Objective: To determine the association between a leak of sour natural gas (more than 30% hydrogen sulfide) from a pipeline in a river valley and the health of beef cattle in the intensively ranched surrounding area.
Design: Prospective cohort study.
Sample population: 13 herds of cattle within 4 km (2.5 miles) of the leak and 10 herds outside the 4-km zone.
Procedure: Distance of herds from the leak site was determined, using geographic information system technology. Information about speed and direction of winds was obtained from a local meteorologic station and an ambient air-quality monitoring trailer. Health and productivity data for surrounding beef herds, as well as exposure information, were collected and analyzed.
Results: An association was not found between total herd calf mortality and herd distance from the leak, wind-aided exposure, location in the river valley, signs of irritation consistent with exposure to the gas, or reports of odors of gas at the time of the leak. Management changes reported in response to the gas leak were identified as risk factors for total herd calf mortality. Other herd-level risk factors associated with increased calf mortality ratio included a median calving date in February and percentage of twin births for a herd.
Clinical implications: In this example, we did not detect an association between productivity of cattle and exposure to sour natural gas. Several methods can be used for ranking potential exposure after discovery of a leak.