by: Caitlin McAllister
What is Heart Worm?
“Why do I have to pay for a heartworm test?” Is a question posed by many pet owners staring down the barrel of an ever climbing vet bill, counting the dollar signs of everything we’re doing today. A heartworm test feels quite suspiciously like a convenient little add-on designed to line the veterinarian’s pockets. However, if you’d ever seen a bundle of threadlike worms being pulled from the heart through the jugular vein like angel-hair pasta, you may revise your opinion on why this is necessary.
Dirofilaria immitis is the true name of heartworms. Originating from latin, dīrus meaning “Fearful” and fīlum meaning “thread” you can understand that this is not a parasite one wishes to mess with. Transmitted by over 70 different species of mosquitos, it is considered endemic to 48 states, and has been reported in all 10 Canadian provinces. Wherever mosquitos thrive, and there exists a temperate, semi-temperate, or tropical climate, one is likely to find heartworm (2,7).
How do Pets Get Heartworm?
The mosquito serves as what is referred to as an “intermediate host” or a disease “vector”, the mosquito’s anatomy and physiology do not permit heartworm maturation, they are instead used as a transport mechanism by the parasite to infect susceptible animals whose bodies provide a more accommodating environment from the perspective of the worm (1,2). Domestic species infected by heartworms are primarily dogs, but also cats and ferrets. For the purpose of this blog I will only look at dogs, as the disease affects cats and ferrets much differently. However, the disease behaves somewhat differently in all three so when discussing heartworm prevention and treatment one may treat them almost as separate conditions.
The transfer of heartworms between susceptible animals is fairly intuitive. Microscopic pre-larval stage worms called microfilariae circulate in an infected animal’s blood stream. An unsuspecting mosquito accidentally consumes them when it feeds, and becomes a temporary host in doing so. Within the mosquito the microfilariae spend the next two weeks undergoing three maturations, transitioning from the first to third larval states (1,2). At this point they prepare to infect a new host by migrating into the lower lip of the mosquito, the labium. When the mosquito lands on an unprotected animal, a dog for instance, the L3 worms pierce through the labium and exit the mosquito’s body, landing on the skin. From there they burrow into the bite wound, entering the subcutaneous tissues of the animal and mature to stage 4 larvae and then to young adults at which time they migrate to the heart and lungs. They reach maturity about 6 months after the animal first became infected. The adults can be 15-25 cm long on average with females being longer than males, and they live for about 3-5 years.
What are the Signs and Symptoms?
Unlike intestinal worms, the signs and symptoms of heartworm tend to be less obvious. An animal can have an infection for years and have few if any signs of disease to the untrained eye. You won’t notice worms passed in feces, your animal may behave normally, indeed if it is sedentary you may not notice early symptoms such as lethargy and exercise intolerance. Symptoms begin to appear in full force when the worms begin to die. During life, heartworms exhibit an immunosuppressive effect which allows them to somewhat evade becoming targets of your dog’s immune system. However, heartworms only live for so long. When they die and begin to decay, the animal begins to exhibit an immune response, and as bits of worm corpses break off and find their way into smaller vessels where they disrupt blood flow and potentiate pulmonary thromboembolism.
“Most infections are diagnosed in medium- to large-sized dogs that are 3–8 years old.”
The Merck Veterinary Manual 11th Edition.
The result is an impedance of blood and reduced transport of nutrients and vital gasses like oxygen. Symptoms include reduced tolerance to exercise from the inability to deliver oxygen to tissues and remove waste appropriately, coughing with or without the presence of blood (hemoptysis), epistaxis (nosebleed), difficulty breathing, abdominal swelling, and cyanotic (bluish) or pale gums (1,2). Pulmonary thromboembolism occurs when a blood clot blocks a blood vessel. This causes ischemia (the restriction of oxygen from a tissue) and can lead to infarction, where blood flow is restricted from a tissue or organ to the point where that tissue dies. Hemorrhage or bleeding is also a risk with embolism. Blood test may show evidence of anemia. Hemolysis can occur, that is the breaking of blood cells, as they pass through masses of heartworms.
