Ron Hines DVM PhD 4/28/06

Heartworm disease (Dirofilaria immitis) is the most dangerous parasite that occurs in dogs worldwide. It also occasionally occurs in cats, ferrets and man – all of which are considerably more resistant to the parasite than dogs.

Heartworms are transferred from animal to animal through the bite of an infected mosquito. Initially the disease appeared to be confined to temperate, moist regions of the world, but through the years it has spread to areas where it was previously absent. The extent of the disease depends on a number of factors: the age of the pet, the number of mature worms present in the heart and great blood vessels, the specific area where the worm(s) are present, the sex of the worm(s) (male worms are much smaller than female worms), the extent of reaction of the pet to the presence of the worms and how long the worms have been present. Ambient temperature is also very important. Even an infected mosquito cannot spread the disease unless the average daily air temperature is above 57 F. (13.9 C). So only in South Florida and the Texas Rio Grande Valley might mosquitoes transmit the disease the entire year. The peak months for transmission of heartworms are July August and September. Despite this data, it is still commonly recommended that dogs, cats and ferrets receive monthly heartworm preventative medicine until the first frost of the year. The drug, ivomectin, is sufficiently mild that this will do no harm.

There are two ways this disease can be diagnosed. A crude, older method relied on fining the immature larva of these worms (microfilaria) in the pet's blood. This was called a "difil" test and relied on passing a blood sample through a porous filter. When the larval parasites, were present, the test was accurate. However, approximately twenty to forty percent of dogs with heartworms did not have microfilaria. And those that occasionally received ivomectin were uniformly negative for microfilaria. It also took approximately 7 months after a dog was bitten by an infected mosquito for these microfilaria to appear in the blood. In the 1980's an extremely accurate diagnostic test was developed that could identify as few as two to six adult female worms in the heart. This test is called and occult heartworm test or ELISA test (Enzyme Linked Immunoassay, Snap Test by IDEXX Labs). The ELISA test is accurate six and one half months after the bite of an infected mosquito if several female worms were deposited by the mosquito. The diagnosis of heartworm disease in cats and ferrets is more difficult. In these species, microfilaria rarely persist more than a few days in the blood and the number of adult heartworms in the heart is usually less than five - too few to make the ELISA test accurate. In these animals we must rely on the later clinical signs of heart failure and lung obstruction or on visualizing the parasites using ultrasound. An additional complication in cats and ferrets is that treatment is much more risky than in dogs.
The heartworm preventative which has proven most effective is ivermectin (Heartguard, Iverheart, Ivomec, etc.). There are a number of other monthly or every six-month preventatives on the market but none has proven as effective as ivermectin. Ivermectin also has the important advantage of "reaching back" and killing migrating microfilaria even if a few months of medication were missed. It is also the only product that has also been found to kill adult parasites within the heart over a one-year period. Its only drawbacks are that it should not be used in collie-type breeds of dogs. In these dogs, another product, Interceptor (milbemycin oxime Ceba) should be used. Puppies should be placed on one of these preventatives as soon as they are weaned. Cats are less susceptible to heartworms but if they get them it is often fatal. That is why Merck markets an ivermectin product for cats. Less than 5% of the cat population of the United States receives this product.

At one time, diethylcarbamazine citrate (DEC, Filaribits, etc.) were given daily as a heartworm preventative. If days were skipped, the dog could develop heartworms and circulating microfilaria. If that dog was then given D.E.C., ivermectin or another newer heartworm preventative, a systemic reaction could occur when all the larval (baby) heartworm were suddenly killed. That was the rational for testing all dogs before placing them on D.E.C. This is not required before placing dogs on the newer products. In fact, Ivermectin is the chief medicine used to treat and eliminate heartworm larva from the blood. There is also little scientific rational for testing dogs yearly that have received ivermectin on a regular monthly basis. I know of no dogs that received ivermectin every 60 days that ever developed heartworms.

Treating Dogs that have Heartworms:

About one hundred days after being bitten by an infected mosquito, the heartworm arrives in the heart. For the first few years of infection, your dog may appear perfectly normal. But with time, and increasing number of worms within the right side of the heart, damage begins to occur. Each heartworm lives about two years. The first damage to occur is inflammation and thickening of arteries within the lungs. This is often accompanied by coughing. These dogs also become prematurely gray around the muzzle and lack energy and enthusiasm. Later in the disease, the parasites damage the right valves and portion of the heart causing blood to backflow into the venous system. This causes edema (fluid) retention in the lungs and enlargement of the liver. Dogs in this later stage of the disease often have pear-shaped abdomens - the result of an enlarged liver and fluid accumulation (ascites) - all signs of cardiac (heart) failure. A particularly severe form of heartworm disease is called "caval syndrome". This condition is due to large numbers of heartworms being transferred to the dog through mosquito bits in rapid succession. Because of the rapid large worm burden that occurs, the disease progresses very quickly to cardiac failure.

