April 28, 2006

Heartworms in Dogs

The heartworm ( Dirofilaria immitis ) is a deadly parasitethat is carried by mosquitoes. The highest infection rates arein areas within 150 miles of the Atlantic and Gulf coasts andalong the Mississippi River and its major tributaries. Otherareas with large mosquito populations also have a high rate ofinfestation. Heartworm disease is present on every continentexcept Antarctica.

Dogs should be on heartworm prevention wherever mosquitoes arepresent. A variety of effective types of heartworm preventionare available for your dog. Some treatments are oral whileothers are topical. Some treatments are taken daily while someare taken monthly. There are even treatments out there that mayprevent other types of worms while preventing heartworms in yourdog. All treatments are available only from a veterinarian. Themonthly treatment is more popular and actually offers moresafety and protection than the daily treatment. It doesn’t stayin the dog’s system for a month, but instead acts on aparticular stage in the heartworm’s development. A monthly doseprevents heartworms from maturing in your dog. Your dog may onlyneed to use a prevention treatment during the warmer months.Your veterinarian can advise you about when your puppy shouldstart and if year-round prevention is necessary in your area.

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

If you forget to give the prevention treatment prescribed by avet, your dog may get heartworms. If you think that your dog hasheartworms, do not give a daily prevention treatment, the dogcould die. It must be tested for worms by the veterinarian firstbefore starting the treatment, unless the dog is less than 7months old. Puppies should be started on Heartworm preventativeby 8 weeks of age, depending on the product being used, and thenblood tested at 7 months of age.

The parasite

Heartworm parasites go through several life stages beforeemerging as adults. There are a total of four molts before theparasite may mature into an adult heartworm. The first two moltsoccur inside the mosquito and the last two occur inside the dog.A mosquito serves as the intermediate host for the larval stageof the worm, also known as the microfilariae. Development of themicrofilariae in the mosquito, requires a temperature at orabove 80 degrees Fahrenheit for about two weeks. No larvaldevelopment takes place in the mosquito below 57 degrees F. Themosquito ingests the larva when it bites an infected dog. Onceinside the mosquito’s body, the microfilaria goes through twomolts over 14 days or longer, depending on the environment’stemperature. As they go through their first two molts, theychange from an L1 to an L2 and then from an L2 to an L3. As anL3, they have reached the third stage of development and havemigrated into the mosquito’s salivary glands. The mosquito thendeposits the microfilariae it ingested into an uninfected dogwhen seeking another meal. The microfilariae will then burrowdown into the dog and undergo several changes before reachingtheir adult form as a heartworm.

Once inside, the L3 larva goes through its first molt to theL4, within the first 15 days and as early as 2-5 days afterinfection. The second molt, from the L4 to the L5, occurs withinthe next 2 months. The L5 larva is considered a juvenile adultand works its way through the dog’s tissues to the heart asearly as 70 days after first entering the dog’s body. Themajority of L5 larvae arrive in the heart by 90 days where theystay and grow rapidly in length and size. After reachingmaturity, the heartworm then travels to the right side of theheart 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 inthe heart. The worms continue to grow and multiply, infestingthe chambers on the right side of the heart, arteries in thelungs, and sometimes the liver. The females start to passthousands of microfilaria per day into the blood. These youngmicrofilariae can circulate in the bloodstream for up to threeyears, before finding another mosquito to pass them on to thenext dog.

A soft cough, the first sign of heartworm infestation, may notshow up for a year after infection. The cough will worsen untilthe dog is weak, loses weight and condition, and may even coughup blood. Breathing will become worse and the dog will no longerbe able to enjoy walks without respiratory distress. Congestiveheart failure will then ensue, resulting in the dog’s death.

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


The most common way of checking for heartworms is to check theblood for circulating microfilarae, but this method may fail todetect the presence of adult heartworms in as many as 20 percentof all tested dogs. Another test, the occult heartworm test, isslightly more expensive, but more accurate. The occult heartwormtest, tests for the presence of antigens to heartworms in theblood. Many veterinarians prefer to do both tests because theabsence of microfilariae in the blood does not necessarily meanthat there are no adult worms in the heart. Both tests are donewith a single blood draw, preferably in the early spring beforedaily temperatures warm above 57 degrees F.

With either test, the presence of heartworms will not bedetectable until nearly seven months after infection. Heartwormsare treatable in their early stages, but the treatment isexpensive and is not without risks, treated dogs go throughweeks of discomfort while the worms are killed and expelled fromtheir bodies. If left untreated, heartworms will kill your dog.

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


Adult heartworms are treated by injecting a drug into the dogtwice a day, for two days. This kills the adult heartworms inthe heart and adjacent vessels over a period of about 30 days.Some adult worms die in a few days and start to decompose, whileothers remain and will die within a month. As they break up,they are carried to the lungs, where they lodge into small bloodvessels and are eventually reabsorbed by the dog’s body. This isa dangerous period, where plenty of rest and quiet should beprovided for the dog. The dog should not be allowed to exercisefor 1 month following treatment, to give the dog’s system achance to absorb the dead worms. Exertion can cause the deadworms to dislodge, travel to the lungs, and cause death. A coughis noticeable for 7 to 8 weeks after treatment in many heavilyinfected dogs. If the dog shows loss of appetite, shortness ofbreath, severe coughing, coughing up blood, fever, and/ordepression, you should notify your dog’s veterinarian.Antibiotics, cage rest, and intravenous fluids, are usuallyrecommended in these cases.

Microfilaria are treated approximately one month after killingthe adult heartworms, if the infection is not occult (meaning nomicrofilariae were present). The infected dog would need to stayin the hospital for the day after receiving the treatment. Sevento ten days later, a test is performed to determine ifmicrofilariae are present. If they have all been killed, thetreatment is complete. If there are still some present in theblood, treatment for microfilariae is repeated.

Some dogs may be diagnosed with advanced heartworm disease.This means that the heartworms have been present long enough tocause substantial damage to the heart, lungs, blood vessels,kidneys, and liver. A few of these cases will be so far advancedthat it will be safer to just treat the organ damage rather thanrisk treatment to kill the worms. Dogs in this condition are notlikely to live more than a few weeks or months. If the dog doessurvive, it may need lifetime treatment for the failing heart,even after the heartworms have been killed. This includes theuse of diuretics, heart drugs, aspirin, and special low salt,low protein diets.

About the author:

Jennifer Bryant breeds American Pit Bull Terriers and buildswebsites in her spare time.

Bryant’s RedDevils

Puppies and Dogsfor Sale

This article may be reprinted but the content and signaturemust remain intact.

Jennifer Bryant

%d bloggers like this: