Organophosphates and the Gulf War Syndrome

CHAPTER

6

Organophosphates and the Gulf War Syndrome LINDA A. McCAULEY University of Pennsylvania, Philadelphia, Pennsylvania

following Iraq's invasion of Kuwait on August 2, 1990. There was a rapid buildup of Coalition forces primarily from the United States, Britain, and Canada. The conflict, referred to as Operation Desert Storm, began on January 17, 1991, when the U.S.-led Coalition air forces attacked Iraqi targets, and intense ground warfare was of a remarkably short duration, lasting from February 24 toFebruary 28, 1991. During the postcombat period, military posts were dismantled, munitions depots in Iraq were inspected and destroyed, and by August 1991 all U.S. service members who participated in the ground war had returned to the United States. The military conflict was viewed as a major success with few casualties; however, in the months following the military conflict, disturbing reports emerged from military units of unexplained health problems that were persistent and worsening. By 1992 and 1993, these complaints had caught the attention of scientists, the U.S. govemment, the Department of Defense, and the Department of Veterans Affairs. Reports of unexplained illness also emerged from other countries involved in the GW. The environmental exposures during the GW were numerous and potentially toxic to human health. Troops were exposed to temperature extremes; the threat of chemical warfare, including detection alarms sounding; combat stress; severe sand storms; insects, including sand flies carrying leishmaniasis; petrochemicals; oil well smoke; pesticides; munitions combustion products; depleted uranium; pyridostigmine bromide; and biological warfare agent vaccines (anthrax, plague, and botulinum toxoid vaccine). Approximately 35-45% of troops who served in Desert Storm received the CM anticholinesterase drug PB, a soman antidote enhancer commonly used in the treatment of myasthenia gravis. Not all servicemen who were dispensed packets of PB were compliant in taking them as ordered. Approximately 150,000 U.S. troops received anthrax vaccine and 8000 received botulinum toxoid vaccine (Presidential Advisory Committee, 1996a). Controversy emerged regarding the possible etiology of the multisymptom complexes seen in ill veterans, with many

I. I N T R O D U C T I O N More than a decade after the end of Operation Desert Storm in 1991, a large number of veterans who served during the war continue to experience an array of chronic health symptoms, including memory loss, fatigue, cognitive problems, somatic pain, skin abnormalities, and gastrointestinal difficulties. Since reports of unexplained illness among veterans of the 1991 Gulf War (GW) first appeared, organophosphate (OP) and other chemical exposures were believed by many to have contributed to the symptoms that were being reported. Now, more than 15 years after this military conflict, some scientists strongly believe the unexplained illness can be attributed to military combat and stress-related conditions, whereas others contend that OPs and carbamates (CMs) are implicated. In addition to possible exposure to chemical warfare agents, veterans were also potentially exposed to the OP insecticide chlorpyrifos and non-OP insect repellants such as N,N-diethyl m-toluamide (DEET) and the CM pyridostigmine bromide (PB) as a nerve agent antidote enhancer. This chapter examines the OP and CM exposures and health effect associations found in epidemiological studies and the methodological issues inherent in these study designs. Animal studies contributing to the conception model of how OPs and CMs mechanistically resulted in neurological damage are reviewed. The Khamisiyah detonations of nerve gas stores, the military incident in which there was overwhelming evidence of exposure to chemical warfare agents, is discussed. Finally, this chapter discusses current research that continues to strive to uncover the link between OP and/or CM exposure and unexplained neurological disease.

II. H I S T O R I C A L P E R S P E C T I V E The first U.S. troops arrived in the southwest Asia theater of operations in August 1990 during the preparatory period referred to as Operation Desert Shield, the period immediately Toxicology of Organophosphate and Carbamate Compounds

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scientists and advisory groups believing that the symptoms were related to the rapid deployment and combat stress. Although combat stress diagnoses such as posttraumatic stress disorder were not seen in excess, there was intense psychological stress associated with the severe environmental conditions and the ever-present threat of chemical warfare.