Diagnosis of Heartworm
Antigen Testing
Dirofilaria immitis is generally diagnosed using a simple rapid blood-antigen test, which detects the presence of a glycoprotein specific to adult female heartworms. This test is somewhat similarly to how one performs a covid-19 self test, but with blood instead of mucous. A blood sample is mixed with a buffer solution and added to the sample well of the test kit, in a lateral flow test you then wait to see if a test line and control line appear. In a SNAP test, such as those made by IDEXX, bidirectional flow is utilized to give the antigen in the blood two chances to bind to antibody in the test kit. In a SNAP test, the person performing the test will wait for the blood to flow all the way across the testing window before ‘snapping’ the kit, pressing hard at one end such that the kit flattens. This allows for a wash solution to be released and removes the debris from the window so you can clearly see the distinctive test indicator dots. However, these tests only show a positive result when adult females are present. If there are only male heartworms or if the worms have not yet reached maturity, then the test may give a false negative. Modern Veterinary Therapeutics claims to have developed a test that can detect both male and female adult worms; however, their test has the same pit fall in that if there are no adults present, the test may read as negative despite circulating microfilariae.
Microfilariae Tests
To confirm presence of heartworm a microfilariae test may be performed. A direct smear test can be done where a drop of blood mixed with anticoagulant (a substance that prevents clotting) is placed on a microscope slide and covered with a cover slip. The slide is then examined under the microscope for signs of movement. If positive you would see a translucent worm, barely visible, wiggling about and displacing red blood cells (6). It is sometimes possible to see microfilariae but not have a positive antigen test, either because there are no adult worms present (which can occur for a number of reasons), or because you are looking at a different blood parasite such as Dipetalonema reconditum which is considered relatively harmless. To differentiate the two a Modified Knott’s test is considered the gold standard of microfilariae testing. In this test a blood sample is collected in a tube containing an anticoagulant such as heparin or EDTA. A milliliter of blood is then mixed with 10 milliliters of 2% formalin and spun in a centrifuge for 5-8 minutes. This straightens and condenses the microfilariae in the sediment which is then mixed with a stain like methylene blue, placed on a slide the microfilariae can be easily differentiated when measured. Microfilariae of D. immitis, are longer and fatter than D. reconditum at 307-322 x 6.7-7.1 microns (one thousandth of a millimeter) (6).
Confirming Stage of Disease
The stage of the disease can be determined using a combination of blood tests, x-rays, and ultrasounds to confirm the extend of the damage to organs and worm burden. The more blood vessels become blocked the more likely organs are to fail. Indicators of inflammation and organ damage can be observed in blood tests, chest x-rays may show areas of increased density fluid in the lungs, and enlargement of the heart and major blood vessels such as the pulmonary vessels and the aorta and vena cava (10, 11). In cases of high worm burden, where there are lots of worms in the heart, they may be observed on an echocardiogram, a type of ultrasound used to see inside the heart. They appear as thin white lines like equal signs (2).
Risk Categories
Animals may be classed into 4 stages of risk for developing pulmonary thromboembolisms and death, depending on the amount of worms present, the age of the animal, the severity of damage to organs, and to a lesser extent the symptoms they present with. It is possible for there to be many worms in an animals heart and few noticeable symptoms, or for there to be few worms and an abundance of symptoms as symptoms are generally dependent on worm death, not number. Class I represents the lowest risk whereas class IV is the highest risk.
