“What a beautiful world it would be if people had hearts like dogs.” All dog lovers will absolutely agree with this statement… And speaking of canine hearts, there are unfortunately some health issues related with the hearts of our furry friends which can cause very serious and even fatal consequences.
One of the most serious parts of this topic certainly is the heartworm disease in dogs. Facing the fact that the dog is suffering from such a serious disease can be heartbreaking for most of the owners… But, let’s start from the beginning and let’s see what causes this disease, what can we do to prevent it, what is the best heartworm medicine for dogs and what to do if the dog has been diagnosed with heart worm disease.
The role of your trusted veterinarian
The heart worm chemoprophylaxis is recommended for every dog since eight weeks of age. Depending on the medications which are used in this purpose, the prophylaxis treatment is repeated monthly, or at every six months. Annual heartworm testing is highly recommended for every dog, especially for the ones living in the endemic heartworm areas.
Always consult your trusted veterinarian and ask for advice which should help you perform the most effective mosquito protection of your dog, because the transmission of the infection is directly dependent on the mosquitoes as vectors. If you notice some unusual changes in your dog’s behavior such as exercise intolerance, lethargy, especially if dry cough is present, you should notify your veterinary doctor as soon as possible.
The diagnosis can be confirmed by heartworm antigen testing and microfilaria test which detects the juvenile forms of the parasite in the dog’s bloodstream. The veterinarian will first make a general clinical examination of the dog, paying particular attention to the lungs and the heat. In this purpose he can recommend additional diagnostic methods such as blood analysis, x-ray and echocardiography trying to determine the condition of the affected organs.
You must be aware that this is a life threatening condition for your dog so you should take your vet’s recommendations very seriously, especially the ones related with the maximum possible limitation of the dog’s activity. The heartworm treatment protocol is consisted of few steps, and your role as a owner is a very important in the parts related with the proper home care, so make sure your veterinarian gives you a clear and detailed explanation for everything you need to know.
The pathology behind the heartworm disease
This serious and complex health issue is actually caused by parasite infestation with nematodes (roundworms) known as Dirofilaria immitis. Dogs, as well as some wild canids, are hosts and main reservoirs for the spread of this parasite, and the role of the mosquito as a vector is of unique importance for its life cycle and its transmission. This parasite may also infect cats, ferrets and other animals, as well as humans in some very rare cases.
At the outset we must be sure to fully understand the life cycle, the mode of transmission, the reproductive cycle and the maturation process of these parasites because it is essential for the preventive measures and the medical treatment of this disease.
The lifespan of Dirofilaria immitis is quite long, actually much longer than the rest of the nematodes, and it usually lasts 7 to 9 months. The mosquito, as an unique vector of the disease, becomes infected with the juvenile forms of the parasite (called microfilariae) during sucking the blood of an infected dog. Larval stages of parasite development are taking place precisely in the body of the mosquito until they gradually get to the stage when they become infectious for the animals.
The duration of this process in the body of the mosquito depends on some environmental conditions related with the temperature and the humidity. Favorable temperature enables rapid development, while the lower temperature makes the development last longer.
Next, when the mosquito takes a blood meal from uninfected dogs it transmits the infectious forms of the parasites and thus enables the spread of the infection among the canine population. Here we can conclude that the transfer of parasites from animal to animal and from animals to human, is absolutely impossible without the role of the mosquito as a transmitter. Infectious larva forms of the parasite enter the dog’s body through the wound on the skin made by a mosquito bite.
They penetrate the muscle fibers and the blood vessels and they start their migration to their final destination – the heart and the lungs of the dog. At least 67-85 days are required for first juvenile forms to get to the pulmonary blood vessels. These parasites reach their full sexual maturity 120 days after the infection, and the first microfilariae in the canine circulation can be found 6-9 months post-infection.
The symptoms of heartworms in dogs
The symptoms of heartworms in dogs are connected with the organs that are most affected by the parasites – the heart and the blood vessels, the lungs and the kidneys. The number of worms, the duration of the infestation and the condition of the infested animal are directly affecting the severity of the disease. According to the severity of the symptoms the animals diagnosed with Dirofilariasis are divided in three classes. The first class includes asymptomatic dogs without visible signs of the disease.
