Dirofilaria immitis

{{Short description|Species of worm that causes parasitic disease in animals}} {{Redirect|Heartworm|other uses}} {{Speciesbox | image = Heartworm.JPG | image_alt = A German Shepherd dog heart infested with "Dirofilaria immitis" | image_caption = A German Shepherd dog heart infested with ''Dirofilaria immitis'' | status = | status_system = | status_ref = | genus = Dirofilaria | species = immitis | authority = (Leidy, 1856) | synonyms =''Filaria immitis'' Leidy, 1856 | synonyms_ref =<ref>{{GBIF|taxon=''Dirofilaria immitis'' (Leidy, 1856)|id=5188737|access-date=21 August 2023}}</ref> | subdivision_ranks = | subdivision = }}

'''''Dirofilaria immitis''''', also known as '''heartworm''' or '''dog heartworm''', is a parasitic roundworm that is a type of filarial worm, a small thread-like worm, and which causes dirofilariasis. It is spread from host to host through the bites of mosquitoes. Four genera of mosquitoes transmit dirofilariasis, ''Aedes'', ''Culex'', ''Anopheles'', and ''Mansonia''.<ref>{{Cite web|title=CDC – Dirofliariasis – Biology – Life Cycle of D. immitis|url=https://www.cdc.gov/parasites/dirofilariasis/biology_d_immitis.html|author=((Global Health, Division of Parasitic Diseases))|publisher=CDC-Centers for Disease Control and Prevention, U.S. Department of Health & Human Services|date=2012-02-08|website=www.cdc.gov|language=en-us|url-status=dead|archive-url=https://web.archive.org/web/20190615084805/https://www.cdc.gov/parasites/dirofilariasis/biology_d_immitis.html|archive-date=2019-06-15}}</ref> The definitive host is the dog, but it can also infect cats, wolves, coyotes, jackals, foxes, ferrets, bears, seals, sea lions and, under rare circumstances, humans.<ref name="ahs faq">{{cite web|url=http://www.heartwormsociety.org/pet-owner-resources/faqs.html |title=American Heartworm Society | FAQs |publisher=Heartwormsociety.org |access-date=2014-07-04}}</ref>

Adult heartworms often reside in the pulmonary arterial system (lung arteries) as well as the heart, and a major health effect in the infected animal host is damage to its lung vessels and tissues.<ref name=Ettinger_2010>{{Cite book|author1=Ettinger, Stephen J. |author2=Feldman, Edward C. |title=Textbook of Veterinary Internal Medicine|edition=7th|publisher=W.B. Saunders Company|year=2010|isbn= 978-1-4160-6593-7}}</ref> Heartworm infection may result in serious complications for the infected host if left untreated, eventually leading to death, most often as a result of secondary congestive heart failure.

Heartworms go through several life stages before they become adults infecting the pulmonary artery of the host animal. The worms require the mosquito as an intermediate host to complete their lifecycles. The rate of development in the mosquito is temperature-dependent, requiring about two weeks of temperature at or above 27 °C (80 °F). Below a threshold temperature of 14 °C (57 °F), development cannot occur, and the cycle is halted.<ref>{{Cite journal|last1=Knight|first1=D. H.|last2=Lok|first2=J. B.|date=May 1998|title=Seasonality of heartworm infection and implications for chemoprophylaxis|journal=Clinical Techniques in Small Animal Practice|volume=13|issue=2|pages=77–82|doi=10.1016/S1096-2867(98)80010-8|issn=1096-2867|pmid=9753795|doi-access=free}}</ref> As a result, transmission is limited to warm weather, and duration of the transmission season varies geographically. The period between the initial infection when the dog is bitten by a mosquito and the maturation of the worms into adults living in the pulmonary arteries takes six to seven months in dogs and is known as the "prepatent period".<ref>{{cite book |last=Bowman |first=Dwight|date=2021 |title=Georgis' Parasitology for Veterinarians |url=https://www.sciencedirect.com/book/9780323543965/georgis-parasitology-for-veterinarians |location= |publisher=W. B. Saunders |page=135-260 |edition=Eleventh |isbn=978-0-323-54396-5}}</ref>

The first larval stage (L1) and second larval stage (L2) of heartworm development occurs within the body of a mosquito. Once the larvae develop into the infective third larval stage (L3), the mosquito locates and bites a host, depositing the larvae under the skin at the site of the bite. After a week or two of further growth, they molt into the fourth larval stage (L4) . Then, they migrate to the muscles of the chest and abdomen, and 45 to 60 days after infection, molt to the fifth stage (L5, immature adult). Between 75 and 120 days after infection, these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Over the next three to four months, they increase greatly in size. The female adult worm is about 30 cm in length, and the male is about 23 cm, with a coiled tail.<ref>{{cite web| last = Johnstone | first = Colin | title = Heartworm | work = Parasites and Parasitic Diseases of Domestic Animals | publisher = University of Pennsylvania | year = 1998 | url = http://cal.vet.upenn.edu/parasit/heartworm/hw_1.html | access-date = 2007-02-26 | archive-url = https://web.archive.org/web/20001206085300/http://cal.vet.upenn.edu/parasit/heartworm/hw_1.html | archive-date = 2000-12-06 }}</ref> By seven months after infection, the adult worms have mated and the females begin giving birth to live young, called microfilariae. Heartworms can live for 5 to 7 years in a dog.<ref name="heartwormsociety.org">{{Cite web|title=Heartworm Basics – American Heartworm Society|url=https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics|website=www.heartwormsociety.org|access-date=2020-05-07|archive-url=https://web.archive.org/web/20200523232358/https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics|archive-date=2020-05-23|url-status=dead}}</ref>

