Spikes in Ala Wai infectious bacteria are strongly influenced by the amount of rainfall in surrounding areas, according to a recently published study by oceanographers at the University of Hawaii at Manoa and Hawaii Pacific University.

A catalyst for the study was the April 6, 2006, death of Oliver Johnson, an Oahu man who died from a Vibrio vulnificus infection after his cuts and scrapes were exposed to Ala Wai Harbor water. Days later his body had swelled to about three times his normal size, his left leg was amputated and he suffered organ failure before his family took him off life support.

Following his death, friends of Johnson peppered the media with warnings to stay out of the water. His family maintained that he had contracted the infection while in the boat harbor, which was allegedly contaminated with feces after the city diverted 48 million gallons of raw sewage into the Ala Wai Canal when a 42-inch main broke about two weeks earlier.

Study authors Grieg Steward, professor in the UH School of Ocean and Earth Science and Technology, and Olivia Nigro, a former UH graduate student in oceanography and now an assistant professor at HPU, began assessing Vibrio in 2006 after Johnson’s death.

A quick pilot study in 2006 showed that the bacteria are present in high but not unexpected numbers, and inspired a yearlong study from 2008 to 2009. The project was supported by funding from the National Oceanic and Atmospheric Administration through the Hawaii Sea Grant program and from the National Science Foundation.

Steward said in a news release that Johnson’s exposure “occurred after many days of heavy rainfall, which, given our results, suggests this may have been an exacerbating factor.”

The researchers observed that rainfall has a variable effect depending on the amount of precipitation.

“Too little rainfall means much of the canal is saltier than optimum for V. vulnificus growth; too much rainfall and the canal is fresher than the bacterium likes,” Nigro said in the release. “We also found that, because of the flushing effect of intense rainfall, the highest abundance of V. vulnificus shifted from within the canal out into the harbor.”

The oceanographers hope that additional years of data will make it possible to provide real-time predictions of when and where the bacterium is likely to be at unusually high concentrations, a component in assessing infection risk.