Description

Heartworm is a parasitic roundworm (Dirofilaria immitis) that is spread from host to host through the bites of mosquitoes. The heartworm is a type of filaria, a small thread-like worm. The definitive host is the dog but it can also infect cats, wolves, coyotes, foxes and other animals, such as ferrets, sea lions and even, under very rare circumstances, humans.  The parasite is commonly called "heartworm" because the adult reproductive stage of its life cycle resides primarily in the right ventricle of its host where it can live for many years. Heartworm infection may result in serious disease for the host.

History

The first published evidence of disease in the dog in the United States was in The Western Journal of Medicine and Surgery in 1847. Feline heartworm was first described in the 1920s.

Distribution

Although at one time confined to the southern United States, heartworm has now spread to nearly all locations where its vector, the mosquito, is found. Transmission of the parasite occurs in all of the United States (except Alaska) and the warmer regions of Canada. The highest infection rates are found within 150 miles of the coast from Texas to New Jersey, and along the Mississippi River and its major tributaries.  It has also been found in South America, southern Europe, Southeast Asia, the Middle East, Australia, and Japan.

Course of infection

Heartworms go through several life stages before they become adults infecting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle. Development in the mosquito is temperature dependent, requiring approximately two weeks of temperature at or above 27 °C (80 °F). Below a threshold temperature of 14 °C (57 °F), development cannot occur, and the cycle will be halted.  As a result, transmission is limited to warm months, and duration of the transmission season varies geographically. 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 6 to 7 months in dogs and is known as the "prepatent period".

After infection, the third stage larval heartworms (L₃) deposited by the mosquito grow for a week or two and molt to the fourth larval stage (L₄) under the skin at the site of the mosquito bite. Then they migrate to the muscles of the chest and abdomen and 45 to 60 days after infection, molt to the fifth stage (L₅, immature adult). Between 75 and 120 days after infection these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. 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 giving birth to live young, called microfilariae.

The microfilariae 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 third larval stage and then migrate to the salivary glands of the mosquito, where they wait to infect another host. The incubation period required to reach the stage where the microfilariae become transmittable to another host can be as little as two weeks or as long as six weeks, depending on the warmth of the climate, and the larval life cycle ceases entirely if the ambient temperature drops below 14° Celsius (57° Fahrenheit).

Clinical signs of infection

Dogs show no indication of heartworm infection 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. But normally, until the larvae mature and congregate inside the heart, they produce no symptoms or signs of illness.

Many dogs will show little or no sign of infection even after the worms become adults. These animals usually have only a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially on exercise and early exhaustion upon exercise. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, fainting, coughing up blood and, finally, congestive heart failure.

Role of Wolbachia

Wolbachia is an intracellular bacterium that is an endosymbiont of Dirofilaria immitis. It is thought that all heartworms are infected with Wolbachia to some degree. Research indicates that the inflammation that occurs at the die-off of adult heartworms or larvae is in part due to the release of Wolbachia bacteria into the tissues. This may be particularly significant in cats, in which disease seems to be more related to larval death than living adult heartworms (see below). Pre-treating heartworm positive animals with an antibiotic such as doxycycline to remove Wolbachia may prove to be beneficial, but further studies are necessary.

Diagnosis

A blood test to detect the antigens released from the adult female reproductive tract is the most common method of diagnosis. It takes approximately 6 – 7 months from the time of infection until the female is mature and the antigen test becomes positive. For example, a dog can be infected with 5 month-old worms, and the antigen test will still be negative. The specificity of these tests is close to 100 percent and the sensitivity is more than 90%.  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 concentrating filter test. X-rays are used to evaluate the amount of lung damage caused by the presence of heartworms.

Treatment

If an animal is diagnosed with heartworms, treatment may be indicated. Before the worms can be treated, however, the dog must be evaluated for heart, liver, and kidney function to evaluate the risks of treatment. Usually the adult worms are killed with an arsenic-based compound. The currently approved drug in the US, melarsomine dihydrochloride, is marketed under the brand name Immiticide.  It has a greater efficiency and fewer side effects than previously used drug (thiacetarsamide sodium, sold as Caparsolate) which makes it a safer alternative for dogs with late-stage infections.

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 are 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 year round at three times the dose normally used for heartworm prevention (see "Prevention") will eventually kill adult heartworms. However, this is not the treatment of choice for removal of adult heartworms for two reasons. First, this treatment is not as effective as melarsamine. More importantly, adult heartworms do not begin to die until 18 months of treatment have elapsed, which is not acceptable for dogs with high-volume infestations.

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 choice of using them should be evaluated carefully.

Prevention

Prevention of heartworm infection can be obtained through a number of veterinary drugs. The drugs approved for use in the US are ivermectin (sold under the brand name Heartgard and several other generic versions), milbemycin (Interceptor Flavor Tabs and Sentinel Flavor Tabs) and moxidectin (ProHeart) administered as pills or chewable tablets. Moxidectin is also available in both a 6-month and 12-month sustained release injection, ProHeart 6, ProHeart 12, administered by veterinarians. 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 12 is used extensively in Australia and Asia as a 12-month injectable preventive. Topical treatments are available as well. Advantage Multi (imidacloprid + moxidectin) Topical Solution, which utilizes moxidectin for control and prevention of roundworms, hookworms, heartworms, whipworms, as well as imidacloprid to kill adult fleas. Selamectin (Revolution), is a topical preventive that is likewise administered monthly and also controls fleas, ticks, and mites.

Preventive drugs are highly effective and when regularly administered will protect more than 99 percent of dogs and cats from heartworm. Most compromises in protection result from failure to properly administer the drugs during seasonal transmission periods.  In regions where the temperature is consistently above 14 °C (57 °F) year round, a continuous prevention schedule is recommended.

It has been shown that lapses of up to 4 months between doses of Ivermectin-based products still provides 95% protection from adult worms. This 'safety net' period is called the 'Reach Back Effect'.  Annual heartworm testing is highly recommended for pet owners who choose to use minimal dosing schedules.

Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats).

Wikipedia and GNU

The content of this entry is from the Wikipedia article "Heartworm" licensed under the GNU Free Documentation License.