Pesticide Exposure and Childhood Development in the Yaqui Valley
Monitoring Type: Human Health

Problem Exposure to pesticides has been linked to developmental health effects, but this is rarely demonstrated with field studies and the linkage can be difficult to document.
Objective To develop and demonstrate the use of simple methods for detecting subtle health impacts, and to document these health impacts in a comparative way in two communities with different pesticide exposure levels.
Monitoring Type Human Health
Community Involvement The Yaqui community where the study took place did not participate extensively in the study design or execution.
Notable Feature Clear evidence of motor skill and developmental impairment gathered by simple diagnostic methods. There is a striking contrast between drawings made by children less exposed to pesticides and by children who were more exposed to pesticides. Also, there is a manual widely available to community researchers showing how to employ the simple, low-cost methods used by Dr. Guillette in Sonora Mexico and elsewhere. The manual can be downloaded at:


BACKGROUND: Pesticides in two Yaqui Communities in Sonora Mexico

In the 1950s, the indigenous Mayan people living in the Yaqui Valley in the Mexican state of Sonora hit a philosophical divide. Some wanted to adopt the so-called „Green Revolutionš modern farming techniques, including the use of pesticides and tractors, while others wanted to continue with traditional farming and ranching methods. Those wishing to adopt the modern techniques (including use of pesticides and tractors) moved to the valley of their region, and those who did not occupied the foothills.

Yaqui Valley is now one of the largest agricultural areas in Mexico (31,000 hectares / 76,600 acres) and annually produces 1/2 million tons of fruits and vegetables annually, mainly for export to the U.S. and Canada. Along with increased production has come greatly increased pesticide use in all other respects, even genetic, the communities of the valley and the foothills do not differ.

In the late 1980‚s, the Institute for Technology in Sonora (ITSON) conducted studies in the Yaqui Valley and found high levels of pesticides in cord blood and breast milk. In the early 1990s, Dr. Jose Luiz Perez Gonzales conducted a health effects study in the Yaqui Valley and found that children had extremely high levels of pesticides in their porous organs that in some cases had led to death. Unfortunately, because of the political climate of the time, his team was not allowed to expand or even continue the study. Elizabeth Guillette, an anthropologist and research scientist, learned about these studies in the mid-1990s and obtained permission from the Mexican government to continue testing on the condition that she not use the word őpesticide,‚ which the Mexican government thought would create undue alarm among the residents of the communities. She agreed to conduct a generic „environmental study.š

THE STUDY: Yaqui Valley Mothers Suspect Chemicals Are Harming Children

Dr. Guillette and Maria Mercedes Meza, a chemist for Institute of Technology in Sonora (ITSON) carried out the study, entitled „An Anthropological Approach to the Evaluation of Preschool Children Exposed to Pesticides in Mexico,š funded by the U.S. Government and printed in the scientific journal, Environmental Health Perspectives in June 1998.

Their study was conducted in farming communities of the Yaqui Valley and in Tescopaco. Tescopaco, in the Sierra Madre foothills, is the 'control site' because it is the town furthest from the farms, about 80 km away (approximately 50 miles). These towns are considered interesting communities to compare because the people in these areas are very similar genetically.
They also eat the same food source, have the same education, same economy, customs, and the same housing facilities, yet there are significant differences in the health of the children in the two communities.

Those who farm in the valley use a wide mix of pesticides so it is impossible to tell what specific pesticides they have been exposed to. There is strong evidence of DDT usage and exposure, among dozens of other chemicals. Farmers would not reveal information about specific pesticides, in part because pesticides are tied to bank loans, and the banks are not willing to reveal what is being used with certain crops. However, they did tell Dr. Guillette and Ms. Meza that pesticides were applied thirty to forty times a season - about three times the number of applications reported by experts in the U.S. and Canada. The families under study lived in towns surrounded by farms. The children are exposed to aerial and ground spraying in their environment where they play and from the food they eat. Many of the valley families told the researchers that they are often showered with pesticides that come into their homes and cause problems like headaches and diarrhea.

Dr. Guillette was initially interested in cancer - the dramatic, obvious outcome, like that found in laboratory rats subjected to huge amounts of chemicals. But mothers in the valley insisted that the researchers search for broader, perhaps subtle effects. The mothers already suspected that chemicals were affecting their children, but could not identify specific health effects. To uncover hidden impacts, the researchers used common intelligence and developmental diagnostics Ų for example asking children, ages four and five, to perform a series of play activities corresponding to developmental levels. Tests consisted of simple tasks such as catching a ball, and dropping raisins into a bottle cap. Pediatricians to measure development in children use these same tests. Children of age three should have been able to accomplish these tasks. The research team noticed right away that the Valley children showed deficits in energy levels and in hand and eye coordination. The 4 and 5-year-old Valley children weren't performing at the level of a three year old.

