies without affecting their other functions.

A pilot trial using flies sterilised in Mexico was carried out in 2009 in Brazil and Uruguay. The results, which have just been evaluated, are promising.

Between Jan. 23 and May 15, 2009, low-flying aircraft dispersed over 200 million sterilised male flies, brought from Chiapas, over a strip of land 100 kilometres long and 60 kilometres wide along the Uruguayan-Brazilian border, centred on the towns of Artigas in Uruguay, and Quaraí in Brazil.

'The experiments showed that the technique worked with native strains of fly, and they were useful for training personnel, ' the director general of animal health services at the Uruguayan Ministry of Livestock, Agriculture and Fisheries (MGAP), Francisco Muzio, told IPS. 'This was a good pilot study for evaluating the possibility of carrying out a regional eradication programme.'

The reproductive cycle was interrupted in 40 percent of the fly population on the Brazilian side of the border, where the greatest effect was demonstrated. Sterility over the whole area studied was 25 percent.

'All the evidence indicates that this was due to the higher animal density on the Uruguayan side of the border, and especially to the large number of sheep (which are more susceptible to the parasite), whereas on the Brazilian side more land is devoted to farming,' Muzio said.

At a meeting in August this year, researchers concluded that if the programme is continued, the sterile population will effectively outnumber fertile flies.

The pilot trial cost 2.6 million dollars and was supported by the Inter-American Development Bank with a contribution of one million dollars.

Rianni, whose property is within the pilot study area, said the incidence of parasitic disease caused by the fly had fallen among his livestock since the trial. 'If it is continued, it could provide a solution. The ministry should continue to invest in this technique,' he said.

Now the researchers want to assess the total size of the native fly population, the usefulness of natural barriers, livestock density and movements, and the incidence of screw-worm in humans, in order to design a regional eradication plan. Representatives from Paraguay and Argentina also took part in the meeting.

Screw-worm not only causes illness and death among livestock, but also spoils leather and creates enormous expense in insecticides, treatments, and the closure of markets to livestock from countries where the infection is rife.

According to the United Nations Food and Agriculture Organisation (FAO), in 2000 estimated annual losses were 210 million dollars in Uruguay, 1.77 billion dollars in Brazil and 103 million dollars in Paraguay.

In spite of the fact that the biological control technique uses highly radioactive material, FAO affirms it is not harmful to the environment and is innocuous in human beings.

'There has been no evidence of any impact at all on biodiversity, perhaps because a multitude of fly species occupy the same biological niche,' COMEXA's Parra said.

After the United States eradicated screw-worm and closed its own sterilisation plant, Mexico became 'a world reference centre for technological progress in this area,' he said.

In addition to Mexico and the United States, screw-worm has been eradicated in Belize, Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua, and is on the way to extinction in Panama, where there is another facility for sterilising flies. It has also disappeared from Aruba, Curaçao and the Virgin Islands in the Caribbean. However, in South America, only Chile is free of the parasite.

In 1988 there was an outbreak in Libya, where the fly was introduced with infected South American sheep. Two hundred cases of the parasite in humans were reported. With help from FAO and the World Bank, COMEXA eradicated the Libyan screw-worms in 1992, after carrying out 50 flights with 1.3 billion sterile flies.

© Inter Press Service (2010) — All Rights ReservedOriginal source: Inter Press Service

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