Heartworm (Dirofilaria immitis) is a serious threat to dog wellness. Over the last 2 decades its recognition worldwide has increased dramatically. Heartworm infection is quite debilitating, even deadly. Furthermore, the treatment is costly and difficult. Thus, prevention of heartworm is a key goal in wellness. The risk of contracting heartworm in the Okanagan is not high, but with a number of cases reported it is a disease to be taken seriously.
Heartworm is able to infect wild animals such as coyotes, wolves, fox etc. As a result, once heartworm is introduced to an area it becomes permanent, as wild animals are not able to be treated with a preventive and transmission goes unchecked.
Infective heartworm larvae are transmitted between animals. Via mosquitos, and their continued transmission is directly related to environmental conditions within an area. The correct species of mosquito is necessary (about 70 are known to carry the disease) and a temperature of roughly 21 degrees for at least 14 days is required for the heartworm larvae to complete the required moults with the mosquito. Typically, during a blood meal, a mosquito will pick up1 or 2 microfilarie, and rarely 3. A moderate rate of infection, both in the intermediate host (mosquito) and the final host (dog) is most conducive to heartworm transmission. If there are too many circulating microfilaria the mosquito will be killed by the number it ingests.
Adult worms range in size from 6-12 inches and generally live in the right ventricle of the heart and pulmonary arteries, where the vessels of infected dogs begin to thicken and expand. Eventually, the dog develops increased blood pressure (hypertension) as a result of the damage to the arteries. In very severe cases, the worms can actually enter right side of the heart. If the number of worms or “worm burden” becomes significant, they can cause congestive heart failure as well as kidney and liver failure.
Heartworm life cycle
- Adult heartworms in the heart and lungs of infected animals (domestic or wild)
- Fertile female heartworm release microfilarie into the bloodstream
- Mosquito swallows the micofilarie when it bites
- Microfilarie grow to the infective stage in the mosquito
- The mosquito serves as an intermediate host for 2 moults of the larvae
- In the infective third stage, the larvae enter the mosquito’s mouth parts, ready to be transmitted.
- The mosquito bites the dog and the larvae are deposited via the saliva
- The 3rd moult to L4 occurs 3-4 days after the infection when the larvae begin their journey under the dogs skin
- The 4th moult takes place 50-70 days after the infection. During that time the larvae have been working their way toward the pulmonary arteries
- Larvae become adults in heart and lungs
- Within 70-100 days after infection, the worms arrive in the pulmonary arteries. After an additional 90 days the worms reach sexual maturity, mate and release microfilarie, completing the cycle
The time from infection by a mosquito to an active adult is completed in approximately 6 months
The technical term for canine heartworm disease is dirofilariasis. It is only fairly recently that it was recognised as a serious problem. Until the early 1970’s, it was confined primarily to the costal areas of the south-eastern and southern sections of the USA. Since that time the disease has spread so rapidly that it is found in all but in the mountainous and desert areas of the country. This spread is due to human migration from the heartworm –endemic parts of the country (with then heartworm infected dogs) to other areas where heartworm was previously rare or unrecognized. The heaviest infestations of heartworm continue to be found along the major waterways and tributaries, due to the high prevalence of mosquitos, but the incidence is increasing across Canada.
The rate of transmission of heartworm is directly proportional to the number of mosquitos in the area. It is also dependant on the number of infected dogs. Dogs are increasing susceptible if they live outdoors, and susceptibility increases with years of exposure. Numerous mosquito species have demonstrated the ability to carry the heartworm larvae.
Dogs with extremely high concentrations of micofilaria (20, 000 per millilitre or greater) are actually less efficient disease carriers than dogs with lower concentrations, because the mosquito can take in so many microfilaria that it dies.
For the past 65 years, heartworm disease has also been know to occur in cats, although only in the past 20 years has it been diagnosed with any regularity. The incidence is believed to be 10% or less of that of unprotected dogs living in the same area. Cats are less susceptible to the heartworm than dogs; however, arterial damage and clotting can be severe in some cats and may be a cause of sudden death.
Generally, infections in cats consist of a very light worm burden (1 to 3 adult worms), which may be of the same sex; therefore, no microfilaria are generated. Diagnosis of heartworm infection in cats is difficult due to ambiguous test results. However, chronic vomiting and coughing can be signs of an infection. Significant pulmonary disease can be identified from radiographs.