III. C H A L L E N G E S W I T H E X P O S U R E ASSESSMENT AMONG GULF WAR VETERANS Many investigators argued that the symptoms of the unexplained illnesses could be related to environmental exposure, and the role of exposure to insecticides, repellents, and PB was examined in most epidemiological studies of Gulf War veterans. Distribution and use of nonpersonal-use pesticides applied to military compounds were strictly controlled by the military in the theater of operations and restricted to certified personnel or contractors. Although the actual shipment amounts of personal-use insecticides and repellents were kept in military records, no records are available on personal use, but the use of insecticide repellent, insecticide spray (permethrin) applied to uniforms, and unauthorized use of flea collars did frequently occur. There was frequent combination use of these substances. Area fogs were regularly sprayed in camp areas for vector control. A 2003 Department of Defense report indicated that 37 individual pesticide ingredients were probably used during the GW (Table 1) and

that a potential link with illness in GW veterans could not be eliminated (U.S. Department of Defense, 2003). In the first decade after the conflict, most investigations of the role of chemicals and other exposures were evaluated through surveys of self-reported health symptoms and exposures (Fukuda et al., 1998; Iowa Persian Gulf Study Group, 1997; Spencer etal., 2001; Steele, 2002; Unwin et al., 1999). The symptoms reported in these study populations were remarkably similar. The veterans also reported a large array of environmental exposures, many of which occurred simultaneously. The lack of objective data on environmental and other exposures resulted in severe limitations in linking adverse health outcomes detected by epidemiologic studies to specific exposures and risk factors (Presidential Advisory Committee, 1996a). No biomarker of exposure has been available in epidemiological studies of this cohort and records of exposures do not exist. McCauley et al. (1999) first reported on the significant methodological issues associated with the dependence on self-reported exposure data. Using the time of employment and records of when PB, vaccines, and other exposures could have occurred, these investigators found significant overreporting of exposures during the Desert Shield and Desert Storm time periods. For example, 8.6% of surveyed veterans who were deployed to Southwest Asia only during noncombat periods reported that they took PB tablets. Twenty-eight percent of veterans deployed only during the months preceding combat reported that they believed they had been exposed to chemical warfare agents. Forty-seven percent of veterans who served after the combat period was over

TABLE 1. Products Used during the Gulf War Identified by the Deployment Health Support Directorate as Pesticides of Potential Concern a Pesticide DEET, 33% cream/stick DEET, 75 % liquid Permethrin, 0.5% spray d-Phenothrin, 0.2% aerosol Methomyl, 1% crystals Azamethiphos, 1% crystals Dichlorvos, 20% pest strip Chlorpyrifos, 45% liquid Diazinon, 48% liquid Malathion, 57% liquid Propoxur, 14.7% liquid Bendiocarb, 19% liquid Chlorpyrifos Malathion, 91% liquid Lindane, 1% powder

Class

Purpose

Application method

Repellnt Repellnt Repellnt Area spray Fly bait Fly bait Pest strip Sprayed liquid Sprayed liquid Sprayed liquid Sprayed liquid Sprayed powder Fog Fog Delouser

Repel flies and mosquitoes Repel flies and mosquitoes Repel flies and mosquitoes Knock down, kill flies and mosquitoes Attract and kill flies Attract and kill flies Attract and kill mosquitoes Kill flies, mosquitoes, flying insects Kill flies, mosquitoes, flying insects Kill flies, mosquitoes, flying insects Kill flies, mosquitoes, flying insects Kill flies, mosquitoes, flying insects Kill flies, mosquitoes Kill flies, mosquitoes Kill lice'

By hand to skin By hand to skin, uniform, netting Sprayed on uniforms Sprayed in area Placed in pans outside latrines, tents Placed in pans outside latrines, tents Hung in tents, working area, dumpsters Sprayed in comers, cracks, crevices Sprayed in comers, cracks, crevices Sprayed in comers, cracks, crevices Sprayed in comers, cracks, crevices Sprayed in comers, cracks, crevices Large area fogging Large area fogging Dusted on prisoners, also for personal use

aFrom the U.S. Departmentof Veterans Affairs (2004).