The microfilariae circulate in the bloodstream for as long as two years, and are ingested by bloodsucking mosquitos, where development occurs and the cycle repeats.{{citation needed|date=January 2023}}

==Clinical signs of infection in dogs== Dogs show no indication of heartworm infection during the six-month prepatent period prior to the worms' maturation, and current diagnostic tests for the presence of microfilariae or antigens cannot detect prepatent infections. Rarely, migrating heartworm larvae get "lost" and end up in aberrant sites, such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures, and lameness, but normally, until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.<ref>{{Cite web |title=Dirofilaria immitis - Learn About Parasites - Western College of Veterinary Medicine |url=https://wcvm.usask.ca/learnaboutparasites/parasites/dirofilaria-immitis.php |access-date=2024-10-02 |website=wcvm-learnaboutparasites |language=en}}</ref>

Many dogs show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially during or after exercise, and exercise intolerance. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, fainting, coughing up blood, and finally, congestive heart failure.{{citation needed|date=January 2023}}

There are four different classes of symptoms: * Class 1 – no or mild symptoms with occasional cough. * Class 2 – mild symptoms with occasional cough and tiredness after moderate activity. * Class 3 – more severe symptoms, including a generally sick appearance, persistent cough, difficulty breathing, and tiredness after mild activity. Heart and lung changes may be seen with a chest x-ray. * Class 4 – also called caval syndrome. The blood flowing back to the heart is blocked due to the large mass of worms. This is life-threatening and the only treatment option is surgery.<ref>{{Cite web|author=Center for Veterinary Medicine|date=2022-12-22|title=Keep the Worms Out of Your Pet's Heart! The Facts about Heartworm Disease|url=https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease|archive-url=https://web.archive.org/web/20190912044707/https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease|url-status=dead|archive-date=September 12, 2019|website=U.S. Food & Drug Administration|language=en|access-date=2024-06-11}}</ref>

==Role of ''Wolbachia pipientis''== ''Wolbachia pipientis'' is an intracellular bacterium that is an endosymbiont of ''D. immitis''. All heartworms are thought to be infected with ''Wolbachia'' to some degree. The inflammation occurring at the die-off of adult heartworms or larvae is in part due to the release of ''Wolbachia'' bacteria or protein into the tissues. This may be particularly significant in cats, in which the disease seems to be more related to larval death than living adult heartworms. Treating heartworm-positive animals with an antibiotic such as doxycycline to remove ''Wolbachia'' may prove to be beneficial as it does for the filariae that cause elephantiasis,<ref name="pmid12684759">{{cite journal | vauthors = Hoerauf A, Mand S, Fischer K, Kruppa T, Marfo-Debrekyei Y, Debrah AY, Pfarr KM, Adjei O, Büttner DW | display-authors = 6 | title = Doxycycline as a novel strategy against bancroftian filariasis-depletion of Wolbachia endosymbionts from Wuchereria bancrofti and stop of microfilaria production | journal = Medical Microbiology and Immunology | volume = 192 | issue = 4 | pages = 211–216 | date = November 2003 | pmid = 12684759 | doi = 10.1007/s00430-002-0174-6 | s2cid = 23349595 }}</ref> but further studies are necessary.<ref name="forum">{{Cite journal| last = Todd-Jenkins | first = Karen | title = The Role of ''Wolbachia'' in heartworm disease | journal = Veterinary Forum | volume = 24 | issue = 10 | pages = 28–30 |date=October 2007}}</ref>

==Diagnosis in dogs== thumb|Microfilaria of ''Dirofilaria immitis'' (heartworms) in a lymph node of a dog with lymphoma. This baby nematode is in a pillow of intermediate-to-large, immature lymphocytes, exhibiting multiple criteria of cancer.<ref>{{cite web|last1=Wheeler|first1=Lance|title=Photostream|url=https://www.flickr.com/gp/146435388@N03/0670TX|website=Flickr|access-date=2 December 2017|language=en-us}}</ref>

Microfilarial detection is accomplished by the using one of the following methods:

===Direct blood smear=== A blood sample is collected and viewed under the microscope. The direct smear technique allows examination of larval motion, confirming the presence of microfilaria. It also helps in the distinction of ''D. immitis'' from ''Acanthocheilonema reconditum''. This distinction is important because the presence of the latter parasite does not pose a health risk to the host. ''D. immitis'' usually has stationary body movement, while ''A. reconditum'' has progressive movement. However, this method often misses light infections because only a small amount of blood sample is used.<ref>Sirois, M. (2015) Laboratory Procedures for Veterinary Technicians. St. Louis, MO: Elsevier.</ref>

