Your dog’s well-being is paramount, and understanding health threats is part of responsible pet ownership. Heartworm disease is a serious concern for dog owners. Knowing how dogs contract heartworms is the first step in protecting your furry friend. This article provides a comprehensive guide on heartworm disease in dogs, focusing on transmission, prevention, and keeping your canine companion healthy.
Heartworm Disease: The Basics
Heartworm disease is a severe and potentially fatal condition affecting dogs, cats, and ferrets. It leads to significant lung disease, heart failure, organ damage, and even death. The culprit behind this disease is a parasitic worm called Dirofilaria immitis. These worms are transmitted through the bite of an infected mosquito. Dogs are considered the definitive host for heartworms. This means that inside a dog, heartworms mature into adults, reproduce, and release offspring. Mosquitoes act as intermediate hosts, playing a crucial role in the heartworm lifecycle by allowing the worms to develop into an infective stage. The term “heartworms” comes from the fact that adult worms reside in the heart, lungs, and associated blood vessels of infected animals.
Heartworm disease is prevalent in the United States, particularly along the Atlantic and Gulf Coasts, extending from the Gulf of Mexico to New Jersey, and along the Mississippi River and its tributaries. However, it’s important to note that heartworm has been reported in dogs across all 50 states, highlighting the widespread risk.
The Heartworm Lifecycle and How Dogs Get Infected
To understand how dogs get heartworm, it’s crucial to understand the heartworm lifecycle.
- Microfilariae Stage: In a dog already infected with heartworms, adult female heartworms release microscopic offspring called microfilariae into the dog’s bloodstream.
- Mosquito Ingestion: When a mosquito bites an infected dog, it ingests these microfilariae along with the dog’s blood.
- Development in Mosquito: Inside the mosquito, over a period of 10 to 14 days, the microfilariae develop into infective larvae. This development is temperature-dependent, requiring warm conditions.
- Transmission via Mosquito Bite: When the infected mosquito bites another dog, it transmits these infective larvae through its saliva into the new dog’s bloodstream. This is how dogs get heartworm disease – through the bite of an infected mosquito.
- Maturation in Dog: Once inside the new dog, the infective larvae migrate through the tissues and eventually reach the heart and lungs. It takes approximately 6 to 7 months for these larvae to mature into adult heartworms within the dog.
- Adult Heartworm Stage: Adult heartworms can live for 5 to 7 years in a dog. They grow to resemble strands of spaghetti, with males reaching 4 to 6 inches and females 10 to 12 inches in length. Adult heartworms reproduce, and the females release new microfilariae, restarting the cycle if another mosquito bites the infected dog.
It’s vital to remember that heartworm disease is not contagious directly from dog to dog. Dogs cannot contract heartworms by simply being near an infected dog. The disease is exclusively spread through the mosquito vector.
The number of heartworms in an infected dog is called the worm burden. While the average worm burden is around 15 worms, it can range dramatically from just one to as many as 250 worms in severe cases.
Testing for Heartworm in Dogs
Veterinarians use blood tests to diagnose heartworm infection in dogs. There are two main types of tests:
- Antigen Test: This is the most common test. It detects heartworm antigens, specific proteins released by adult female heartworms, in the dog’s blood. Antigen tests are highly accurate in detecting infections with one or more adult female heartworms, typically around 5 months post-infection when these proteins become detectable.
- Microfilariae Test: This test identifies microfilariae directly in a dog’s blood sample. A positive microfilariae test confirms adult heartworm infection, as only adult worms can produce offspring. Microfilariae are usually detectable around 6 months after infection, after the worms have matured and started reproducing.
Heartworm Testing Schedule for Dogs
Regular heartworm testing is crucial for all dogs. The timing and frequency depend on several factors:
- Age at Prevention Start: Dogs 7 months and older should be tested before starting heartworm prevention.
- Missed Prevention Doses: If doses are missed or delayed, testing might be needed.
- Changes in Prevention: Switching between different types of preventatives may warrant testing.
- Travel History: Travel to heartworm-prevalent areas increases risk and the need for testing.
