Introduction
Heartworm disease is a serious parasitic disease that affects dogs and other canines. It’s caused by foot-long worms that live in the heart, lungs, and associated blood vessels of affected pets[1]. Heartworm infection is a major threat to dogs’ health and wellbeing. If left untreated, it can lead to lung disease, heart failure, organ damage, and even death[1]. This debilitating disease is transmitted from animal to animal by mosquitoes and can have long-lasting impacts on a dog’s health and quality of life, even after the parasites are eliminated[2].
Cause
Heartworm disease in dogs is caused by a parasitic worm called Dirofilaria immitis. This worm is spread through the bite of an infected mosquito. When an infected mosquito bites a dog, it transmits the microscopic heartworm larvae into the dog’s bloodstream. Over the next 6-7 months, these larvae mature into adult heartworms that lodge in the heart, lungs, and associated blood vessels, causing severe disease.
As stated by the Heartworm Society, “It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body.” (https://www.heartwormsociety.org/heartworms-in-dogs)
Life Cycle
The heartworm life cycle begins when a female mosquito bites an infected dog and ingests heartworm microfilariae during a blood meal [1]. Inside the mosquito, the microfilariae develop further for 10 to 30 days before becoming infective third stage larvae. When the mosquito bites another dog, the larvae infect the new host by entering through the bite wound [2].
Once inside a dog, the larvae migrate for about 3 to 4 months before maturing into adult heartworms. The adults live in the heart, pulmonary artery, and lungs, with females growing up to 12 inches long and males about half that size. Inside the heart and blood vessels, the worms mate and females release new microfilariae into the dog’s bloodstream to continue the cycle [1].
Transmission
Heartworms are spread through the bite of an infected mosquito. Mosquitoes become infected with heartworm larvae, known as microfilaria, when they feed on an infected animal. Inside the mosquito, the microfilaria mature into infective larvae over the course of about two weeks.
These infected mosquitoes then spread heartworms by biting another dog, cat, or susceptible wild animal. When an infected mosquito bites a dog, the larvae enter through the bite wound and migrate under the skin to connective tissues and small blood vessels. Within about three months, they mature into adult heartworms capable of reproducing and completing the life cycle.
Adult female heartworms living inside a dog produce new microfilaria, which circulate in the blood. If a mosquito bites an infected dog again, it can pick up these microfilaria and spread them to another animal (Heartworm in Dogs). This is how heartworms are transmitted between dogs via the mosquito vector.
Symptoms
Some of the most common symptoms of heartworm disease in dogs include coughing, fatigue, and trouble breathing. A persistent, dry cough is often one of the first signs of infection. As the disease progresses and the heartworm burden increases, dogs may experience fatigue and shortness of breath after mild activity or exercise. In advanced cases, heartworm disease can cause significant damage to the heart, lungs, and pulmonary blood vessels. Severely infected dogs may cough up blood and experience weight loss, fainting spells, and heart failure.
According to the Heartworm Society, signs of heartworm disease may include “a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss.” https://www.heartwormsociety.org/heartworms-in-dogs
As Heartgard notes, “A persistent, dry cough is a common sign seen in dogs with advanced heartworm disease.” Other signs of an infection may include difficulty breathing, weakness, swollen belly, jaundice, and fainting. https://heartgard.com/signs-of-heartworms-in-dogs
Diagnosis
Heartworm disease is typically diagnosed through blood testing at the veterinarian’s office. The two main blood tests look for signs of adult heartworms or microfilariae, their offspring. The antigen test is most common and detects proteins from adult female heartworms in the bloodstream. This test is very accurate when positive but may miss early infections.
If the antigen test is negative but heartworms are still suspected based on symptoms, vets may run additional blood tests like the microfilariae test. This detects offspring heartworms circulating in the blood. However, many infected dogs may not test positive if few offspring are present. Given the limitations of blood tests, the American Heartworm Society recommends using multiple testing methods to conclusively diagnose heartworms.
