How do dogs get Heartworm? 4 Factors that Increases the Risks

Heartworm disease represents one of the most serious threats to canine health across America, causing potentially fatal damage to a dog’s heart, lungs, and blood vessels. This parasitic infection often progresses silently, with symptoms appearing only after significant damage has occurred.

Understanding how dogs contract the disease, recognizing early warning signs, and implementing effective prevention strategies are essential responsibilities for every dog owner. With proper knowledge and preventative care, this devastating disease can be avoided entirely.

What is Heartworm?

Heartworm disease is a serious and potentially fatal condition caused by parasitic worms (Dirofilaria immitis) that primarily infect dogs and other canines. These long, spaghetti-like worms live in the heart, lungs, and associated blood vessels, causing severe lung disease, heart failure, and damage to other organs. Adult worms can grow up to 12 inches in length, and in severe cases, a dog can harbor hundreds of these parasites, leading to complete blockage of blood flow and organ failure.

7 Clinical Signs of Heartworm in Dogs

• Persistent Cough – Dogs with heartworm often develop a mild, consistent cough as the parasites affect the lungs and surrounding blood vessels.
• Fatigue and Exercise Intolerance – Infected dogs typically show a reluctance to exercise and become tired easily, even after mild physical activity.
• Weight Loss and Decreased Appetite – As the disease progresses, dogs may experience unexplained weight loss despite normal food intake or show reduced interest in eating.
• Swollen Belly- In advanced cases, fluid accumulation in the abdomen creates a swollen, pot-bellied appearance due to right-sided heart failure.
• Labored Breathing – Dogs may exhibit difficulty breathing or rapid, shallow breathing patterns, especially after minimal exertion.
• Pale Gums – Reduced blood oxygen levels can cause pale or bluish gums, indicating poor circulation.
• Collapse – In severe cases, dogs may collapse suddenly due to blockage of blood flow by adult worms.

4 Factors That Increase the Risk of Heartworm

Understanding the Heartworm Cycle

Mosquito Bites Infected Animal

– The cycle begins when a mosquito takes a blood meal from a dog (or other host) already infected with heartworm. During this feeding, the mosquito ingests microscopic baby worms (microfilariae) that circulate in the infected animal’s bloodstream.

Development Inside Mosquito

– Over the next 10-14 days, these microfilariae develop inside the mosquito, passing through several larval stages (L1, L2, and finally to L3). This development can only occur within the mosquito, making these insects essential to the parasite’s life cycle.

Transmission to New Host

– When the infected mosquito bites another dog, it deposits the now-infective L3 larvae onto the animal’s skin. These larvae then enter the new host through the mosquito’s bite wound, beginning the infection process.

Migration Through Host Tissues

– After entering the dog’s body, the larvae spend the next 3-4 months migrating through subcutaneous tissues, muscle layers, and eventually into the bloodstream. During this time, they molt twice more, developing into L4 and finally L5 stage immature adult worms.

Arrival at Final Destination

– Approximately 6 months after infection, the immature worms reach the heart and pulmonary arteries. Here, they continue growing to their full adult length of 6-12 inches, settling primarily in the right ventricle of the heart and adjacent blood vessels.

Reproduction and Completion of Cycle

– Adult female worms begin producing microfilariae about 6-7 months after infection. These microfilariae enter the dog’s bloodstream, where they can survive for up to two years, waiting to be ingested by another mosquito to continue the life cycle.

Can Heartworm Infection be cured?

Yes, heartworm disease can be cured in most dogs, though the treatment process is complex, expensive, and not without risk. The standard treatment protocol eliminates adult worms over several months through a series of injections and careful management.

However, dogs with advanced heartworm disease may suffer permanent damage to their heart, lungs, and blood vessels even after the successful elimination of the parasites. The longer the worms remain in a dog’s body, the more severe and potentially irreversible the damage becomes.

