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Heartworm is a parasitic roundworm (Dirofilaria immitis) that is spread from host to host through the bites of mosquitoes. The heartworm affects dogs, cats, wolves, coyotes, foxes, and some other animals, such as ferrets, sea lions, and even humans. The parasitic worm is called a "heartworm" because the parasite, in the final reproductive stage of its life cycle, resides in the heart of its host where it can stay for many years, until it kills its host through congestive failure of the heart.

History of the disease

Heartworm was first discovered in dogs over a century ago and documented in cats by the 1920s. Since then, diagnostic tests and treatments for heartworm as well as preventative measures have been developed. Heartworm infestation may be extremely serious for the infected host; infected dogs that go untreated can die and even treated dogs must go through a long period of uncomfortable treatment (sometimes requiring surgery) to kill the worms and remove them from the body. The best defense against heartworm is the use of prophylactic treatment given regularly during the mosquito season.

A course of heartworm prevention begins with a blood test to see if the parasite is present. If the dog is parasite free, a prophylactic medication can be used to prevent heartworm infection. A positive test result, on the other hand, usually requires treatment to eradicate the worms.

Heartworm in North America

Although at one time confined to more southern climates, heartworm has now spread to nearly all climates where its vector, the mosquito, occurs (a vector is an intermediate host for the developing parasite, that spreads the disease from host to host). Transmission of the parasite from dog to dog occurs in all of the United States (except Alaska) and the warmer regions of Canada. The highest infection rates in North America occur in dogs within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries. However, high rates of infections are found in any area with large mosquito populations.

The parasite


Heartworms go through several life stages before they become adults infesting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle and so at least two animal hosts other than the mosquito are required for the heartworm to reproduce. A mosquitoe ingests heartworm larvae, called microfilariae, from an infected host. The mosquitoe then transfers the larvae to another uninfected host when next it feeds. The microfilariae then go through several changes to reach adult form, eventually traveling to the right side of the heart to reproduce. Reproduction results in the dispersal of microfilariae into the bloodstream where ingestion by another feeding mosquito spreads the microfilariae to another host.

At this stage, the host dog will likely be asymptomatic. Once the infestation reaches a certain concentration in the lungs, the now adult worms migrate from the pulmonary artery to the right side of the heart and begin to reproduce in ernest, filling the blood with microfilariae. At this point the host will begin to show symptoms of infestation. These symptoms can manifest earlier or increase in severity depending on the activity level of the animals as infestation reduces cardio-pulmonary capacity. Very active animals may experience symptoms at lower heartworm concentrations and have more severe symptoms than less active animals.

Heartworms can reach up to 12 inches in length and can remain in the host's heart for several years.

Course of infestation

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 heart takes some 6.5 to 7 months in dogs and is known as the prepatent period.

Heartworms bear live young, known as microfilariae, producing thousands of them every day. The microfilariae then circulate in the bloodstream for as long as two years, waiting for the next stage in their life cycle in the gut of a bloodsucking mosquito. When ingested by a mosquito, the microfilariae undergo a series of molts to the infective, or third, larval stage and then migrate to the head of the mosquito, where they wait to infect another host. These changes can occur in as little as two weeks and as long as six weeks, depending on the warmth of the climate, and generally cease entirely at ambient temperatures below 14 Celsius (57 Farenheit).

After infection, the third stage larval heartworms deposited by the mosquito grow for a week or two and molt to the fourth larval stage under the skin at the site of the mosquito bite. Then they migrate to the muscles of the chest and abdomen and, some 45 to 60 days after infection, molt to the fifth stage (immature adult). Between 75 and 120 days after infection these immature heartworms then enter the bloodstream and are carried through the heart to lodge in the arteries of the lungs. Over the next 3 to 4 months they increase greatly in size, growing backwards until they fill the right atrium and ventricle of the heart. By approximately 6.5 to 7 months after infection the adult worms have mated and the females begin producing microfilariae.

Symptoms of infestation

Dogs show no indication of heartworm infestation during the 6 month long 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 unusual sites such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures and lameness.

Many dogs will show little or no sign of infection even after the heartworms have matured. To some degree these dogs may be described as seeming to age slightly faster than normal as the worms slowly damage the lungs, kidneys and liver. These animals usually have a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections will quickly show the classic symptoms of heartworm disease. Early symptoms include a cough, especially on exercise, and early exhaustion upon exercise. More advanced cases progress to severe weight loss, fainting, coughing up blood, and, finally, congestive heart failure.


Heartworm is present on every continent except Antarctica, where the mosquito vector is noticeably absent. The presence of heartworm in a geographic region is dependent on the following factors:

  • susceptibility of host population
  • stability of the disease reservoir
  • population stability of vector species
  • proper climate conditions

Dogs are considered the definitive susceptible host for the parasite. Untreated dogs also provide a stable disease reservoir. (Cats, on the other hand, are considered a resistant host and a poor disease reservoir. However, cats are more difficult to treat and so prevention is even more critical with felines.) Mosquitoes of several different species are the vectors. Development of the microfilariae in the mosquito ideally requires a temperature at or above 80 Fahrenheit for about two weeks. No larval development takes place in the mosquito below 57 F.


