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Heartworm (Dirofilaria immitis) Brad and Tiffany
Where It's Found & What It Does:
is a parasite which feeds on its host organism's blood exclusively.
The nematode usually occupies the right chamber of the heart and the pulmonary arteries of dogs, foxes, wolves, coyotes, and cats.
, belong to the same class of worms as roundworms. In fact, they look a bit like roundworms, but that is where the similarity ends. Heartworms spend their adult life in the right side of the heart and the large blood vessels connecting the heart to the lungs.Heartworms are found in dogs, cats, and ferrets. They also occur in wild animals such as California sea lions, foxes and wolves. They have rarely been found in people.
Adult heartworms in the heart lay very tiny
called microfilariae, which then live in the bloodstream. These microfilariae enter a mosquito when it sucks blood from an infected animal. In 2-3 weeks, the microfilariae develop into larger larvae in the mosquito and migrate to the mosquito's mouth.
There are more than 60 different species of mosquitoes that can transmit heartworms.
When the mosquito bites another animal, the larvae enter the animal's skin. The larvae grow and after about three months finish their migration to the heart, where they grow into adults, sometimes reaching a length of 14 inches. The time from when an animal was bitten by an infected mosquito until adult heartworms develop, mate, and lay microfilariae is about 6-7 months in dogs and 8 months in cats. (Remember this – it is important when we talk about diagnosis.)
Severely infected dogs can have up to several hundred heartworms in their hearts and vessels. Adult worms in dogs usually live up to 5-7 years. Thirty to eighty percent of infected dogs have microfilariae, and the microfilariae can live up to 2 years. Microfilariae cannot mature into adult heartworms unless they pass through a mosquito
has an indirect life cycle.
The adult parasite sexually reproduces in its vertebrate host, and the offspring are transferred to the intermediate host, which is usually a mosquito or a flea.
The larva develops inside the intermediate host and molts twice.
When the intermediate host feeds, the larvae enter the new vertebrate host through the wound.
The parasite remains dormant in the vertebrate hosts muscle tissue for 85 to 120 days. After this time period, the parasites enter the host's blood stream, where they are carried to the heart.
Completion of the life cycle in the heart requires 7 to 9 months.
Most dogs with heartworm infection do not show signs of disease. Some dogs may show decreased appetite, loss of weight, and listlessness. Often, the first sign of the disease is a cough. Animals with severe heartworm disease will start to show lack of endurance during exercise. Some will accumulate fluid in their abdomen (ascites) that makes them look pot-bellied. In rare situations in which animals have many adult worms, the animals may die of sudden heart failure.
Blood testing is performed to identify dogs infected with
. Because blood tests are not always accurate, we need to interpret test results in relation to the history and the symptoms the animal is showing. Radiographs (x-rays) and ultrasound (echocardiography) are often performed to look for typical changes in the heart and lungs caused by
, and determine the severity of the infection. Changes include enlargement of the pulmonary artery and the right ventricle. Certain types of cells (
) may increase in the blood or secretions from the lungs in heartworm infections. These additional findings can all help support the diagnosis.
Medications used to prevent heartworm infections are called preventives. The first thing to remember is that preventives are NOT used to kill the adult worms. Special drugs called adulticides must be used to kill the adults. These drugs will be discussed in the treatment sections. Some preventives can cause severe problems if given to animals with adult heartworms or microfilariae. Follow the recommendations of your veterinarian and the manufacturer of your preventive in regard to testing prior to giving the preventive.
A number of monthly heartworm preventives for dogs are on the market. Some heartworm preventives, or drugs that are combined with them, will control other parasites. Preventive products should be used year-round, even in areas where mosquitoes only occur seasonally. Even if doses are accidentally skipped, preventive products are still beneficial to the pet. If given consistently over a 12-month period, it's possible to actually stop worms from developing into adults. Also, monthly heartworm preventives have activity against intestinal parasites, which inadvertently infect millions of people every year. These preventives protect pets and people.
The daily preventive, diethylcarbamazine is available with a prescription through compounding pharmacies. Two main disadvantages are that it can produce severe reactions if given to a dog with a heartworm infection, and that missing even two or three days of administration could result in a lapse of protection.
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 [[/wiki/Arsenic|arsenic]]-based compound. The currently approved drug in the US, [[/w/index.php?title=Melarsomine_dihydrochloride&action=edit&redlink=1|melarsomine dihydrochloride]], is marketed under the brand name
[[home#cite_note-13|]]] It has a greater efficiency and fewer side effects than previously used drug ([[/w/index.php?title=Thiacetarsamide_sodium&action=edit&redlink=1|thiacetarsamide sodium]], sold as
) 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 [[/wiki/Respiratory_failure|respiratory failure]] and death. According to the American Heartworm Society, use of [[/wiki/Aspirin|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 [[/wiki/Ivermectin|ivermectin]] year round at three times the dose normally used for heartworm prevention (see "[[/wiki/Heartworm#Prevention|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 infections. Long term treatment over a year with doxycycline daily and heartguard plus has been shown to be effective in early heartworm who are asymptomatic.
From time to time various [[/wiki/Homeopathic|homeopathic]], natural or [[/wiki/Organic_product|organic]] products are used as 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.
Heartworms: colonialterraceanimalhospital &
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