===Hematocrit tube method=== This method uses a microhematocrit (or capillary tube) filled with a blood sample that has been centrifuged, separating the plasma from the red blood cells. These layers are divided by the buffy coat. The buffy coat consists of the leukocytes and platelets that are in the sample. The tube is snapped at the buffy coat and added to a slide for microscopic examination. Adding methylene blue stain to the sample may allow greater visibility of any microfilariae. However, the hematocrit tube method will not allow for species differentiation.<ref>{{Cite web|date=2013-07-01|title=Diagnosis of Internal Parasites|url=https://todaysveterinarypractice.com/todays-technician-diagnosis-of-internal-parasites/|access-date=2021-03-13|website=Today's Veterinary Practice|language=en}}</ref>

===Modified Knott's test=== The modified Knott's test is more sensitive because it concentrates microfilariae, improving the chance of diagnosis.<ref name="Ettinger_2010" /> A blood sample is mixed with 2% formalin and centrifuged in a tube. The supernatant is removed and methylene blue stain is added to the pellet remaining in the tube for microscopic examination. It allows microfilariae species differentiation based on morphology. Microfilariae can be differentiated between ''D. immitis'' and ''Acanthocheilonema reconditum'' because of small differences in morphology. The Modified Knott's test is the best method of visual examination when determining presence of microfilaria because it preserves their morphology and size. It is easy to perform, quick, and inexpensive.<ref>{{Cite journal|last1=Magnis|first1=Johannes|last2=Lorentz|first2=Susanne|last3=Guardone|first3=Lisa|last4=Grimm|first4=Felix|last5=Magi|first5=Marta|last6=Naucke|first6=Torsten J.|last7=Deplazes|first7=Peter|date=2013-02-25|title=Morphometric analyses of canine blood microfilariae isolated by the Knott's test enables Dirofilaria immitis and D. repens species-specific and Acanthocheilonema (syn. Dipetalonema) genus-specific diagnosis|url= |journal=Parasites & Vectors|volume=6|issue=1|pages=48|doi=10.1186/1756-3305-6-48|issn=1756-3305|pmc=3598535|pmid=23442771 |doi-access=free }}</ref>

The potential for a microfilaremic infection is 5 – 67%. The number of circulating microfilariae does not correlate with the number of adult heartworms, so is not an indicator of disease severity.<ref name="Ettinger_2010" />

===Antigen testing=== In most practices, antigen testing has supplanted or supplemented microfilarial detection.{{citation needed|date=June 2024}} Combining the microfilaria and adult antigen test is most useful in dogs receiving diethylcarbamazine or no preventive (macrolides like ivermectin or moxidectin typically render the dog amicrofilaremic). Up to 1% of infected dogs are microfilaria-positive and antigen-negative.<ref name="Ettinger_2010" /> Immunodiagnostics (ELISA, lateral flow immunoassay, rapid immunomigration techniques) to detect heartworm antigen in the host's blood are now regularly used. They can detect occult infections, or infections without the presence of circulating microfilariae. However, these tests are limited in that they only detect the antigens released from the sexually mature female worm's reproductive tract. Therefore, false-negative results may occur during the first five to eight months of infection when the worms are not yet sexually mature.<ref name="Ettinger_2010" /> The specificity of these tests is close to 100%, and the sensitivity is more than 80%.<ref>{{cite web | last = Atkins | first = Clarke | title = Heartworm Disease in Dogs: An Update | work = Proceedings of the 30th World Congress of the World Small Animal Veterinary Association | year = 2005 | url = http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2005&PID=10900&O=Generic | archive-url = https://web.archive.org/web/20071024113047/http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2005&PID=10900&O=Generic | url-status = dead | archive-date = 2007-10-24 | access-date = 2007-02-26 }}</ref> A recent study demonstrated a sensitivity of only 64% for infections of only one female worm, but improved with increasing female worm burden (85%, 88%, and 89% for two, three, and four female worms, respectively). Specificity in this study was 97%.<ref name="Ettinger_2010" /> False-negative test results can be due to low worm counts, immature infections, and all-male infections.

===X-rays=== '''X-rays '''are used to evaluate the severity of the heartworm infection and develop a prognosis for the animal. Typically, the changes observed are enlargement of the main pulmonary artery, the right side of the heart, and the pulmonary arteries in the lobes of the lung. Inflammation of the lung tissue is also often observed.<ref>{{cite web|title=American Heaartworm Society|url=http://www.heartwormsociety.org/pet-owner-resources/canine-heartworm.htm|url-status=dead|archive-url=https://archive.today/20131104095619/http://www.heartwormsociety.org/pet-owner-resources/canine-heartworm.htm|archive-date=2013-11-04}}</ref>

