Environmental Health Perspectives

Menace in the Mix
New research underway at Duke University could yield clues to how chemicals used by U.S. soldiers during the Persian Gulf War may have intermingled and caused neurotoxic effects in some veterans. The combined, or synergistic, effects from three chemicals: pyridostigmine bromide, DEET, and permethrin, may have caused some of the symptoms reported by Gulf War veterans, including chronic fatigue, rashes, headaches, weight loss, and joint pain, according to Mohamed Abou-Donia, a professor of pharmacology at Duke University who is spearheading the research.

"It's a plausible hypothesis that synergism occurred," says Ernest Hodgson, head of the toxicology department at North Carolina State University, who has devoted much of his work to studying the synergism of chemicals. "That's not to say [the hypothesis] is an appropriate lead for further investigation," Hodgson cautions. "Dramatic cases of synergism are really not that common."

Abou-Donia's preliminary findings are arresting, however: they show that, when introduced alone, the chemicals caused no harmful effects on laboratory animals. However, when the chemicals were administered two or more at a time, the animals underwent significant neurological damage. Abou-Donia and his colleagues, toxicologist Ken Wilmarth and biochemist John Locklear, tested the chemicals on chickens because they are more sensitive than rats to chemicals that harm the central nervous system and because federal agencies call for the use of chickens when screening chemicals for possible neurological effects.

Findings from Abou-Donia's research, due to be published soon, may supply at least one missing link in the chain that may one day conclusively tie a number of symptoms reported by Gulf War veterans to environmental exposures they suffered during the war. Soldiers there endured environmental hardships ranging from oil well fires and infectious parasites to pesticides, insecticides, and anti-nerve gas pills originally intended to protect them. Abou-Donia's research is being funded by a grant from former presidential candidate and veterans' advocate H. Ross Perot.

Ironically, the three chemicals being evaluated at Duke were issued by the Department of Defense to protect soldiers. At the outset of the conflict, U.S. and British troops were given a 21-count package of 30-mg pyridostigmine bromide, anti-nerve gas pills that would counter the effects of potential Iraqi chemical warfare. "It shields an enzyme present in the brain and peripheral nervous system in a reversible manner, for a short period of time," says Abou-Donia, explaining how the chemical functions.

Though the DOD and a Defense Science Board Task Force on Gulf War Effects concluded in their 1994 report that the Iraqis did not use chemical or biological weapons against coalition forces, rumors of chemical warfare apparently circulated widely among soldiers throughout the conflict. According to Abou-Donia, fear prompted many soldiers to take more than the recommended dosage of pyridostigmine bromide pills. "During the war, over 50 percent of U.S. service personnel seen in the health service complained of symptoms relating to pyridostigmine bromide," Abou-Donia says.

The other two chemicals Abou-Donia and his colleagues are studying are N,N-diethyl-m-toluamide (DEET), an insect repellent, and permethrin, a liquid insecticide. According to Abou-Donia, DEET (used in a 90% concentration) was used due to concern about insect-borne tropical illnesses. Soldiers' uniforms were impregnated with permethrin, says Abou-Donia. Some veterans have reported that combat uniforms were doused with the liquid, then distributed in plastic bags. Because soldiers would presumably wear the uniforms for an extended time--a period of days, perhaps--extensive dermal contact with the permethrin would have occurred. Both DEET and permethrin have low acute toxicity, Abou-Donia says. "If the two chemicals had been given alone, they would not have caused harm," he quickly points out. In 1992, New York state banned the use of insect repellents containing more than 30% concentrations of DEET because of concerns over health effects (see EHP, vol. 102, no. 11, p. 910).

To test the combination of chemical exposures in the lab, the researchers administered pyridostigmine bromide orally and both the DEET and permethrin dermally via subcutaneous injection. Though soldiers in the battlefield would have absorbed the latter two chemicals dermally, Abou-Donia and his colleagues had to inject the chickens subcutaneously to deliver precisely measured and statistically viable quantities.

Abou-Donia's team is also investigating the hypothesis that the chemicals the Gulf War soldiers were exposed to generated a delayed toxic impact known as organophosphate-induced delayed neurotoxicity (OPIDN). OPIDN assaults both the central and peripheral nervous systems, producing symptoms such as weakness, lack of coordination, and even paralysis. After analyzing brain tissue samples from laboratory chickens, the researchers found that nerve damage was linked to the decreased activity of an enzyme present in tissues, neurotoxic esterase. The team is searching for biomarkers present in the animals' blood that will show nervous system damage, and are comparing animal blood samples with those from affected veterans. "If we could find a biomarker, then we could find a treatment for existing populations," Abou-Donia said.

