Pesticide Interferes With Enzyme’s Job in Nervous System

People who have eaten watermelons contaminated with the pesticide aldicarb get sick because the chemical blocks an enzyme involved in the transmission of nerve impulses that help regulate a wide variety of organ functions.

The symptoms of aldicarb poisoning include nausea, diarrhea, vomiting, abdominal pains and excessive sweating. The pesticide disrupts the delicate balance between stimulation and inhibition that is built into the body’s nervous system for the normal control of organs that are regulated involuntarily, such as the brain, heart, lungs and gastrointestinal tract.

Carries Nerve Impulses

While there are dozens of neurotransmitters, the one involved in aldicarb poisoning is called acetylcholine. It carries nerve impulses across the gap between a nerve ending and the next cell in line. This open space is called a synapse, and its job is to act like an on-off switch.

Immediately after initiating this “on” action, the switch is turned off by the release of a specific enzyme that rapidly clears out the acetylcholine, thereby stopping its stimulatory action. In the case of acetylcholine, the enzyme is called cholinesterase.

But when aldicarb, which is known as an anticholinesterase, gets into the system, it destroys the enzyme. This allows the nerve cell to continue to pump out the acetylcholine without any chemical to inhibit it.

Several classes of pesticides affect the so-called cholinergic nervous system. Fortunately for the victims of the current watermelon outbreak, aldicarb, which chemically is called a carbamate, is not as dangerous at a given dose as another widely used class of pesticides called organophosphates.

Some of the organophosphates also destroy cholinesterase, but their effect on the enzyme action is said to be irreversible and may call for drug treatment. When aldicarb strikes the nervous system, it inhibits the cholinesterase for only about 20 minutes and usually does not require medical attention.

Can Cause Problems

But if the exposure is great enough, even carbamates can cause severe breathing problems and convulsions, according to a spokesman for the Los Angeles County Medical Assn. Regional Poison Control Center.

A spokesman said the service has received 1,300 calls from the public and physicians since the watermelon-associated outbreak began last Thursday, but that none was life-threatening.