Mosquitoes at an All-Time High: Western States Harbor Heartworm Disease

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American Heartworm Society recommends yearly protection and annual testing to ward off heartworm disease, even in temperate climates.

According to the American Heartworm Society (AHS), heartworm disease is one of the leading pet health problems in the United States, and has been diagnosed in all 50 states. It is estimated that approximately one million U.S. pets are heartworm-positive.

California and Arizona may not be known as hotbeds of heartworm transmission, but the American Heartworm Society (AHS) is reminding pet owners that dogs and cats here remain at risk – even in the summer months. Environmental conditions that foster proliferation of mosquitoes carrying the heartworm parasite, such as monsoon season in the southwest, combined with the mobility of infected pets, has increased the threat of heartworm infection, as well as the need for protection.

According to the AHS, heartworm disease is one of the leading pet health problems in the United States and has been diagnosed in all 50 states. It is estimated that approximately 1 million U.S. pets are heartworm-positive. An incidence survey conducted by AHS revealed that both California and Arizona have a number of counties where heartworm disease has been diagnosed.
Both dogs and cats can develop heartworm disease. Left untreated, heartworm infection can lead to severe lung disease, heart failure, organ damage and death.

Heartworm has no address
Heartworm disease has long been endemic in the southern United States and along the Eastern Seaboard, where both mosquitoes and infected pets are prevalent. With its considerably more arid climate, many veterinarians and pet owners in states such as California and Arizona are unaware that heartworm could be a problem in their area.

“Heartworm disease is under the radar here. I call it ‘heartworm denial,’” said Robert Stannard, DVM, Adobe Pet Hospital, Livermore, Calif., and AHS executive board member. “The problem is, many veterinarians don’t see heartworm because they don’t test for it. And if you don’t look, you’ll never find it.”
Since he began testing patients for heartworm in 2004, Stannard, whose practice is located in the Bay Area, has had only three dogs test positive for heartworm, but had 40 cats test positive for heartworm — 10 of which had never lived outdoors.
Mosquitos get inside homes and live there a long time, the veterinarian warned. “A sedentary cat makes an easy meal for a mosquito. It’s not even a moving target. And this doesn’t just happen here in California, it can happen anywhere.”

A growing problem

Kellie Barrett, DVM, staff veterinarian at the Humane Society of Southern Arizona in Tucson, says that her shelter treated 11 heartworm-positive dogs last year and has treated three heartworm-positive dogs so far in 2013.

“Pet owners think that heartworm is not an issue here in the desert,” said Barrett. She cites the state’s monsoon season, which runs from June 15 to September 15, as one reason heartworm disease is found in her usually water-starved state. These months can bring heavy rain and flash flooding, followed by lots of standing water, which is a magnet for mosquitoes.

“We also have a high population of snowbirds who travel here for the winter. If their dogs happen to be heartworm-positive, there is greater potential of the disease spreading,” said Barrett. As evidence of these phenomena, Stannard notes that an increase in the incidence of heartworm was noted in southern Canada, when dogs were adopted in the wake of Hurricane Katrina. Many were heartworm-positive and were sent to homes in southern Canada, causing an increase in the disease there.

“Climate patterns shift and change. Now there is water where there wasn’t before in the West — on golf courses, for example,” said Stannard. “For many species of mosquitoes, all they need to reproduce is the amount of water you’d find in the bottom of a flowerpot. That makes the threat of heartworm disease real.”

Testing and prevention are vital

Because of the unpredictability of heartworm disease across the country, the American Heartworm Society recommends annual testing and year-round prevention. Stannard noted that many heartworm preventives also protect against parasites, such as hookworm and roundworm, which can be harmful to humans as well as pets.

“With prevention so readily available, there’s no reason we can’t significantly increase the percentage of protected pets,” he stated. “Pet owners in all areas of the country should strongly consider asking their veterinarians to recommend a heartworm preventive for their dogs and cats.”

For more information on heartworm disease, visit

About the American Heartworm Society

The mission of the American Heartworm Society is to lead the veterinary profession and the public in the understanding of heartworm disease. Founded during the Heartworm Symposium of 1974, the American Heartworm Society aims to further scientific progress in the study of heartworm disease, inform the membership of new developments, and encourage and help promote effective procedures for the diagnosis, treatment and prevention of heartworm disease.

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Kaylee Arostegui