Treatment
As outlined by the Merck Veterinary Manual & AHS
When treating an active case of heartworm the risk level of the animal must be considered. In dogs, the only treatment for adult worms is an arsenic-based drug called melarsomine dihydrochloride, sold under the brand names Immiticide (Boehringer Ingelheim), or Diroban (Zoetis). To avoid rapid die-off and subsequent thromboembolisms the drug is administered in 2 to 3 doses injected intramuscularly. The 2-dose method involves giving 1 dose, waiting a day, and then administering the second dose. For this method 2 months of restricted exercise or cage rest is recommended to reduce risk of embolism as the worms die. In the 3-dose method, a dose is given, and after a month or so the animal is given the remaining two doses 24 hours apart. Sometimes a vet may pair the three dose or two dose method with a heartworm preventative and or doxycycline antibiotic pre-treatment to kill the microfilariae (the baby worms) to further reduce risk of injury to the heart and lungs along with steroids to reduce inflammation. A month of exercise restriction is recommended with the 3-dose method. Throughout treatment testing should be repeated to ensure the success of the treatment and monitor the animal’s physical condition. This treatment regimen is quite expensive. In some cases only for owners who have significant price limitations is it recommended for an animal to spend 12-30 months on strict rest while being treated with ivermectin and doxycycline alone. This so called “slow-kill” method is considered very, very, risky and not recommended except in extenuating circumstances.
Note: Melarsomine injection is not appropriate to be done by an owner and should only be done by a licensed veterinarian. Side effects may be noticed at the injection site and location of injection is very important.
For high risk animals that develop a condition called Caval Syndrome, where worms block the right side of the heart and traverse the tricuspid valve, an animal must be stabilized and then have the worms surgically removed. (1, 2) Damage to the heart and lungs at this point is generally extensive and an animal will need to be treated for heart failure after surgery as well as with melarsomine to kill any remaining worms once it recovers sufficiently from the surgical procedure.
Prevention
Dirofilariasis is much easier to prevent than to treat. A class of macrolide drugs, so named for their macrocyclic-lactone rings chemically, are sufficient for prevention (1,2). These include things like ivermectin, selamectin, moxidectin, and milbemycin sold under brand names like Heartgard, Nexgard Spectra, and Revolution or Revolution+ and Advantage Multi. A full list of FDA approved preventatives can be found on the American Heartworm Society Website. They are generally given monthly either topically or in pill form. Many of these drugs have the benefit of controlling or preventing other parasites as well such as fleas and or ticks, so check with your veterinarian what the best one looks like for your pet and your financial state.
In general it is recommended to begin prevention at 6-8 weeks of age, begin testing at around a year old, or if prophylaxis has not been started, test first at 7 months and then again in 6-7 months time (1). Testing should be done annually at the discretion of the veterinarian based on the regional prevalence and risk to the animals health (8). As long as there are mosquitos around, heartworm is a concern. Your veterinarian is always your best resource.
In compiling this information, the following resources were used.
- The Merck Veterinary Manual 11th Edition. pgs 127-136. & https://www.merckvetmanual.com/circulatory-system/heartworm-disease/heartworm-disease-in-dogs,-cats,-and-ferrets
- https://www.heartwormsociety.org/component/finder/search?q=prevention&Itemid=178
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4677615/ Grimes JA, Scott KD, Edwards JF. Aberrant heartworm migration to the abdominal aorta and systemic arteriolitis in a dog presenting with vomiting and hemorrhagic diarrhea. Can Vet J. 2016 Jan;57(1):76-9. PMID: 26740703; PMCID: PMC4677615.
- http://modernveterinarytherapeutics.com/canine-heartworm-antigen-test-kit/
- https://www.idexx.com/files/using-snap-test-kits-en.pdf
- Coles Veterinary Clinical Pathology 4th Edition Chapter 20 (pg 379-386).
- McGill E, Berke O, Weese JS, Peregrine A. Heartworm infection in domestic dogs in Canada, 1977-2016: Prevalence, time trend, and efficacy of prophylaxis. Can Vet J. 2019 Jun;60(6):605-612. PMID: 31156260; PMCID: PMC6515813.
- https://www.canadianveterinarians.net/related-resources/heartworm/
- https://www.up.ac.za/veterinary-tropical-diseases/news/post_2859735-diagnostics-of-filarial-infections-of-carnivores-in-the-helminthology-laboratory-of-the-department-of-veterinary-tropical-diseases
- https://vcacanada.com/know-your-pet/testing-for-heartworm-disease-in-dogs#:~:text=Radiographs%20(X%2Drays).,presumptive%20evidence%20of%20heartworm%20disease.
- https://www.frontiersin.org/articles/10.3389/fvets.2019.00440/full
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