Dirofilaria immitis, the dog heartworm, is a parasitic roundworm that affects dogs and other canids (foxes, wolves, coyotes, etc.) and less frequently also cats. Very occasionally humans can be infected as well.

Heartworm microfilaria in blood under the microscope. Picture from Wikipedia Commons

Dirofilaria immitis is found worldwide in regions with a mild and warm climate: the Americas (up to South Canada), Europe (especially around the Mediterranean), Southeast Asia and Middles East, Japan, Australia, Africa, etc.  Since it is transmitted by mosquitoes, heartworm prevalence follows that of mosquitoes. In colder regions mosquitoes disappear in winter and thrive between late spring and early autumn, and this is also the season for heartworm infections in such regions.

Incidence in dogs some is regions is very high (e.g. southern US, the Po valley in Italy, parts of Australia, etc.): up to 60% of the dogs can be infected in such regions. Infected dogs can carry hundreds of worms.

Heartworm infection is much less frequent in cats than in dogs. In the US it is estimated that only 1 to 5% of all the heartworm in pets concern cats.

Dirofilaria repens is a related species that affects dogs in southern Europe and North Africa.

In the last decades heartworm incidence in dogs and cats has generally increased and spread into regions where it was uncommon. This seems to be related to the global warming that extends the mosquito range and prolongs its season, but also to the increased human mobility, which makes it more likely for pets living in cold regions to become infected during a stay in warmer regions, where their owners take them for holidays.

The disease caused by Dirofilaria worms is called dirofilariasis.

Are dogs or cats infected with Dirofilaria heartworms contagious for humans?

  • NO. The reason is that heartworms are transmitted through mosquito bites, not through direct contact between animals and/or humans (hair coat, feces, body fluids, etc.). In addition, human infections with Dirofilaria heartworms are extremely rare. For additional information read the chapter on the life cycle below.

These worms do not affect cattle, sheep, goats, pigs, horses or poultry

You can find additional information in this site on the general biology of parasitic worms and/or roundworms.


Final location of Dirofilaria heartworms

Predilection sites of adult Dirofilaria heartworms are the pulmonary arteries and the right ventricle of the heart. Larvae can be found in the skin, in some muscles and in the blood.


Anatomy of Dirofilaria heartworms

Adult Dirofilaria immitis are rather big worms: they can be up to 30 cm long and 1.5 mm thick, whereby males are smaller than females. Larvae (called microfilariae) are no longer than 0.5 mm. The worm's body is covered with a smooth cuticle, which is flexible but rather tough. The worms have no external signs of segmentation. They have a tubular digestive system with two openings, the mouth and the anus. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. The female ovaries are large and the uteri end in an opening called the vulva. Males have simple chitinous spicules for attaching to the female during copulation.

Dirofilaria females do not lay eggs, but releases already hatched L1 larvae, i.e. they are viviparous.


Life cycle and biology of Dirofilaria heartworms

Mosquito female (Aedes spp). Picture from Jarmo Holopainen taken at www.pbase.com/holopain/flies

Dirofilaria heartworms have an indirect life cycle with dogs, cats and other canids as final hosts, and mosquitoes of various genera (e.g. Aedes, Anopheles, Culex) as intermediate hosts.

Mosquitoes become infected with microfilariae when they bite an infected host and suck its contaminated blood. Inside mosquitoes microfilariae develop to infective L3-larvae in about 2 weeks if the temperature is >27°C. At lower temperatures it takes longer, and below 14°C they do not develop at all. These L3 larvae get into the salivary gland of the infected mosquito and wait until it bites a host. This is one of the reasons for the higher incidence of heartworms during the hot summer months.

