Heartworm disease is a significant illness in pets that if left untreated, can eventually lead to death. The
disease is caused by worms settling in the heart, lungs and related blood vessels of affected pets. Over
time, heartworms, which can grow up to a foot long, cause heart failure and lung disease. Even after
treatment, damage to the heart and other affected organs can be permanent causing lifelong
complications. Dogs, cats and ferrets can contract heartworm disease. Other mammals, including foxes
and coyotes, can be affected by heartworms and are considered carriers of the disease.
Transmission of Heartworms
Mosquitos serve as the vector for heartworm disease in pets. Adult female heartworms live in the
infected animal and produce baby worms called microfilaria. These baby worms circulate in an infected
animal’s bloodstream. When a mosquito bites an infected animal, the baby worms are transferred to it
and mature into infective stage larvae. When the infected mosquito then bites a dog, the infective
larvae enter the dog’s bloodstream. Approximately six months after infection, the larvae have matured
into adult heartworms and settle in the vessels around the dog’s heart and lungs. Mature heartworms
can live up to 7 years in dogs and with each mosquito season, the risk of additional adult heartworms
Preventing Heartworm Disease in Dogs
Since dogs are a natural host for heartworms and considering the high incidence of disease in the
southeast, heartworm prevention should be given to dogs every 30 days year-round. Puppies have the
same risk of heartworm infection as adult dogs and should be started on prevention as soon as possible.
Depending on the product, this can be as early as 4-6 weeks of age. When used properly, heartworm
prevention is safe and extremely effective.
Heartworm prevention kills the infective larvae that entered the dog’s bloodstream in the previous 30-
45 days. Any infective larvae present in the dog longer are likely to survive the treatment and will
remain in the bloodstream to mature into adult heartworms. Therefore, it is extremely important to
administer prevention to your dog every 30 days.
Types of Monthly Heartworm Prevention
There are many products available to pet owners that prevent heartworm disease. Yearly heartworm
tests and a prescription from your vet is required to purchase any type of heartworm prevention. Some
dog breeds should not be administered certain products. Consult your veterinarian to determine the
best product for your dog. Popular oral preventions include Heartgard Plus, Interceptor Plus and
Sentinel. Revolution and Advantage Multi are common topical preventions. Depending on the weight of
the dog, Heartgard Plus can be purchased with a prescription from online retailers for a little as $7/month.
A heartworm antigen test typically cost anywhere from $20-$35 and detects the presence of adult
heartworms. A small drop of blood is taken from the dog’s forearm and results are available in as little
as 10 minutes. As mentioned earlier, puppies should be started on prevention no later than 6 weeks of
age and then tested for heartworms at one year of age and then yearly. Adult dogs not on heartworm
prevention should be tested prior to starting a monthly medication to prevent potential complications if
heartworms are present. All dogs should be tested yearly even if preventions are given on time every 30
days. Spay/Neuter clinics. Wellness Clinics, and Animal Hospitals all offer heartworm screenings and
Heartworm Positive Dogs
Dogs with early stages of infection may not show any signs or symptoms of heartworm disease for
months making annual testing extremely important. Initial symptoms of heartworm disease may include
a mild cough that does not go away, fatigue, reluctance to go for a run or walk and loss of appetite. As
the disease progresses, heart failure develops leading to extreme fatigue and worsening cough. A
swollen belly is often seen with advanced disease. Dogs with a large presence of heart worms may
experience cardiovascular collapse. Cardiovascular collapse results from a sudden blockage of blood
flow within the arteries of the heart and is a life-threatening emergency.
For information on treating heartworm positive dogs, please visit American Heartworm Society online
and contact a local veterinarian about treatment options, protocols and pricing. National Spay Alliance
in Calhoun is recommended for low cost, high quality heartworm treatment.
American Heartworm Society, 2020, May 23 rd , 2020, Heartworm Basics,
American Heartworm Society, 2020, May 20 th , 2020, Heartworm Incidence,
Cummings Veterinary Medical Center, Tufts University, December 31 st , 2014, Heartworm Basics and
Prevention: Prevention is the Best Medicine, https://news.vet.tufts.edu/2014/12/heartworm-basics-and-