Malathion’s Threat to Humans and Animals Is Negligible, Officials Say : Health: All are susceptible to the pesticide. But the amount used is called too small to be dangerous.

The pesticide malathion, which will be sprinkled over Camarillo and Somis neighborhoods beginning Wednesday to combat a Medfly infestation, kills the pest by attacking its nervous system.

Animals with nervous systems--including humans and pets as well as many insects--are susceptible to the pesticide’s effects, if the substance is breathed, eaten or absorbed through the skin in sufficient quantities.

“That’s the way all those organophosphates work,” said Albert Heier, a spokesman for the Pesticides Division of the U. S. Environmental Protection Agency.

But the amount that will be mixed with the sugary fly bait is so small--only 1.2 ounces per acre--that health effects are expected to be negligible for healthy people, state officials said. Nevertheless, it is a substance designed to be toxic to the Medfly and other pests.

“There isn’t a pesticide made that is intended for people to be covered with it,” Heier said. “They are made to kill insects.

Unlike other organophosphates, like parathion, malathion is broken down much more quickly in mammals than it is in insects. “So it is much more toxic to insects than to people,” said Phil Phillips, an entomologist with the UC agriculture extension office in Ventura.

Malathion inhibits the enzyme cholinesterase, which is essential to proper communication between nerve cells and between the nerves and muscles. Without it, the nerves fire continuously, Phillips said.

“It sort of sends the system into a frenzy, with the nerves telling the muscles, ‘Move, move, move,’ ” Phillips said. “The system gets overloaded and everything shuts down.”

Although the malathion can be broken down quickly by humans, it is still quite toxic to fish, Phillips said.

Studies submitted to the EPA do not show any evidence that the pesticide has caused birth defects, and early studies showed no evidence that the substance causes cancer in humans.

Federal officials still await the results of a new study on carcinogenic effects of malaoxon, a product that emerges when malathion breaks down. Results of that study should be made public within a few months, said EPA spokesman Dave Schmidt.

Studies on malathion have shown that it may cause allergic reactions in some people, and that each subsequent exposure could produce a stronger skin reaction, according to the California Department of Food and Agriculture.

In Riverside County, where the pesticide was sprayed by air, a small number of residents experienced skin rashes, allergic reactions and asthma after the applications, said Dr. Bradley Gilbert, Riverside County public health officer.

Still, he said, “you would have to lick an entire football field of malathion” to get sick.

Health officials advise the very young, the elderly and the ill to stay out of the pesticide spray and minimize contact with recently sprayed areas.

Gilbert said some problems may be inevitable when malathion is sprayed in the 16-square-mile eradication zone, with an estimated population of 29,000.

“When you spray over a huge area and expose a lot of people, you’re almost guaranteed through bad luck of something freaky happening . . . like a baby crawling through a just-sprayed area,” he said.

Miller is a Times staff writer and Fields is a correspondent.

Aerial Spraying WHERE: A 16-square-mile area of eastern Camarillo roughly bounded by California 118 to the north and the Ventura Freeway to the south.

WHEN: Beginning at 9 p.m. Wednesday and continuing three to four hours, and recurring once every two weeks for about six months.

WHAT: The pesticide malathion mixed with corn syrup to attract Medflies.

PRECAUTIONS: Stay out of the spray, especially the ill, very young and elderly. Cover shallow ponds and wading pools. Wash lawn furniture and playground equipment before using. Cover vehicles to prevent damage to paint.