==Treatment in dogs== If an animal is diagnosed with heartworms, treatment may be indicated. Before the worms can be treated, however, the dog's heart, liver, and kidney function must be evaluated to determine the risks of treatment. Usually, the adult worms are killed with an arsenic-based compound. The currently approved drug in the US, melarsomine, is marketed under the brand name Immiticide.<ref>{{cite web| title = Product Information| url = http://ph.merial.com/pet_owners/dogs/product_immiticide.asp| access-date = 2008-07-01| url-status = dead| archive-url = https://web.archive.org/web/20080828103952/http://ph.merial.com/pet_owners/dogs/product_immiticide.asp| archive-date = 2008-08-28}}</ref> It has a greater efficacy and fewer side effects than the previously used drug thiacetarsamide, sold as Caparsolate, which makes it a safer alternative for dogs with late-stage infections.{{citation needed|date=January 2023}}

After treatment, the dog must rest, and exercise is to be heavily reduced for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, if the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure and sudden death. According to the American Heartworm Society, the administering of aspirin to dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit, and aspirin may be contraindicated in several cases. Aspirin had previously been recommended for its effects on platelet adhesion and the reduction of vascular damage caused by the heartworms.{{citation needed|date=January 2023}}

The course of treatment is not completed until several weeks later, when the microfilariae are dealt with in a separate course of treatment. Once heartworm tests are negative and no surviving worm is detected, the treatment is considered a success, and the patient is effectively cured.{{citation needed|date=January 2023}}

Surgical removal of the adult heartworms as a form of treatment may also be indicated, especially in advanced cases with substantial heart involvement and damage.<ref>{{cite web|title=2005 Guidelines For the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Dogs |work=American Heartworm Society |year=2005 |url=http://www.heartwormsociety.org/article.asp?id=48 |access-date=2008-07-01 |archive-url=https://web.archive.org/web/20080616144710/http://www.heartwormsociety.org/article.asp?id=48 |archive-date=2008-06-16 |url-status=dead }}</ref>

==Prevention of infection in dogs== Prevention of heartworm infection can be obtained through a number of veterinary drugs. The drugs approved for use in the US are ivermectin (sold under the brand names Heartgard, Iverhart, and several other generic versions), milbemycin (Interceptor Flavor Tabs and Sentinel Flavor Tabs) and moxidectin (Simparica Trio) administered as chewable tablets. Moxidectin is also available in both a six-month and 12-month sustained-release injection, ProHeart 6 and ProHeart 12, respectively, administered by veterinarians. This injectable form of moxidectin was taken off the market in the United States due to safety concerns in 2004, but the FDA returned a newly formulated ProHeart 6 to the market in 2008. ProHeart 6 remains on the market in many other countries, including Canada and Japan. Its sister product, ProHeart 12, is used extensively in Australia and Asia as a 12-month injectable preventive. It was approved for use in the United States by the FDA in July 2019.<ref>{{Cite journal |last=Medicine|first=Center for Veterinary |date=2019-07-02 |title=FDA Approves ProHeart 12 (moxidectin) for Prevention of Heartworm Disease in Dogs |url=https://www.fda.gov/animal-veterinary/cvm-updates/fda-approves-proheart-12-moxidectin-prevention-heartworm-disease-dogs |archive-url=https://web.archive.org/web/20190912062437/https://www.fda.gov/animal-veterinary/cvm-updates/fda-approves-proheart-12-moxidectin-prevention-heartworm-disease-dogs |url-status=dead |archive-date=September 12, 2019 |journal=FDA |language=en}}</ref> Topical treatments are available, as well. Advantage Multi (imidacloprid plus moxidectin) Topical Solution, uses moxidectin for control and prevention of roundworms, hookworms, heartworms, and whipworms, as well as imidacloprid to kill adult fleas. Selamectin (Revolution) is a topical preventive likewise administered monthly, and can also be used to control fleas, ticks, and mites.{{citation needed|date=January 2023}}

Preventive drugs are highly effective, and when regularly administered, have been shown to protect more than 99% of dogs and cats from heartworm. Most compromises in protection result from the failure to properly administer the drugs during seasonal transmission periods.<ref name="Knight 1998">{{cite web |last=Knight |first=David |date=1998-05-01 |title=Heartworm |url=http://www.journals.elsevierhealth.com/periodicals/ysvms/article/PIIS1096286798800108/abstract|work=Seasonality of Heartworm Infection and Implications for Chemoprophylaxis |publisher=Topics in Companion Animal Medicine}}</ref> In regions where the temperature is consistently above 14 °C (57 °F) year-round, a continuous prevention schedule is recommended.{{citation needed|date=January 2023}}

Due to newly emerging resistant strains of heartworms, which no macrocyclic lactone (heartworm prevention) can protect against, the American Heartworm Society recommends dogs be on a repellent and a heartworm preventive. The repellent, such as Vectra 3-D, keeps mosquitoes from feeding on the dog and transmitting the L3 stage worms. If a dog is bitten, the heartworm preventive takes over when administered. If a mosquito feeds on a heartworm positive dog on a repellent, they do not live long enough for the microfilaria they ingested to molt into the infective L3 larva. Vectra 3-D was tested using thousands of mosquitoes infected with the resistant heartworm strain JYD34. In the control group that was given only a placebo, every dog contracted heartworms. In the experimental group that was given only Vectra 3-D, two of eight dogs contracted heartworms and had an average of 1.5 adult worms each. In the experimental group given both heartworm prevention and Vectra 3-D, one dog was infected with L3 stage larvae that did not mature into adulthood due to the heartworm prevention. Using a repellent and a prevention is at least 95% effective.<ref>{{cite web |url=https://fightheartwormnow.com/ |title=Fight Heartworm Now |website=Fight Heartworm Now |access-date=2018-08-17 |archive-url=https://web.archive.org/web/20180818020840/https://fightheartwormnow.com/ |archive-date=2018-08-18 |url-status=dead}}</ref><ref>{{Cite web |date=2016-10-04 |title=Yes, there's a new way to protect dogs from heartworm |url=https://goodnewsforpets.com/new-way-protect-dogs-heartworm/ |access-date=2021-03-28 |website=Goodnewsforpets |language=en-US}}</ref>

