Heartworms are exactly what they sound like: worms, up to 8 inches long, like adults, that develop and live in the blood vessels of the dog’s heart. Although they prefer the right side of the heart when the infection is severe, they also develop and live in the lungs or larger artery – wherever there is room with access to blood.
Heartworm illness refers to the constellation of negative effects that the dog suffers from the growth of this population within him. Heartworm disease has the highest morbidity and mortality of any insect-borne disease in the United States.
At one point, heartworm disease was viewed as an area-specific problem (the southeast is notorious for it), climate change, dogs crossing state lines (e.g., dogs relocated from Louisiana after Hurricane Katrina), and changes in wildlife areas led to fluctuations in the distribution patterns of the disease. As a result, heartworm disease in dogs is no longer restricted to certain parts of the country.
The good news is that with the advent of modern parasiticides and routine surveillance (checking for heartworms), heartworm disease can be prevented. If prevention is not taken and a dog becomes infected with heartworms, they can be treated for the infection – but the treatments can put the dog at risk. Prevention is the best approach!
WHAT HAPPENS IF A HEARTWORM INFECTION?
Dogs become infected with heartworms from the bites of infected mosquitoes. A mosquito that has taken a meal of blood from an infected canid (dog, coyote, wolf, fox) transfers heartworm larvae to the next canid, which it bites.
The microscopic heartworm larvae penetrate the dog from the mouth parts of the mosquito and begin a developmental journey during which they pass through various larval forms and migrate into the dog’s bloodstream. They are looking for a hospitable place where they can lodge and grow into adult worms.
The pulmonary arteries, in which the adult worms eventually settle, become inflamed, expand, and deformed as the heart tries to force blood past the clogging worms. Blood clots and aneurysms (a dangerously bloating lump in a blood vessel) may develop.
The heavier the “stress” on the heartworm – the more worms the dog infects – the worse the dog’s symptoms and prognosis. Dogs can become infected with so many parasites that the heartworms can obstruct blood flow through the heart, causing the working muscles on the right side of the heart to widen and thicken. The thickened heart muscle is exposed to disturbances in the heart’s electrical impulses that lead to arrhythmias.
If the worm population is not activated, the heart becomes too distorted to function at all and the dog can suffer from right-sided heart failure. Or “cavalry syndrome,” in which the entire right side of the heart is filled with heartworms, which interfere with the closing of the tricuspid valve and impede blood flow through the heart, causing cardiovascular collapse.
As the infection progresses, the blood vessels in the lungs (pulmonary vessels) become inflamed and unhealthy. This leads to pulmonary hypertension (high blood pressure in the lungs). And because the worm-laden heart has to work harder, it can pump less blood around the body, which creates oxygen and expels carbon dioxide into the lungs. As the dog develops, it becomes increasingly intolerant of physical activity – or, in severe cases, even any movement. The dog will develop a chronic cough that will get worse over time, and the dog’s abdomen may expand with fluid (ascites).
The heart, lungs, and even the kidneys and liver eventually show signs of illness – all caused by the increasingly inefficient work of the stressed cardiovascular system.
It usually takes at least a year for a dog bitten by an infected mosquito to show signs of heartworm disease – and it can be even longer. The symptoms dogs may show due to their heartworm infection depend on a number of factors:
• How many heartworms does a dog become infected with (dogs can be infected with as little as one or dozen worms; the number of worms will not increase without repeated reinfections caused by bites from more infected mosquitoes).
• The exact location in the dog’s circulatory system where the worms lodge (worms, for example near heart valves, can cause more problems than anywhere else).
• How long have the worms been in the dog (the longer, the more damage they can cause; worms can live up to 5 to 7 years old and grow 12 inches or longer).
• The dog’s health (some dogs may tolerate a limited infection better than others).
Early signs of a heartworm infection can be a slight cough or a “slowdown” during exercise (exercise intolerance). You may find that your dog is reluctant to run, play or get tired, especially in the heat. A dog that is only slightly infected (with only one or two worms) can never have symptoms worse than these.
However, dogs that have a more severe worm load and / or are repeatedly re-infected will experience worsening and severe symptoms. Eventually, there is a complete failure of the right side of the heart. Bloody fluid may drip from your nose. A dramatic weight loss called cardiac cachexia can be noted. This is end-stage heart failure and successful treatment is difficult. At this point death is not far away.
HEARTWORM LIFE CYCLE
To understand how best to prevent and treat heartworm infections, it is helpful to understand the heartworm life cycle.
