Dirofilaria immitis
Dirofilaria immitis | |
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A German Shepherd dog heart infested with Dirofilaria immitis | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Nematoda |
Class: | Chromadorea |
Order: | Rhabditida |
Family: | Onchocercidae |
Genus: | Dirofilaria |
Species: | D. immitis
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Binomial name | |
Dirofilaria immitis (Leidy, 1856)
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Synonyms[1] | |
Filaria immitis Leidy, 1856 |
Dirofilaria immitis, also known as heartworm or dog heartworm, is a
Adult heartworms often reside in the pulmonary arterial system (
Distribution and epidemiology
Although at one time confined to the southern United States, heartworm has now spread to nearly all locations where its mosquito
Course of infection
Heartworms go through several life stages before they become adults infecting the pulmonary artery of the host animal. The worms require the mosquito as an intermediate host to complete their lifecycles. The rate of development in the mosquito is temperature-dependent, requiring about 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 is halted.[13] As a result, transmission is limited to warm weather, 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 pulmonary arteries takes six to seven months in dogs and is known as the "prepatent period".[citation needed]
The first larval stage (L1) and second larval stage (L2) of heartworm development occurs within the body of a mosquito. Once the larvae develop into the infective third larval stage (L3), the mosquito locates and bites a host, depositing the larvae under the skin at the site of the bite. After a week or two of further growth, they molt into the fourth larval stage (L4) . Then, they migrate to the
The microfilariae circulate in the bloodstream for as long as two years, and are ingested by bloodsucking mosquitos, where development occurs and the cycle repeats.[citation needed]
Hosts
Hosts of Dirofilaria immitis include:[3]
Reservoir hosts for D. immitis are coyotes and stray dogs.[18]
Clinical signs of infection in dogs
Dogs show no indication of heartworm infection during the six-month prepatent period prior to the worms' maturation, and current diagnostic tests for the presence of
Many dogs 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 during or after exercise, and exercise intolerance. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss,
There are 4 different classes of symptoms:
- Class 1 – no or mild symptoms with occasional cough.
- Class 2 – mild symptoms with occasional cough and tiredness after moderate activity.
- Class 3 – more severe symptoms, including a generally sick appearance, persistent cough, difficulty breathing, and tiredness after mild activity. Heart and lung changes may be seen with a chest x-ray.
- Class 4 – also called caval syndrome. The blood flowing back to the heart is blocked due to the large mass of worms. This is life-threatening and the only treatment option is surgery.[19]
Role of Wolbachia pipientis
Diagnosis in dogs
Microfilarial detection is accomplished by the using one of the following methods:
Direct blood smear
A blood sample is collected and viewed under the microscope. The direct smear technique allows examination of larval motion, confirming the presence of microfilaria. It also helps in the distinction of D. immitis from Acanthocheilonema reconditum. This distinction is important because the presence of the latter parasite does not pose a health risk to the host. D. immitis usually has stationary body movement, while A. reconditum has progressive movement. However, this method often misses light infections because only a small amount of blood sample is used.[23]
Hematocrit tube method
This method uses a microhematocrit (or capillary tube) filled with a blood sample that has been centrifuged, separating the plasma from the red blood cells. These layers are divided by the buffy coat. The buffy coat consists of the leukocytes and platelets that are in the sample. The tube is snapped at the buffy coat and added to a slide for microscopic examination. Adding methylene blue stain to the sample may allow greater visibility of any microfilariae. However, the hematocrit tube method will not allow for species differentiation.[24]
Modified Knott's test
The modified Knott's test is more sensitive because it concentrates microfilariae, improving the chance of diagnosis.[4] A blood sample is mixed with 2% formalin and centrifuged in a tube. The supernatant is removed and methylene blue stain is added to the pellet remaining in the tube for microscopic examination. It allows microfilariae species differentiation based on morphology. Microfilariae can be differentiated between D. immitis and Acanthocheilonema reconditum because of small differences in morphology. The Modified Knott's test is the best method of visual examination when determining presence of microfilaria because it preserves their morphology and size. It is easy to perform, quick, and inexpensive.[25]
