The Earth is getting hotter
Not everyone in the veterinary world is convinced human-caused climate change is one of the main drivers of the movement of animal diseases.
Warren Hess, assistant director in 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 included disaster preparedness.
He said that while 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."
To be sure, making the link between the expansion or shift of ticks that carry diseases, infection rates and local 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 that appear to be on the move didn't exist even 10 years ago, so it's difficult to judge their historic range.
Even so, many scientists are seeing patterns and links that point them toward climate change as a significant part of the changes they're seeing.
"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.
And yet he's documented significant changes to the tick populations in the Midwest, both in tick infestation intensity, area and when ticks are active. For example, 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 yet. 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.
Over the last century, temperatures in the contiguous United States are on average 1.5 degrees warmer than they were the century before, according to the U.S. National Oceanic and Atmospheric Administration. In addition, rainfall and humidity levels have changed in some areas. All of these factors are affecting where insects that can carry disease thrive.
For veterinarians and biologists who study diseases spread by insects, it's not just where the diseases are now striking that's changing, but when. The times of year when dogs are potentially at risk is changing in some areas where summers are simply becoming too hot to support the insects or the diseases they carry. But that doesn't stop the spread.
"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 Princeton, New Jersey.
Rocky Mountain Spotted Fever is spreading
That means more dog owners have to pay attention to illnesses such as Rocky Mountain Spotted Fever, a bacterial disease carried by ticks that can sicken and even kill both humans and their canine companions.
The bacteria initially invades the bloodstream but then settles into the cells that line blood vessels. Blood can then seep out of the blood vessels and pool under the skin or even in the brain. It can be treated with antibiotics if caught in time.
At UC Davis, Foley is studying its spread. Historically, most cases were spread by the American Dog Tick and occurred in the southern Atlantic states and the southcentral states, with North Carolina and Oklahoma accounting for the largest proportion.
However, she has been tracking a new tick strain that is 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 United States in Florida, Texas, Arizona and Southern California, where it may have been introduced from Brazil and Mexico.
It also can carry Rocky Mountain Spotted Fever. Cases are now appearing along the U.S.-Mexico border in areas that have never had to deal with the disease before. The new tick has now 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 we are used to in the United States.
"It bites more the hotter it gets. So the hotter it is, the more infections there are," Foley said.
Heartworm cases on the rise
Dog owners are also having to do more to keep their dogs protected against heartworm. The parasitic worms called Dirofilaria immitis are spread through the bite of a mosquito that carries them in a larval state.
It is an especially grisly disease. Once a dog is infected with the larva, it can grow into a foot-long 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 between 16 and 20 days. If it's 82 degrees, it takes just 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 has historically been a problem in the south and southeast. Likely due to climate change and other factors, environments farther north are now able to support the mosquitoes that transmit it and the larvae that cause it.
Infections are rising. Between 2013 and 2016 there was a 21.7% increase in heartworm infections in the number of dogs per veterinary clinic testing positive for heartworm, 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 heartworm kills each year, but it's well known that untreated infections shorten a dog's lifespan.
"Based on my own anecdotal experience, I would conservatively estimate that heartworm-infected dogs lose one-third of their life span if not treated properly and in a timely manner," said Rehm.
As the parasite moves into new areas, owners may not always be aware they need to be on the lookout for it. It's also a problem for more months of the year, said Foley.
"A hot winter means the mosquitos don't die back, so they're raring to go as early as January and start spreading heartworm," she said.
This means that pet owners across a wider swath of the United States need to give their dogs preventative medicine to keep them from getting heartworm. In addition, people in areas where heartworm infections were only a problem in the summer now must treat their dogs for more months out of the year.
Since 2010, the American Heartworm Disease Association and the federal Food and Drug Administration have both recommended year-round preventative treatment, because the disease is more prevalent and because it's so devastating to dogs who get it, said Kornreich.
Even if heartworm 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," said Rehm.
Lyme disease moving north
On top of being an enormous health hazard to humans, Lyme disease can also 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 southern United States.
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, Georgia.
Unfortunately, even as Lyme disease moves northward, 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 found to be positive for Lyme disease. In Worcester County, Massachusetts, it was 21% and in Ulster County, New York, it was 20%, according to data collected by the Companion Animal Parasite Council.
'Fast and ugly' changes
Some even fear that the changing climate might bring new diseases never before seen in canine companions. When ticks expand into new areas, they come into contact with new hosts, and those hosts may carry new diseases — which they could spread to the animals they bite.
This already may have happened with two human diseases. The Heartland virus was discovered in 2009 and has so far infected about 20 people in the Midwest. It can cause fever, fatigue, nausea and diarrhea. Almost all patients have been hospitalized and some have died, according to the U.S. Centers for Disease Control and Prevention.
The Bourbon virus was first identified in 2014 and has infected a limited number of people in the Midwest and the South, some of whom have died, according to the CDC. It can cause fever, rash, tiredness, body aches and vomiting.
It's a reminder that the shifting climate is going to affect people and their pets in ways they may not be prepared for, Dobson said.
"There's no debate about whether it's happening or not," he said. "It's happening fast and it's ugly."