The second class includes dogs with more severe symptoms such as dry coughing and exercise intolerance. The third class describes the most severe cases of dirofilariasis which include symptoms such as anemia, labored breathing, high blood pressure, tachycardia, exercise intolerance and fainting. Chronic right-heart failure can also be present in some of the most severe cases. Dirofilariasis is potentially lethal disease mainly because dead parasites can cause a block of the blood flow in the branches of the blood vessels, actually causing thromboembolism.
Diagnostic heartworm screening
Annual routine heartworm antigen and microfilariae testing is highly recommended for every dog. These diagnostic methods are often used for screening of the population of asymptomatic dogs as well as for verification of the suspected heartworm cases. Antigen tests detect proteins secreted by adult Dirofilaria immitis females in the dog’s body, and microfilaria test (the modified Knott test) concentrates microfilariae and allows greater sensitivity of the test.
The earliest that heartworms can be detected is 5 to 6 months after the infection. Additional radiographic and echocardiographic diagnostic methods are recommended in order to define the cardiopulmonary status of the infected dog and to determine the consequences caused by the disease.
The heartworm disease has been diagnosed around the globe and it is actually spread in all the areas of the world where environmental conditions allow adequate temperature and humidity for development of the mosquitoes as main vectors of Dirofilaria Immitis parasites.
Aedes albopictus (known as Asian tiger mosquito), Aedes sticticus, Aedes trivittatus, Aedes vexans and Aedes canadensis are some of the mosquitoes which are the most “responsible” vectors for the transmission of the heartworm parasites. Keeping this on mind we understand why proper mosquito control provided by using proven repellent products has a very important role in the management of this infection.
Mosquito nets for windows can also be used to prevent the entry of mosquitoes in the house from the outside. In order to reduce the dog’s exposure to the transmitters of the disease it is recommended to avoid long walks in a peak mosquito season, especially near areas with expected large presence of mosquitoes, such as ponds or lakes.
The heartworm infection is preventable!
The heartworm disease is preventable. Dog heartworm prevention is actually a priority in the management of this disease and it is implemented true chemoprophylactic medications which can be administered monthly (oral and topical administration) or at every 6 months (parenteral administration of the medications).
The chemoprophylaxis is recommended for every dog since a very early age (8 weeks) and it is specially recommended for dogs which are living in the endemic heartworm areas. It is believed that the reducing of the reservoirs of the disease through increasing the number of the dogs receiving proper chemoprophylactic medications will help decrease the prevalence of the disease among the unprotected dogs too.
Macrocyclic Lactones are group of drugs (ivermectin, milbemycin oxime, selamectin and moxidectin ) which affect the third and the fourth larva stage of microfilarieae. They are designed for oral use (ivermectin and milbemycin oxime) and topical use (selamectin and moxidectin) in a 30-day dosing intervals. It must be notified that these medications should not be used in some dog breeds such as Collies, because they are sensitive to this type of drugs.
Moxidectin-impregnated lipid microspheres administered subcutaneously in a slow-release formulation provide protection for 6 months, so therefore it is recommended to repeat the treatment every 6 months.
Treatment of heartworm disease in dogs
If your dog has been diagnosed with heartworm disease, first thing you should do is to make sure to restrict the dog’s activity as much as possible. It is of utmost importance in order to avoid the cardiopulmonary complications which can be caused by the presence of dead parasites in the blood vessels.
The heartworm treatment is provided in order to improve the clinical condition of the infected dog and to eliminate all the parasites from the dog’s body with minimal post-treatment complications. Symptomatic therapy consisted of fluid therapy, corticosteroids, vasodilatators and diuretics is required to normalize the condition of the dogs with more severe symptoms before specific heartworm adulticide drugs can be administered.
The heartworm treatment is provided through a three – dose regimen protocol of deep intramuscular injection of the adulticidal drug Melarsomine dihydrochloride. One month after the first dose, two more doses of the drug should be administered in a 24 hours interval in order to ensure greater success of the treatment. You should be aware that your dog must stay in the veterinary hospital for monitoring after every single treatment with melarsomine, because there is a risk of possible occurrence of post – treatment complications that require immediate veterinary attention.