Ivermectin, even with lapses up to four months between doses, still provides 95% protection from adult worms. This period is called the reach-back effect.<ref>{{cite web |last = Atwell |first = R. |title = Heartworm |work = Dirofilariasis (CRC Press, 1988:30–34) |year = 1988 |url = http://www.northside-vet.com/pages/?coords=005,013 |access-date = 2009-05-09 |archive-url = https://web.archive.org/web/20110714195359/http://www.northside-vet.com/pages/?coords=005,013 |archive-date = 2011-07-14 |url-status = dead }}</ref> Since dogs are susceptible to heartworms, they should be tested annually before they start preventive treatment.<ref name="Paul, Mike.">Paul, Mike. 10 Things You Need to Know About Heartworm and Your Dog. Pet Health Network, 10 Sept. 2015 [cited 3 Apr 2019]. www.pethealthnetwork.com/dog-health/dog-diseases-conditions-a-z/10-things-you-need-know-about-heartworm-and-your-dog.</ref> Annual heartworm testing is highly recommended for pet owners who choose to use minimal dosing schedules. Testing a dog annually for heartworms and other internal parasites is a fundamental part of a complete heartworm prevention program, and is also recommended for dogs who are already on a monthly prevention program.<ref name="Paul, Mike." />

==Heartworm infection in cats== While dogs are a natural host for ''D. immitis'', cats are atypical hosts. Because of this, differences between canine and feline heartworm diseases are significant. The majority of heartworm larvae do not survive in cats, so unlike in dogs, a typical infection in cats is two to five worms. The lifespan of heartworms is considerably shorter in cats, only two to three years, and most infections in cats do not have circulating microfilariae. Cats are also more likely to have aberrant migration of heartworm larvae, resulting in infections in the brain or body cavities.<ref name="AHS_2007">{{cite web|title=2007 Guidelines For the Diagnosis, Treatment and Prevention of Heartworm (''Dirofilaria immitis'') Infection in Cats |publisher=American Heartworm Society |year=2007 |url=http://www.heartwormsociety.org/article.asp?id=47 |access-date=2007-07-26 |archive-url=https://web.archive.org/web/20070614130430/http://www.heartwormsociety.org/article.asp?id=47 |archive-date=2007-06-14 |url-status=dead }}</ref>

The infection rate in cats is 1–5% of that in dogs in endemic areas.<ref>{{Cite journal|vauthors=Berdoulay P, Levy JK, Snyder PS |title=Comparison of serological tests for the detection of natural heartworm infection in cats |journal=Journal of the American Animal Hospital Association |volume=40 |issue=5 |pages=376–84 |year=2004 |pmid=15347617 |doi=10.5326/0400376|display-authors=etal}}</ref> Both indoor and outdoor cats are infected. The mosquito vector is known to enter homes.<ref>{{Cite journal|vauthors=Atkins CE, DeFrancesco TC, Coats JR, Sidley JA, Keene BW |title=Heartworm infection in cats: 50 cases (1985–1997) |journal=J. Am. Vet. Med. Assoc. |volume=217 |issue=3 |pages=355–8 |year=2000 |pmid=10935039 |doi=10.2460/javma.2000.217.355|citeseerx=10.1.1.204.2733 }}</ref>

===Pathology=== The vascular disease in cats that occurs when the L5 larvae invade the pulmonary arteries is more severe than in dogs. A reaction has been identified in cats: heartworm-associated respiratory disease, which can occur three to four months after the initial infection, and is caused by the presence of the L5 larvae in the vessels. The subsequent inflammation of the pulmonary vasculature and lungs can be easily misdiagnosed as feline asthma or allergic bronchitis.<ref name="forum_246">{{Cite journal| last = Yin | first = Sophia | title = Update on heartworm infection | journal = Veterinary Forum | volume = 24 | issue = 6 | pages = 42–43 |date=June 2007}}</ref>

Obstruction of pulmonary arteries due to emboli from dying worms is more likely to be fatal in cats than dogs because of less collateral circulation and fewer vessels.<ref name="Foley_2005">{{Cite book|editor=August, John R.|chapter=Heartworm Disease|author1=Atkins, Clarke E. |author2=Litster, Annette L. |title=Consultations in Feline Internal Medicine Vol. 5|publisher=Elsevier Saunders|year=2005|isbn=978-0-7216-0423-7}}</ref> Heartworms can live for 2 to 3 years in cats.<ref name="heartwormsociety.org" />