CHAPTER 6 9 Gulf War Syndrome (during the cleanup period) also believed they were exposed to chemical or biological warfare agents. Test-retest reliability estimates indicated inconsistency in the frequency and rate of self-reported exposures. Veterans were found to have poor recall reliability for use of insecticide cream, taking more than three PB pills per day, reports of use of insecticide cream or sprays, and the belief that they had been exposed to chemical warfare agents. Nevertheless, the majority of reports with associated links between exposure and illness were dependent on self-reported exposures, although some investigations included physical examinations of veterans and verification of self-reported health information (Bourdette etal., 2001; Coker etal., 1999; Joseph, 1997; Murphy, 1999). Tests that were used in these clinical studies did not indicate any major differences between symptomatic and nonsymptomatic veterans. These negative clinical studies and the frequency of mental health diagnoses among symptomatic veterans led to many reports that the symptom patterns were suggestive of posttraumatic stress syndrome or other combat-related mental health problems (Hyams et al., 1996; Presidential Advisory Committee, 1996a; Storzbach et al., 2001; Vlahov and Galea, 2004).

IV. T H E R O L E OF P Y R I D O S T I G M I N E B R O M I D E IN G U L F W A R I L L N E S S Packets of 21 PB tablets were distributed to personnel in the GW for use as a nerve gas antidote enhancer. Personnel were instructed to take one 30-mg tablet three times per day at 8-hr intervals. PB is routinely administered to patients with myasthenia gravis in the United States at doses up to 17-fold greater than suggested for the short-term pretreatment against chemical warfare agents (Presidential Advisory Committee, 1997). The use of PB for protection in the event of an imminent attack with chemical warfare agents was not Food and Drug Administration (FDA) approved, and the FDA granted waivers of informed consent during military exigencies. Cook et al. (1992) conducted one of the few clinical trials of PB use among military populations and described the side effects that were most likely. Keeler et al. (1991) reported that approximately one-half of U.S. soldiers who took PB pills reported side effects associated with the medicine, the most common being anticholinesterase effects such as vision problems, headache, nausea, and abdominal cramping. Veterans report that use of PB was frequently discontinued due to the uncomfortable side effects. In 2000, the Institute of Medicine reported that there is inadequate evidence to determine whether an association exists between PB and long-term adverse health effects. Assessment of PB use has been included in all epidemiological studies of GW illness. Haley et al. (1997) reported an association between symptoms while taking PB and the likelihood of having GW illness. Although Spencer et al. (2001) also found this association, further analysis found that any

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combination of symptoms, regardless of their specificity with PB side effects, was strongly related to the likelihood of having unexplained illness after the war. When PB is examined as a singular exposure, it is almost consistently associated with the risk of having GW unexplained illness. However, PB exposure always occurred in combination with other environmental exposures during the GW. The possibility of a synergistic reaction with other exposures cannot be disregarded.

