The heartworm (Dirofilaria immitis) is a deadly parasite that is carried by mosquitoes. The highest infection rates are in areas within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries. Other areas with large mosquito populations also have a high rate of infestation. Heartworm disease is present on every continent except Antarctica.

Dogs should be on heartworm prevention wherever mosquitoes are present. A variety of effective types of heartworm prevention are available for your dog. Some treatments are oral while others are topical. Some treatments are taken daily while some are taken monthly. There are even treatments out there that may prevent other types of worms while preventing heartworms in your dog. All treatments are available only from a veterinarian. The monthly treatment is more popular and actually offers more safety and protection than the daily treatment. It doesn’t stay in the dog’s system for a month, but instead acts on a particular stage in the heartworm’s development. A monthly dose prevents heartworms from maturing in your dog. Your dog may only need to use a prevention treatment during the warmer months. Your veterinarian can advise you about when your puppy should start and if year-round prevention is necessary in your area.

Diethylcarbamazine is given daily. Ivermectin (Heartguard, Milbmycin (Interceptor) and Moxidectin (ProHeart) are given monthly. Selamectin (Revolution) is a new preventive applied topically that also prevents and treats fleas, ticks, and mites.

If you forget to give the prevention treatment prescribed by a vet, your dog may get heartworms. If you think that your dog has heartworms, do not give a daily prevention treatment, the dog could die. It must be tested for worms by the veterinarian first before starting the treatment, unless the dog is less than 7 months old. Puppies should be started on Heartworm preventative by 8 weeks of age, depending on the product being used, and then blood tested at 7 months of age.

The parasite

Heartworm parasites go through several life stages before emerging as adults. There are a total of four molts before the parasite may mature into an adult heartworm. The first two molts occur inside the mosquito and the last two occur inside the dog. A mosquito serves as the intermediate host for the larval stage of the worm, also known as the microfilariae. Development of the microfilariae in the mosquito, requires a temperature at or above 80 degrees Fahrenheit for about two weeks. No larval development takes place in the mosquito below 57 degrees F. The mosquito ingests the larva when it bites an infected dog. Once inside the mosquito’s body, the microfilaria goes through two molts over 14 days or longer, depending on the environment’s temperature. As they go through their first two molts, they change from an L1 to an L2 and then from an L2 to an L3. As an L3, they have reached the third stage of development and have migrated into the mosquito’s salivary glands. The mosquito then deposits the microfilariae it ingested into an uninfected dog when seeking another meal. The microfilariae will then burrow down into the dog and undergo several changes before reaching their adult form as a heartworm.

Once inside, the L3 larva goes through its first molt to the L4, within the first 15 days and as early as 2-5 days after infection. The second molt, from the L4 to the L5, occurs within the next 2 months. The L5 larva is considered a juvenile adult and works its way through the dog’s tissues to the heart as early as 70 days after first entering the dog’s body. The majority of L5 larvae arrive in the heart by 90 days where they stay and grow rapidly in length and size. After reaching maturity, the heartworm then travels to the right side of the heart through a vein and awaits the opportunity to reproduce. The worm can live here, sometimes reaching 14 inches in length, for 5-7 years.

Sexual maturity is achieved about three months after arrival in the heart. The worms continue to grow and multiply, infesting the chambers on the right side of the heart, arteries in the lungs, and sometimes the liver. The females start to pass thousands of microfilaria per day into the blood. These young microfilariae can circulate in the bloodstream for up to three years, before finding another mosquito to pass them on to the next dog.

A soft cough, the first sign of heartworm infestation, may not show up for a year after infection. The cough will worsen until the dog is weak, loses weight and condition, and may even cough up blood. Breathing will become worse and the dog will no longer be able to enjoy walks without respiratory distress. Congestive heart failure will then ensue, resulting in the dog’s death.

The smaller the dog, the fewer worms it takes to cause big problems. Once the number of worms grows too large, based on the size and activity level of the dog, the adult worms move into the heart and the symptoms begin to occur.

Testing

The most common way of checking for heartworms is to check the blood for circulating microfilarae, but this method may fail to detect the presence of adult heartworms in as many as 20 percent of all tested dogs. Another test, the occult heartworm test, is slightly more expensive, but more accurate. The occult heartworm test, tests for the presence of antigens to heartworms in the blood. Many veterinarians prefer to do both tests because the absence of microfilariae in the blood does not necessarily mean that there are no adult worms in the heart. Both tests are done with a single blood draw, preferably in the early spring before daily temperatures warm above 57 degrees F.

With either test, the presence of heartworms will not be detectable until nearly seven months after infection. Heartworms are treatable in their early stages, but the treatment is expensive and is not without risks, treated dogs go through weeks of discomfort while the worms are killed and expelled from their bodies. If left untreated, heartworms will kill your dog.

Radiographs (X-rays) can also detect the presence of adult heartworms in the heart and lungs.

Treatment

Adult heartworms are treated by injecting a drug into the dog twice a day, for two days. This kills the adult heartworms in the heart and adjacent vessels over a period of about 30 days. Some adult worms die in a few days and start to decompose, while others remain and will die within a month. As they break up, they are carried to the lungs, where they lodge into small blood vessels and are eventually reabsorbed by the dog’s body. This is a dangerous period, where plenty of rest and quiet should be provided for the dog. The dog should not be allowed to exercise for 1 month following treatment, to give the dog’s system a chance to absorb the dead worms. Exertion can cause the dead worms to dislodge, travel to the lungs, and cause death. A cough is noticeable for 7 to 8 weeks after treatment in many heavily infected dogs. If the dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, you should notify your dog’s veterinarian. Antibiotics, cage rest, and intravenous fluids, are usually recommended in these cases.

Microfilaria are treated approximately one month after killing the adult heartworms, if the infection is not occult (meaning no microfilariae were present). The infected dog would need to stay in the hospital for the day after receiving the treatment. Seven to ten days later, a test is performed to determine if microfilariae are present. If they have all been killed, the treatment is complete. If there are still some present in the blood, treatment for microfilariae is repeated.

Some dogs may be diagnosed with advanced heartworm disease. This means that the heartworms have been present long enough to cause substantial damage to the heart, lungs, blood vessels, kidneys, and liver. A few of these cases will be so far advanced that it will be safer to just treat the organ damage rather than risk treatment to kill the worms. Dogs in this condition are not likely to live more than a few weeks or months. If the dog does survive, it may need lifetime treatment for the failing heart, even after the heartworms have been killed. This includes the use of diuretics, heart drugs, aspirin, and special low salt, low protein diets.

About the Author:

Jennifer Bryant is a life-long lover of dogs. Jennifer loves and breeds American Pit Bull Terriers in her North Georgia home with her 2 young children. Her love of dogs has inspired her to create a website for herself (as well as other dog breeders) to advertise their occasional litter of puppies and to educate others with dog breed information and useful articles.

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