===Signs and symptoms=== Acute heartworm disease in cats can result in shock, vomiting, diarrhea, fainting, and sudden death. Chronic infection can cause loss of appetite, weight loss, lethargy, exercise intolerance, coughing, and difficulty breathing. Some cats' immune systems are able to clear a heartworm infection, though the immune system response can cause many of the same symptoms. Also, even if the infection resolves, respiratory damage can cause some symptoms to persist beyond it.<ref name="forum_246" />{{request quotation |date=June 2022}}<ref>{{cite web |title=The Myths and Facts of Heartworm Disease in Cats |website=WhitesburgAnimalHospital.com |last=Reno LVT |first=Tabitha |url=http://www.whitesburganimalhospital.com/blog/heartworm-disease-cats/ |url-status=dead |archive-url=https://web.archive.org/web/20210508044925/http://www.whitesburganimalhospital.com/blog/heartworm-disease-cats/ |archive-date=2021-05-08 |date=2017-05-05 |access-date=2022-06-07}}</ref>

===Diagnosis=== Diagnosis of heartworm infection in cats is problematic. Like in dogs, a positive ELISA test for heartworm antigen is a very strong indication of infection. However, the likelihood of a positive antigen test depends on the number of adult female worms present. If only male worms are present, the test will be negative. Even with female worms, an antigen test usually only becomes positive seven to eight months after infection. Therefore, a cat may have significant clinical signs long before the development of a positive test. Heartworm-associated respiratory disease can be found in cats that never develop adult heartworms and therefore never have a positive antigen test.{{citation needed|date=January 2023}}

An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to ''D. immitis'', so a cat that has successfully eliminated an infection may still be positive for up to three months. The antibody test is more sensitive than the antigen test, but it does not provide direct evidence of adult infection.<ref>{{Cite journal|author=Atkins C |title=The diagnosis of feline heartworm infection |journal=Journal of the American Animal Hospital Association |volume=35 |issue=3 |pages=185–7 |year=1999 |pmid=10333254 |doi=10.5326/15473317-35-3-185}}</ref> It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for heartworm-associated respiratory disease.

X-rays of the chest of a heartworm-infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries.<ref>{{Cite journal|vauthors=DeFrancesco TC, Atkins CE, Miller MW, Meurs KM, Keene BW |title=Use of echocardiography for the diagnosis of heartworm disease in cats: 43 cases (1985–1997) |journal=J. Am. Vet. Med. Assoc. |volume=218 |issue=1 |pages=66–9 |year=2001 |pmid=11149717 |doi=10.2460/javma.2001.218.66}}</ref>

===Treatment and prevention=== [[File:Parasite140076-fig1 Dirofilaria repens removed from a subcutaneous nodule - Photos.png|thumb|Comparison of probable ''Dirofilaria repens'' (left column, from human) and ''D. immitis'' (right column, from dog) ]]

Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) topical solution. Ivermectin, milbemycin, and selamectin are approved for use in cats in the US.{{citation needed |date=June 2022}}

Arsenic compounds have been used for heartworm adulticide treatment in cats, as well as dogs, but seem more likely to cause pulmonary reactions. A significant number of cats develop pulmonary embolisms a few days after treatment. The effects of melarsomine are poorly studied in cats. Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended, and no drugs are approved in the US for this purpose in cats.<ref name="Foley_2005" />

Treatment typically consists of putting the cat on a monthly heartworm preventive and a short-term corticosteroid.<ref name="AHS_2007" /> Surgery has also been used successfully to remove adult worms. The prognosis for feline heartworm disease is guarded.<ref>{{Cite journal |last1=Garrity |first1=Sarah |last2=Lee-Fowler |first2=Tekla |last3=Reinero |first3=Carol |date=September 2019 |title=Feline asthma and heartworm disease: Clinical features, diagnostics and therapeutics |journal=Journal of Feline Medicine and Surgery |volume=21 |issue=9 |pages=825–834 |doi=10.1177/1098612X18823348 |issn=1532-2750 |pmid=31446863|pmc=10814146 }}</ref>{{clarification needed |date=June 2022}}{{citation needed |date=June 2022}}

==Heartworm infection in humans== Dirofilaria are important medical parasites, but diagnosis is unusual and is often only made after an infected person happens to have a chest X-ray following granuloma formation in the lung. The nodule itself may be large enough to resemble lung cancer on the X-ray, and requires a biopsy for a pathologic assessment.<ref name="TumolskayaPozio2016" /> This has been shown to be the most significant medical consequence of human infection by the canine heartworm. Patients are infected with the parasite through the bite of an infected mosquito, which is the same mechanism that causes heartworm infection in dogs.<ref>{{Cite web |title=Heartworm Basics |url=https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics?highlight=WyJ3aGF0Iiwid2hhdCdzIiwiaXMiLCJoZWFydHdvcm0iLCJ3aGF0IGlzIiwid2hhdCBpcyBoZWFydHdvcm0iLCJpcyBoZWFydHdvcm0iXQ== |access-date=2024-10-02 |website=American Heartworm Society |language=en-gb}}</ref>{{medical citation needed|date=July 2023}}

