Heartworm is a parasitoid 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
Although at one time confined to more southern climes, the heartworm has now spread to all climates where its vector, the mosquito, can reach, and can now be found in all 50 states of the United States. It appears that 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 have been noted in any area with large mosquito populations.
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 is extremely serious for the infected host; infected dogs that go untreated will 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 preventive medication can be used to prevent heartworm infection. A positive test result, on the other hand, requires immediate treatment to eradicate the worms.
A canine heart with heartworms.
The parasite
Heartworms go through several life stages before they become adults infesting the hearts 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. Mosquitoes ingest heartworm larvae, called microfilariae, from an infected host. The mosquitoes then transfer 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. Heartworms can reach up to 12 inches in length and can remain in the host’s heart for several years.
Even without the worms reaching the heart, host dogs can have some microfilariae in their blood. Heartworms begin their infestation in the lungs and may be able to reproduce some at this time. At this stage, however, the host dog will likely be asymptomatic. Once the infestation reaches a certain concentration in the lungs, the now adult worms migrate to the heart through the pulmulnary vein 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.
Course of infestation
The period between the initial infestation with microfilariae and reproduction of microfilariae by adult worms living in the heart takes some six-to-seven months in dogs.
Heartworms bear live young, producing thousands of them every day. The microfilariae then circulate in the bloodstream for as long as three years, waiting for the next stage in its life cycle in the gut of a bloodsucking mosquito. The microfilariae undergo changes in the mosquito that allow them to infect another host and become reproducing adults. These changes can occur as quickly as two weeks and as long as six weeks depending on the warmth of the climate.
Symptoms of infestation
After infection, the microfilariae grow into worms which infest first the lungs and then the heart. Beginning in the veins of the lungs, the worms move by way of the pulmonary vein to the heart, filling the right atrium and ventricle. They can also clog the veins of the liver. Heartworm infestation may not become symptomatic until a year or more after the initial infection. At some point though, the sheer size and number of the worms creates physical obstructions that start to interfere mechanically with the function of the heart and lungs, and even liver.
Signs of infestation often begins with a slight cough that increases as the dog continues to exert itself in exercise. Eventually, as the cough worsens, the dog may even faint from heavy exercise. In advanced stages, breathing becomes increasingly labored even during only moderate exersion, such as walking. There may also be blood expectorated in the cough. The now thoroughly infested dog will tire quickly, display general weakness, and behave in a listness manner. Loss of appetite is also a symptom as is the concommitant loss of wieght and reduced physical condition. In the final stages, heartworm-infested dogs are subject to congestive heart failure and sudden death.
Epidemiology
Heartworm disease has now inflitrated every continent except Antarctica, where the vector, mosquitoes, are noticeably absent. The occurance of heartworm disease is dependent on the following factors:
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Susceptibility of host population
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Stability of the disease reservoir
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Population stability of vector species
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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 (or intermediate host for the developing parasite and spreading the disease from host to host). Development of the microfilariae in the mosquito ideally requires a temperature at or above 80 degrees Fahrenheit for about two weeks. No larval development takes place in the mosquito below 57 degrees F.
Testing
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 as 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 degrees F.
X-ray radiographs can also detect the presence of adult heartworms in the heart and lungs.
Treatment
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 caught 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 make 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.
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.
Prevention
Prevention of heartworm infection can be obtained through a number of veterinary drugs. Most popular are Ivermectin (sold under the brand name Heartguard), Milbmycin (Interceptor) and Moxidectin (ProHeart). These drugs are given monthly during the preventative season of the year. Selamectin (Revolution), on the other hand, is a topical preventive. Some of these drugs also kill other parasites, including other kinds of worm infestations. Selamectin, for instance, can also be used to control fleas, ticks, and mites.
Moxidectin is also available in a six-month sustained release injection administered by veterinarians or a monthly oral dosage form.
Those living in seasonal areas with mild winters may want to engage in year-round heartworm prevention to guard against the occasional mosquito outbreak during an especially warm snap.
Cats may be treated with ivermectin (Heartguard for Cats) or the topical selamectin.