Inside the hosts, L3 larvae remain a few weeks under the skin and molt to L4 larvae. Subsequently they migrate to the thoracic and abdominal muscles and molt to L5-larvae (immature adults) in 6 to 9 weeks after infection. Afterwards they get into the blood stream and move to the pulmonary arteries, their predilection site, where they arrive 12 to 20 weeks after infection. There they grow quickly and complete development to adults in 2 to 3 months. If there are many worms they occupy the right ventricle of the heart as well. Adult worms can live for up to 7 years in their host. After mating, female worms release microfilariae into the host's blood, which can survive for up to 3 years waiting to be picked by a mosquito bite and continue development inside the mosquito. Very occasionally migrating larvae do not reach the pulmonary artery and end elsewhere (e.g. in the abdominal cavity, in the brain, in the eyes, etc.).

In cats, many of the microfilariae injected with the mosquito bite do not survive. This explains why rather few worms (usually 2 to 6) are found in infected cats. The life span of heartworms in cats is 2 to 3 years, significantly shorted than in dogs. It is also usual not to find microfilariae in the blood of infect cats, and abnormal migration to the abdominal cavity or the brain are more frequent in cats than in dogs.


Harm caused by Dirofilaria heartworms, symptoms and diagnosis

In dogs, infections with a few heartworms are usually rather benign, often without clinical signs. Lack of appetite, weight loss and apathy, difficult breathing, cough and fatigue upon exercise, and fluid in the abdomen (ascites) can occur.

Massive infections can be much more harmful. Mechanical irritation of the arterial wall can cause swellings (edemas) that favor deposition of thrombotic material. Too many worms can block the pulmonary arteries. Dead worms or their remains can block pulmonary vessels and cause acute local inflammations. The chronic narrowing of the arteries increases the resistance to blood flow, which can cause a condition called cor pulmonale, a hyperthropy of the right ventricle, often followed by right ventricular insufficiency, sometimes fatal. Massive infections can also cause a serious condition called caval syndrome due to obstruction of the veins that bring blood to the heart.

Symptoms in cats are similar to those in dogs. Acute cases show mainly respiratory signs.

Diagnosis is not trivial in dogs or cats. Microfilariae can be detected by microscopic examination of blood samples, better after centrifugation or filtration. However, false negatives are possible, because female worms do not release larvae continuously, or there may not be adult female worms in the host. 

Immunoassays are also available (e.g. ELISA) that detect specific antigens produce by heartworms. However, these assays are often based on uterine proteins of female worms, which can result in false negatives if such adult females are not in the host. Other assays are based on specific antibodies produced by the host as a reaction to the heartworms. They are capable of detecting a single male heartworm. This assay is the one most used in cats. Unfortunately, this assay can also produce false positives

Human heartworm infections are very infrequent, usually benign and without clinical signs. In most cases microfilariae do not complete development to adult worms and are sooner or later killed by the human immune system.


Prevention and control of Dirofilaria infections

In endemic regions, i.e. where it is known that this parasite is regularly present, basic heartworm prevention includes three major elements that have to be adequately combined by an experienced veterinary doctor:

  • Periodic diagnostic tests to find out whether the pet is infected or not
  • Administration of an adequate heartworm preventative
  • Protection against mosquitoes, which can bite the pets anytime, also indoors in urban environments.

FIRST WARNING! Heartworm preventatives stop development of microfilariae to adult worms but do not cure infections with adult worms. These preventative medicines are different from those curative anthelmintics that kill the adult worms. Preventatives may kill a few adult worms, but won't kill all of them. If this happens, such dead worms may block lung vessels, which can be seriously harmful, even fatal for the pet. Consequently, heartworm preventatives are usually not administered to pets that are already infected with adult worms (hence the periodic diagnostic tests), because the risk of serious complications is real. The infection has first to be treated with adequate curative anthelmintics before preventative products are administered. This is however not trivial, and also risky for the same reason.

Most heartworm preventatives contain macrocyclic lactones at a dose that kills microfilariae and ensures adequate protection for about 1 month, i.e. treatment has to be repeated monthly. In endemic regions with mild to warm climate it is recommended to treat the pets during the whole year, because mosquitoes can be infective the whole year through.