''D. immitis'' is one of many species that can cause infection in dogs and humans. It was thought to infect the human eye,<ref name="TumolskayaPozio2016" /> with most cases reported from the southeastern United States. However, these cases are now thought to be caused by a closely related parasite of raccoons, ''Dirofilaria tenuis''. Several hundred cases of subcutaneous infections in humans have been reported in Europe, but these are almost always caused by another closely related parasite, ''Dirofilaria repens'', rather than the dog heartworm. There are proven ''D. immitis'' infections,<ref name="TumolskayaPozio2016" /> but humans rarely get infected with heartworms due to the larvae never fully maturing.{{medical citation needed|date=July 2023}} When the heartworm infective larvae migrate through the skin, they often die since heartworms cannot survive in a human host, even if they make it into the bloodstream. Once the heartworms die, the immune system in the human body reacts to their tissue with inflammation as it tries to destroy the heartworms.{{medical citation needed|date=July 2023}} When this happens, the condition is called pulmonary dirofilariasis.

==Distribution and epidemiology== Although at one time confined to the southern United States, heartworm has now spread to nearly all locations where its mosquito vector is found. In the southeast region of the United States, veterinary clinics saw an average of more than 100 cases of heartworm each in 2016.<ref>Heartworm Incidence Maps. American Heartworm Society. 1970 Apr 10 [cited 2019 Apr 16]. https://www.heartwormsociety.org/pet-owner-resources/incidence-maps {{Webarchive|url=https://web.archive.org/web/20190423021800/https://www.heartwormsociety.org/pet-owner-resources/incidence-maps|date=2019-04-23}}.</ref> Transmission of the parasite occurs in all of the United States (cases have even been reported in Alaska), and the warmer regions of Canada. The highest infection rates are found within {{convert|150|mi|km}} of the coast from Texas to New Jersey, and along the Mississippi River and its major tributaries.<ref name="Ettinger_2010" /> It has also been found in South America,<ref>{{Cite journal |vauthors=Vezzani D, Carbajo A |year=2006 |title=Spatial and temporal transmission risk of ''Dirofilaria immitis'' in Argentina |journal=Int J Parasitol |volume=36 |issue=14 |pages=1463–72 |doi=10.1016/j.ijpara.2006.08.012 |pmid=17027990|hdl=11336/244927 |hdl-access=free }}</ref> southern Europe,<ref name="Merck">{{cite web |year=2006 |title=Heartworm Disease: Introduction |url=http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/11300.htm |access-date=2007-02-26 |work=The Merck Veterinary Manual /}}</ref><ref name="VieiraVieira2014">{{cite journal |last1=Vieira |first1=Ana Luísa |last2=Vieira |first2=Maria João |last3=Oliveira |first3=João Manuel |last4=Simões |first4=Ana Rita |last5=Diez-Baños |first5=Pablo |last6=Gestal |first6=Juan |year=2014 |title=Prevalence of canine heartworm (''Dirofilaria immitis'') disease in dogs of central Portugal |url= |journal=Parasite |volume=21 |pages=5 |doi=10.1051/parasite/2014003 |issn=1776-1042 |pmc=3927308 |pmid=24534524}} {{open access}}</ref> Southeast Asia,<ref>{{cite web |last=Nithiuthai |first=Suwannee |year=2003 |title=Risk of Canine Heartworm Infection in Thailand |url=http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2003&PID=6702&O=Generic |access-date=2007-02-26 |work=Proceedings of the 28th World Congress of the World Small Animal Veterinary Association}}</ref> the Middle East,<ref>{{cite web |last=Rafiee |first=Mashhady |year=2005 |title=Study of Prevalence of ''Dirofilaria immitis'' Infestation in Dogs were Examined in Veterinary Clinics of Tabriz Azad University (Iran) during 1992–2002 |url=http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2005&PID=11172&O=Generic |access-date=2007-02-26 |work=Proceedings of the 30th World Congress of the World Small Animal Veterinary Association}}</ref> Australia, Korea, and Japan.<ref name="Ettinger_1995">{{Cite book |author1=Ettinger, Stephen J. |title=Textbook of Veterinary Internal Medicine |author2=Feldman, Edward C. |publisher=W.B. Saunders Company |year=1995 |isbn=978-0-7216-6795-9 |edition=4th}}</ref><ref>{{Cite journal |last1=Oi |first1=M. |last2=Yoshikawa |first2=S. |last3=Ichikawa |first3=Y. |last4=Nakagaki |first4=K. |last5=Matsumoto |first5=J. |last6=Nogami |first6=S. |year=2014 |title=Prevalence of ''Dirofilaria immitis'' among shelter dogs in Tokyo, Japan, after a decade: comparison of 1999–2001 and 2009–2011 |journal=Parasite |volume=21 |page=10 |doi=10.1051/parasite/2014008 |pmc=3937804 |pmid=24581552}}</ref>