Children‚s Drawings Dramatically Illustrate Problem
One of Dr. Guillette‚s findings stood out hauntingly above the others; the ability to draw a person (see figures below). This is a pediatrician's method to measure a child's development of perceptual and motor abilities. The foothills children at ages 4 and 5 could draw a complete person. Among the exposed children, most 4-year-olds just scribbled, and the 5-year-olds could draw a head and a line or a circle and a line. She also noted evidence that the valley children were getting sick more often

There were also differences in behavior. The foothill children were observed to be busy with group play, whereas the valley children were more apt to play alone. Local teachers also complained that the valley children were much more difficult to teach, as they have trouble remembering and often have behavioral problems. They Yaqui mothers from the valley also reported more problems getting pregnant and higher rates of miscarriage, stillbirth, neonatal death and premature birth.

Dr. Guillette made a second trip to the region two years after the first to follow up on the developmental progress and health of the children. In the second trip she noted that the valley children had continued developmental and mental deficits, including poor, sense of balance, and poorer general health. The exposed children lacked the balance to turn around on a 2x4 plank and walk back to the starting location whereas the lesser exposed could. She also found that the 7-year-old valley (exposed) children were only drawing at the 4-year-old level. The 7-year-old lesser-exposed were identifying people by gender, with a dress or pants, appropriate hair, fingers, facial features and shoes.

During this second trip to Sonora, she also surveyed the children for illness during the past three months and found that the valley children had been ill about six times more often than the lesser-exposed children in the foothills. The valley children had high rates of upper respiratory infection and other symptoms such as allergies. Some of the illnesses in the valley children were associated with direct pesticide exposure, such as rashes and gastrointestinal problems.

Endocrine Disruption Suspected
Elizabeth Guillette‚s husband, Louis, was one of the first scientists to discover that many pesticides can disrupt the endocrine system that controls animal hormone production (see case study on Lake Apopka). Scientists now suspect that pesticides have similar effects on humans. Knowing this, Elizabeth Guillette noticed that the Valley children appeared to be maturing more rapidly than those in the foothills. . She has recently completed a study on breast development in 8 to 10 year old girls and found the valley girls generally have a decrease, or lack of, mammary tissue in the developing breast. Dr. Guillette found that mothers experienced higher levels of birth defects, spontaneous abortion, premature birth, and stillborns that were more than double in the valley town than in the foothill town of Tescopaco. Although Elizabeth couldn't prove conclusively that pesticides were the reasons for the deficiencies, the results are alarming and strongly suggest the possibility of a connection to the pesticide contamination, as other variables are limited.

REFLECTIONS ON THE PROJECT: Simple Methods Generate Powerful Data

The methods used in this study are attractive because they are simple and inexpensive. For example, the team Dr. Guillette is now working with in a similar study using three community researchers. A diagnostic test for a single child takes about 30 minutes. An interview with the parent of the child also takes about 30 minutes. Compared to other monitoring techniques discussed in this handbook, these methods require little technical expertise. Essentially, one merely has to follow the directions in the handbook developed by Dr. Guillette (see below). Dr. Guillette will review your interview forms, data analysis and help solve related problems using e-mail. Typical equipment includes: tape measure, tennis balls, a 2x4 plank, raisins to drop into a bottle cap, and gallon jugs.

Because Dr. Guillette‚s study was so widely read and well received, many researchers are now looking at how children function, especially mentally. The studies have been used mainly in the context of decreasing exposures (for example, pesticide bans in Canada resulted from this type of study). Also, through the UN convention for the elimination of POPS (Stockholm Convention), similar diagnostic methods are being used in India and elsewhere in the developing world.

It is important to keep in mind that the diagnostic techniques described in this case study are not a magic bullet. As with all health effects monitoring, these methods are not sufficient evidence that the results seen are the result of any particular contaminant. They can, however, be used in conjunction with other evidence to build such a case. Researchers must also be careful not to exclude the community in designing or executing the study. Lack of community involvement in this study occurred because this monitoring study was the first of its kind and basic testing methodologies were being adapted for use at the community level. And, of course, the study was directed in response to mothers‚ concerns about their children‚s health. Collaboration with the community at all phases will enrich any similar studies and ensure that the best possible results are achieved.

Dr. Guillette has produced a manual outlining the simple and elegant diagnostic methods used in her Yaqui research and elsewhere. It is available for download at:

Web links and contact info

Accounts of Elizabeth‚s work in Mexico:

Elizabeth A. Guillette, Ph.D.
Asst. Research Scientist, Anthropology
University of Florida
32 SW 43rdTerrace
Gainesville, FL 32607
(352) 375-5929 (tel)
(352) 392-6929 (fax)

Figures: These are pictures of people drawn by Yaqui children. Notice the incredible contrast in the representations performed by the lesser-exposed foothill children (left) and more-exposed valley children (right). (Back to text)

4 Year Olds

5 Year Olds

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