The potential for a microfilaremic infection is 5 – 67%. The number of circulating microfilariae does not correlate with the number of adult heartworms, so is not an indicator of disease severity.[4]
Antigen testing
In most practices,
X-rays
X-rays are used to evaluate the severity of the heartworm infection and develop a prognosis for the animal. Typically, the changes observed are enlargement of the main pulmonary artery, the right side of the heart, and the pulmonary arteries in the lobes of the lung. Inflammation of the lung tissue is also often observed.[27]
Treatment in dogs
If an animal is diagnosed with heartworms, treatment may be indicated. Before the worms can be treated, however, the dog's heart, liver, and kidney function must be evaluated to determine the risks of treatment. Usually, the adult worms are killed with an
After treatment, the dog must rest, and exercise is to be heavily reduced for several weeks so as to give its body sufficient time to absorb the dead worms without ill effect. Otherwise, if the dog is under exertion, dead worms may break loose and travel to the lungs, potentially causing respiratory failure and sudden death. According to the American Heartworm Society, the administering of aspirin to dogs infected with heartworms is no longer recommended due to a lack of evidence of clinical benefit, and aspirin may be contraindicated in several cases. Aspirin had previously been recommended for its effects on platelet adhesion and the reduction of vascular damage caused by the heartworms.[citation needed]
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 and no surviving worm is detected, the treatment is considered a success, and the patient is effectively cured.[citation needed]
Surgical removal of the adult heartworms as a form of treatment may also be indicated, especially in advanced cases with substantial heart involvement and damage.[29]
Prevention of infection in dogs
Prevention of heartworm infection can be obtained through a number of veterinary drugs. The drugs approved for use in the US are
Preventive drugs are highly effective, and when regularly administered, have been shown to protect more than 99% of dogs and cats from heartworm. Most compromises in protection result from the failure to properly administer the drugs during seasonal transmission periods.[31] In regions where the temperature is consistently above 14 °C (57 °F) year-round, a continuous prevention schedule is recommended.[citation needed]
Due to newly emerging resistant strains of heartworms, which no macrocyclic lactone (heartworm prevention) can protect against, the American Heartworm Society recommends dogs be on a repellent and a heartworm preventive. The repellent, such as Vectra 3-D, keeps mosquitoes from feeding on the dog and transmitting the L3 stage worms. If a dog is bitten, the heartworm preventive takes over when administered. If a mosquito feeds on a heartworm positive dog on a repellent, they do not live long enough for the microfilaria they ingested to molt into the infective L3 larva. Vectra 3-D was tested using thousands of mosquitoes infected with the resistant heartworm strain JYD34. In the control group that was given only a placebo, every dog contracted heartworms. In the experimental group that was given only Vectra 3-D, two of eight dogs contracted heartworms and had an average of 1.5 adult worms each. In the experimental group given both heartworm prevention and Vectra 3-D, one dog was infected with L3 stage larvae that did not mature into adulthood due to the heartworm prevention. Using a repellent and a prevention is at least 95% effective.[32][33]
Ivermectin, even with lapses up to four months between doses, still provides 95% protection from adult worms. This period is called the reach-back effect.[34] Since dogs are susceptible to heartworms, they should be tested annually before they start preventive treatment.[18] Annual heartworm testing is highly recommended for pet owners who choose to use minimal dosing schedules. Testing a dog annually for heartworms and other internal parasites is a fundamental part of a complete heartworm prevention program, and is also recommended for dogs who are already on a monthly prevention program.[18]
Heartworm infection in cats
While dogs are a natural host for D. immitis, cats are atypical hosts. Because of this, differences between canine and feline heartworm diseases are significant. The majority of heartworm larvae do not survive in cats, so unlike in dogs, a typical infection in cats is two to five worms. The lifespan of heartworms is considerably shorter in cats, only two to three years, and most infections in cats do not have circulating microfilariae. Cats are also more likely to have aberrant migration of heartworm larvae, resulting in infections in the brain or body cavities.[35]
The infection rate in cats is 1–5% of that in dogs in endemic areas.[36] Both indoor and outdoor cats are infected. The mosquito vector is known to enter homes.[37]