V. I N C R E A S E D T O X I C I T Y F R O M CHEMICAL COMBINATIONS Some investigators have postulated that the unexplained illness among GW veterans is related to the interaction of stress, PB, and OP exposures. The interaction of PB and insecticides has received considerable attention (Table 2). PB is a rapidly reversible inhibitor of acetylcholinesterase (ACHE) and blocks the binding of agents to ACHE. Investigators have hypothesized that the insect repellent DEET and the insecticide permethrin can access and damage the nervous system if peripheral cholinesterase binding sites are occupied by PB (Abou-Donia et al., 1996a,b; McCain et al., 1997). Multiple animal studies have helped elucidate potential pathways of neurological damage from low-level exposure to OPs and CMs. A review of 28 animal studies was conducted by the Department of Veterans Affairs Research Advisory Committee on GW veterans' illness (2004). These studies consistently demonstrate synergistic effects of different combinations of GW-related exposures. Contrary to previous assumptions, both animal a n d human studies suggest that exposure to these agents at levels too low to produce acute symptoms can result in chronic adverse effects on the nervous and immune systems. Controlled studies on rodents and hens administered single or repeated doses of DEET with or without PB and an OP or pyrethroid insecticide suggest that the toxic potency of these substances may differ when they are administered in various combinations. Some investigators have suggested that the combination of chemical exposures affects the ability of one or more of the substances to penetrate the blood-brain barrier; however, this has not been a consistent finding (Buchholz et al., 1997). Haley and Kurt (1997) began an intensive research program that has linked clinical syndromes among GW veterans with self-reported combinations of chemical warfare agents, PB, pesticides, and DEET. However, McCauley et al. (2001) found that in a multivariate analysis of a case-control study of GW veterans, combined self-reported PB use and insecticide use were not associated with an increased risk of unexplained illness. The possibility that stress induces increased susceptibility to chemical agents was reported by Kaufer et al. (1998) and Friedman et al. (1996). From animal studies, Friedman et al.

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TABLE 2. Study

Animal Studies Evaluating Synergistic Effects of Gulf War-Related Exposures a

Animal model

Exposures studied

Major finding(s)

Abou-Donia et al. (1996a)

Hen

PB, DEET, permethrin

Neurotoxicity greater when two exposures combined, further enhanced when all three exposures combined

Abou-Donia et al. (1996b)

Hen

PB, DEET, chlorpyrifos

Combined exposures enhanced inhibition of brain ACHE, BuChE, and NTE, neurologic dysfunction, and neuropathologic lesions

Baynes et al. (1997)

Rodent, pig skin

DEET, permethrin, carbaryl

DEET does not enhance dermal absorption of permethrin

Buchholz et al. (1997)

Rat

DEET, permethrin

PB ingestion resulted in lower central nervous system levels of permethrin

McCain et al. (1997)

Rat

PB, permethrin, DEET

Significantly greater lethality from combinations of exposures than single exposures

Chaney et al. (2000)

Rat

DEET, PB

DEET + PB significantly inhibited brain AChE activity but not peripheral AChE activity

van Haaren et al. (2000)

Rat

PB, permethrin

Serum permethrin levels were increased by coexposure to PB, but the combination did not affect behavioral responses

Hoy et al. (2000a,b)

Rat

PB, DEET, permethrin

Permethrin in combination with either PB or DEET affected locomotion rates in male but not female rats

Abou-Donia et al. (2001a)

Rat

PB, DEET, permethrin

Combined exposures at physiologically relevant doses led to neurological and behavioral deficits and alterations in brain AChE receptors

Abou-Donia et al. (2001b)

Rat

DEET, permethrin

Combined dermal exposures decreased blood-brain barrier permeability in cerebral cortex and produced impaired sensorimotor performance

Abu-Qare and Abou-Donia (2001a)

Rat

Satin, PB

Combined exposures led to increases in urine levels of markers of oxidative stress

Abu-Qare and Abou-Donia (200 lb)

Rat

DEET, permethrin

Single dermal dose of combined exposures produces significant increase in mitochondrial release of cytochrome c

Abu-Qare et al. (2001)

Rat

PB, DEET, permethrin

Oral PB combined with dermal application of DEET resulted in maximum urine levels of markers of oxidative stress

Peden-Adams et al. (2001)

Mouse

DEET, PB, JP-8 jet fuel

Exposure combination altered selected immunological end points, including delayed hypersensitivity and suppression of IgM response, but did not affect other immune measures

van Haaren et al. (2001)

Rat

PB, permethrin, DEET

Small doses of chemicals disrupted behavioral responses,with synergistic effects observed in some measures

