Twenty years ago, when veterinarian Colleen Duncan arrived in Fort Collins, Colorado, there were no signs of heartworm in the region’s dogs and cats.

“We didn’t test for it,” said Duncan, a professor of veterinary medicine at Colorado State University. “We didn’t put our animals on prevention. We now will do it.”

Colorado’s not alone in this shift. Across the West, veterinarians are seeing an increase in the mosquito-borne disease that can cause symptoms like coughing and decreased appetite in dogs and cats, and even lead to death in advanced cases. 

Infected pets can bring the disease with them when they move to new regions, where a lack of preventative treatment can contribute to an uptick in heartworm infections. But many experts like Duncan also point to warmer weather and shifting precipitation patterns that are helping the mosquitoes that spread the disease to expand their ranges and populations.

“Vector-borne disease is the easiest, the least-disputed discussion of climate-associated diseases,” said Duncan, who studies the connection between climate change and animal health. 

“Climate change moves vectors into areas, geographic locations that they weren’t before,” she explained. “It allows them to survive longer periods of time. Typically, they show up earlier in the spring and they last later into the fall. Or even really winter.”

“We keep dogs on heartworm preventative year-round now,” she said.

“The Mosquito Season Seems to Start Earlier”

Mosquitoes thrive in warm, wet conditions. Historically, the Southeastern United States—places like the Gulf Coast, Arkansas, Tennessee and the Carolinas—has been a hotbed for heartworm, which starts when a mosquito bites a pet and passes on the larvae that grow into the worm. Though the Southeast still leads the nation in heartworm prevalence, the disease can now be found in all 50 states.

In recent years, Western cities like Salt Lake City, Vancouver, Wash. and Albuquerque, N.M., have appeared on the Companion Animal Parasite Council’s monthly report of the top 10 cities for the disease. And in 2022, the American Heartworm Society’s latest assessment found that Western states with historically low heartworm rates such as Washington and Oregon, along with cities like Seattle, Tucson and Boise, all experienced unexpected increases in heartworm during the previous three years.

Shari DePauw, shelter veterinarian at Longmont Humane Society in Colorado, about an hour south of Fort Collins, has seen an upward trend in heartworm cases since 2011, though she’s not sure if this is related to more infected animals moving into the state or a shifting climate. That said, she’s witnessed changes in mosquito populations in the area.

“As years go by, the mosquito season seems to start earlier,” she said. “Every once in a while we’ll see a mosquito in March and still see one in November.”

Down in Albuquerque, experts are seeing similar trends with mosquito populations as the city experiences more mild winter days.

“The seasons are getting longer, especially in the fall,” says Nick Pederson, Urban Biology Division manager with Albuquerque’s Environmental Health Department. “We aren’t seeing some of those species go down quite as fast, so we can have some lingering numbers, especially with species that we suspect are involved in transmission with heartworm.”

The city has seen an increase in Culex Quinquefasciatus, also known as the southern house mosquito, one species suspected to transmit heartworm. Meanwhile, Aedes aegypti, another heartworm-associated mosquito species, first emerged in Albuquerque about seven years ago and has since expanded its range across the entire city.

Albuquerque’s warmer winters are part of a trend across the entirety of the Mountain West. One report from Climate Central found that climate change slashed the number of days that dipped below freezing in states across the region from 2014 to 2023, with Arizona losing 11 days annually, New Mexico nine, Nevada eight, Utah six and other states losing between two and four.

But it’s not just the heat. Mosquitoes also need standing water to reproduce. And though much of the West has been suffering from ongoing drought, Pederson said this hasn’t stopped mosquitoes from thriving. In fact, Aedes aegyptiare increasing their numbers in Las Vegas “pretty significantly,” he said. Colorado recorded its first incidence of the species last summer, he added.

One reason is that property owners often create perfect mosquito habitats in their backyards, Pederson says. For instance, discarded tires can provide ideal breeding grounds, according to several studies.