Pathology
The vascular disease in cats that occurs when the L5 larvae invade the
Obstruction of pulmonary arteries due to emboli from dying worms is more likely to be fatal in cats than dogs because of less collateral circulation and fewer vessels.[39] Heartworms can live for 2 to 3 years in cats.[15]
Signs and symptoms
Acute heartworm disease in cats can result in shock, vomiting, diarrhea, fainting, and sudden death. Chronic infection can cause loss of appetite, weight loss, lethargy, exercise intolerance, coughing, and difficulty breathing. Some cats' immune systems are able to clear a heartworm infection, though the immune system response can cause many of the same symptoms. Also, even if the infection resolves, respiratory damage can cause some symptoms to persist beyond it.[38][need quotation to verify][40]
Diagnosis
Diagnosis of heartworm infection in cats is problematic. Like in dogs, a positive ELISA test for heartworm antigen is a very strong indication of infection. However, the likelihood of a positive antigen test depends on the number of adult female worms present. If only male worms are present, the test will be negative. Even with female worms, an antigen test usually only becomes positive seven to eight months after infection. Therefore, a cat may have significant clinical signs long before the development of a positive test. Heartworm-associated respiratory disease can be found in cats that never develop adult heartworms and therefore never have a positive antigen test.[citation needed]
An antibody test is also available for feline heartworm infection. It will be positive in the event of exposure to D. immitis, so a cat that has successfully eliminated an infection may still be positive for up to three months. The antibody test is more sensitive than the antigen test, but it does not provide direct evidence of adult infection.[41] It can, however, be considered specific for diagnosing previous larval infections, and therefore fairly specific for heartworm-associated respiratory disease.
X-rays of the chest of a heartworm-infected cat may show an increased width of the pulmonary arteries and focal or diffuse opacities in the lungs. Echocardiography is a fairly sensitive test in cats. Adult heartworms appear as double-lined hyperechoic structures within the heart or pulmonary arteries.[42]
Treatment and prevention
Heartworm prevention for cats is available as ivermectin (Heartgard for Cats), milbemycin (Interceptor), or the topical selamectin (Revolution for Cats) and Advantage Multi (imidacloprid + moxidectin) topical solution. Ivermectin, milbemycin, and selamectin are approved for use in cats in the US.[citation needed]
Arsenic compounds have been used for heartworm adulticide treatment in cats, as well as dogs, but seem more likely to cause pulmonary reactions. A significant number of cats develop pulmonary embolisms a few days after treatment. The effects of melarsomine are poorly studied in cats. Due to a lack of studies showing a clear benefit of treatment and the short lifespan of heartworms in cats, adulticide therapy is not recommended, and no drugs are approved in the US for this purpose in cats.[39]
Treatment typically consists of putting the cat on a monthly heartworm preventive and a short-term corticosteroid.[35] Surgery has also been used successfully to remove adult worms. The prognosis for feline heartworm disease is guarded.[clarification needed][citation needed]
Heartworm infection in humans
Dirofilaria are important medical parasites, but diagnosis is unusual and is often only made after an infected person happens to have a chest X-ray following granuloma formation in the lung. The nodule itself may be large enough to resemble lung cancer on the X-ray, and requires a biopsy for a pathologic assessment.[17] This has been shown to be the most significant medical consequence of human infection by the canine heartworm. Patients are infected with the parasite through the bite of an infected mosquito, which is the same mechanism that causes heartworm infection in dogs.[medical citation needed]
D. immitis is one of many species that can cause infection in dogs and humans. It was thought to infect the human eye,
References
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Further reading
- Traversa, D.; Di Cesare, A.; Conboy, G. (2010). "Canine and feline cardiopulmonary parasitic nematodes in Europe: emerging and underestimated". PMID 20653938.
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.
- Preventing Heartworm Infection in Dogs (VeterinaryPartner.com)
- Overview and main concepts of Dirofilaria immitis (heartworm) infection (MetaPathogen.com)
- Mosquito-borne Dog Heartworm Disease (University of Florida Extension Bulletin)
- Case Study of Canine Heartworm Disease (from the University of California, Davis)
- Case Study of Feline Heartworm Disease (from the University of California, Davis)
- Case Study of Canine Heartworm Disease (from the University of California, Davis)