Q & A: Heartworm “Season”
By Dr. Kim Everson, DVM
Question: Why does my vet recommend using a heartworm preventative for my dog during the winter. It is so cold now that there can’t possibly be any mosquitoes!
Answer: Heartworm disease is an infection in dogs and cats caused by a worm parasite transmitted by mosquitoes. A heartworm-carrying mosquito bites a pet, then the worm larvae enter into the pet’s tissues and migrate to the lungs. There they develop over the course of 6-9 months into adult heartworms and begin reproducing inside the pet’s lungs and heart. Over time the heartworms clog up the vessels and chambers in the heart and occupy vessels in the lungs. Symptoms of infection may take years to show up. In dogs symptoms include coughing, lack of stamina, unexplained weight loss, raspy breathing, abdominal bloating and sudden death. Feline heartworm disease is slightly different with vomiting, coughing, seizure or collapse, breathing difficulty and sudden death being seen.
Treatment for heartworm disease in dogs is costly and the side effects are risky. There is no approved treatment to eliminate heartworms in cats! Depending on the severity of infection in dogs, different protocols are used to kill off the heartworms. Chest x-rays, bloodwork and urine tests may all be needed to determine which protocol is appropriate for an infected dog. The medication used to kill heartworms is costly and often requires hospitalization during administration. Probably the hardest part of treatment is strict cage rest for weeks following treatment to reduce the risk of decaying worm fragments lodging in blood vessels leading to potentially fatal embolism.
Fortunately, heartworm is one of the most preventable deadly diseases affecting pets. The most typical preventative is a pill given monthly by mouth, although topical and long-acting injectable medicines are also available. Before starting a pet on heartworm disease preventative, a heartworm test must be done to ensure there is not already infection present. This is because the “preventatives” actually kill off immature heartworm larvae that entered the pet’s body during the previous month stopping them from maturing into reproducing adult worms. Not knowing a dog’s internal systems might be showered with dead worm bits is very risky!
Heartworm disease has been diagnosed in all 50 states, with the highest number of cases developing in tropical and subtropical climates where “mosquito season” lasts all year. In order for heartworms to be infective, temperatures must be in the high-50°Fs for a period of time. Therefore, during the dead of Wisconsin winter, a heartworm preventative is technically not necessary to prevent acquisition of heartworm disease. However, most heartworm medicines also include dewormers for intestinal parasites which should continue to be given every month to purge intestinal worms that dogs are exposed to even during frigid months.
Other reasons to give heartworm disease preventatives all year include:
- ensuring continued protection for your pets if you travel with them to warmer, higher-risk climates during the winter
- avoiding getting a late start or stopping prematurely because heartworm “season” in northern states varies dramatically year by year; sometimes it begins as early as March or as late as May and can last until December (preventatives should be given for 30 days after the first killing frost)
- improving owner compliance by developing a habit of giving the preventative at the same time of the month every month; the most common reason pets who are “on a preventative” develop heartworm disease is because owners forget to give it consistently
Heartworm disease preventatives are generally well-tolerated, safe and effective. Your veterinarian should welcome a frank, educated conversation about starting and keeping your pet on a heartworm disease preventative. For more information, please check out American Heartworm Society.
Originally printed in Pet Journal, January 2014