Updated March 21, 2024 – Over 400 years ago, researchers first identified heartworms (Dirofilaria immitis) in animals. For more than 100 years, veterinarians have known that D. immitis causes heartworm disease in dogs and cats. Despite advances in our knowledge of heartworm disease, including improved diagnostic tests and safer, more effective treatments, heartworm remains a significant health threat for pets.
Heartworm Lifecycle 101
Until recently, people thought that mosquitoes transmitted heartworms directly into the pet’s bloodstream via a bite. We now know that heartworm larvae (microfilariae) are deposited onto the surface of a dog or cat’s skin when the mosquito feeds. The larvae continue to mature on the skin, sometimes for several months.
If male and female heartworms are present in an infected dog, they can mate and produce offspring. These tiny microfilariae release into the dog’s blood system. Female mosquitoes then ingest microfilariae during feeding, undergo further maturation in the mosquito and pass on when the mosquito feeds again. The transmitted larvae enter the dog’s or cat’s body and finish maturing in their pet host.
Adult heartworms eventually reach the blood vessels of the heart and lungs. Microfilariae can be found in the blood of infected pets as early as six months post-infection, although, in most dogs, microfilariae appear seven to nine months post-infection. Adult heartworms have a life expectancy of five to seven years in dogs and up to two to three years in cats. Unlike dogs, most worms in cats do not survive to the adult stage, and infected cats are less likely sources of additional infections.
Two factors influence larval molts and maturation: ambient temperature and Wolbachia pipientis, a bacterium that lives within the heartworm and is needed for normal maturation of heartworm larvae. The discovery of Wolbachia was crucial in improving heartworm treatment efficacy and successfully eliminating D. immitis.
Understanding Heartworm Diagnostic Tests
Even among heartworm experts, recommendations for diagnostic testing vary. We know that the earlier heartworm infections are detected in your pet, the better the chance of recovery with fewer complications.
The current guidelines from the American Heartworm Society recommend antigen and microfilaria testing be performed annually on all dogs, regardless of geographic location. This might be new information for some pet owners located in areas that historically had low heartworm infections. These tests also help diagnose heartworms in cats.
Veterinarians categorize diagnostic tests for heartworm in pets into two main categories:
- Primary diagnostic tests include microfilaria and antigen tests.
- Secondary diagnostic tests involve imaging and clinical pathology.
Primary testing for microfilaria includes wet-mount direct examination and the modified Knott test or millipore filter test. It is essential to look for diagnostic purposes and assess microfilaria burden. In addition, some heartworm-infected pets can make antibodies that result in false negative antigen tests. Microfilaria can help confirm a diagnosis of heartworm infection in these patients.
Heartworm antigen tests are the most sensitive method for diagnosing heartworm disease in dogs. Antigen tests detect the actual presence of adult worms and are almost 100% specific for infection. They’re also crucial in identifying dogs that have heartworms but don’t have circulating microfilaria. The tests are sensitive enough to detect one female worm!
Antigen tests are more challenging in cats. That’s because cats tend to have far fewer worms in their systems, sometimes as few as one or two, compared to 30 or more in dogs. A positive antigen test in cats confirms heartworm infection; a negative test only lowers the possibility of infection but doesn’t 100% rule out infection.
Other diagnostic tests include imaging such as X-rays and heart ultrasounds. Although neither test can definitively diagnose heartworm infection, veterinarians find radiography and echocardiography extremely useful for diagnosing heartworm infection and assessing its severity once diagnosed.
Further diagnostic tests, such as a complete blood panel, biochemistry profile and urinalysis, are essential for evaluating a patient before treatment and detecting underlying diseases that could affect treatment. However, the intention of these tests is not to diagnose heartworm disease.
Best Friends - Wolbachia and D. immitis
In the 1970s, scientists discovered bacteria living in the bodies of both adult and immature heartworms. It took another 20 years for scientists to understand that Wolbachia are not just passive passengers but play a crucial role in a worm’s survival.
Due to this co-dependency, the discovery of Wolbachia has changed how veterinarians treat heartworm disease in dogs. The American Heartworm Society recommends that doxycycline, an antibiotic that kills Wolbachia, be given for one month before treatment for adult heartworms in dogs.
Unfortunately, due to safety concerns, regulatory agencies have not approved any drugs for treating adult heartworms in cats. However, antibiotic therapy directed against Wolbachia leads to decreases in all stages of immature heartworms. It might decrease the number of adult heartworms in dogs and cats and reduce the potential for respiratory complications.
