Heartworm disease can be deadly and is becoming increasingly common around the world. While dogs are the primary hosts for heartworms, cats and ferrets can also catch the disease, and all pets are at risk even if they remain indoors. Read on to learn:
Dogs, cats, and ferrets should remain on a heartworm preventative year-round to protect them from contracting the parasite suffering the side effects of serious infestation. Pet owners should know when and how often to have their pets tested for heartworms, what symptoms to be aware of, and what types of preventive medications are available.
Heartworm disease is a serious infection that primarily affects dogs but can also infect cats, ferrets, and other mammals. The condition is caused by Dirofilaria immitis, a spaghetti-sized parasitic worm that lives in the blood vessels of its host. Heartworms affect dogs, cats, and ferrets differently but can lead to heart failure, severe lung disease, and other organ damage throughout the body in all three species. Heartworms can survive in the body for years and the presence of even one adult worm can damage the sensitive lining of blood vessels around the heart and lungs. Large numbers of heartworms do even more damage and are much more likely to lead to heart failure.
Heartworms are spread by infected mosquitoes. Baby worms are ingested by a mosquito when it feeds on an animal infected with adult worms. The baby worms, or microfilariae, mature in the mosquito for 10 to 30 days, and are then transmitted to another animal through mosquito bites. Once the worms are transmitted to a new host, they take about six months to mature into adult worms.
Heartworm infections are found in all 50 U.S. states, all parts of Canada, and other countries. The disease is most common in regions with large populations of infected mosquitos, particularly along the Atlantic and Gulf coasts in warmer climates. Unfortunately, heartworm disease is becoming more common and widespread due to climate change, increased population density, and the migration of rescued animals in need of treatment.
Heartworm disease affects dogs, cats, and ferrets differently, so there is a range in clinical signs and how the disease affects the body. In all three species, the beginning stage of a heartworm infection causes little to no identifiable symptoms. Dogs are the most natural hosts for heartworms and have higher rates of infection, but cats and ferrets are more likely to die from the disease. Heart damage, lung damage, and other organ damage are often irreversible.
Many infected dogs are asymptomatic when they are first diagnosed. Symptoms begin to show with the onset of heart disease, congestive heart failure, lung disease, and damage to other organs. Symptoms of heartworm infection in dogs include:
There are four classes of heartworm disease in dogs based on severity. A change in the heart and lungs can often be seen on x-rays when the disease reaches stage two or three. Stage four refers to caval syndrome, which is a rare, acute, and severe form of heartworm disease. Caval syndrome occurs when there are so many adult-stage worms that they block the major blood vessels near the heart and lungs. Emergency surgery is the only option when caval syndrome develops, and even with prompt treatment, the mortality rate in dogs with caval syndrome is very high. Some dogs live with stage one heartworm disease for years, while others develop advanced stages rapidly depending on the number of heartworms in the bloodstream.
Heartworm infection in cats is primarily a respiratory disease with wheezing and coughing as symptoms. Heartworm disease in cats is easily confused with feline asthma and frequently goes undiagnosed. Clinical signs in cats include:
In dogs, heartworms often go undetected as they mature and move toward the large blood vessels around the heart and lungs. Since cats are not the usual host for heartworms, an infected cat’s immune system usually destroys all of the worms before they reach adulthood. This process creates a lot of inflammation in the blood vessels and lungs, producing scarring and long-lasting respiratory symptoms.
Routine heartworm screening tests rely on the presence of adult worms, so diagnosing feline heartworm disease is complicated by the fact that there are usually no surviving worms. Standard treatment protocols focus on killing adult worms, so they are also of little use. Since diagnosis and treatment of heartworm infection in cats is so challenging, prevention is the best course of action.
Signs of heartworm disease in ferrets are similar to signs in cats, but infection in a ferret is more likely to be deadly. Even one adult worm can be fatal for a ferret due to the small size of its heart. Like with cats, testing and treatment for heartworms is of limited usefulness, so prevention is the best approach to avoid severe heartworm disease in ferrets.