Abdel-Rahman et al. (2002)

Rat

Restraint stress, low-dose PB, DEET, permethrin

Combined exposures disrupted blood-brain barrier in some brain areas and led to increased neuronal death

Baynes et al. (2002)

Pig skin

PB, permethrin

PB significantly enhanced dermal absorption and distribution of permethrin

Vogel et al. (2002)

Mice

DFP, PB, permethrin, parathion

PB reduced DFP binding; permethrin and parathion increased DFP binding in the brain

Abou-Donia et al. (2003)

Rat

Stress, PB, DEET, permethrin

Combined exposure to physiologically relevant doses of chemicals caused destruction of testicular germ cells; effect enhanced by stress

Husain and Somani (2003)

Mouse

Satin, PB, exercise

Exercise augmented synergistic effects of chemical exposures on AChE activity and NTE activity in various tissues (continues)

CHAPTER 6 9 Gulf War Syndrome

TABLE 2.

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(continued)

Study

Animal model

Exposures studied

Riviere et al. (2003)

Pig skin

Scremin et al. (2003)

Rat

DEET, PB, DFR permethrin, sulfur mustard PB, satin

Abdel-Rahman et al. (2004a)

Rat

Malathion, DEET, permethrin

Abdel-Rahman et al. (2004b)

Rat

Stress, PB, DEET, permethrin

Abou-Donia et al. (2004)

Rat

PB, DEET, permethrin

Olgun et al. (2004)

Mouse

Lindane, malathion, permethrin

Major finding(s) DEET absorption enhanced by PB, mustard, and DFP PB reduced delayed neurological and behavioral effects of sarin but did not reverse delayed effects on brain muscarinic receptors Combination of pesticides induced neurobehavioral deficits and neuron degeneration, with no overt signs of neurotoxicity Combined exposures produced damage to areas of the brain associated with motor and sensory function, learning and memory, and coordination Combined exposure produced sensorimotor deficits and altered brain AChE activity levels Exposure of thymus cells to combinations of chemicals resulted in significantly higher levels of apoptosis and necrosis

aFrom the U.S. Departmentof VeteransAffairs (2004). found that the crossing of PB through the blood-brain barrier is increased more than 100-fold in stressed animals. Exposing animals to increased stress apparently increases access of anticholinesterases and circulating viruses to the brain. However, this finding has not been replicated in other animal models (Lallement et al., 1998; Sinton et al., 2000).

VI. C H E M I C A L W A R F A R E E X P O S U R E Prior to and during the GW, the threat of enemy use of chemical warfare agents against Allied troops was seriously considered and troops were issued Mission Oriented Protective Posture (MOPP) protection, PB pill packets, and syringes of atropine. Several types of chemical warfare agent detectors were deployed; however, all these detectors and warning systems detected only nerve agent concentrations that would cause acute symptoms or death, not subclinical concentrations (Presidential Advisory Committee, 1996b). Detectors were reported by veterans to frequently alarm. Veterans would use their MOPP protection, only to be told that there was a false alarm. Not surprisingly, a large number of veterans in repeated epidemiological studies reported the belief that they were exposed to low levels of nerve agents. In general, the nerve gas warning systems used during the GW have been viewed as inadequate to protect military personnel from low levels of exposure and recommendations have been made to improve this surveillance system (Presidential Advisory Committee, 1996a). The most definitive potential exposure to OP agents occurred during the aftermath of the GW with the detonation of Iraqi munitions at the Bunker 73 Khamisiyah site in Iraq. Munitions containing 8.5 metric tons of sarin/cyclosarin

