Environmental Factor, May 2010, National Institute of Environmental Health Sciences
Study Well Positioned to Look at Age-Related Disease
By Colleen Chandler
"As the third phase of data collection in the Agricultural Health Study (AHS)(http://www.niehs.nih.gov/research/atniehs/labs/epi/studies/ahs/index.cfm) draws to a close, the established aging cohort will likely continue to be a prime resource for studying associations between pesticides and diseases of aging, such as those that involve cognitive function and late-in-life development of autoimmune diseases or Parkinson's disease," said NIEHS Epidemiology Branch Staff Scientist Jane Hoppin, Sc.D.(http://www.niehs.nih.gov/research/atniehs/labs/epi/chronic/staff/hoppin/index.cfm)
Hoppin is the study's NIEHS co-principal investigator. NIEHS Epidemiology Branch Chief Dale Sandler, Ph.D.(http://www.niehs.nih.gov/research/atniehs/labs/epi/chronic/index.cfm), is NIEHS principal investigator on the study. Together with their collaborators at the National Cancer Institute, they recently compiled an impressive overview of since the launch of the study 20 years ago.
Field data collection for the third phase of the study concluded earlier this year. Hoppin said new data will be available to researchers this summer. Researchers are preparing to delve into the wealth of new information that can be gleaned from the data - especially as it relates to the health effects of pesticides from occupational and general population exposures.
Drawing on resources of the AHS, Hoppin and colleagues at NIEHS and elsewhere are also involved in two additional studies - the Farming and Movement Evaluation (FAME) Study and the AHS Neurobehavioral Study. FAME is a case-control study of Parkinson's disease nested in the AHS, while the extramurally funded AHS Neurobehavioral Study will examine results of neurobehavioral tests and biological samples to evaluate whether long-term, low-level exposures to pesticides have neurological consequences.
"We are observing the world as it happens," Hoppin explained. Hoppin has been deeply involved with the AHS since joining the Institute in 1999 - nine years after the study was launched by NIEHS and the National Cancer Institute (NCI), along with collaborators from the Environmental Protection Agency (EPA) and the National Institute for Occupational Safety and Health (NIOSH). Hoppin and colleagues at NIEHS and NCI have been authors on more than 120 papers(http://aghealth.nci.nih.gov/publications.html) published so far by researchers working with the AHS data (see text box).
Families with known exposures to pesticides
One of the basic premises of the AHS was that, as a group, farmers are not only routinely exposed to pesticides, but they also know what chemicals they use. And that, epidemiologists say, makes them a near perfect group to follow to determine what health effects may be expected in the general population as a result of pesticide exposure. The average person is incidentally exposed to pesticides in daily life, but often knows little about the chemicals he or she is using.
NCI, of course, is specifically interested in cancers related to pesticide exposure. Not only are they conducting cancer analyses using the questionnaire-based data, they are also using the buccal cell DNA to assess whether specific individuals may be more susceptible to pesticide exposure.
EPA and NIOSH have been part of the research team to contribute exposure assessment expertise to the study. EPA researchers conducted a field monitoring study of 2,4-D and chlorpyrifos applicators and NIOSH investigators monitored fungicide exposure in orchard farmers. Results of both these studies have contributed information to the predictors of pesticide exposure and help to refine the algorithm used in the AHS to assign exposure intensity.
Mining the data for associations with complex diseases
Hoppin said the potential disease processes related to pesticide exposure are not very well understood. While much is known about the potential neurotoxic effects of insecticides at high levels of exposure, low-level neurotoxic effects and effects in other organ systems are not well characterized. For cancer, no specific pesticides, with the exception of arsenicals, have been classified as known human carcinogens, although IARC currently considers "occupational exposures in spraying and application of non-arsenical pesticides" as a probable human carcinogen. Researchers in the AHS have looked at respiratory effects, diabetes, thyroid, Parkinson's Disease, neurologic effects, and endocrine disruption as well as cancer.
(Colleen Chandler is a writer/editor in the NIEHS Office of Communications and Public Liaison and a regular contributor to the Environmental Factor.)
Farm Families Provide a Rich Database for Health Studies
Following its launch in 1990, researchers with the AHS recruited 89,000 pesticide applicators, mainly farmers, and spouses in Iowa and North Carolina between 1993 and 1997. Certified and trained pesticide applicators and their spouses were recruited as they sought to renew their state licenses or took refresher courses on pesticide handling. The pesticide exposure and lifestyle information collected is much more detailed than any previous study of agricultural workers, and the AHS itself is the largest and most comprehensive health study of its kind, according to the study Web site(http://www.aghealth.org/") .
Both North Carolina and Iowa have cancer databases. Researchers check the registries each year to determine if any participants have been diagnosed with cancer and use the database to confirm the type and extent of the cancer. Researchers also search the National Death Index and death certificate databases in both North Carolina and Iowa to determine if any participants have died. Hoppin said the information in the cancer registries is backed by pathology confirmation, providing solid data on incidence of cancer. The death records provide cause-of-death information on participants who die during the study, she said.
The average age of farmers and spouses who enrolled in the study in its infancy was 45. That means those individuals are now around 60, but they range in age from younger than 30 to more than 90 years. Researchers have already collected a variety of demographic, medical history, and pesticide-use data as well as buccal cells for DNA. Hoppin said this information will enable researchers to look at associations and establish connections between agricultural exposures and health effects. The study has been flexible enough to change as needed during the course of the last 15 years. For example, by collecting information on all pesticides used since enrollment, the AHS has the ability to identify new trends in pesticide use, such as the increased use of fungicides in Iowa to combat soybean rust.
According to Hoppin, the stage is now set to look at effects of chronic, long-term exposure by assessing the new cases of disease identified since the study began. As researchers evaluate the data looking for new ways to use the pesticide exposure-related health effects data set, they will likely collect more targeted data from participants. These sub-studies allow for more detailed characterization of disease or exposure status than can be done through questionnaires and record linkage alone. Currently, There are already a number of add-on studies using sub-cohorts of the AHS being conducted by both intramural researchers and extramural grantees. These studies leverage the strength of the AHS to enhance the study of pesticides and diseases of aging.