Researching synergism. Duke's Mohammed Abou-Donia is investigating whether the illnesses plaguing Gulf War soldiers are the result of exposure to chemical mixtures.

While Abou-Donia's research sheds light on the connection between environmental exposures and some medical symptoms associated with what is unofficially referred to as Gulf War Syndrome, it does not explain why some individuals appear to be more sensitive to the chemical effects.

"We would like to try to see if, in fact, we could identify a population segment that is naturally predisposed to chemical sensitivity," says Abou-Donia describing MCS, a medical phenomenon that is widely acknowledged, yet little understood, even by scientists and physicians closest to the issue. "Maybe there is a genetic variant we need to identify," Abou-Donia suggests. A more complete understanding of genetically dependent chemical sensitivity could be useful should the DOD deploy soldiers in future military operations.

One scientist interested in studying Persian Gulf veterans with symptoms resembling multiple chemical sensitivity is physician Claudia Miller, an expert on MCS and an assistant professor of environmental and occupational medicine at the University of Texas Health Science Center at San Antonio. As a staff physician at the Houston VA's Persian Gulf Regional Referral Center, Miller evaluates the health of Gulf War veterans.

According to Miller, certain people may have genetically determined metabolic differences that make them susceptible to chemical sensitivity--a phenomenon that falls into a new area of study called ecogenetics. "We rely heavily on epidemiological studies, which are based on crude estimates of past exposure," Miller says of her work with Gulf War veterans and other MCS patients. "We're trying to correlate [chemical exposures] with health problems," a process that is error-prone because it relies on anecdotal information from patients, but which Miller calls "invaluable" for the information it supplies.

Of the veterans she has evaluated who have unexplained illnesses following their service in the Persian Gulf, a large portion have reported the onset of new chemical intolerances which commonly include diesel exhaust, solvents, gasoline, tobacco smoke, hairspray, and fragrances. Miller refers to this as a toxin-induced loss of tolerance (TILT), a term she prefers to chemical sensitivity "because available data on MCS patients and Gulf War veterans point away from MCS as a syndrome, but perhaps toward what may be an emerging new mechanism or theory of disease." Describing TILT, Miller says, "Once their tolerance level is exceeded, they don't respond normally to low-level exposures." These veterans exhibit the typical two-phase response observed in MCS patients: during the induction phase, loss of tolerance occurs following an acute chemical event or a less-acute series of events. This loss of tolerance can involve any of a wide range of chemicals such as medications, caffeine, foods, and other chemically unrelated substances. In the triggering phase, patients experience symptoms when they are exposed to tiny amounts of such common substances, but these responses overlap in timing, thereby masking any individual reaction to a single chemical. What is relevant to the Gulf veterans' illnesses about TILT, says Miller, is the fact that a wide range of environmental agents (solvents, pesticides, combustion products, etc.) appear to be capable of initiating this process.

According to Miller, further research on chemically sensitive Gulf War veterans will require studies conducted in a controlled environmental medical unit--a hospital-like environment built and furnished with materials that don't emit chemical vapors and equipped with an efficient air filtration system. Only in such an environment, Miller says, could subjects be observed to see whether they improve, and if so, then be re-exposed to very low levels of chemicals, one at a time, under double-blind, controlled conditions and then evaluated for symptoms.

Studies on Gulf War health effects are as politically significant as they are scientifically important. Confirmation of a link between chemical exposures and negative health effects would allow affected Gulf War veterans to receive compensation under the veterans' disability compensation program.

The U. S. Department of Veterans Affairs established three research centers--in Boston; East Orange, New Jersey; and Portland, Oregon--in October 1994 to study how environmental and toxic hazards may affect health. Among six research projects underway at the Boston center is a study to examine the relationship between war-time exposure and chronic fatigue, chemical hypersenstitivity, and post-traumatic stress disorder. At the New Jersey center, scientists are gathering information on illnesses suffered by Gulf War veterans, hoping to examine a characteristic symptom profile and connect certain risk factors with the development and progression of unexplained illnesses. Researchers at the Portland center are screening veterans for medical, chemical, or biological markers that may confirm exposure and disease, as well as studying how chemical agents, including pyridostigmine and pesticides, may affect the nervous system.