12 min read
Heartworm disease is a serious and potentially fatal disease for dogs who contract it. It’s important for dog owners to understand what causes the disease, how to prevent it, and how it is treated by a veterinarian. Read on if you have ever wondered:
Dogs and other canines are the natural hosts for heartworms and can catch them easily. Severe heartworm disease can lead to heart failure and multi-organ damage. Fortunately, heartworms can be prevented and the disease can be caught early.
Heartworm disease occurs when a parasitic worm, called Dirofilaria immitis, is contracted through an infected mosquito bite. The adult parasites can grow up to one foot (30.5cm) long worms, roughly the size and appearance of a spaghetti noodle. A heartworm infection can be deadly with irreversible damage to the heart, lungs, kidneys, and widespread inflammation of the blood vessels. Dogs are the natural host for heartworms and are affected more often than other types of pets.
Heartworms are spread by infected mosquitoes. When a mosquito takes a blood meal from an infected animal, it ingests immature heartworms, or microfilariae. The microfilariae mature over 10 to 30 days into infectious larvae. When a mosquito carrying heartworm larvae bites a dog, the larvae are transferred to the dog’s bloodstream. Once infected, it takes about six months for the larvae to travel to the large blood vessels around the heart and lungs and mature into adult heartworms. Once the heartworms have matured into adults, they reproduce, sending large numbers of microfilariae into the bloodstream to be picked up by another mosquito. Heartworm infections have been found in all 50 U.S. states, all parts of Canada, and many other countries around the world. The disease is most common in dogs that live along the Atlantic or Gulf Coasts, especially in warm climates. Unfortunately, due to climate change, population density, and the movement of infected dogs between states and countries, heartworm disease rates are increasing.
Heartworm symptoms vary based on the damage that the heartworms have caused in the body. Many dogs diagnosed with heartworm disease show no symptoms. “Heartworm infections are largely asymptomatic in the early stages,” explains Dr. Jo Myers, a Vetser vet. “Annual testing is important to catch it early for more successful treatment and recovery.” Symptoms occur with the onset of heart failure, kidney disease, and respiratory disease due to the presence of the heartworms. There are four classes of heartworm disease in dogs based on the severity of infection and the number of worms present.
Early infections are usually asymptomatic when the worms are still growing or are few in number. A dog may experience a mild cough.
Class 2 infections occur when the worm burden increases enough to produce mild to moderate symptoms and changes in the heart and lungs on x-rays. Clinical signs of a class 2 heartworm infection include:
Class 3 severity results from even higher worm burdens relative to the size of the dog. Symptoms become more severe and widespread, and the disease becomes more difficult to treat. Some heart and organ damage at this stage is usually irreversible. Clinical signs include:
The fourth class is a rare, acute form of canine heart disease called caval syndrome. In caval syndrome, the number of mature heartworms in the large arteries around the heart and lungs becomes so high that blood flow is blocked. The only option at this point is the surgical removal of the worms to restore blood flow from the heart and to the lungs. However, dogs who have developed the condition are considered critically ill and surgery is extremely dangerous. Dogs with caval syndrome usually die within a couple of days of developing the condition, and the survival rate of the surgery is low.
Wolbachia pipientis is a bacteria that is released from within a heartworm every time one molts, reproduces, or dies. A dog’s immune system attacks the bacteria, causing even more inflammation around the heart, lungs, and throughout the body’s blood vessels. The inflammation can restrict blood flow in smaller veins, arteries, and capillaries throughout the body and is responsible for many of the symptoms associated with heartworm disease.
Regular testing is the only way to diagnose the early, most treatable stages of heartworm disease. It takes six months for infected larvae to develop into adult worms, so puppies under six months of age cannot be accurately tested until they are older. Puppies as young as eight weeks, however, can begin heartworm prevention. Dogs start receiving annual heartworm testing during their first adult wellness exam, around the age of 16 months.
Adults receiving preventive medication should be tested every year, as no form of heartworm preventive is 100% effective. Adult dogs who are not on regular prevention should be tested every six months until they have been regularly receiving heartworm prevention for at least one year. A heartworm test involves either a small blood draw for a quick response in-clinic test or a blood sample sent to a lab for antibody testing.
If your dog tests positive for heartworms, the next step is to have some additional tests to confirm the diagnosis as well as identify how advanced the disease is. These tests may include a complete blood profile, chest x-rays, and an abdominal ultrasound. False positives, while rare, can occur. As soon as the diagnosis is suspected, your dog should be put on immediate exercise restriction to reduce the chances of additional damage to the heart and lungs. Critically ill dogs, such as those who have developed congestive heart failure, severe lung disease, kidney failure, or caval syndrome may require stabilization prior to treatment focused on eliminating the adult worms.
Treatment options for heartworms depend on the severity of the disease and the symptoms. After stabilization, treatment for heartworm disease involves anti-inflammatory medications, heartworm prevention to treat young heartworms, antibiotics to eliminate Wolbachia bacteria, and a multi-injection series of adulticides to kill adult heartworms. Dogs with heartworms go through a treatment process that can take six months to multiple years and are limited to leash walks only for the duration. The treatment is also expensive and can involve multiple negative side effects in dogs.
When it comes to heartworms, year-round prevention and routine testing are key. Pet owners have a variety of preventive medications to choose from. In areas that have a high population of mosquitoes, mosquito control around your home and yard during peak season can help, but is not a replacement for prevention. No heartworm preventive medicine is 100% effective. Some common reasons for breakthrough infections include:
Regular testing is essential even when your dog is regularly taking heartworm prevention. It only takes one infected mosquito to transmit heartworms to your dog. Always give size and species-appropriate prevention as recommended by your veterinarian.
There are many websites that promote the use of essential oils or other natural preventions or remedies for mosquitoes that spread heartworm disease. However, only FDA-approved heartworm medications have been shown to work. In addition, many essential oils are toxic or irritating to dogs. Luckily, all FDA-approved heartworm prevention options belong to a family of naturally occurring compounds and ingredients. They are safe, effective, and more affordable than treating heartworm disease down the road.
There are oral, topical, and injectable prevention options. Most prevention options are given monthly, and many protect against fleas, ticks, and intestinal parasites as well. The best prevention option ultimately depends on the owner’s preference and what their dog prefers, as long as it’s an FDA-approved preventive. Year-long dosing is recommended for all dogs, even if you live in a cold winter climate. If you have questions about heartworm medicine, symptoms, or treatment, you can connect to an online vet with Vetster who will be happy to walk you through your options.
The treatment for a heartworm infection can cost hundreds to thousands of dollars depending on the severity of disease and its symptoms. Heartworm prevention medication and testing is, in the long run, cheaper and safer than treatment after infection.
Not directly. Dogs can only catch heartworms from infected mosquitoes. However, if there are a large number of dogs with heartworms or other infected animals in an area, it is likely there are populations of infected mosquitoes in the area. A dog cannot catch heartworms by playing with or even through a bite wound by another infected animal.
FDA-approved heartworm preventatives have a natural origin. Other homeopathic remedies and preventions, such as essential oils, can be dangerous to give to dogs and do not offer protection from heartworm disease.
Yes. Heartworm-positive dogs usually have no symptoms in the early stages and are only caught with regular testing. The earlier the disease is caught, the easier it is to treat successfully. No prevention is 100% effective, so even dogs on regular year-round prevention should receive annual heartworm tests.
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