The initial phase of our investigation consisted of confirming the diagnosis of each reported case and conducting an interview with each case family to identify any potentially common characteristics or environmental exposures that might point to a preventable cause. We are indebted to the Centers for Disease Control and Prevention as well as the Massachusetts Department of Public Health for their assistance in providing us with model interview instruments.
The case family interviews were conducted face-to-face with each family. This involved a detailed review of the family's residential history from the date of diagnosis back to a point in time two years prior to conception of the ill child. For each residence we inquired as to the source of water, in-home treatment of water, and uses of water. We also inquired about known exposures to chemicals from agricultural or home use of herbicides and pesticides as well as indoor uses of chemicals and solvents. For each parent, we also inquired about occupation and occupation-related exposures to chemicals, fumes, dust, or radiation. We conducted a detailed review of the child's medical history and the mother's pregnancy and breast-feeding histories. Finally, we asked case families about any hobbies, sports activities, or typical travel destinations that might have brought them into contact with chemicals, fumes, dust, or radiation.
From this interview process we learned that half of the case families had spent 2 years or more in the Fallon / Churchill County area. The others had resided in the area for shorter periods of time. The 12 case families had resided in a total of 88 different homes over their respective time periods of interest. Of these, 22 were located within Churchill County. Of these 22 local residences, half were served by public water systems while the others obtained their water from domestic wells.
Our initial analysis of the occupational, medical, environmental, and other historical information provided by the case families has not suggested any particular common denominator that would link these cases together. We recognize, however, that some of our data is subject to recall limitations on the part of the families. Specifically, they may not have known of an environmental exposure that did, in fact, exist, or may have forgotten about it. For this reason we are currently taking steps to obtain additional data through objective environmental sampling. This constitutes a second phase of the investigation.
We are now in the process of obtaining water samples from those current and former case residences in Churchill County that are served by domestic wells. These samples are being subjected to the analyses that are routinely done for public water systems. In other words, any test required by the safe drinking water act for public water systems is also being conducted on the water samples obtained from the wells of residences where case families have lived. The results of these analyses are pending at this time.
We have also invited the Centers for Disease Control and Prevention as well as the Agency for Toxic Substances and Disease Registry to assist us in identifying and analyzing completed pathways for other sources of environmental contamination. This would include industrial, agricultural, military, or other sources.
On a parallel tract with these environmental studies we are also collecting data on the overall population dynamics in Churchill County. This includes looking at size of various age cohorts over the last 10 years, school enrollment information, and military populations. This analysis will help to determine if Churchill County matches the profile of other communities around the world where population mixing has been suggested as a possible explanation for increased rates of childhood leukemia.
In closing, I would like to make some brief comments as to the importance of cancer registries in the conduct of cancer cluster investigations. Nevada has maintained a population-based cancer registry since 1979. This activity has been funded, in part, through a grant from the Centers for Disease Control and Prevention since 1995.
All disease reporting systems, including cancer registries, experience a lag in time between the diagnosis of a case and the reporting of that case. With a disease such as cancer, the patient record may not be complete enough to warrant abstracting information until about six months from the date of diagnosis. Additional delays in obtaining information beyond this six month time period relate to work load and staffing. In the more rural parts of Nevada, this situation is made even more difficult due to the distances involved and the relatively low number of acute hospital beds in each facility making it a costly and time consuming process to collect rural data. For these reasons, if a cancer cluster is identified through a cancer registry it is likely to have been going on for some time.
The increased incidence of childhood leukemia in Churchill County was not identified through analysis of cancer registry data. The local hospital, physicians, and community leaders noted the cases and perceived the numbers to be unusually high. Nevertheless, Nevada's cancer registry has been invaluable in helping to place the observed number of childhood leukemia cases in historical and geographic context. Only through analysis of cancer registry data have we been able to calculate the usual rate of childhood leukemia and determine that the local cases represent a significant excess over the expected.
I hope this overview of our investigation to date and the role of cancer registries has been helpful. I would be happy to answer any questions the committee may have.