The only drugs that are approved for treatment of heartworm disease in the United States are arsenic-containing drugs and only one of these, melarsomine dihydrochloride (Immiticide, Meriel Ltd.) is currently on the market. The older formulation, caparsolate is no longer marketed. They both have relatively low therapeutic indexes. That is, the amount of drug that kills the heartworm in only slightly less than the amount that can kill the dog. Because of this, dogs that are clinically ill with the disease should first be treated with anti-inflammatory drugs, aspirin (to prevents clots, thromboembolism), antibiotics, exercise restriction and cardiac drugs (enalapril maleate) when appropriate. Dogs that do not yet show signs of heartworm damage should be treated with a single daily injection of Immiticide given daily for two consecutive days. The injection is quite painful and must be given with a long needle deep into the muscles of the loin. After these injections, the dog’s activity must be severely restricted for sixty days - the period of time that dead worms are disintegrating and showering into the lungs. This treatment will kill approximately 90% of the worms four months old and older. I treat dogs that are heartworm dog heartworm
clinically ill in a different manner. I give them a single dose of Immiticide and thirty to sixty days later, I place them directly on an oral monthly ivermectin heartworm preventative (2.7 micrograms/pound/month). If pretreatment blood work shows evidence of a failing liver, I do not give Immiticide. I simply begin the dog on monthly ivermectin. Studies have shown that within four month of beginning oral ivermectin treatment, seventy percent of the adult worms are dead (McCall, J. DVM Best Practices, March 2003 16-21). A few remaining heartworms – should any remain – are not a threat to the dog’s health.g heartworm

Heartworm Disease in Ferrets:
Although ferrets are susceptible to heartworms, the majority of them live indoors away from the mosquitos that
Cary (vector) the disease. Only a few worms are necessary to cause severe illness in ferrets. The signs of heartworms in ferrets are difficult breathing (dyspnea), listlessness, raspy respiration and cough, abdominal breathing (labored breathing), prominent rib cage, weight loss and lack of appetite. None of these signs are specific for heartworm disease. Heartworm diagnostic tests are often falsely negative. Treatment consists of aspirin to prevent blood clots (thromboemboli), diuretics to improve heart function (furosemide, Lasix) and ivermectin 0.025 to 0.05mg/lb/month.

Heartworm Disease in Cats:
The clinical signs of heartworm disease in cats are more severe than in dogs even though they tend to have considerably less worms. They rarely have larval heartworms (microfilaria) in their blood and ELISA tests are often falsely negative. In the southern gulf states and low4er Midwestern states, the incidence of heartworms in untreated cats is approximately 7%. Heartworm disease is hard to diagnose in cats because heart problems are often lacking, blood eosinophil counts are often normal and all diagnositic tests often give false negatives (missing about 50% of the posative cases). Some of the signs seen in cats with heartworm disease are chronic or periodic weight loss, coughing, labored breathing, lethargy, intolerance to exercise, or asthma-like symptoms. X-rays often show chronic bronchial pneumonia-like signs and enlarged pulmonary blood vessels. Because cats are extremely susceptible to Imitacide, the best treatment is to begin them on feline Heartguard (ivermectin 50 micrograms/pound body weight). Please read Threads at bottom of article.

Most Recent Information:

Also see Treads below. The March 2005 issue of Veterinary Practice News updates our information on heartworm disease. Only 55% of dogs in the United States take heartworm preventative. Down from 66% in 1998. It is estimated that 250,000 dogs have the disease in the United States at this time. A single mosquito bite can transfer 8-10 heartworm larva to your dog. It takes approximately 100 days from an infected bite for the parasites to mature in the heart. Cats are naturally resistant to heartworms and many less mosquito-injected larva reach the heart to mature. Thirty to forty percent of cats living in coastal regions of the US have been exposed to heartworms. The most common signs of infection in dogs are persistent cough, pervasive tiredness, reduced appetite and reluctance to exercise. It may take up to seven months after infection for a dog to reliably test posative with the diagnostic kits available today . Currently, the American Heartworm Society recommends a three-dose treatment schedule for infected dogs.

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