were detonated on March 4, 1991. On March 10, additional rockets were destroyed in a pit at Khamisiyah. Evidence of chemical warfare release during these detonations was considered overwhelming and exposure was assumed for nearby troops (Presidential Advisory Committee, 1996a). The Presidential Advisory Committee on Gulf War Veterans' Illnesses reported that the intelligence community clearly possessed information prior to and during the GW that constituted reasonable cause for concern that Khamisiyah was a chemical munitions storage facility. Current evidence indicates that this knowledge was obtained at least as early as the mid-1980s. However, no serious effort was made to examine the possibility of chemical warfare agent exposure to U.S troops at Khamisiyah until late 1995. When the detonations were made public in 1996, multiple projects modeling the atmospheric dispersion of the chemical warfare agents were begun. As a result of the modeling, more than 100,000 troops were believed to be potentially exposed and received notification to seek medical attention and register in the health registries that had been established at the Department of Defense and the Department of Veterans' Affairs (Walpole and Rosttker, 1997). A General Accounting Office (GAO) report concluded that the number of troops potentially exposed to chemical warfare agents as a result of the Khamisiyah detonations may have been underestimated and that the epidemiological studies using the plume modeling estimates are likely to be unreliable (GAO, 2004). The possibility of long-term health effects associated with low-dose exposure to chemical warfare agents has been a controversial issue. A 1982 National Academy of Sciences report could not rule out the possibility of longterm effects. The Japanese terrorist attacks with sarin during the 1990s provided evidence of the long-term effects of

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toxic exposures to sarin, but individuals who did not present with evidence of acute toxicity at the time of the attacks have not been followed in longitudinal studies (Murata et al., 1997; Nakajima et al., 1999). The Department of Veteran Affairs Research Advisory Committee (2004) reviewed the animal studies of chronic effects of low-level satin exposure and concluded that low-dose sarin exposure is associated with chronic indicators of both neurological and immunological impairment. These animal studies report a number of effects, including decreased immune function, downregulation of muscarinic receptors in the hippocampus, chronic depression of AChE activity, memory loss, and cognitive changes and persistent changes on electroencephalograph readings in different animals. The disclosure from the Department of Defense of a definitive release of nerve gas agents at Khamisiyah, the animal studies indicating the potential for residual effects from low-dose exposure to these agents, and the prevalence of multisymptom complexes among GW veterans all point to the need for more studies of this phenomenon.

VII. HEALTH OUTCOMES ASSOCIATED WITH H U M A N EXPOSURE TO O R G A N O P H O S P H O R U S CHEMICALS A growing body of research indicates that veterans deployed in Desert Storm have increased health complaints compared to veterans who were either not deployed or deployed to other conflicts. Most often, investigators have found that selfreported health symptoms are significantly associated with diverse and multiple exposures rather than just PB- or UPassociated exposures (Iowa Persian Gulf Study Group, 1997; Spencer et al., 2001; Steele, 2002; Unwin et al., 1999). The possibility of residual neurological symptoms associated with the Khamisiyah exposure was investigated using a computer-assisted telephone survey of 2918 GW veterans from Oregon, Washington, California, North Carolina, and Georgia to evaluate the prevalence of self-reported symptoms (2001) and medical diagnoses and hospitalizations (2002) among a subsample of troops in close proximity to the Khamisiyah site and comparison groups of veterans deployed and nondeployed to the Southwest Asia theater of operations. Veterans who had participated in or witnessed the demolition were more likely to report historical or extant symptoms than were veterans from other military units. Five current neurological symptoms were reported in excess: tingling or burning sensations of the skin, changes in memory, difficulty sleeping, persistent fatigue, and depression. A subsequent factor analysis of this study sample found that in addition to the previously mentioned symptoms, veterans who witnessed the Khamisiyah detonations were also more likely to report dysesthesia (Shapiro et al., 2002). McCauley et al. (2002) reported that troops reported to be within 50 km of the Khamisiyah site did not differ from

other deployed troops with regard to reports of any medical conditions or hospitalizations in the 9 years following the GW. Other investigators have also studied the health of veterans who were exposed to chemical agents at Khamisiyah. Gray et al. (1999) also failed to detect an increase in hospitalization among active military personnel; however, when the atmospheric modeling of the dispersion of the chemical warfare agents was improved, they found an increased risk of hospitalization for dysrhythmias among exposed personnel (Smith et al., 2003).

