Pet Connection: How deadly heartworm disease spreads

You’ve probably seen a dusty jar of long, spaghettilike worms in your veterinarian’s office. They’re heartworms (Dirofilaria immitis), and they are deadly to dogs and cats. The internal parasites make themselves at home in the heart and lungs, causing heart failure and lung disease and potentially migrating to the brain, eye and spinal cord.

Here are seven things you might not know about heartworm disease:

▪  Heartworms are transmitted by more than 70 species of mosquitoes. Some of these mosquitoes don’t need standing bodies of water to reproduce. They thrive in small areas, such as downspouts, gutters and flowerpots, and adapt well to cold weather.

▪  The incidence of heartworm disease is rising. Between 2013 and 2015, the Companion Animal Parasite Council saw a 166 percent increase in reported positive heartworm cases. That’s because nationwide only about 35 percent of dogs are on preventive medication, says C. Thomas Nelson, DVM, who practices in Anniston, Alabama, and is a spokesman for the American Heartworm Society. On the West Coast, it’s only 16 to 18 percent. In the Southeast, where heartworms are especially prevalent, it’s about 26 percent.

▪  Heartworm disease has been found in pets in all 50 states. “Owners carry their dogs with them a lot,” says parasitologist John W. McCall, Ph.D., professor emeritus in the department of infectious diseases at the University of Georgia College of Veterinary Medicine. “They go from the North to the South, and they just don’t really think that they’re going into an area where there’s mosquitoes. Many owners don’t even know heartworm is transmitted by mosquitoes.”

▪  Cats can get heartworm disease. They are not as susceptible as dogs, but the worms can cause more serious problems in cats. Larvae in the lungs lead to what’s called heartworm-associated respiratory disease (HARD), which has signs similar to feline asthma. It’s worse if worms manage to develop to the adult stage. “The clinical signs associated with the presence of a couple of adult worms in the cat’s pulmonary artery are usually very severe, ranging from acute respiratory distress to sudden death as a result of severe inflammation and pulmonary embolism,” says Romain Pariaut, DVM, an internal medicine specialist and associate professor of cardiology at Cornell University College of Veterinary Medicine in Ithaca, N.Y.

▪  Preventive medication is recommended year-round for dogs and cats in all areas. One reason is because mosquitoes are more widespread. Another is that longer bouts of warm weather and shorter bouts of cold weather mean mosquitoes are seen year-round in most areas.

▪  Heartworms are becoming resistant to preventive products. Bacteria, viruses and parasites such as heartworms eventually become resistant to drugs used against them, McCall says. The current drugs have been used for almost 30 years. “We don’t really know how much of a problem it is,” he says, “but the longer we use the products, the more likely it is to occur.”

▪  Experts recommend combining preventive with a dog-safe mosquito repellent. Even though preventive medication is highly effective, it’s not failproof, especially in areas where heartworms have become resistant. Based on a study McCall did using Vectra 3D, which repels and kills mosquitoes, combining heartworm preventive with the topical parasiticide was 100 percent effective in blocking transmission of microfilariae (immature heartworms) from dogs to mosquitoes – a necessary part of the heartworm lifecycle – and more than 95 percent effective in repelling and killing mosquitoes for 28 days after treatment. “If the dog is treated, the mosquito can’t bite the dog and it can’t transmit the infective larvae to the dog,” McCall says. “It will pretty much keep the dog protected, even when there’s a high degree of resistance.”

The product is not safe for use on cats, but cats who live with dogs who are protected share the benefit.

Pet Connection is produced by a team of pet-care experts headed by veterinarian Dr. Marty Becker and Kim Campbell Thornton, author of many pet-care books. The two are affiliated with