“That birdbath dish or a child’s toy might be collecting sprinkler water, and it might not rain for a couple weeks, but that has water, and that is suitable for those mosquitoes to use,” he explained. “And so even in these drought years, we’ve seen huge numbers of these mosquitoes.”

More than Mosquitoes

Duncan, at Colorado State University, warned that an increase in heartworm is just one of several health threats pets are facing in a warming world. 

For instance, “we also really want to think about tick-borne diseases and what the opportunities are for our pets to be exposed to ticks,” she said, adding that Lyme disease is probably the biggest threat within this vector. 

Back in Longmont, DePauw has noticed an increase in ticks and fleas, but said the particular deer tick species that carries Lyme disease hasn’t been recorded in the area yet.

“We are basically just waiting for when it is going to migrate here,” she said. “But it has not, knock on wood, happened yet.”

Scientists also expect Valley fever, a fungal disease endemic to the Southwest that affects humans and animals, to expand its reach due to climate change. The number of infections had already skyrocketed in recent years in areas where it’s common—cases tripled from 2014-2018 in California, according to the California Department of Public Health—and now the disease has spread as far north as Washington, where it was once rare. 

Experts report climate change is supercharging the spread of the lung disease, as weather events like drought-fueled dust storms and wildfires spread the spores in the air, while warmer temperatures create more favorable conditions for the fungus to grow. In dogs, Valley fever causes symptoms like dry cough, fever, lethargy and loss of appetite while cats most often experience swelling or a skin wound along with fever, decreased appetite and weight loss.

Pets are also vulnerable to other climate-related risks such as air pollution from wildfire smoke, which Duncan called “one of the most pressing climate associated illnesses” in the Rocky Mountain West for pets. In 2024, wildfires burned over a million acres in California alone, according to Cal Fire. Canadian wildfires burned over 13 million acres that year, according to the Canadian Interagency Forest Fire Centre. And that smoke travels.

“So even here in Colorado, we get the smoke from California, we get the smoke out of Canada,” Duncan said. And, “it affects everyone that breathes.”

Plus, she said warmer temperatures contribute to the proliferation of blue-green algae in water bodies, which can be deadly to dogs and cats. Hotter weather can also spur heat-related illnesses like heat stroke, which can be life-threatening to pets, particularly when they’re active outside.

Keeping Pets Safe in a Changing Climate

One of the easiest ways to help decrease heartworm risk is to put pets on preventative treatments like Heartgard Plus or Iverhart Plus, chewable tablets that are commonly given monthly. However, Duncan warns that it’s important for pet owners to talk with their veterinarians about which medication is most effective in their region. 

Many cities are also trying to decrease mosquito populations, which will help combat heartworm along with other vector-borne diseases that impact humans such as West Nile virus. Albuquerque’s integrated mosquito management plan, for instance, includes trapping mosquitoes on a weekly basis during mosquito season and bringing them back to a lab to identify them and sometimes test for diseases like West Nile virus. The plan also involves reducing or removing habitats where mosquitoes breed, larval control—primarily through the use of bacterial-based pesticides—and spraying other pesticides to kill adult mosquitoes.

But the public is also critical to slowing the proliferation of mosquito-borne diseases. 

“It’s going to take a community-wide effort and people really kind of looking at what’s in their yard and where these mosquitoes might be coming from,” Pederson said. “Because it only takes one five-gallon bucket to really impact things around your neighborhood.”

As for other other climate-related threats to our pets, Duncan said awareness and preparation are key. For instance, she encourages people to learn how to check the air quality index on their phone so they can adjust their dog’s outdoor exercise schedule accordingly. And people living in disaster-prone areas should include their pets in their advanced evacuation plans, she said.

“Your veterinary community wants to have those conversations with you, too,” she added, encouraging people to ask questions. “They want your pets to be healthy, they want your pets to be prepared.”