Climate change may endanger your dog
Insect pests and diseases they carry spread into new parts of USA
SAN FRANCISCO – As if storms, floods and heat waves weren’t enough, some experts fear climate change is expanding the distribution of diseases that can sicken or even kill dogs, putting more pets at risk for diseases their owners have never had to deal with before.
Though diseases in dogs are not tracked as intensively as those in humans, veterinary epidemiologists and biologists said Rocky Mountain spotted fever, a bacterial disease that can cause fever, joint pain and vomiting, is moving into California and Texas. Heartworm disease, which can damage the cardiovascular system and clog the heart, is spreading beyond its traditional home in the South and Southeast. Lyme disease, which can cause joint swelling and lameness, affects dogs as far north as Canada.
“The veterinarians need to know what’s local. But what’s out there is changing so fast, how are you going to keep up?” said Janet Foley, a professor of epidemiology at the School of Veteri
nary Medicine of the University of California, Davis.
Many of these diseases also affect humans. But dogs are especially at risk because they spend a lot of time outdoors and in vegetation.
Warren Hess, assistant director of the American Veterinary Medical Association, said the spread of heartworm disease is increasing because of the changes in how frequently dogs are moved across the country.
“With the increased social pressure to restrict the sale of dogs in pet stores, this has resulted in a dramatic increase in the movement of dogs from pet shelters to fill the demand,” he said.
Natural disasters also play a part. “The biggest spread in heartworm disease in the United States certainly followed the 2005 national distribution of dogs due to Hurricane Katrina,” said Hess, whose responsibilities include disaster preparedness.
He said that although climate change is happening, and will continue to happen, “it is important that we properly frame the discussions and use all available science as we further the discussion.”
Linking the expansion or shift of ticks that carry diseases, infection rates and dog populations is not an easy task. There are no mandated reporting requirements as there are for some human diseases. Data on tick and mosquito distribution is piecemeal in many areas. Tests for some of the diseases didn’t exist 10 years ago, so it’s difficult to judge their historic range.
Even so, many scientists see patterns and links that point them toward climate change.
More ticks
“There’s no smoking gun, and there will never be a smoking gun. We’re trying to connect two things that operate at very different scales both in time and space,” said Ram Raghavan, a professor of spatial epidemiology at the College of Veterinary Medicine at Kansas State University in Manhattan, Kansas.
He documented significant changes to the tick populations in the Midwest – in infestation intensity, the areas and when ticks are active. His team’s surveillance in western parts of Kansas and Oklahoma found Lone Star ticks that didn’t use to live there. These ticks can carry ehrlichiosis, a disease that in dogs can cause bruising of the gums, bleeding from the nose and lameness.
“There is this belief that these ticks do not exist in these areas, but increasingly over the last five years, we’re constantly finding them. So I’m pretty sure they’ve expanded” their habitat, he said. “Tick-borne diseases have really gone up. We go out into the field, and we see and find ticks more easily than we used to do in the past.”
To get to the bottom of it will require data that doesn’t exist. Raghavan has written several grant proposals to the U.S. National Institutes of Health for funding to do long-term studies, broad testing and analysis.
“Regardless of who caused climate change, climate has changed. Let’s take the emotion out of the debate and get some answers,” he said.
Temperatures in the contiguous USA are on average 1.5 degrees warmer than they were the century before, according to the U.S. National Oceanic and Atmospheric Administration. Rainfall and humidity levels have changed in some areas. All of these factors affect where insects that can carry disease thrive.
Veterinarians and biologists who study diseases spread by insects observed that it’s not just where but when the diseases strike that’s changing.
“Diseases like Lyme disease that used to be transmitted in the peak summer months could now be peaking in the spring and fall because it’s too hot in the summer. So you get a longer transmission window,” said Andrew Dobson, a professor of ecology and evolutionary biology at Princeton University in New Jersey.
Rocky Mountain spotted fever
Rocky Mountain spotted fever, a disease carried by ticks, can sicken and even kill humans and their canine companions. The bacteria initially invades the bloodstream, then settles into the cells that line blood vessels. Blood can seep out of the vessels and pool under the skin or even in the brain. The disease can be treated with antibiotics if caught in time.
At UC Davis, Foley studies its spread. Historically, most cases were spread by the American dog tick and occurred in the southern Atlantic states and the south-central states. North Carolina and Oklahoma accounted for the largest proportion.
Foley has tracked a new tick strain making its way north. This tropical strain of the brown dog tick has been found in many parts of the world and is known in the USA in Florida, Texas, Arizona and Southern California, where it may have been introduced from Brazil and Mexico. It can carry Rocky Mountain spotted fever. Cases are appearing along the U.S.-Mexican border in areas that have never had to deal with the disease before. The new tick has gotten as far north as Los Angeles. Foley expects it to make its way up through California’s Central Valley as far north as Sacramento.
It’s much more aggressive than tick species Americans are used to.
“It bites more, the hotter it gets. So the hotter it is, the more infections there are,” Foley said.
Heartworm cases
The parasitic worms called Dirofilaria immitis are spread through the bite of a mosquito that carries them in a larval state. They cause an especially grisly disease. Once a dog is infected with the heartworm larva, it can grow into a footlong parasitic worm that invades the dog’s cardiovascular system, damages the arteries that carry blood from the heart to the lungs and blocks blood flow to the lungs by their presence and the clots they can cause.
To spread from one dog to another, the larvae have to develop to a specific infective stage inside the mosquito. The hotter it gets, the more quickly the larvae mature into a form that can transfer from the mosquitoes to the dogs. When it’s 71 degrees out, that process can take 16 to 20 days. If it’s 82 degrees, it takes 11 to 12 days, said Bruce Kornreich, a cardiologist and professor of veterinary medicine at the Cornell University College of Veterinary Medicine in New York.
Heartworm disease has historically been a problem in the South and Southeast. Environments farther north are now able to support the mosquitoes that transmit it and the larvae that cause it.
Infections are rising. From 2013 to 2016, there was a 21.7% increase in heartworm infections in the number of dogs per veterinary clinic testing positive for heartworms, said Christopher Rehm, a veterinarian who practices in Mobile, Alabama, and is president of the American Heartworm Society.
There are no solid figures on how many dogs heartworms kill each year, but untreated infections shorten a dog’s lifespan.
Even if heartworm disease is caught and treated in time, it takes its toll on dogs. “Once they’ve ever had a heart infection, they’re never the same,” Rehm said.
Lyme disease
On top of being an enormous health hazard to humans, Lyme disease can harm dogs, causing lameness, fever and lethargy. It’s carried primarily by the blacklegged tick, or deer tick, in the Northeast and the western blacklegged tick in the South.
Both are on the move.
“With Ixodes (blacklegged ticks) moving northward from the United States into Canada, it’s a clear example of how things are changing,” said Michael Yabsley, a professor in the College of Veterinary Medicine at the University of Georgia in Athens.
Even as Lyme disease moves north, it’s not decreasing in its historic area. In fact, infection rates in dogs are getting worse, said Yabsley, who studies wildlife diseases.
In 2018 in Columbia County, New York, 30% of dogs tested were positive for Lyme disease, according to data collected by the Companion Animal Parasite Council.