VIII. EMERGING RESEARCH ON NEUROLOGICAL DISEASE IN GULF WAR VETERANS Although initial studies of GW veterans focused on selfreported symptoms and exposures, recent reports have investigated specific diseases and biological mechanisms for the neurological deficits seen in GW veterans. An advisory panel of the Department of Veterans Affairs concluded that the growing body of evidence indicates that an important component of GW veterans' illnesses is neurological in character. The most sobering indication of neurological disease among GW veterans comes from two studies indicating an increased risk of amyotrophic lateral sclerosis, commonly referred to as ALS or Lou Gehrig's disease, at approximately twice the rate of comparison populations in the years since the GW (Haley, 2003; Homer et al., 2003). Haley collected cases of ALS diagnosed from 1991 through 1998 from military registries and a publicity campaign in 1998. Diagnoses were established from neurologists' medical records. The expected incidence was estimated from the age distribution of GW veterans, weighted by age-specific death rates of the U.S. population. During the 8 years postwar, the incidence among veterans younger than age 45 increased from 0.93 cases/year in 1991 to 1.57 cases/year in 1998 and the observed incidence increased from one to five cases per year. Homer et al. conducted a nationwide epidemiologic case study to ascertain all occurrences of ALS for the 10-year period since August 1990 among active duty military and mobilized reservists who served in the GW. The ALS diagnosis was confirmed by medical record review. These investigators found a significant elevated risk of ALS among all deployed personnel (RR = 2.15, 95% confidence interval, 1.38-3.36). Although the number of cases is small, these study results indicate a need to closely monitor this military cohort for further ALS diagnoses. These reports are of serious concern; however, neither group of authors attempted to speculate if UP and CM exposure contributed to this increased risk. The initial clinical studies of GW veterans were largely negative, and no pathology could be detected using routine physical examinations and diagnostic tests (Bourdette et al., 2001; Fukada et al., 1998; Haley et al., 1997). However, in recent years Haley and others have begun to integrate

CHAPTER 6 9 Gulf War Syndrome objective measures of neurological pathology and impairment using specialized neuroimaging techniques, tests of autonomic nervous system function, and audiovestibular testing (Haley et al., 2000a,b, 2004; Roland et al., 2000). These physiological findings have not been widely replicated by other investigators but findings are intriguing. Haley et al. (2000b) found physiological measures of functional brain mass to differ between ill GW veterans and matched veteran controls. As part of the MRS study reported previously, Haley et al. (2000a) also assessed the level of central dopamine activity in the basal ganglia of ill GW veterans and controls and found evidence suggesting an injury of dopaminergic neurons in the basal ganglia. A separate team of investigators reported the findings of a study using MRS that indicated that GW veterans have evidence of neuronal damage in the hippocampus (Menon et al., 2004). Vojdani and Thrasher (2004) reported significant immune alterations in veterans 2-8 years after participation in the GW but did not attribute these findings to the specific action of OPs or CMs. Three investigative teams have explored the possibility of autonomic nervous system disorders in GW veterans. GW veterans have been found to have abnormal responses to tilt-table testing when compared to healthy controls (Davis et al., 2000). Haley et al. (2004) found other indications of autonomic dysfunction, including heart rate blunting during sleep. Peckerman et al. (2000) also reported blunted cardiovascular response among ill GW veterans. Other investigations of neurophysiological assessments have been less consistent. Audiovestibular assessments have only been studied by Roland et al. (2000). Study findings on peripheral nerve function have been inconsistent (Jamal et al., 1996; Sharief et al., 2002). Multiple investigators have examined the potential role of polymorphisms in veterans with unexplained illness, but the results have been mixed (Haley et al., 1999; Mackness et al., 2000; Hotoph et al., 2003). Haley et al. reported that the most severely symptomatic GW veterans exhibited particularly low activity of paraoxonate (PON1) type Q, the type that would be most active in neutralizing nerve gases. Mackness et al. found that veterans' decreased capacity to metabolize OP chemicals may have contributed to their likelihood of developing GW illness. Hotoph et al. found that PON1 activity, which is a major determinant of OP's toxicity in humans, was significantly decreased in British veterans deployed to the GW compared to nondeployed veterans. Low PON1 activity is associated with several diseases with an inflammatory component; however, a link with OP exposures during the GW has not been established.