Treatment Concepts
Although living heartworms can trigger a significant immune response and damage blood vessels, dead and dying worms are even more destructive. Veterinarians need to customize the treatment of heartworms for each patient based on the presence or absence of clinical signs. We can simplify our understanding of heartworm treatments by breaking it down into three categories:
- Adulticide treatment (including antibiotics against Wolbachia)
- Microfilaricide treatment
- Adjunct therapy
Melarsomine is the only adulticide approved by the FDA. Although other treatment regimens have been described, the American Heartworm Society (2018) recommends a three-dose protocol as the most effective drug for eliminating adult heartworms in dogs.
In contrast to older protocols, therapy aimed at killing microfilaria is initiated before adulticide therapy. Before starting therapy, veterinarians must determine the microfilaria burden, and for dogs with large numbers of microfilaria, they recommend pre-treatment with steroids and antihistamines.
The use of adjunctive therapy with prednisone and antihistamines, as well as supportive care measures, are dictated by pre-treatment diagnostic tests, clinical signs and worm burden for dogs. One month after completing adulticide therapy, veterinarians should resume appropriate heartworm chemoprophylaxis for dogs and retest them for microfilaria. Another antigen and microfilaria test should be performed six months after the last dose of melarsomine to monitor treatment success.
If this seems complicated, it is! That’s why the prevention of infection is so important.
Treatment for Cats Is Different
As noted, the FDA has not approved any drugs for heartworm infections in cats. Heartworm-positive cats with mild symptoms respond well to small doses of prednisone to reduce inflammation. Cats with more severe infections will require supportive therapies, including intravenous fluids, drugs to treat lung and heart symptoms, antibiotics and, in some cases, surgical removal of the heartworms.
Prevention is Key
Although treatment of heartworm disease in dogs and cats has come a long way in the last 30 years, prevention is still best. There are many choices when it comes to heartworm preventives. Working with your veterinarian to decide which is best for your pet is essential. Have your pet tested annually to ensure the heartworm preventives work and provide optimal protection against infection. As a bonus, many heartworm medications may also prevent other parasites.
You can find more details and strategies for treating heartworm-infected dogs and cats at the American Heartworm Society.
Science to Save Pets with Heartworm – What’s New at the Foundation?
Researchers recently discovered some mosquito species naturally carry Wolbachia bacteria, making them less likely to transmit viruses like dengue fever and Zika. Ongoing Morris Animal Foundation-funded research looks at the possibility of the same being valid for mosquitoes that transmit heartworms. They hope infecting mosquitoes with naturally occurring bacteria will help reduce the mosquitoes’ ability to spread heartworm disease. Another ongoing study examines which mosquitoes can harbor and successfully transmit the larval parasite that causes heartworm disease in dogs in the United States.
Together, these studies are valuable to heartworm disease's long-term regional management.
No vaccine is commercially available to prevent heartworm disease in dogs or cats. However, this is a focused area of heartworm research today. Foundation-funded researchers are working to identify antigens needed to develop a vaccine against heartworm disease in dogs. If successful, this vaccine will protect against multiple D. immitis strains. Vaccination against the disease could also address another pressing problem – drug resistance to available heartworm medications in North America. A vaccine would provide an alternate way to protect your pets and reduce the growing threat of heartworm drug resistance.
Lastly, a new paper published this past year using data from the Golden Retriever Lifetime Study showed some startling findings about heartworm preventive use in the Study cohort. The researchers found less than 40% of dogs in the Study were on heartworm-preventive medications at baseline. The team also reported that dogs were less likely to be on preventive if they were sexually intact, larger in height or receiving supplements. Conversely, if dogs had received vaccinations, been diagnosed with an infectious disease (other than heartworm), or had an ear, nose, or throat condition diagnosed at their last wellness check, they were more likely to be on preventive medications. The team’s findings are surprising since heartworm infections are spreading to new areas of the country, putting more dogs at risk for infection.
We Need Your Help
Science starts with donors passionate about advancing animal health. Science can be slow, frustrating and demanding, but we’re in it for the long haul – will you join us?
Learn how you can help us find the best, most impactful scientific studies to advance the health of animals worldwide.
Additional Resources:
To learn more about Morris Animal Foundation and our work on heartworm disease, listen to our Fresh Scoop Podcast, Unravelling Heartworm Disease.
To learn more about heartworms in people (and yes, it does happen), check out the Centers for Disease Control and Prevention. To get more details on preventives and treatment for heartworm in dogs and cats, check out the American Heartworm Society.