The heartworm goes through several physical changes in its life and requires hosts of two different species to complete its life cycle: the mosquito and a mammal. The life cycle begins with an infected animal. When an adult male and female adult heartworm are present in a dog, they can produce microfilariae, a type of motile embryo. The microfilariae circulate freely in the blood of the infected animal and can only develop further if they are sucked into a mosquito by their bite.
The next stages in the development of a heartworm can only take place in the intestines of a mosquito – strange, right? As soon as a mosquito ingests a blood meal loaded with microfilariae, the microfilariae begin to mature. Over a period of 10 to 30 days (average is around two weeks), the microscopic embryos develop in three larval stages – still very microscopic, but each different. When the mosquito bites another dog, all of the third-stage larvae (called L3 larvae) leave the mosquito through their mouth parts and enter the dog. Only when the L3 larvae are back in a mammal can they develop further.
The larvae live and develop in the dog’s subcutaneous tissue for about 50 to 70 days at a stage called L4 – the fourth instar larvae. Once this stage is complete, the larvae begin migrating through the dog’s tissues in search of circulation. Once they reach a blood vessel and enter the bloodstream, they become sexually immature adults. These juvenile worms move toward the heart and lungs, where they lodge and mature into sexually capable adults who can mate so the females can produce viable embryos (microfilariae), starting the life cycle again.
This entire process, from the bite of the mosquito that steals the microfilariae from an infected dog, through the larval stages in the mosquito, through the larval stages in a new mammalian host, to adulthood, takes at least six months.
It is important to understand this cycle as veterinary technology can prevent your dog from becoming infected or, if it does not, detect its infection in order to provide timely treatment at certain times in the heartworm’s life cycle and under certain conditions.
Heartworm preventive drugs belong to a class of drugs called macrocyclic lactones. They have existed for about 30 years and come from the soil microorganism Streptomyces.
Macrocyclic lactones are used to treat both human and animal parasitic diseases. These drugs inhibit nerve transmission within parasites, causing paralysis and death. They effectively kill the third stage larvae that infected mosquitoes may have newly implanted in your dog, as well as the fourth stage larvae that develop under his skin.
These drugs do not kill juvenile or adult heartworms that may already be in circulation (although the drugs may weaken them). Once the larvae reach the bloodstream, it is too late for the preventive drugs to kill them. To be effective, the drugs Got to given between being bitten by an infected mosquito and reaching young adulthood. Because of this, these medicines must be given exactly on the schedule recommended by their manufacturers (monthly for many of the medicines, or according to the directions on the label).
DIAGNOSIS OF HEARTWORM INFECTION
Most dog owners have had their dogs tested for heartworm infections, but many are unaware of the limitations of these tests.
The most sensitive and easily available antigen detection test is a snap test that can be done in your veterinarian’s office. A blood sample is drawn, mixed with a conjugated solution, and applied to the test. The results are available within 10 minutes.
The most common heartworm tests used by veterinarians detect the antigens released into the dog’s bloodstream by adult female heartworms. In most cases, antigen tests can accurately identify infections with one or more adult female heartworms.
There are currently no USDA-licensed serological tests that can detect male heartworms. That said, if a dog happens to be infected only with male heartworms, the test won’t catch them. And if a dog is infected but the heartworms are not yet adult, the test won’t catch them.
If a dog is infected with only one or two adult female heartworms, the tests will only detect antigens produced by this tiny female population 60% to 70% of the time. The dog may test negative or inconsistently positive.
According to Michael W. Dryden, DVM, MA, PhD, of the Kansas State Veterinary Diagnostic Laboratory, recent studies have documented that antigen testing in up to 7% of dogs is due to the presence of “antigen-antibody complexes” formed in the dog’s blood .
To counter these rather sobering test stats, in an ideal world, puppies would start heartworm prevention on their first visit to the vet and receive their first heartworm tests in or about a year ago. Since tests cannot detect heartworms until the heartworms are 6 to 7 months old, there is little reason to test much earlier. (That is, if I were looking at a rescued dog of indeterminate age that lived primarily outdoors before rescuing, especially in a warm climate where mosquitoes are active, I would test it sooner.)
Your vet can also examine a blood sample under a microscope. The microfilariae are easy to spot because the active embryonic larvae are much larger than the blood cells. They also “whip” from side to side. A positive antigen test and the presence of microfilariae definitely mean a dog has a heartworm infection.
If the antigen test is positive but no microfilariae are detected, a second heartworm test should be performed to confirm the positive result. Treatment of the infection should not be started until positive has been confirmed.
