Alabama’s hot, humid weather is the perfect environment for mosquitoes, and these blood-thirsty parasites can transmit deadly heartworm disease to your pet. Your pet’s health and wellbeing mean everything to our Countryside Veterinary Hospital team, so we are providing information about heartworm disease and explaining how the condition can easily be prevented.

Heartworm transmission to pets

Canine species, including coyotes, foxes, raccoons, and domestic dogs, are the heartworms’ natural hosts, which means that the parasite can complete their life cycle while parasitizing these animals. When a mosquito takes a blood meal from an infected canid, they ingest heartworm larvae. After a few weeks, these microfilariae (i.e., baby heartworms) mature enough to be passed to another host, which occurs when the infected mosquito bites your pet. The microfilariae are deposited on your pet’s skin in the mosquito’s saliva, and they swim through the bite wound to infect your pet.

Heartworm disease in dogs

Heartworms affect dogs and cats differently. Dogs are a natural host for the parasite, and once the microfilariae are inside, they gradually migrate to the arteries that nourish the lungs and grow to about 12 to 14 inches. The dog’s immune system recognizes the parasites as foreign, and creates inflammation in response, damaging the arteries and adjacent lung tissue. As the inflammation progresses, scarring and fibrosis in the lung vasculature impede the heart’s ability to pump blood effectively throughout the body, eventually leading to congestive heart failure. Other important information about heartworm disease in dogs includes:

  • Signs — Most dogs don’t exhibit signs in the initial stages, but as their condition progresses, signs include exercise intolerance, excessive panting, a soft and persistent cough, and weight loss.
  • Kidney inflammation — The dog’s immune system cannot clear the large heartworms, but their body continues to produce antibodies to combat the infection. This results in a large number of antibodies that lodge in the kidneys that the kidneys must clear. The resulting inflammatory cells can damage the sensitive tissue, resulting in a condition called glomerular disease. 
  • Caval syndrome — When numerous heartworms are present, they fill the right heart chamber and can obstruct blood flow through the heart, leading to collapse, shock, and potentially sudden death, although the dog may survive with emergency surgery to remove the worms.

Heartworm disease in cats

Cats are atypical heartworm hosts, and their immune system responds strongly to the microfilariae, which seldom survive to adulthood. However, immature parasites can cause significant problems for an infected cat. Important information about heartworm disease in cats includes:

  • Signs — Many cats never show noticeable signs, and about 10% to 20% experience sudden death. When present, signs include coughing, wheezing, vomiting, and extreme nosebleeds.
  • Heartworm-associated respiratory disease (HARD) — When migrating larvae reach the cat’s lung vasculature, many are killed by the cat’s strong immune response, causing severe inflammation that results in lung disease that closely mimics feline asthma. 
  • Vascular disease — In most cases, heartworm disease in cats is a lung disease caused by the inflammatory reaction to the parasites, but heartworms do grow to adulthood in some cases. Since cats are atypical hosts, the adult worms don’t live as long or grow as large, and usually only one or two worms are present. However, an adult worm can quickly impede blood flow through the cat’s small heart. 

Heartworm diagnosis in pets

Diagnostics used to determine if your pet has heartworms include:

  • Knott’s test — This test looks for circulating microfilariae in the blood, and is often used in conjunction with other tests to determine if your pet has heartworm disease.
  • Antigen test — Antigen tests detect a protein produced by mature female heartworms that typically appears about seven months post infection. The American Heartworm Society currently recommends that dogs be tested annually with antigen and microfilariae tests. 
  • Antibody test — Antibody tests detect the pet’s immune response to the parasite and are especially useful for cats, who often don’t have mature worms. The preferred screening methods for cats include antigen and antibody testing.
  • Imaging — X-rays and ultrasound can be helpful for diagnosing heartworms in pets.

Heartworm treatment in pets

If your pet tests positive for heartworm disease, they must be strictly confined, because physical activity can exacerbate the condition and lead to more heart and lung damage. Treatment varies for dogs and cats.

  • Dogs — Treatment for dogs involves stabilizing their condition and administering an appropriate treatment protocol. The process can take several months, and your dog must be monitored closely for complications. Steps include:
  • Pre-adult heartworm treatment — Medications to decrease inflammation and control bacterial infections, which commonly accompany heartworm disease, are often prescribed before addressing the adult heartworms. Heartworm preventives are also typically used to kill the circulating microfilariae.
  • Treatment for adult worms — After completing the initial treatment, our team will administer a series of drug injections to kill the adult worms. This step takes at least 60 days.
  • Retest — We will retest your dog about six months after treatment to ensure the infection has cleared.
  • Cats — No approved medication is available to treat heartworms in cats, and treatment focuses on supportive care.

Heartworm disease can lead to fatal consequences, but the condition is easily prevented by administering year-round heartworm prevention medications. Contact our Countryside Veterinary Hospital team to discuss what product is best for your pet.