Today there are a lot of heartworm preventatives for monthly administration. The reason is that it is a very lucrative business for Animal Health companies, and all want to have their own brand. In fact, heartworm prevention is probably the largest veterinary anthelmintic sub-market worldwide. All began in the late 1980's with the discovery and introduction of ivermectin, the first macrocyclic lactone. Before, prevention required daily treatments, mostly with diethylcarbamazine, a piperazine derivartive still available today as a cheap alternative to the monthly preventatives. After the introduction of ivermectin numerous other original active ingredients as well as generics have been introduced. In addition, numerous mixtures with other active ingredients extend the spectrum of activity to simultaneously control other pet parasites: tapeworms, fleas, mites, ticks, lice, etc.

The most common brands found worldwide are the following ones:

Efficacy af all these brands as heartworm preventatives is usually excellent. The choice is often based on the need to simultaneously protect the pets against other parasites, or on the preferred delivery form (tablet, spot-on, injectable).

Other macrocyclic lactones (abamectindoramectin and eprinomectin) used in livestock are not approved for use on pets in most countries.

Most generic brands approved as heartworm preventatives contain generic ivermectin, but there are also a few ones with other generic active ingredients.

SECOND WARNING! All these heartworm preventatives contain macrocyclic lactones, (e.g. ivermectin, milbemycin oxime, moxidectin, selamectin) which must be handled very carefully on dogs. The reason is that dogs of some breeds do not tolerate macrocyclic lactones or other medicines (e.g. emodepside) that can cross the blood-brain barrier. They can suffer more or less serious adverse effects if treated at dose rates slightly higher than the recommended ones. Consequently dosing must be as accurate as possible. This is the case for Collies and related breeds, which have a mutation in the MDR-1 gene that affects the blood-brain barrier and makes it more permeable to such compounds than in dogs without this mutation. Besides Collies, other dog breeds have shown similar problems, although the MDR-1 mutation has not been confirmed in all of them. The breeds more affected by this mutation are (% frequency): Collie (70%), Long-haired Whippet (65%), Australian Shepherd (50%, also mini),  McNab (30%), Silken Windhound (30%), English Shepherd (15%), Shetland Sheepdog (15%), English Shepherd (15%), German Shepherd (10%), Herding Breed Cross (10%). Other less affected breeds are: Old English Sheepdog, Border Collie, Berger Blanc Suisse, Bobtail, Wäller. The only way to be sure that a dog is affected or not, is to test for it. As more dogs are tested it is likely that the mutation is discovered in other breeds, or that the frequencies change.

There are very few anthelmintics approved for the control of adult heartworms in dogs and cats. Most used is melarsomine dihydrochloride (IMMITICIDE). Its use is rather complex and exceeds the purpose of this site. 

The bottom line regarding heartworm prevention and control in dogs and cats is that it has to be supervised always by an experienced veterinary doctor.

There are so far no true vaccines against Dirofilaria heartworms. To learn more about vaccines against parasites of livestock and pets click here.

Direct biological control of Dirofilaria heartworms (i.e. using its natural enemies) is so far not feasible. An indirect option is the biological control of mosquitoes. However, this is not something a single person can achieve, but needs a global regional approach by governments or municipalities.

You may be interested in an article in this site on medicinal plants against external and internal parasites.  


Resistance of Dirofilaria heartworms to anthelmintics

There are reports on resistance of Dirofilaria heartworms to macrocyclic lactones (ivermectin, milbemycin oxime, moxidectin, selamectin etc.) in the USA, reported particularly in Louisiana. In this region, this means that if a heartworm preventative fails to achieve the expected efficacy, chance is real that it is due to resistance. Elsewhere if a heartworm preventative fails to achieve the expected efficacy, chance is very high that either the product was unsuited for the control of Dirofilaria heartworms, or it was used incorrectly. Considering the massive use of these chemical class worldwide for heartworm prevention, it wouldn't be surprising that resistance emerges in other regions in the next years.

Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Dirofilaria control.

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