Heartworm disease is a potentially fatal, but preventable, infection caused by a worm parasite, Dirofilaria immitis. The organism is transmitted by mosquitoes, which carry the heartworm larvae (called microfilariae) from an infected animal host to a new animal host. Once the larvae arrive in a new host, they grow into adult worms in several months and live in the blood vessels that serve the heart and lungs. In advanced infections, the heartworms enter the heart as well. The presence of parasites stresses the dog’s heart and causes inflammation of the blood vessels and lungs. In addition, severe complications are possible when the number of worms present becomes high or when the heartworms die. Susceptible animals can be reinfected numerous times, so different stages of heartworm infections may be present in the same animal.
The life cycle of the heartworm
At least 70 species of mosquitoes can transmit heartworm disease. Heartworm disease has been reported in most countries with temperate, semitropical, or tropical climates around the world, including the United States, Canada, Australia, Latin America, and southern Europe. The infection risk is greatest in dogs that are housed outdoors. Any dog, whether it is an indoor or an outdoor pet, is capable of being infected; all it takes is a bite from a mosquito carrying the infective heartworm larvae. Most infections are diagnosed in medium- to large-sized dogs that are 3–8 years old.
The severity of a heartworm infection and the signs depend on the number of worms present, the immune response of the infected dog, the duration of infection, and the activity level of the animal. Most dogs are highly susceptible to heartworm infection, and the majority of infective larvae develop into adult worms. The presence of heartworms in the vessels serving the heart and lungs, and in the heart itself, causes irritation and inflammation of the affected vessels or heart chamber. In addition, the dog's immune system does further damage to the blood vessels, especially in response to dead worms. With longterm infection (over 1 year in duration), the constant irritation will lead to scarring and reduced flexibility of the blood vessels. Heartworms typically live for 3–5 years in an animal's body.
In general, small dogs do not tolerate heartworm infections or treatments as well as large dogs do. This is mostly because small dogs have smaller blood vessels and heart chambers, so they can tolerate fewer worms without vessel damage or blockage.
The bacterial organism Wolbachia pipiens lives within heartworms. It is thought to contribute to the damage caused by heartworm disease. Importantly, the bacteria is necessary for the normal development and reproduction of heartworms. As such, your veterinarian will likely include medication to kill these bacteria as part of the treatment plan.
Signs and Diagnosis
Common signs of heartworm infection include coughing, exercise intolerance, failure to grow, labored breathing, a blue or purplish discoloration of the skin and gums, spitting up blood, fainting, nose bleeding, and the accumulation of fluid in the abdominal cavity. The severity of the signs is often related to the amount of lung damage and the dog’s activity level. Active dogs (such as hunters and performers) will typically show more dramatic signs of infection than will less active dogs. Even though they may have many worms, sedentary dogs may show few or no signs. In addition, clinical signs can be seen as worms die or if blood clots or worm fragments block blood vessels.
Your veterinarian has a number of options for diagnosing heartworm infection. The antigen detection test is the preferred method for diagnosis verification of a suspected heartworm infection. This test is easy to perform and is considered the most accurate test available for dogs. However, this test cannot identify early infections because it can only identify adult, female worms. This is part of the reason your veterinarian will likely recommend that this test be performed every year. Other diagnostic tools include echocardiography (a type of ultrasonography), blood tests, and chest x-rays. Your veterinarian will select tests based on your history of use of preventive medication and the overall health of the dog.
Prior to treating your dog for heartworm infection, your veterinarian will want a comprehensive medical history. Selecting the most appropriate treatment regimen will depend on many factors, including whether the dog has another disease that may influence the course of treatment, the number of worms present, and the extent of damage they have caused. Severely affected dogs may need medications to stabilize heart and lung disease before heartworm treatment.
The only drug currently available to treat infection with adult heartworms is melarsomine dihydrochloride, an arsenical compound. Appropriate treatment kills both mature and some immature heartworms. There are 2 approved treatment protocols, or methods, used for treating existing infections; a 2-dose protocol or a 3-dose protocol. In both cases the medication is delivered by injection deep into the muscles of the dog’s back, alternating sides of the back between treatments. About one-third of dogs will have some local pain, swelling, soreness with movement, or rarely a sterile abscess at the sites of the injections. The 2-dose protocol consists of 2 doses, 24 hours apart. The 3-dose protocol introduces a delay in the treatment schedule. Under this schedule, the dog would first receive a single injection. One month later, the second and third injections will be administered 24 hours apart. Many veterinarians choose to use the 3-injection treatment, regardless of the dog’s stage of heartworm disease, because it may be safer for the dog and more efficient at killing all the parasites.
Some veterinarians will prescribe medications before and after melarsomine. The antibiotic doxycycline can kill Wolbachia pipiens bacteria and young heartworms and reduce lung injury associated with melarsomine treatment. Preventive heartworm medications are also used to kill young larval stages. Corticosteroids can reduce inflammation associated with dying worms. Follow your veterinarian's instructions closely, and give medications as directed.
As a result of treatment, there is a risk that dead heartworms will cause severe respiratory problems, especially if dogs are not properly confined to restrict activity following treatment. These problems can occur from several days to 6 weeks after treatment of a heartworm-infected dog. The signs of post-treatment complications include coughing, spitting up blood, labored or rapid breathing, lethargy, lack of appetite, and fever. In dogs with complications of heartworm treatment, cage confinement, with several days of oxygen treatment and drugs to control inflammation and reduce blood clotting, may help alleviate the problem. If properly cared for, most dogs begin to recover from treatment complications within 24 hours.
To reduce the potential for such “dead worm reactions,” all dogs should be confined throughout treatment and for 4–6 weeks after the final injection of melarsomine. Dogs treated for heartworm should be placed on heartworm preventive drugs (usually before treatment with melarsomine) and tested after 8–12 months to be sure all worms were killed. For those dogs that test positive, retesting might be required and, if infection is confirmed, a new round of melarsomine will be needed.
Severely affected dogs can have worms in the heart that significantly block blood flow. These dogs may need surgery to manually remove the worms.
Heartworm infection is preventable. Several medications that are both safe and effective at preventing infection are available from your veterinarian. Preventive treatment in dogs is recommended beginning at 6 to 8 weeks of age. No pretesting is necessary at this age. When beginning preventive treatment in dogs 7 months of age or older, an antigen test (to make sure the dog is not already infected) is recommended, followed by an additional negative test 6–7 months later (due to the 6-month development period of the worms) to ensure that the dog is not infected. Year-round preventive treatment is recommended for most dogs; however, you should check with your veterinarian about the best schedule for your pet.
The most important thing that pet owners can do to protect their companions from heartworm infection is to be absolutely sure their pet receives the prescribed dose of medication at the correct time. Because the most common preventive drugs for dogs are given only once a month, many pet owners may forget to administer the medication. Pet owners may find it helpful to post the heartworm medication dates on their refrigerator door and then check off the administration dates as the prescription is provided. Free e-mail reminder services are also available from most manufacturers of heartworm preventive drugs.
Missing an administration date may have serious consequences. If you miss a dose, you should contact your veterinarian regarding administration of the medication.