Over 400 years ago, researchers first identified heartworms (Dirofilaria immitis) in animals. For more than 100 years, veterinarians have known that D. immitis causes heartworm disease in dogs and cats. Despite advances in our knowledge of heartworm disease, including improved diagnostic tests and safer, more effective treatments, heartworm remains a significant health threat for pets.
Heartworm Lifecycle 101
Until recently, people thought that mosquitoes transmitted heartworms directly into the pet’s bloodstream via a bite. We now know that heartworm larvae (microfilariae) are deposited onto the surface of a dog or cat’s skin when the mosquito feeds. The larvae continue to mature on the skin, sometimes for several months.
If male and female heartworms are present in an infected dog, they can mate and produce offspring. These tiny microfilariae release into the dog’s blood system. Female mosquitoes then ingest microfilariae during feeding, undergo further maturation in the mosquito and pass on when the mosquito feeds again. The transmitted larvae enter the dog’s or cat’s body and finish maturing in their pet host.
Adult heartworms eventually reach the blood vessels of the heart and lungs. Microfilariae can be found in the blood of infected pets as early as six months post-infection, although, in most dogs, microfilariae appear seven to nine months post-infection. Adult heartworms have a life expectancy of five to seven years in dogs and up to two to three years in cats. Unlike dogs, most worms in cats do not survive to the adult stage, and infected cats are less likely sources of additional infections.
Two factors influence larval molts and maturation: ambient temperature and Wolbachia pipientis, a bacterium that lives within the heartworm and is needed for normal maturation of heartworm larvae. The discovery of Wolbachia was crucial in improving heartworm treatment efficacy and successfully eliminating D. immitis.
Understanding Heartworm Diagnostic Tests
Even among heartworm experts, recommendations for diagnostic testing vary. We know that the earlier heartworm infections are detected in your pet, the better the chance of recovery with fewer complications.
The current guidelines from the American Heartworm Society recommend antigen and microfilaria testing be performed annually on all dogs, regardless of geographic location. This might be new information for some pet owners located in areas that historically had low heartworm infections. These tests also help diagnose heartworms in cats.
Veterinarians categorize diagnostic tests for heartworm in pets into two main categories:
- Primary diagnostic tests include microfilaria and antigen tests.
- Secondary diagnostic tests involve imaging and clinical pathology.
Primary testing for microfilaria includes wet-mount direct examination and the modified Knott test or millipore filter test. It is essential to look for diagnostic purposes and assess microfilaria burden. In addition, some heartworm-infected pets can make antibodies that result in false negative antigen tests. Microfilaria can help confirm a diagnosis of heartworm infection in these patients.
Heartworm antigen tests are the most sensitive method for diagnosing heartworm disease in dogs. Antigen tests detect the actual presence of adult worms and are almost 100% specific for infection. They’re also crucial in identifying dogs that have heartworms but don’t have circulating microfilaria. The tests are sensitive enough to detect one female worm!
Antigen tests are more challenging in cats. That’s because cats tend to have far fewer worms in their systems, sometimes as few as one or two, compared to 30 or more in dogs. A positive antigen test in cats confirms heartworm infection; a negative test only lowers the possibility of infection but doesn’t 100% rule out infection.
Other diagnostic tests include imaging such as X-rays and heart ultrasounds. Although neither test can definitively diagnose heartworm infection, veterinarians find radiography and echocardiography extremely useful for diagnosing heartworm infection and assessing its severity once diagnosed.
Further diagnostic tests, such as a complete blood panel, biochemistry profile and urinalysis, are essential for evaluating a patient before treatment and detecting underlying diseases that could affect treatment. However, the intention of these tests is not to diagnose heartworm disease.
Best Friends - Wolbachia and D. immitis
In the 1970s, scientists discovered bacteria living in the bodies of both adult and immature heartworms. It took another 20 years for scientists to understand that Wolbachia are not just passive passengers but play a crucial role in a worm’s survival.
Due to this co-dependency, the discovery of Wolbachia has changed how veterinarians treat heartworm disease in dogs. The American Heartworm Society recommends that doxycycline, an antibiotic that kills Wolbachia, be given for one month before treatment for adult heartworms in dogs.
Unfortunately, due to safety concerns, regulatory agencies have not approved any drugs for treating adult heartworms in cats. However, antibiotic therapy directed against Wolbachia leads to decreases in all stages of immature heartworms. It might decrease the number of adult heartworms in dogs and cats and reduce the potential for respiratory complications.
