Carbon pollution isn’t just boosting hurricanes, heatwaves and wildfires. It could be increasing hazards like ticks, pollen, mosquitoes and poison ivy. Science writer Melissa Gaskill surveys the situation in Texas.


Eastern poison ivy (Toxicodendron radicans)

During the now six-months-and-counting of Covid-19, spending time outdoors has been key to keeping many of us sane. Going outside offers a break from staring at four walls – and screens – and a low-risk way to socialize with other humans.

The outdoors isn’t without hazards, of course, and across the country, climate change may be increasing some of them. Think more potent poison ivy, longer growing seasons for pollen-producing plants, changes in the timing of “mosquito days,” and higher risk of tick-borne diseases.

Here in Texas, the news is mixed, with some risks increasing and others decreasing.

Leaves of three: Worse

Rising carbon dioxide levels are associated with more increased abundance of poison ivy (Toxicodendron radicans) and a 173 percent increase in levels of its toxic oil, urushiol. Given the plant’s wide distribution, this represents a growing health problem.

Poison ivy grows throughout North America, occurs in Central America, parts of Asia, Bermuda, and the Bahama Islands, and has been introduced into Europe, South Africa, Australia and New Zealand.

The plant is particularly common in the southeastern U.S. and widespread over Texas. “It can pop up anywhere, in a garden or park, winding up a tree,” says William M. Johnson, a Texas A&M AgriLife county extension agent for horticulture. “In particular, it likes semi-shaded areas such as along the edge of a tree line or a trail.” A perennial, it is around all year.

Birds that feed on its seeds spread the plant through their droppings. “It also can spread underground, so if you have a plant around your home or someplace where you’ll come in contact, you need to take care of it,” Johnson says. “It will do nothing but spread and get bigger.”

Urushiol is absorbed through the skin in as little as 10 minutes and just four one-hundredths of a drop of the oil can cause a reaction in the skin. A leaf has to be broken to release the oil, Johnson says, but that doesn’t take much.

Urushiol is invisible and can transfer from hard surfaces, such as a garden tool, clothes or hiking boots, and may remain active on those surfaces for several years. Pets can transfer the oil if they get it on their fur and you rub it. A serious reaction can occur if the vine is burned and someone breathes in the compound.

Roughly 80 percent of adults are susceptible to the allergic reaction. Generally, a rash develops within 24 hours, but can take up to a week.

Johnson recommends flushing an affected area with rubbing alcohol and then rinsing with “a lot of running water, like a hose. You can wash the oil off, but you have less than 15 minutes before it binds to the skin and washing has no effect.”

Poison ivy is commonly confused with other vining plants, including Virginia creeper and, on the upper Gulf Coast, peppervine. An old saying, “leaves of three, let it be” is quite appropriate, Johnson points out, as the plant has clusters of three leaves on each stem. He adds that it pays to be cautious: “Even if you aren’t sure it is poison ivy, leave it alone.”

Some pollen: Worse

In the contiguous 48 states, the average length of the growing season – the time between the last spring frost and the first fall one – has increased by nearly two weeks since the beginning of the 20th century, particularly during the past 30 years.

This sounds like good news for farmers and gardeners, but it could increase problems for those who suffer from pollen-related allergies. If your tormenter is ragweed, though, a bit of good news: The number of ragweed pollen days in Texas decreased by one between 1995 and 2015.

Ticks: Mixed news

Biting insects such as mosquitoes and ticks do more than annoy people in the outdoors; some carry serious diseases. Changes in the length and timing of seasons, increasing temperatures and increased rainfall can affect the likelihood of exposure to these vectors.

For example, warmer temperatures help ticks develop faster and emerge earlier, which could increase their odds of surviving to adulthood and biting someone.

The incidence of Lyme disease, carried by the blacklegged tick, has doubled over the past two decades, to 30,000 cases per year in the U.S. One long-term study found peak activity for juvenile blacklegged ticks (the ones most likely to transmit Lyme disease) moving three weeks earlier. That moves the risk of exposure earlier as well.

In Texas, hot summers actually reduce the chances of blacklegged ticks spreading Lyme disease to humans, according to Pete Teel, regents professor of entomology at Texas A&M University.

These ticks are most likely to bite humans during the immature stages, which are active in spring and early summer. When temperatures climb, though, they stay down in the leaf-litter to reduce the risk of dying from desiccation or loss of water, Teel says, which reduces the chance of exposure to humans. In northern regions of the U.S., these immature ticks climb vegetation and wait for hosts to pass, increasing the likelihood of biting a human.

Low incidence of Lyme disease in Texas also can be attributed to the fact that blacklegged ticks here feed on a wider array of hosts. Many Texans with Lyme disease may have contracted it elsewhere – something people need to consider when traveling to parts of the country more conducive to exposure.

Avoid tick habitats such as tall grass and shrubs and walk in the center of trails to avoid contact with adjacent vegetation. Wear long-sleeved shirts, long pants, and high boots, and tuck shirts into pants and pants into socks to keep ticks out. Light-colored clothing makes it easier to see ticks. After time outdoors, check your entire body for ticks and promptly remove attached ticks (without squeezing).

Texas residents can submit ticks removed from a human for free testing for pathogens through a partnership between the University of North Texas and the Texas Department of State Health Services. Of the 40 species of ticks known in Texas, only a handful affect humans and outdoor activities. A free online resource from Texas A&M provides information on identification, biology, prevention and more.

Mosquitoes: Mixed news

Increasing temperatures actually could mean some good news for Texas, mosquito-wise. The insects survive best at temperatures between 50 and 95 degrees Fahrenheit and a relative humidity of 42 percent or more. Warmer temperatures mean longer mosquito seasons in some parts of the U.S., but here in the South, more of our summer is becoming simply too hot for mosquitoes (but of course for us as well).

Introduced to North America in 1999, the mosquito-borne West Nile virus (WNV) spread rapidly, and future climate scenarios project expansion of suitable conditions for the disease. The virus spreads most efficiently at temperatures between 75.2 and 77 F, according to another study, so transmission could increase in some areas and decrease in others, with an overall increase.

In the past 10 years, Texas reported more than 3,300 cases of West Nile disease and 172 deaths. So far in 2020, more than a dozen counties have had mosquitoes test positive for the virus. The number of cases here varies from year to year, based on factors such as rainfall.

About 20 percent of people bitten by an infected mosquito develop symptoms, which include headaches, fever, muscle and joint aches, nausea and fatigue. In less than 1 percent of people, WNV affects the nervous system.

Research suggests that disease-carrying mosquito species fare better in human-altered environments than in more natural ones. Most of us, of course, live and spend much of our time in the former.

Mosquitoes breed in standing water and it generally takes eggs at least 7 days to develop into adults. Remove all standing water around your house, even tiny amounts. When outdoors, wear light-colored long-sleeved shirts, long pants and socks. If you use a repellent, choose one registered with the EPA. Remember insecticides kill beneficial insects as well.

No need to stop going outside, but it pays be informed and take proper precautions.


Melissa Gaskill is a contributing editor of Texas Climate News.

Image credit: SWMNPoliSciProject via Wikimedia Commons