In February of 2013, my English Shepherd, Nell, and I traveled to North Carolina for a herding camp. There were a couple of rainy days but the rest of the time the weather was sunny and very typical for a southern spring. It was a nice break from the cold and snow back home in Massachusetts. Nell and I had a great time and learned a lot, but we came home with more than just knowledge.
In August 2013 Nell was due for her annual visit to the veterinarian. While there she had a heartworm and tick panel test done. 
The test results consist of a series of four dots. One blue dot is good, as that is the control and means the test worked correctly. Two or more blue dots mean a positive result and the location of the dot identifies for which disease the dog tests positive. On Nell’s test, two blue dots showed up – one being the control, the other signified heartworm. However, the heartworm dot was so faint it was barely visible, so we decided it was an anomaly of the test. After all, Nell was on heartworm preventative and hadn’t missed a dose.
Living in the Northeast, the mosquito season usually runs from May through November. Since mosquitoes are the carriers of heartworm, the recommendation is to give a monthly heartworm preventative from April through December. Nell had received all of her doses.
In April 2014 Nell and I went to a Health Clinic sponsored by the local Kennel Club. It was held in our veterinarian’s office. While Nell was there for the CERF eye exam I also did her annual heartworm and tick panel test. Since she is out in the fields and woods a lot I worry about the tick borne diseases, so I wanted to have her checked. We ran the little blue dot test. Fortunately, she tested negative for the tick borne diseases, but that pesky blue dot for heartworm was back again and this time it was a much brighter blue and could not be ignored. The vet suggested we send a blood sample to an outside laboratory for further testing. The results came back positive for heartworm. It appears we also brought back heartworm from our trip south for “winter” herding camp.
How could this have happened? Nell was on heartworm preventative. However she was on the recommended dosing for dogs in the Northeast where mosquitoes are not a problem during the cold winter months. Nell did not receive heartworm preventative during January, February, and March. She was able to become infected in February while we were in North Carolina. Mosquitoes there don’t die off from the cold like they do during the winters in the Northeast. It takes six months after being infected for the test to show a positive result. So that very faint dot we saw last August was actually the result of being infected six months prior, which would have been February.
Nell’s treatment for heartworm consisted of a series of Immiticide injections, over several month deep into the muscle in the lumber region of the spinal column. After each injection it was vitally important that she rest for a month and not be involved in any strenuous activities. So Nell missed out on a lot of activities and a summer of herding. This has been a hard and scary lesson, but from now on each of my dogs will be receiving heartworm preventative all year long, so no matter when and where we travel they will be protected. I don’t want to bring back anything but knowledge from future herding trips.
Heartworm Disease
Sourced from the FDA, American Heartworm Society, and DVM360
Canine heartworm disease develops when a dog is bitten by a mosquito carrying microscopic larvae of a parasite called Dirofilaria immitis. Adult female heartworms living in an infected dog, fox, coyote, or wolf produce microscopic larvae called microfilaria that circulate in the bloodstream. 
When a mosquito bites a dog with microfilaria in the blood, it ingests the microfilariae along with the blood. Over the following 10 to 14 days, these microfilariae develop and mature into infective larvae inside the mosquito. When the mosquito bites another dog, the larvae are left behind to enter the fresh wound. In 6 to 7 months, these infective larvae migrate inside the dog, eventually reaching the heart and vessels of the lungs, where they continue to grow to full maturity. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. 
They make their home in the right side of the heart and vessels of the lungs (pulmonary arteries), often causing lung disease and heart failure. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats.
In the United States, heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states.
Heartworm disease is not contagious, meaning that a dog cannot catch the disease from being near an infected dog. Heartworm disease is only spread through the bite of a mosquito.
Inside a dog, a heartworm’s lifespan is five to seven years. Adult heartworms look like strands of cooked spaghetti, with males reaching about 4 to 6 inches in length and females reaching about 10 to 12 inches in length. 
The number of worms living inside an infected dog is called the worm burden. The average worm burden in dogs is 15 worms, but that number can range from 1 to 250 worms.
The severity of heartworm disease is directly related to how many worms are living inside the dog (the worm burden), how long the dog has been infected, and how the dog’s body is responding to the presence of the heartworms. The dog’s activity level also plays a role in the severity of the disease and in when symptoms are first seen. Symptoms of heartworm disease may not be obvious in dogs that have low worm burdens, have been recently infected, or are not very active. Dogs that have heavy worm burdens, have been infected for a long time, or are very active often show obvious symptoms of heartworm disease.
There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms.
- Class 1: No symptoms or mild symptoms such as an occasional cough.
- Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
- Class 3: General loss of body condition, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
- Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die.
Not all dogs with heartworm disease develop caval syndrome. However, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.
The treatment for heartworm can be potentially toxic to the dog’s body and can cause serious complications, such as life-threatening blood clots to the dog’s lungs. Treatment is expensive because it requires multiple visits to the veterinarian, bloodwork, x-rays, hospitalization, and a series of injections.
Annual testing of all dogs on heartworm prevention is recommended. Dogs older than six to seven months of age should be tested for heartworms before starting heartworm prevention. A dog may appear healthy on the outside, but on the inside, heartworms may be living and thriving. Although they may shorten the lifespan of the worms, heartworm preventives do not kill adult heartworms. If a heartworm-positive dog is not tested before starting a preventive, the dog will remain infected with adult heartworms until it gets sick enough to show symptoms. Also, giving a heartworm preventive to a dog that has an adult heartworm infection may be harmful or deadly. If microfilariae are in the dog’s bloodstream, the preventive may cause the microfilariae to suddenly die, triggering a shock-like reaction and possibly death in some dogs.
The Best Treatment Is Prevention! Talk to your dog’s veterinarian to decide which preventive is best for your dog.
Marianne Dwight’s Nell