IX. C O N C L U S I O N S Fifteen years after the GW, the etiology behind the large number of veterans who have unexplained illnesses remains a mystery. Perhaps the role of OP and CM chemicals in

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the risk of unexplained illness will never be delineated. The lack of objective data on exposure and the inability of routine physical examinations and tests to detect subtle neurological dysfunction compound the inability to find answers to this problem. A large number of scientists and clinicians believe that the unexplained illness is not chemical in origin but results from a psychological reaction to the war trauma, although there is little objective evidence to support this assumption. What is clear is that the symptoms are disabling and more than $1 billion has been spent in the United States on health evaluations and research on this phenomenon (Gray, 2004). As recently as 2004, a scientific advisory panel review recommended increased studies on the chronic health effects of low-dose exposures to OP substances (U.S. Department of Veterans Affairs, 2004). Most important, the GW syndrome has resulted in many improvements in the surveillance of environmental exposures that occur during military deployments and has increased the number of research studies aimed at finding effective treatments for the many veterans who still suffer from unexplained illness.

References Abdel-Rahman, A. A., Shetty, A. K., and Abou-Donia, M. B. (2002). Disruption of the blood-brain barrier and neuronal cell death in cingulate cortex, dentate gyrus, thalamus, and hypothalamus in a rat model of Gulf War syndrome. Neurobiol. Dis. 10, 306-326. Abdel-Rahman, A. A., Dechkovskaia, A. M., Goldstein, L. B., et al. (2004a). Neurological deficits induced by malathion, DEET, and permethrin, alone or in combination in adult rats. J. Toxicol. Environ. Health A 67, 331-356. Abdel-Rahman, A. A., Abou-Donia, M. B., E1-Masry, E. M., Shetty, A. K., and Abou-Donia, M. B. (2004b). Stress and combined exposure to low doses of pyridostigmine, bromide, DEET, and permethrin produce neurochemical and neuropathological alterations in cerebral cortex, hippocampus, and cerebellum. J. Toxicol. Environ. Health A. 67, 163-192. Abou-Donia, M. B., Wilmarth, K. R., Jensen, K. E, Oehme, E W., and Kurt, T. L. (1996a). Neurotoxicity resulting from coexposure to pyridostigmine bromide, DEET, and permethrin: implications of Gulf War chemical exposures. J. Toxicol. Environ. Health. 48, 35-56. Abou-Donia, M. B., Wilmarth, K. R., Abdel-Rahman, A. A., Jensen, K. E, Oehme, E W., and Kurt, T. L. (1996b). Increased neurotoxicity following concurrent exposure to pyridostigmine bromide, DEET, and chlorpyrifos. Fundam. Appl. Toxicol. 34, 201-222. Abou-Donia, M. B., Goldstein, L. B., Jones, K. H., et al. (2001a). Locomotor and sensorimotor performance deficit in rats following exposure to pyridostigine bromide, DEET, and permethrin, alone and in combination. Toxicol. Sci. 60, 305-314. Abou-Donia, M. B., Goldstein, L. B., Dechkovskaia, A. M., et al. (2001b). Effects of daily dermal application of DEET and permethrin, alone and in combination, on sensorimotor performance, blood-brain barrier, and blood-testis barrier in rats. J. Toxicol. Environ. Health A 62, 523-541.

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