=== Influence of climate change === {{See also|Climate change and infectious diseases}}Warming temperatures caused by climate change will allow mosquitoes that spread the disease to appear in new areas.<ref>{{cite journal |vauthors=Lacetera N |date=January 2019 |title=Impact of climate change on animal health and welfare |journal=Animal Frontiers |volume=9 |issue=1 |pages=26–31 |doi=10.1093/af/vfy030 |pmc=6951873 |pmid=32002236 |doi-access=free}}</ref> Heartworm larvae also need temperatures above 14°C for maturation within mosquitos. As climate change increases the temperature, heartworm will be able to expand the geographical range where it is able to mature and transmit.<ref>{{Cite journal |last=Atkinson |first=Peter Jonathon |last2=Nielsen |first2=Torben Dahl |last3=Caraguel |first3=Charles |date=15 December 2025 |title=Historical and Projected Impact of Global Climate Change on the Extrinsic Incubation of Dirofilaria immitis |url=https://onlinelibrary.wiley.com/doi/10.1002/ece3.72525 |journal=Ecology and Evolution |language=en |volume=15 |issue=12 |doi=10.1002/ece3.72525 |issn=2045-7758 |pmc=12705338 |pmid=41409073}}</ref>

=== Hosts === Hosts of ''Dirofilaria immitis'' include:<ref name="ahs faq" /> {{div col|colwidth=20em}} * Dog * Cat * Jackal * Wolf * Coyote * Fox * Ferret * Raccoon * Bear * Sea lion * African leopard<ref>{{cite journal | last1 = Mazzariol | first1 = S. | last2 = Cassini | first2 = R. | last3 = Voltan | first3 = L. | last4 = Aresu | first4 = L. | last5 = di Regalbono | first5 = A. F. | year = 2010 | title = Heartworm (''Dirofilaria immitis'') infection in a leopard (''Panthera pardus pardus'') housed in a zoological park in north-eastern Italy | journal = Parasites & Vectors | volume = 3 | issue = 1| page = 25 | doi = 10.1186/1756-3305-3-25 | pmid = 20377859 | pmc = 2858128 | doi-access = free }}</ref> * Human (rare)<ref name="TumolskayaPozio2016">{{cite journal|last1=Tumolskaya|first1=Nelli Ignatievna|last2=Pozio|first2=Edoardo|last3=Rakova|first3=Vera Mikhaylovna|last4=Supriaga|first4=Valentina Georgievna|last5=Sergiev|first5=Vladimir Petrovich|last6=Morozov|first6=Evgeny Nikolaevich|last7=Morozova|first7=Lola Farmonovna|last8=Rezza|first8=Giovanni|last9=Litvinov|first9=Serguei Kirillovich|title=''Dirofilaria immitis'' in a child from the Russian Federation|journal=Parasite|volume=23|year=2016|pages=37|issn=1776-1042|doi=10.1051/parasite/2016037|pmid=27600944|pmc=5018928|url=}} {{open access}}</ref> * Beaver * Reptiles {{div col end}}

Reservoir hosts for ''D. immitis'' are coyotes and stray dogs.<ref name="Paul, Mike." />

==References== {{Reflist|2}}

== Further reading == * {{cite journal | last1 = Traversa | first1 = D. | last2 = Di Cesare | first2 = A. | last3 = Conboy | first3 = G. | year = 2010 | title = Canine and feline cardiopulmonary parasitic nematodes in Europe: emerging and underestimated | journal = Parasites & Vectors | volume = 3 | page = 62 | doi = 10.1186/1756-3305-3-62 | pmid = 20653938 | pmc = 2923136 | doi-access = free }}

==External links== {{Commons category|Dirofilaria immitis}} * [http://www.heartwormsociety.org American Heartworm Society] Founded in 1974, the American Heartworm Society is internationally recognized as the definitive authority with respect to heartworm disease in dogs and cats. * [http://www.veterinarypartner.com/Content.plx?A=595 Preventing Heartworm Infection in Dogs] (VeterinaryPartner.com) * [https://web.archive.org/web/20151119044358/http://www.metapathogen.com/heartworm/ Overview and main concepts of ''Dirofilaria immitis'' (heartworm) infection] (MetaPathogen.com) * [http://edis.ifas.ufl.edu/MG100 Mosquito-borne Dog Heartworm Disease] (University of Florida Extension Bulletin) * [https://web.archive.org/web/20070102174236/http://www.vmth.ucdavis.edu/cardio/cases/case27/case27.htm Case Study of Canine Heartworm Disease] (from the University of California, Davis) * [https://web.archive.org/web/20070102160337/http://www.vmth.ucdavis.edu/cardio/cases/case31/case31.htm Case Study of Feline Heartworm Disease] (from the University of California, Davis) * [https://web.archive.org/web/20070102174257/http://www.vmth.ucdavis.edu/cardio/cases/case33/case33.htm Case Study of Canine Heartworm Disease] (from the University of California, Davis)

{{DEFAULTSORT:Dirofilaria Immitis}} Category:Spirurida Category:Parasitic nematodes of mammals Category:Parasites of dogs Category:Parasites of cats Category:Parasitic nematodes of humans Category:Veterinary helminthology Category:Zoonotic parasitic diseases Category:Nematodes described in 1856