Heartworms can be detected by blood test. The filtration test finds microfilariae in the blood; the occult tests (antigen and antibody) are used to detect adult worms. Many veterinarians prefer to do both tests, since the absence of microfilariae in the blood does not necessarily mean that there are no adult worms in the heart. Both tests are done with a single blood draw, preferably in the early spring before daily temperatures warm above 57 F.

X-ray radiographs and, to a lesser extent, ultrasound can also detect the presence of adult heartworms in the heart and lungs. X-rays also can provide a good estimate of the amount of lung damage caused by the presence of heartworms.


If either a blood test or the onset of symptoms betray the presence of heartworms, treatment is indicated. Treatment is highly efficacious if the disease is diagnosed early in the disease process. Before the worms can be treated, however, the dog must be evaluated for good heart, liver, and kidney function to ensure the animal can survive the treatment. Any insufficiencies in these organs must be dealt with first, before treatment, as the eradication process can be taxing on organ function. Usually the adult worms are killed with an arsenic-based compound. The currently recommended compound, Melarsomine dihydrochloride, is marketed under the brand name Immiticide. It has a greater efficacy and fewer side effects than previous formulation (Thiacetarsamide sodium, sold as Caparsolate) which makes it a safer alternative for dogs with late-stage infestations.

After treatment, the dog must rest (restricted exercise) for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, when the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure and death. Aspirin can be used during this time to help prevent further arterial damage and thromboembolism.

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 come back negative, the treatment is considered a success.

Surgical removal of the adult heartworms is also a treatment that may be indicated, especially in advanced cases with substantial heart involvement.

Long term monthly administration of ivermectin (but apparently not moxidectin, milbemycin or selamectin) year round for at least three years at the dose normally used for heartworm prevention (see "Prevention") also removes most adult heartworms from most dogs. However, this is not the treatment of choice for removal of adult heartworms for two reasons. First, not all dogs are completely cleared of heartworms by this treatment. More importantly, adult heartworms do not begin to die until some 18 months of treatment have elapsed, which is not acceptable under most circumstances. This treatment is normally reserved for dogs that are not likely to tolerate treatment with the harsher, but more effective, melarsomine or instances where the owner cannot afford the more expensive melarsomine treatment.

From time to time various "homeopathic," "natural" or "organic" products are touted as cures or preventives for heartworm disease. However, such products have never been proven effective by rigorous scientific methods, and the claims should be viewed with skepticism.


Prevention of heartworm infection can be obtained through a number of veterinary drugs. Most popular are ivermectin (sold under the brand name Heartgard), milbemycin (Interceptor) and moxidectin (ProHeart) administered as pills or chewable tablets. These drugs are given monthly during the local mosquito season. Moxidectin is also available in a six-month sustained release injection administered by veterinarians, but the injectable form of Moxidectin was taken off the market in the United States due to safety concerns. Selamectin (Revolution), on the other hand, is a topical preventive that is likewise administered monthly. Some of these drugs also kill other parasites, including intestinal worms. In addition, Selamectin controls fleas, ticks, and mites.

Preventative drugs are highly effective, and when regularly administered will protect more than 99 percent of dogs from infection. Most failures of protection result from irregular and infrequent administration of the drug. However, the monthly preventives all have a reasonable margin for error in their administration such that if a single month's dose is accidentally missed, adequate protection is usually provided so long as the next two monthly doses are administered on schedule.

Cats may be treated with ivermectin (Heartgard for Cats), or the topical selamectin (Revolution for Cats).

Monthly heartworm prevention should be administered beginning within a month of the onset of the local mosquito season and continued for a month after the cessation of local mosquito activity. In warm climates, such as the warm temperate climate along the immediate Gulf coast of the United States and in tropical and subtropical regions, heartworm prevention must be administered year round. Some authorities recommend year round administration even in colder climates on the theory that mosquito activity may occur during the occasional unseasonable warm spell, but others argue that computer models indicate heartworm transmission is highly unlikely under such circumstances.

Human health considerations

The dog heartworm is a negligible public health risk, because it is unusual for humans to become infected. Additionally, human infections usually are of little or no consequence, although rarely an infected human may show signs of respiratory disease. In most cases, however, the heartworm dies shortly after arriving in the human lung, and a nodule, known as a granuloma, forms around the dead worm as it is being killed and absorbed. If an infected person happens to have a chest X-ray at that time, the granuloma may resemble lung cancer on the X-ray and require a biopsy to rule out the life threatening condition. This may well be the most significant medical consequence of human infection by the dog heartworm.

At one time it was thought that the dog heartworm infected the human eye, with most cases reported from the southeastern United States. However, these cases are now known to be caused by a closely-related parasite of raccoons, Dirofilaria tenuis. Several hundred cases of subcutaneous (under the skin) 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.

Resources and external links

  • 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.

American Heartworm Society Information for Pet Owners:

American Heartworm Society Information for Veterinarians. (Every three years the American Heartworm Society holds an international symposium. Following each triennial symposium, the Society updates its guidelines for veterinarians to incorporate any new discoveries. The following guidelines are widely considered "best practices" and should be closely followed by veterinarians in dealing with this parasite.)

Other Resources

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