A parasite (Dirofilaria immitis) which inhabits the heart chambers and major blood vessels, mainly affecting dogs, with some cases occurring in cats. Individuals with heartworm can develop severe chronic bronchitis, pneumonia, and heart changes. Heavy infestation can lead to an early death. The life cycle of the parasite is dependent upon a mosquito. It is usually restricted to hot humid areas such as the S states of the USA and similar climates, though the geographical scope of the parasite may be increasing. Blood tests are available which can recognize the early stages of development, and preventive treatment is normally effective. Surgical treatment may be the only remedy once adult worms have developed.
For the album by Whipping Boy, see Heartworm (album). 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 and may kill 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.
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, is found.
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 mosquito ingests heartworm larvae, called microfilariae, from an infected host.
At this stage, the host dog will likely be asymptomatic. 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.
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. 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.
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. However, active dogs and those with heavier infections will quickly show the classic symptoms of heartworm disease.
Epidemiology
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 conditionsDogs are considered the definitive susceptible host for the parasite.
Testing
Heartworms can be detected by blood test.
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.
Treatment
If either a blood test or the onset of symptoms betray the presence of heartworms, treatment is indicated.
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. According to the American Heartworm Society, use of aspirin in dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit, and may be contraindicated. It had previously been recommended for its effects on platelet adhesion and reduction of vascular damage caused by the heartworms.
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.
From time to time various "homeopathic," "natural" or "organic" products are touted as cures or preventives for heartworm disease.
Prevention
Prevention of heartworm infection can be obtained through a number of veterinary drugs.
Preventative drugs are highly effective, and when regularly administered will protect more than 99 percent of dogs from infection.
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. 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. 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. 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
Heartworm Society
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:
Quick Review of Heartworm Disease Heartworm Disease in Dogs Heartworm Disease in CatsAmerican Heartworm Society information for veterinarians. (Every three years the American Heartworm Society holds an international symposium. The following guidelines are widely considered "best practices" and should be closely followed by veterinarians in dealing with this parasite.)
Guidelines for the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Dogs Guidelines for the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Cats
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