- Heartworm Season Length: Regions with longer mosquito seasons require more vigilant testing.
Annual heartworm testing is recommended for all dogs, even those on year-round prevention. It’s essential to test before starting prevention because heartworm preventatives are designed to kill larvae, not adult worms. Administering preventatives to a dog with an existing adult heartworm infection can be harmful, potentially causing a severe reaction as the microfilariae die off rapidly. Consult with your veterinarian to determine the best testing schedule for your dog.
Recognizing Heartworm Symptoms in Dogs
The severity of heartworm disease in dogs varies based on worm burden, infection duration, and the dog’s activity level and individual response. Early stages or low worm burdens may show subtle or no symptoms. However, as the disease progresses, symptoms become more apparent.
Heartworm disease is classified into four classes:
- Class 1: Mild or no symptoms, possibly an occasional cough.
- Class 2: Mild to moderate symptoms, including occasional cough and fatigue after moderate exercise.
- Class 3: More pronounced symptoms like general sickness, persistent cough, tiredness after minimal activity, breathing difficulties, and signs of heart failure. Chest X-rays often reveal heart and lung changes in Class 2 and 3.
- Class 4 (Caval Syndrome): A life-threatening stage where a massive worm burden blocks blood flow to the heart. Caval syndrome requires emergency surgery to remove the worms, but even with intervention, survival rates are low.
Untreated heartworm disease progresses, causing damage to the heart, lungs, liver, and kidneys, ultimately leading to death.
Treating Heartworm Disease in Dogs
Treatment for heartworm disease in dogs is available but complex, costly, and carries risks.
- Melarsomine Dihydrochloride: This FDA-approved arsenic-based drug (Immiticide or Diroban) is used to kill adult heartworms in dogs with stabilized Class 1, 2, and 3 disease. It’s administered via deep muscle injections in the back.
- Advantage Multi for Dogs: This topical medication (imidacloprid and moxidectin) is FDA-approved to eliminate microfilariae from the bloodstream after adult heartworm treatment.
Heartworm treatment can cause significant side effects, including lung blood clots and potential toxicity. It requires multiple veterinary visits, blood work, X-rays, hospitalization, and injections, making it an expensive undertaking.
Prevention: The Optimal Approach to Heartworm
Prevention is unequivocally the best strategy to protect your dog from heartworm disease. Numerous FDA-approved heartworm preventatives are available by prescription from your veterinarian. These come in various forms:
- Monthly Topical Liquids: Applied to the skin.
- Monthly Oral Tablets: Chewable and non-chewable options.
- Injectable Preventative: Administered by a veterinarian every 6 or 12 months.
Many preventatives also offer protection against intestinal worms (roundworms, hookworms) and other parasites (fleas, ticks, ear mites).
Year-round heartworm prevention is highly recommended. Mosquito activity can be unpredictable, and consistent prevention ensures your dog remains protected at all times. Consult your veterinarian to determine the most suitable preventative for your dog’s specific needs and lifestyle.
The American Heartworm Society advocates “Think 12”: Give heartworm prevention for 12 months a year and test dogs annually for heartworms every 12 months.
Heartworm in Other Pets and Humans
While this article focuses on dogs, it’s important to know:
- Cats and Heartworm: Cats can get heartworm, though they are less susceptible than dogs. The disease presents differently in cats and is harder to diagnose. Prevention is crucial for cats as well.
- Ferrets and Heartworm: Ferrets are also susceptible to heartworm, with symptoms resembling those in cats. Prevention is vital for ferrets.
- Humans and Heartworm: Humans are not natural hosts for heartworms. While rare human infections can occur via mosquito bites, the worms typically don’t mature into adults in humans and pose a minimal threat.
Seeking Further Heartworm Information
Protecting your dog from heartworm disease requires understanding how transmission occurs and committing to consistent prevention. Regular veterinary check-ups, heartworm testing, and year-round preventative medication are essential for maintaining your dog’s health and well-being. Consult with your veterinarian for personalized advice on heartworm prevention and care for your canine companion.
*Special thanks to David A. Crum, DVM, MPH for sharing his ferret expertise.