Heartworm infection also occurs in wild canines, ferrets, muskrats, raccoons, bears, horses, foxes, and coyotes. In many species, however, the life cycle of the worm ends at the adult stage. It is due to infestation in the wild animals of an area that heartworm becomes a permanent problem once introduced, as wild animals are not treated with preventives and the transmission by mosquitos is allowed to go unchecked.
Heartworm infection in humans usually consists of a single worm, often located just under the skin, although some do reach the pulmonary artery. The incidence in humans is probably far greater than what has been reported, primarily because only countries with advanced medical services are likely to recognize the problem.
The worms generally make a home for themselves in the pulmonary arteries. In the living dog, heartworms are not found in the right side of the heart except when the dog is very heavily infected. They may also be present in the veins connecting to the liver, where large numbers can cause severe damage.
Normal Progression of Clinical Signs
Adult heartworms living in the pulmonary arteries cause inflammation inside the vessels. Blood clots also may form within these arteries, which may completely obstruct segments of the vessels. As the damage progresses, smaller branches of arteries become affected. The inflammation and occlusion of blood vessels leads to increased blood pressure (pulmonary hypertension).
As the pressure increases, the pulmonary arteries try to expand to circulate needed blood. In addition, the right ventricle of the heart must strain to pump blood through the affected arteries. Eventually, the right side of the heart enlarges and may fail. Normally, 3 to 5 years is required for clinical pulmonary hypertension to develop as a result of heartworm disease. However, clinical signs may develop more quickly in active dogs.
Unfortunately, much of the internal damage to the dog occurs before any clinical signs are evident. When clinical signs do develop, they resemble signs of general heart failure consisting of sporadic coughing and lowered exercise tolerance. As time passes, the cough becomes chronic and is accompanied by noticeably difficult respiration.
In advanced cases of the disease, a dog will often collapse after even light physical exertion. Most dogs do develop congestive heart failure and ascites (an accumulation of fluid in the abdomen). When infection becomes this severe, normal health may not be fully restored even after an effective treatment program. Thus, early detection of infection is of critical importance.
Liver Failure or Caval Syndrome
A fatal condition called caval syndrome is encountered in dogs exposed to a massive infection over a short period of time. Caval syndrome occurs when a large mass of worms cluster in the vena cava (the large blood vessel leading to the heart from the liver, kidneys, and abdominal organs). These worms restrict blood flow to the heart and cause anaemia and extensive liver and kidney damage. Two U.S. studies have shown that 16 to 20% of dogs with heartworm disease develop caval syndrome. Most commonly found in young dogs who have acquired little or no protective immunity, the clinical signs include sudden weakness, rapid breathing, and brown urine. If the worms are not eliminated, death occurs in 24 to 72 hours. In the severe cases, immediate surgical removal is the only option for survival.
A combination of methods, including physical examination, diagnostic tests and history of the dog are combined when making a determination. Two extremely important factors are:
- The local incidence of disease
- The accuracy of the tests used
The American Heartworm Society recommends that dogs being seen for the first time should be both Antigen and Microfilarial tested. After the first year they should be antigen tested and this is done every two or three years. Your veterinarian will tell you the recommendation for your area. Depending on the climate of the area of Canada you are in, it may not be necessary to test the dog if it was born after the previous mosquito season ended. If it has never been exposed to mosquitos, it can not have heartworm.
An antigen test is used to detect the presence of adult heartworm in the dog. Adult worms release a substance called and “Antigen” and when combined with elements of a test kit their presence produces a change in the colour of the test medium. This involves taking your dog to the veterinarian for a blood sample, usually a month or so before heartworm season begins in your area. The blood is analyzed and a determination made that the dog is either positive or negative for the presence of adult worms. False positives are rare and usually the result of operator error. A false negative may occur if there are less than 3 adult worms present, (not enough antigen generated to be detected) or if there are only male worms in the heart (as the test only reacts to the antigen of the female).
Another type of test is a microfilaria screening test. This is where the blood is examined for microfilaria under a microscope for the actual microrofilarie. This test is usually done as a confirmation if the antigen test shows positive. A false positive may occur with this test if the microfilarie detected are actually D. reconditum. A microfilaria screen test may show a false negative if:
- Only a single worm is present or all worms are of the same sex;
- The adults are too old or too young to reproduce;
- The dog’s immune system is eliminating microfilarie as they are produced;
- A microfilaricidal drug such as Sentinel® or Interceptor® has destroyed the microfilarie.
X-rays are sometimes taken to confirm heart enlargement due to the presence of adult worms. Radiographs can confirm the presence of heartworm disease, but not whether heartworms are still present. They are best used to assist in assessing the degree of damage that has occurred and the extent to which the lung may be able to cope with the blood vessel blockage that inevitably follows the destruction of adult worms.