Dogs maintaining a schedule of heartworm prevention should be tested every 12 months. Dogs who have not been on regular prevention should be tested every 6 months until they’ve been on regular prevention for a year. Early stages of heartworm infection often have no symptoms at all, so regular testing is crucial for early diagnosis. Once symptoms appear, the disease is more difficult and dangerous to treat.
Pet owners should not count on 100% effectiveness of prevention. Concern over resistance to common preventives is growing, and it’s also possible for your pet on prevention to catch heartworms through:
Dogs don’t test positive until heartworms have grown to adulthood approximately six months after infection, so annual testing often provides early diagnosis. Early diagnosis provides the best chance of making a full recovery and preventing severe illness. A heartworm test involves either testing a small blood sample at your local vet clinic or sending a blood sample out to a lab for antigen testing. Since it takes six months for adult heartworms to develop, young puppies and kittens can begin preventive medications early without testing. After six months of age, pets need to be tested before beginning prevention.
If your pet tests positive for heartworms, your veterinarian will likely recommend additional testing and starting treatment. While rare, snap tests done by vet clinics can produce false positives. A blood sample may be sent to a lab to check for antibodies, heartworm proteins, and the presence of young worms in the bloodstream. Additional blood tests, chest x-rays, and ultrasounds may be needed to determine the stage of disease.
Pets with advanced symptoms or conditions caused by heartworms may need to be stabilized with supportive veterinary care and therapy before heartworm treatment can begin.
Your vet will recommend a treatment based on your pet’s species and the stage of disease. The side effects of heartworm treatment and severe symptoms in advanced cases may require additional supportive care. The treatment for heartworm disease in dogs involves a series of injections of heartworm adulticide and other medications. Dogs undergoing treatment should be limited to only short leash walks to minimize the likelihood of complications. The entire process may take as little as six months but often has to be repeated over one to two years.
Heartworm adulticide is only useful in dogs. There is no approved therapy for cats or ferrets. Treatment options for cats and ferrets are limited to stabilization and treatment of symptoms.
Heartworm prevention is a must for all pets who are at risk. This includes cats and ferrets who live only indoors. It only takes one infected mosquito to cause disease in your pet. If you live in an area that experiences colder winters with limited mosquitoes, your pet should still receive year-round prevention due to when and how preventives work.
Mosquito control around your home and yard is helpful but is not a replacement for prevention. “Heartworm prevention is much safer and less expensive than treatment, ” explains Dr. Jo Myers, a Vetster veterinarian.
There are oral, topical, and injectable preventions available. Some types of oral and topical heartworm prevention also include prevention for fleas, ticks, and intestinal parasites. Most of these options are given monthly. Ultimately, it is up to the owner’s preferences and what your pet tolerates. Always give the dosage and species-appropriate preventative, and never split doses between two animals.
Toxicity can occur if a cat receives prevention meant for dogs or if a pet receives prevention meant for a much larger animal. If you have questions about heartworm prevention, treatment, or symptoms, you can connect to an online vet at Vetster for advice.
Thanks to annual screening, most dogs with heartworms have no symptoms when they are diagnosed. This simplifies treatment and reduces outcomes. When symptoms do appear, they most commonly include coughing, exercise intolerance, and weight loss.
Adult heartworms can be killed in dogs, but in cats and ferrets, the worms themselves have usually died by the time infection is suspected. Surgery to remove adult heartworms does exist, but it is extremely dangerous. Successful treatment of heartworms is possible but can be expensive and dangerous.
Not directly. Pets only catch heartworms from an infected mosquito, and it takes 10 to 30 days before a mosquito can infect a new host after picking up heartworm larvae from an infected animal.
Humans cannot catch heartworms from an infected pet. Like other mammals, humans can only catch heartworms from infected mosquitoes. However, infection in humans is very rare and the worms are unlikely to survive to adulthood or cause significant illness.
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