COVID-19 became the third leading cause of death, behind only heart disease and cancer. Race relations boiled over. The economy may be worst since the great Depression.
But Houstonians were spared one possible scourge this past summer: West Nile disease. With mosquitoes’ prime blood-sucking season now in the rear-view mirror, local health officials are reporting just one 2020 case of the insect-borne disease in Harris County. Case numbers can vary dramatically, topping 100 twice in the past decade, but they typically at least hit double figures.
“We got lucky,” said Kristy Murray, a Baylor College of Medicine professor of tropical medicine. “I was really worried at the start of the season, the idea of a summer of COVID-19 and active West Nile, given that people were going to be spending more time outside at events, working out, just getting out of the house. But it turned out to be super calm in our region.”
Don’t expect any simple explanation for the good fortune: “I wish I knew,” Chris Fredregill, director of Harris County’s mosquito control division, said when asked for his theory. “West Nile is tricky. It’s hard to say why one year is so much better or worse than another.”
The factors include warm weather, rain and immunity or the lack thereof in those infected, including animals. Teasing those factors apart is the hard part.
Unlike COVID-19, the West Nile virus is rarely fatal. But it shares a number of characteristics with the coronavirus — a high percentage of infections don’t cause symptoms; symptoms usually resemble those of the flu; and the virus can cause lingering, serious damage once the acute illness is over.
Murray has documented much of the long-term problems in Houston-area patients, including kidney damage in four of 10 patients she followed for years after they were infected; significant cognitive decline; and hearing loss, abnormal reflexes, muscle weakness and other neurological complications.
West Nile arrived in the Houston area in 2002, three years after the first U.S. cases were reported in New York. It’s spread to people by mosquitoes that become infected when they feed on infected birds, the virus’ primary host.
The state’s worst experience with the disease was 2012, when it recorded 1,868 cases, the most in the nation. That spike was largely the result of widespread transmission in the Dallas-Fort Worth area, which accounted for 872 of the cases. Some 101 of those were reported in Harris County.
Texans braced for another bad West Nile year in 2013, but, characteristically, the number dropped significantly. In 2014, however, Harris County had its most active year ever, with 134 cases, more than a third of the state total.
“It’s very cyclical,” said Paul Grunenwald, communicable disease manager for the Texas department of health. “One year can be quiet, the next year there’s a dramatic increase.”
Indeed, Murray said she wouldn’t be surprised if the virus returns in significantly bigger numbers next year. Bird immunity can keep the spread down, but she noted that house sparrows, which are responsible for a lot of the spread, only live about three years. The birds that replace them are as vulnerable to the mosquito-borne virus as people are to the new coronavirus.
This is the second consecutive year West Nile numbers have been almost non-existent in Harris County. There were just two cases in 2019, down from 39 in 2018.
Dr. Paul Rodrigo Hasbun, an infectious disease specialist with UTHealth, added that the decline in cases is not for a lack of looking. He noted that UTHealth doctors are screening patients in search of West Nile as part of studies they’re conducting, both in collaboration with Murray and independently, to determine the disease’s lingering effects.
But Murray and others said it is way too early to think West Nile may be fading, in the manner of St. Louis encephalitis. That mosquito-borne disease caused appreciable disease in the 1960s and 1970s, but is little seen these days.
However, Zika, a third mosquito-borne disease, does appear to have burned out, said Murray. That virus, which can cause birth defects if it is transmitted to a pregnant woman, caused alarm after it emerged in Latin America in 2015, then spread to the United States in 2016.
In any case, Houston public health officials paused just long enough to celebrate this year’s good fortune with West Nile before warning about complacency.
“It’s great that this year’s numbers have dropped, but it’s hard to predict which way the disease will go next,” said Grunenwald. “But rest assured, this threat hasn’t gone away. People need to keep wearing repellent and long-sleeved clothes.”