Pollen Loads and Allergic Rhinitis in Darwin, Australia: A Potential Health Outcome of the Grass-Fire Cycle

Although the prevalence of asthma and allergic rhinitis has been increasing in tropical regions, little is known about the allergenicity of pollens from tropical plant families or the importance of ongoing environmental changes. We investigated associations between daily average pollen counts of several tropical plant families and sales of medications for the treatment of allergic rhinitis in Darwin, Australia—a tropical setting in which grass abundance has increased due to increased fire frequencies and the introduction of African pasture grasses. Daily pollen counts with detailed identification of plant species were undertaken in conjunction with a weekly survey of flowering plant species from April 2004 to November 2005. Five pharmacies provided daily sales data of selected medications commonly used to treat allergic rhinitis. We used generalized linear modeling to examine outcomes. All analyses accounted for the potential confounding effects of time trends, holidays, respiratory viral illnesses, meteorological conditions, and air pollution. The peak total pollen count was 94 grains/m3. Despite the low levels of Poaceae (grass) pollen (maximum daily count, 24 grains/m3), there was a clear association with daily sales of anti-allergic medications greatest at a lag of 1 day. Sales increased by 5% with an interquartile range rise (3 grain/ m3) in Poaceae pollen (5.07%, 95%CI 1.04%, 9.25%). No associations were observed with pollen from other plant families. Although further testing is required, we suggest that an overlooked aspect of the ‘‘grass-fire cycle’’ that is degrading many tropical landscapes, could be an increase in the prevalence of allergic rhinitis.