|Heartworm is a parasitic roundworm (Dirofilaria immitis) that is spread from host to host through the bites of mosquitoes. The heartworm is a filariasis affecting 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. 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.
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. 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° Fahrenheit).
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. The female adult worms are about 30 cm in length, and males are about 23 cm with a coiled tail. 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.
Blood testing is the most common method of diagnosis. Formerly, ELISA tests were used to detect antibodies to heartworm, but the sensitivity and specificity was low. Current ELISA tests detect female heartworm antigens. The specificity of these tests is close to 100 percent and the sensitivity is about 85 percent. False negative tests can result from low worm counts, immature infections, and all male infections. Heartworm positive dogs should also be tested for the presence of microfilariae, usually using a filter test. X-rays 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. 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. 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, Proheart 6, administered by veterinarians, but the injectable form of moxidectin was taken off the market in the United States due to safety concerns. ProHeart 6 remains on the market in many other countries including Canada and Japan.
Its sister product, ProHeart SR-12 is used extensively in Australia and Asia as a 12 month injectable preventative. Moxidectin has been re-released in the United States as part of Advantage Multi™ (imidacloprid + moxidectin) Topical Solution, which utilizes moxidectin for control and prevention of roundworms, hookworms, heartworms, whipworms, and well as imidocloprid to kill adult fleas. 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), milbemycin (Interceptor), 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.
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