The first freeze of the season earlier this month in the region has killed off mosquito populations carrying the West Nile virus. The good news throughout southwestern Pennsylvania is there were no human cases of the virus reported this year, although the numbers of mosquitoes found with the virus were higher than ever.
"We seem to have dodged that bullet," said Ronald E. Voorhees, acting director at the Allegheny County Health Department.
At the start of the season, Dr. Voorhees said, health officials saw "a higher proportion of pools testing positive. We thought we would end up with human cases."
From a high of 22 human cases, including four deaths, in 2002, Allegheny County has seen a declining trend in the number of cases since. There were 10 human cases in 2003, six in 2005 and one each in the years 2006, 2007 and 2011.
Dr. Voorhees noted that West Nile virus may no longer have the same effect on people in the region as it did when it first appeared a decade ago, and mild infections may have occurred this year without being reported to health officials.
"We only find out about the very serious cases. The vast majority have no symptoms; if they get it, they might get viral symptoms, feel achy. Only a small number might get sick. ... I think it's quite possible that there were mild cases, that people wouldn't have gone to the physician for."
Dr. Voorhees said there is a theory that people and birds might be developing an immunity to the virus, as it has moved from the East Coast to the West Coast, this summer affecting more people from the Midwest and the South and California.
"The only way to find out is to do testing, look for antibodies [in the blood]," Dr. Voorhees said. "It's a brand new disease in the last decade. We want to know what's happening."
He speculated that warm temperatures might have helped curb the amount of standing water in the area, controlling the adult mosquito population. He added that predicting where West Nile might break out next is still very much a challenge for researchers.
National agencies are supporting some of that research. The National Oceanic and Atmospheric Administration monitors how climate and weather events affect human health. In its July "state of the climate" report, NOAA's National Climatic Data Center said warm temperatures on the North American continent combined with stagnant water in flooded areas to set up favorable conditions as breeding ground for mosquitoes.
Responding to concerns that new health risks are appearing because of climate change, in 2010 a National Institutes of Health-led federal interagency group of health experts recommended coordinated research to better understand climate's impact on human health. It called for "integrating human, terrestrial and aquatic animal health surveillance with environmental monitoring ... to better understand emerging health risks like Lyme disease, West Nile virus, malaria and toxins from marine algae."
In Pennsylvania, testing of mosquito breeding sites has ended for the season. However, the state Health Department continues to post human and horse cases on its West Nile virus site: http://www.westnile.state.pa.us.
As of Oct. 18, there were a total of 35 human cases in the state in 2012; 17 females and 18 males. Patients fell into all age groups: under 18 years old, one case; 18-39, five; 40-59, nine; 60-69, eight; 70-79, seven; over 80 years old, five. Twenty-one were diagnosed with encephalitis or meningitis; 14 were listed as cases of West Nile fever. The only Western Pennsylvania reported case was in McKean County, on the northern border with New York.
Of the state's three deaths from the virus, two were reported this month. A Luzerne County man died from the virus on Aug. 27. Two Philadelphia County men have since died, on Oct. 11 and Oct. 16.
Numbers shot up in other parts of the nation. The U.S. Centers for Disease Control and Prevention reported as of Oct. 16, there were a total of 4,531 cases, including 183 deaths, across all 48 contiguous states. Worst hit was Texas, with 1,580 cases and 55 deaths. Other highs in cases and deaths were: Louisiana, 239 (11); California, 285 (11); Illinois, 200 (7); Mississippi, 229 (5); South Dakota 200 (3). Among Pennsylvania's neighbors, Ohio reported 111 cases, five deaths; West Virginia reported six cases, no deaths.
Next year, Dr. Voorhees said he expects county health officials will continue to work with the state Department of Environmental Protection and continue testing standing pools of water for mosquito larva populations, which can then be treated with a chemical that prevents them from becoming adult mosquitoes.
"We have experienced a different climate; it's been warmer. If the climate is getting warmer, that is increasing risk [for West Nile]. We will increase our outreach and we will continue to monitor cases."
Jill Daly: email@example.com or 412-263-1596.