Treatment of heartworm infection is a three stage process because there is no product available which is effective against both adult worms and microfilaria. The treatment program recommended by the American Heartworm Society is to:
- Remove all adult worms (with an adulticide or surgery)
- In 3 to 4 weeks, administer a product to kill the microfilaria (a microfilaricide such as Sentinel® or Interceptor®
- Prevent future re-infection
Eliminating Adult Worms
Thiacetarsamide (Caparsolate®) and Melarsomine (Imidicide®) are the two products approved for adult heartworms. However, because both are arsenic compounds, they are potentially dangerous and can have serious side effects. Your veterinarian will be able to detail the benefits of each drug and will establish the treatment regime for your dog should it become necessary.
In thiacetarsamide treatment, a dog is injected intravenously twice a day for two days. The dosage kills most male and many female heartworms, but less than 50% of infections are eliminated completely. Researchers have found that increasing the recommended dosage by 10% improves the efficacy of the treatment, although the risk of toxic reactions is slightly higher. Occasionally, treatment is suspended before the entire series of injections has been completed if a reaction occurs. Most dogs can be given a full series of injections at a later time without incident.
After thiacetarsamide therapy, adult worms begin to die in 5 to 7 days, but complete elimination of the heartworms may not occur for 30 to 60 days. The dog’s activity is severely restricted during this time to prevent blood vessel blockage as the worms break up into the lungs. Some veterinarians also prescribe other supportive therapy such as aspirin, steroids, and antibiotics; however, studies are not conclusive about the effectiveness of such treatment.
Melarsomine is a newer arsenic compound that is more effective than thiacetarsamide in eliminating adult heartworms at a lower dose. It is given via intramuscular injection; either two injections 24 hours apart or a single injection followed in 30 day by the normal two injection schedule.
Melarsomine is thought to be more effective for adult heartworms, but the resulting side effect from greater numbers of dying worms means a slightly higher chance of reaction due to the fragmenting adults and may be potentially more serious than with thiacetarsamide.
In dogs with caval syndrome, surgery is sometimes necessary if the dog’s life is to be saved. In these cases the worms are extracted by inserting a long forceps through the right jugular vein into the vena cava (the large blood vessel leading to the heart) where a large number of worms are usually found. Once the dog is stable, thiacetarsamide is administered to remove any remaining worms.
When all danger of post-treatment danger has passed, usually in 4 to 6 weeks, the elimination of microfilarie is begun. There are no approved microfilaricides available on the market but the American Heartworm Society has recommended Milbemycin oxime (Interceptor® or Sentinel®) for this purpose.
Although not labelled as a microfilaricide, findings from laboratory tests indicate the regular monthly dose of milbemycin oxime that is used to prevent heartworm has significant microfilaricidal properties, and its use is recommended by the American Heartworm Society for this purpose.
In studies with heartworm infected dogs, microfilaricidal counts showed a marked disease, with no severe reactions as a result of the microfilaricidal effects. Side effects that do occur are generally slight to moderate and can include abdominal respiration, pale mucous membranes, shock, vomiting, and decreased activity.
Heartworm prevention is the goal of every veterinarian and pet owner. Pet “wellness” has thankfully begun the transition from treatment to prevention of most common ailments facing pets today.
Once a dog has been tested for heartworm it’s important that a prevention program suitable for the climate/region of the pet be put in place.
There are several preventive products on the market. Most are simple to administer, although their spectrum of activity may vary.
At K.L.O. Veterinary Hospital we carry 4 different heartworm preventives for you to choose from. They are:
- Interceptor®: This product protects against heartworm disease, roundworms, hookworms, and whipworms, and is offered in a once a month oral flavoured tablet. The active ingredient in Interceptor® kills any heartworm larva that your dog may have contracted in the previous months.
- Sentinel®: A monthly oral medication that protects against heartworm disease, roundworm, hookworm, whipworm, and also fleas. This can be used safely in all breeds of dogs.
- Heartgard Plus®: This product is a once a month chewable tablet. It protects your pet against heartworm disease, roundworm, and hookworm. The active ingredient, Ivermectin, is extremely safe. In doses far greater than those used to treat heartworm disease, it has been associated with toxicity in collies and collie breeds.
- Revolution®: A monthly topical liquid treatment that is used for flea control and prevention, heartworm prevention, control and prevention of ear mites, sarcoptic mange, ticks, and all internal parasites.