Other diagnostic tools may include x-rays, ultrasound scans, or imaging tests of the heart and lungs to check for signs of heart and lung disease associated with heartworms. Bloodwork to evaluate kidney and liver function may also be recommended. Urinalysis can check for protein loss that results from heartworm damage to the organs. While expensive, these combined methods lead to the most accurate heartworm diagnosis.
Treatment
Treatment for heartworm disease in dogs involves killing the adult worms present in the body with a series of injections of an arsenic-based drug called melarsomine. The most common protocol is to give an initial injection, wait 24 hours, give a second injection, wait 24 hours again, and then give a third injection. This sequence allows the drug to kill worms slowly, minimizing the chance of pulmonary embolism from large numbers of dead worms blocking blood vessels.[1]
After the adult worms are killed, dogs still require a recovery period of limited activity to allow their body to break down and absorb the dead worms. Restricted exercise helps prevent complications like respiratory distress or blood clots from worm decomposition. Most vets recommend 4-8 weeks of rest during and after treatment. Additional medications like prednisone and antibiotics may also be prescribed to manage inflammation and prevent secondary infections.[2] Pain medication may be needed to keep dogs comfortable.
Completing the full course of injections and allowing adequate recovery time is crucial for treating heartworms. While the treatment process can take months from start to finish, most dogs are able to make a full recovery and live healthy lives afterwards.
Prevention
The most effective way to prevent heartworm is through the regular and consistent use of monthly heartworm preventive medications (Heartworm Prevention for Dogs). These medications work by killing heartworm larvae before they molt into juvenile worms. By stopping larvae from maturing into adults, monthly preventatives break the heartworm life cycle and prevent disease transmission (Heartworm Disease).
There are many monthly heartworm prevention options for dogs, including tablet, chewable, and topical formulations. These medications require a prescription from a veterinarian and should be given year-round, even in winter when mosquitoes are less active. Consistency is key – just one missed dose can leave a dog vulnerable to infection.
Reducing mosquitoes in the environment can provide additional protection. Use mosquito repellents when taking dogs outside and remove any standing water sources around the home where mosquitoes may breed.
Prognosis
With early detection and prompt treatment, the prognosis for dogs with heartworm disease is good. According to the American Heartworm Society, most infected dogs recover fully when the disease is treated in the early stages.[1] However, the longer treatment is delayed, the more likely there will be irreversible lung damage.[2]
For dogs displaying clinical signs of heartworm disease, the prognosis worsens significantly. Dogs in the later stages of infection are at high risk for potentially fatal complications, including caval syndrome. Caval syndrome occurs when a large number of heartworms die at once, leading to blockages of blood flow to the heart.[3]
Without treatment, most dogs with symptomatic heartworm disease will die within 2-3 years. But even dogs with advanced disease can be successfully treated if the owners and veterinarians are aggressive about therapy.[2] Though treating late-stage heartworm requires months of hospitalization, blood transfusions, and expensive medications, many dogs go on to live happy and active lives afterwards.[1]
Overall, the prognosis for most dogs with heartworm disease is good if the disease is caught early. But heartworm is very serious if left untreated, and can result in severe lung damage, heart failure, and death.[1] This highlights the importance of year-round heartworm prevention.
[1] https://www.heartwormsociety.org/heartworms-in-dogs
[2] https://vcahospitals.com/know-your-pet/heartworm-disease-in-dogs
[3] https://www.petmd.com/dog/conditions/infectious-parasitic/c_dg_heartworm_disease
Conclusion
Heartworm is a dangerous parasitic disease that can affect dogs and potentially lead to severe medical complications or even death if left untreated. The disease is spread through mosquito bites, as infected mosquitoes carry the heartworm larvae from one dog to another. While heartworm is a serious threat, the good news is that it can be effectively prevented through consistent use of heartworm preventative medication prescribed by your veterinarian. The tiny monthly chewable is easy to give and provides protection against the disease. Prevention truly is the best medicine when it comes to protecting your dog against heartworms. Do not wait until after your dog contracts the disease to take action – be proactive and ask your vet about heartworm prevention today. With proper prevention, you can help ensure your dog lives a long and healthy life free of this devastating parasite.