6 Treatment Options

• Melarsomine Injections – This FDA-approved arsenic-based medication is administered via deep intramuscular injections to kill adult worms, typically following a specific protocol of several injections over a 1-2 month period.
• Exercise Restriction – Strict cage rest and exercise limitation are essential during and after treatment to prevent complications as dead worms break down and move through the bloodstream.
• Doxycycline Therapy – This antibiotic targets the Wolbachia bacteria that live symbiotically with worms, weakening the parasites and reducing complications when the adult worms die during treatment.
• Macrocyclic Lactones – Before adult worm treatment begins, dogs typically receive heartworm preventative medications to eliminate immature worms and prevent new infections.
• Corticosteroids and Pain Management – Anti-inflammatory medications may be prescribed to manage inflammation and discomfort during the treatment process.
• Surgical Removal – In severe cases with high worm burdens, specialized veterinary surgeons may physically remove worms from the heart and pulmonary arteries.

Supportive Care – Dogs with heart failure or respiratory complications from heartworm disease often require additional medications and treatments to manage symptoms.

Why Should You Invest in Heartworm Medications

Preventing heartworm disease in dogs is significantly safer, less expensive, and more humane than treating an established infection. Monthly preventatives cost a fraction of heartworm treatment, which can exceed several thousand dollars when including necessary testing, medications, hospitalization, and follow-up care. Additionally, heartworm treatment carries risks of serious complications, including pulmonary thromboembolism (blood clots in the lungs), which can be fatal in some cases. Prevention completely avoids the pain, stress, and potential permanent damage that heartworm infection causes to dogs.

9 Medical Conditions Associated with Heartworm

• Caval Syndrome – This severe form of heartworm disease occurs when a large mass of worms blocks blood flow returning to the heart. Without emergency surgical intervention to physically remove the worms, caval syndrome is almost always fatal within 1-2 days.
• Pulmonary Arterial Disease – Adult worms trigger inflammation and thickening of pulmonary arteries, causing permanent narrowing and scarring. These changes persist even after successful treatment, potentially leading to lifelong breathing difficulties and reduced exercise capacity.
• Right-sided Heart Failure – As the worms multiply in the heart’s right chamber and adjacent vessels, they force the heart to work harder against increasing resistance. This increased workload causes the heart to enlarge abnormally and eventually fail, leading to fluid buildup in the abdomen and extremities.
• Chronic Coughing – Heartworm-infected dogs typically develop persistent coughs as the parasites irritate lung tissue and cause inflammation in the airways. This chronic coughing can persist for months or years, significantly reducing quality of life even after treatment.
• Lung Damage – Dead or dying worms can break apart and cause dangerous blood clots in the lungs. These clots block blood flow to portions of lung tissue, causing tissue death, severe respiratory distress, and sometimes sudden death in heavily infected dogs.
• Kidney and Liver Damage – As heartworm disease progresses, poor circulation and inflammatory responses can cause secondary damage to vital organs. Proteins from dying worms can trigger immune reactions that damage kidney filtration systems, while compromised blood flow affects liver function.
• Exercise Intolerance – Dogs with heartworm disease gradually lose their ability to engage in normal physical activity. Initially manifesting as fatigue after moderate exercise, this condition progressively worsens until even routine walking becomes difficult, severely limiting a dog’s ability to enjoy normal activities.
• Neurological Problems – In rare cases, migrating infective larvae or fragments of adult worms can reach the brain, causing seizures, blindness, circling behavior, or other neurological abnormalities. These complications are particularly difficult to treat and may cause permanent disability.
• Allergic Pneumonitis – Some dogs develop severe allergic reactions to heartworm proteins, resulting in inflammatory lung disease similar to asthma. This condition causes wheezing, labored breathing, and oxygen deprivation that may require intensive emergency treatment.

Conclusion

Heartworm disease poses a serious but preventable threat to dogs across the country. By understanding how heartworm transmission occurs through mosquito blood meals and implementing a comprehensive prevention strategy, pet owners can protect their beloved companions from the devastation of adult worms colonizing their hearts and blood vessels.

Don’t wait until symptoms appear to address heartworm concerns. Schedule an appointment with us at Petsadena Animal Hospital today for personalized heartworm prevention advice and testing. For more educational content about protecting your pet’s health, follow us on Facebook and Instagram, and share your positive experiences by leaving a review on Yelp.