WASHINGTON (Reuters) - A combination of two commonly used agricultural  pesticides, when injected into mice, causes the same pattern of brain  damage seen in Parkinson's disease, researchers said on Thursday.  Mice given the herbicide paraquat and the fungicide maneb showed clear  signs of Parkinson's, a progressive and incurable brain illness, Deborah  Cory-Slechta and colleagues at the University of Rochester School of  Medicine and Dentistry said.

But neither chemical alone works to create the distinctive pattern of  brain damage.

The findings add to a growing body of evidence that exposure to chemicals  such as pesticides may at least contribute to the brain damage seen in  Parkinson's.

"No one has looked at the effects of studying together some of these  compounds that, taken by themselves, have little effect," Cory-Slechta  said in a statement.

"This has enormous implications."

Dr. Eric Richfield, a neurologist who worked on the study, said it may  mean that no one will ever be able to predict who is at risk of  Parkinson's based on exposure to chemicals.

"There is no way to add up how much of any chemical someone is exposed  to," Richfield said in a telephone interview.

"There are so many agents and everybody is a little different. Person A  may have no reaction to a particular compound. How do you test for  interactions between two agents?"

Parkinson's disease, which affects an estimated 500,000 people in the  United States alone, is a progressive and incurable disease that involves  the destruction of brain cells that produce dopamine, an important  message-carrying chemical linked with movement.

Patients start out with tremors and can become paralyzed and die. There  is no cure and treatments can delay the disease for a while but  eventually stop working.

Perhaps the best-known patient is Pope John Paul II, whose doctor  admitted on Wednesday the pontiff had the disease. Actor Michael J. Fox  also has Parkinson's, and boxer Muhammed Ali has symptoms of the disease.

Researchers suspect that a combinations of genetic vulnerability and  exposure to something in the environment may be responsible. One major  suspect is organophosphate pesticides, which are known to affect the  nervous system.

Writing in the Dec. 15 issue of the Journal of Neuroscience,  Cory-Slechta's team said they studied the effects of a mixture of  paraquat and maneb. Both are used on millions of acres of crops such as  potatoes, tomatoes, lettuce, corn, soybeans, cotton and fruit.

Mice injected with one or the other alone showed no ill effects, but when  the combination was given they showed clear patterns of brain damage.

The mice moved around much less than normal and had lower levels of an  enzyme known as tyrosine hydroxylase that is used as a measure of the  health of the dopamine system.

The mice had nearly four times as many "reactive astrocytes," brain cells  that suggest they are damaged, they had about 15 percent fewer dopamine  neurons, and they produced 15 percent less dopamine than normal mice.

Richfield says his team now plans to test mice genetically engineered to  be susceptible to the Parkinson's-like damage, and they may test whether  giving the chemicals orally has the same effect.

He thinks one chemical may act to make the other more damaging. "It could  have to do with the uptake of paraquat," he said.

"If given systemically (as in an injection), very little gets into the  brain. It is possible the maneb compound is promoting transport into the  brain, therefore giving the mice a greater dosage to the brain. That is  something we are planning to experimentally determine."