Treatment Concepts
Although living heartworms can trigger a significant immune response and damage blood vessels, dead and dying worms are even more destructive. Veterinarians need to customize the treatment of heartworms for each patient based on the presence or absence of clinical signs. We can simplify our understanding of heartworm treatments by breaking it down into three categories:
- Adulticide treatment (including antibiotics against Wolbachia)
- Microfilaricide treatment
- Adjunct therapy
Melarsomine is the only adulticide approved by the FDA. Although other treatment regimens have been described, the American Heartworm Society (2018) recommends a three-dose protocol as the most effective drug for eliminating adult heartworms in dogs.
In contrast to older protocols, therapy aimed at killing microfilaria is initiated before adulticide therapy. Before starting therapy, veterinarians must determine the microfilaria burden, and for dogs with large numbers of microfilaria, they recommend pre-treatment with steroids and antihistamines.
The use of adjunctive therapy with prednisone and antihistamines, as well as supportive care measures, are dictated by pre-treatment diagnostic tests, clinical signs and worm burden for dogs. One month after completing adulticide therapy, veterinarians should resume appropriate heartworm chemoprophylaxis for dogs and retest them for microfilaria. Another antigen and microfilaria test should be performed six months after the last dose of melarsomine to monitor treatment success.
If this seems complicated, it is! That’s why the prevention of infection is so important.
Treatment for Cats Is Different
As noted, the FDA has not approved any drugs for heartworm infections in cats. Heartworm-positive cats with mild symptoms respond well to small doses of prednisone to reduce inflammation. Cats with more severe infections will require supportive therapies, including intravenous fluids, drugs to treat lung and heart symptoms, antibiotics and, in some cases, surgical removal of the heartworms.
Prevention is Key
Although treatment of heartworm disease in dogs and cats has come a long way in the last 30 years, prevention is still best. There are many choices when it comes to heartworm preventives. Working with your veterinarian to decide which is best for your pet is essential. Have your pet tested annually to ensure the heartworm preventives work and provide optimal protection against infection. As a bonus, many heartworm medications may also prevent other parasites.
You can find more details and strategies for treating heartworm-infected dogs and cats at the American Heartworm Society.
Science to Save Pets with Heartworm – What’s New at the Foundation?
Researchers recently discovered some mosquito species naturally carry Wolbachia bacteria, making them less likely to transmit viruses like dengue fever and Zika. Ongoing Morris Animal Foundation-funded research looks at the possibility of the same being valid for mosquitoes that transmit heartworms. They hope infecting mosquitoes with naturally occurring bacteria will help reduce the mosquitoes’ ability to spread heartworm disease. Another ongoing study examines which mosquitoes can harbor and successfully transmit the larval parasite that causes heartworm disease in dogs in the United States.
Together, these studies are valuable to heartworm disease's long-term regional management.
No vaccine is commercially available to prevent heartworm disease in dogs or cats. However, this is a focused area of heartworm research today. Foundation-funded researchers are working to identify antigens needed to develop a vaccine against heartworm disease in dogs. If successful, this vaccine will protect against multiple D. immitis strains. Vaccination against the disease could also address another pressing problem – drug resistance to available heartworm medications in North America. A vaccine would provide an alternate way to protect your pets and reduce the growing threat of heartworm drug resistance.
Lastly, a new paper published this past year using data from the Golden Retriever Lifetime Study showed some startling findings about heartworm preventive use in the Study cohort. The researchers found less than 40% of dogs in the Study were on heartworm-preventive medications at baseline. The team also reported that dogs were less likely to be on preventive if they were sexually intact, larger in height or receiving supplements. Conversely, if dogs had received vaccinations, been diagnosed with an infectious disease (other than heartworm), or had an ear, nose, or throat condition diagnosed at their last wellness check, they were more likely to be on preventive medications. The team’s findings are surprising since heartworm infections are spreading to new areas of the country, putting more dogs at risk for infection.
We Need Your Help
Science starts with donors passionate about advancing animal health. Science can be slow, frustrating and demanding, but we’re in it for the long haul – will you join us?
Learn how you can help us find the best, most impactful scientific studies to advance the health of animals worldwide.
Additional Resources:
To learn more about Morris Animal Foundation and our work on heartworm disease, listen to our Fresh Scoop Podcast, Unravelling Heartworm Disease.
To learn more about heartworms in people (and yes, it does happen), check out the Centers for Disease Control and Prevention. To get more details on preventives and treatment for heartworm in dogs and cats, check out the American Heartworm Society.
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