Steroid therapy (Prednisone is the most commonly used corticosteroid drug) is administered as adjunct therapy in order to provide control of the clinical signs of pulmonary thromboembolism as a common complication after adulticide heartworm therapy. Steroid therapy in combination with antihistamine drugs is recommended before administering the micfrofilaricide treatment in dogs diagnosed with large infestation of parasites in order to minimize the potential immune response of the body.
Macrocyclic Lactones/Doxycycline therapy is often recommended in the treatment of the heartworm disease. Doxycycline (or Minocycline) is an antibiotic which is very effective in the reducing of the number of Wolbachia, a gram-negative bacteria which is present in the most of the roundworms. This bacteria helps warms survive and reproduce and therefore it plays a significant role in the pathogenesis of the heartworm disease. It is recommended to provide a 30 days antibiotic treatment using doxycycline, or minocycline as a replacement.
Macrocyclic Lactones (melasormine and topicl moxidectin) are group of drugs which are very effective against the juvenile forms of the parasites (less than 4 months old worms) and are used for elimination of microfilariae. These therapy which is consisted of Doxycycline and Macrocycline Lactones should be provided as soon as the dog has been diagnosed with the disease. Extreme exercise restriction is required during the entire heartworm treatment and few weeks after.
Speaking about the complications that can occur during heartworm infection, the most dangerous and potentially lethal issue is embolism which can be caused by blocking of the blood flow in the pulmonary and cardiac blood vessels by dead parasites or parts of their bodies. Occurrence of competitive bacterial infections of the lungs is also possible due the heartworm disease and their status is most precisely determined with the help of radiographic examination of the respiratory system of the dog.
The adult heartworms in some heavily infected dogs can obstruct the blood flow through the tricuspid valve. This issue is called Caval syndrome and it is potentially fatal for the dogs if the adult forms of the parasite are not extracted by a prompt surgical intervention. The symptoms of the caval syndrome include hemoglobinemia, hemoglobinuria, dyspnea, lethargy and a systolic murmur of tricuspid regurgitation.
The diagnosis is verified by echocardiographic visualization of the parasites located in the posterior vena cava and the tricuspide orifice. The surgical removing of the adult parasites before administering the heartworm therapy decreases the risk of post-treatment complications such as pulmonary thromboembolism. The location of the adult parasites is determined by echocardiogeaphy and the worms are usually removed using a surgical alligator forceps with fluoroscopic guidance.
The Heartworm antigen test and the microfilaria test are the most recommended methods for confirming of the efficacy of the treatment and should be provided 6 months after the heartworm treatment, when the antigen levels are expected to fall below the detectable limit. The dog is considered clear after two consecutive negative antigen test results provided 6 months apart. If the antigen results on these tests are positive the heartworm treatment must be repeated.
Recurrence of the disease is possible and in most of the cases it is caused by incomplete clearance of the adult parasites, maturation of the juvenile forms if the microfilaricide treatment wasn’t successful, or from a new heartworm infection if proper prevention has not been performed.
It is proven that certain surgical interventions, such as spaying and neutering, can be performed safely in heartworm positive dogs with less severe symptoms without increasing the risk of occurrence of postoperative complications. In dogs which are treated with adulticide heartworm therapy it is recommended to perform surgical interventions 6 months after the treatment has been finished.
Protecting the ones we love with all our hearts!
Be sure to perform the best possible protection for your dog from mosquito bites as a unique way of transmission of heartworms, regardless of the season of the year and regardless of the area in which you live. Keep on mind that annual routine heartworm screening is recommended for every dog and should be performed regularly.
Always take very seriously all the recommendations from your trusted veterinarian about the maximum possible restriction of the activity of your dog during and after the treatment because it is one of the key things in the management of the heartworm disease. This includes use of crates and small cages if needed and keeping the dog on leash while going for short, necessary walks.
Be sure you do everything in your power to help your dog overcome this health issue on the best possible way, and do your best to allow your dog healthy and happy life… Because in the end it doesn’t really matter if you own a pure breed or a mixed breed dog, a big dog or a small dog, as long as your dog fits in your heart!