If a dog tests negative but has symptoms compatible with heartworm disease and has not received prevention, more extensive testing is required. This will include a blood smear to check for microfilariae as mentioned above. “Heat fixation” is a more recent test performed by outside laboratories that can dissociate the antigen-antibody immune complexes, releasing the heartworm antigen present so that it can be detected by the antigen tests.
If you are looking to import a dog from another state, make sure it has been thoroughly screened and tested for heartworms – so that it can be treated immediately and there is no possibility of it having a new local reservoir (host) for heartworms in yours Land provides area.
When a heartworm infection is confirmed, the next step is to determine how far the disease has progressed. This may include chest x-rays to check the dog’s heart, lungs, and blood vessels.
Some veterinarians also do an electrocardiogram (EKG) to check for abnormalities in the rhythm of the heart, as well as an echocardiogram – an ultrasound of the heart. This will determine if the heart is enlarged and show heartworms as bright white lines in the echo. Knowing the severity of the condition can predict the likelihood of complications from treatment.
Dogs with minimal cardiovascular changes have a good prognosis. The presence of heart failure, significant lung changes, and cavalry syndrome make treatment significantly difficult.
The treatment of heartworm infections has made significant strides over the past few decades.
First, oral prevention such as milbemycin is given along with doxycycline for two months. Oral prevention kills the larval forms and any circulating microfilariae that may be present. (The microfilariae can’t develop into heartworms in your dog – but you don’t want your dog to be a host any Stage of the heartworm life cycle!)
Doxycycline is an effective antibiotic against Wolbachia, a parasite in heartworm. By killing the parasites in the heartworms, the heartworms themselves are weakened. When doxycycline is given as part of therapy, it reduces the complications of infection.
After two months (day 60), it’s time to give melarsomine (immiticide), the drug that actually kills the adult heartworms. The drug is given as an injection deep into the epaxial muscles along the spine on days 60, 90, and 91. Extremely strict chest rest, with or without sedation, and close monitoring are required during treatment.
A dog’s prognosis depends on the severity of the infection at the time of treatment, as well as how the dog was treated during the treatment period. When the worms die, they break down in the bloodstream. If a dog exercises enough to increase its heart rate, serious complications such as worming or pulmonary embolism (blood clots), leading to shortness of breath and collapse, can occur. Treatment for emboli is hospitalization for oxygen therapy and steroids to reduce inflammation in the lungs. Many dogs are fine when caught and handled quickly. But it cannot be repeated enough: It is important to strictly control the dog’s activity while undergoing treatment for heartworm infection.
Other drugs that may help include steroids to reduce inflammation as the worms die, and pain relievers to relieve discomfort caused by deep intramuscular injection of melarsomine. Sedatives may be needed to keep a dog rested, calm, and physically impaired. According to the American Heartworm Society, “is a critical factor in reducing the risk of thromboembolic complications strict Exercise restriction. “An antihistamine can also be given to reduce the risk of anaphylaxis.
“SLOW KILL” METHOD
The so-called “slow kill” method of treating heartworm infection consists of only one monthly dose of heartworm preventive drugs. No adult drugs are used. Remember that the preventive drugs will kill all circulating microfilariae as well as all L4 forms of the parasite. However, these drugs do not quickly or reliably kill the adult heartworms that damage the dog’s circulatory and pulmonary systems, although they do weaken the adult worms and shorten their lifespan. The result is that heartworms can take up to two years to die, as opposed to a few months.
A 2004 study examining the effectiveness of the slow kill method found that nearly 30% of dogs still tested positive for a heartworm antigen test after 24 months of monthly heartworm prevention.
After all, the slow kill method cannot destroy a subpopulation of worms that are resistant to the drug, resulting in an infection worse than that can not be treated with macrocyclic lactones.
A slow kill protocol is inexpensive in terms of the money the dog’s owner spends, but very costly in terms of the dog’s health. Dogs treated in this way must be strictly rested to avoid the risk of lung blood clots caused by the breakdown of dead heartworms. As long as the heartworms are alive, pathological changes in the lungs and heart tissue persist, and this damage is usually permanent.
In contrast, conventional treatment has been shown to clear heartworm infections 98% of the time – within a few months of starting the treatment protocol.
THE TAKEAWAY MESSAGE
Heartworm disease is a serious, often fatal syndrome. Prevention is easy! Administer monthly prevention to protect your dog, minimize mosquitoes in your area, and work closely with your veterinarian to develop a healthy lifestyle.