Heartworm infection occurs when a pet is infected with a parasite called Dirofilaria immitis. Heartworms are parasites that live in the pulmonary arteries, which are the blood vessels that carry blood from the right side of the heart to the lungs. Dogs are infected with heartworms when an infected mosquito bites them. The heartworm larvae are injected through the bite, after which they migrate through the dog’s body and mature. Ultimately the adult heartworms end up in the pulmonary arteries where they cause damage by physically blocking the arteries, as well as by eliciting an inflammatory immune response by the dog’s body. This then leads to a condition called pulmonary hypertension, in which the pressure within the pulmonary arteries is elevated above normal. Pulmonary hypertension increases the workload on the right side of the heart and may result in right-sided congestive heart failure (R-CHF) and/or cardiac arrhythmias. R-CHF causes fluid to leak from the veins into the body cavities (into the space surrounding the lungs [pleural effusion], into the pericardial sac surrounding the heart [pericardial effusion] or into the abdomen [peritoneal effusion]) or underneath the skin (subcutaneous edema). In small dogs, as well as medium and large dogs with heavy worm burdens, the heartworms may back up into the right side of the heart – this often leads to a condition called caval syndrome in which a dog will develop severe lethargy, discolored urine and often have signs of right-sided congestive heart failure. Dogs with caval syndrome are critically ill and usually require an emergency procedure to manually remove the heartworms from the heart.
When heartworm fragments (or pieces of a blood clot associated with a worm) break off they will travel with blood flow and ultimately obstruct smaller pulmonary arteries – this is known as a pulmonary thromboembolism (PTE). Large PTEs or multiple PTEs can lead to sudden respiratory distress (difficulty breathing), hemoptysis (coughing up blood), collapse, and even sudden death. Since successful treatment with an adulticide (see below) inevitably leads to PTE (as the worms die, they fragment and are swept downstream with blood flow), exercise restriction during treatment is essential to try to limit the consequence of these PTEs.
The American Heartworm Society has published a protocol for the treatment of heartworm infection. This protocol is based on review of the available literature, as well as clinical experience. Since adult heartworms may live for 5-7 years in a dog host, treatment with an adulticide (which kills the adult heartworms) is recommended as soon as feasible. Prior to adulticide treatment it is recommended that dogs receive one month of therapy with doxycycline to reduce the pathology associated with dead worms. Additionally, dogs should receive at least two doses of monthly heartworm preventative to bridge the susceptibility gap during which a heartworm larva/juvenile is not sensitive to either heartworm preventative or adulticide, as well as to prevent new infection. Adulticide treatment consists of 3 injections of melarsomine – an initial injection followed one month later by 2 injections, 24 hours apart. Injections are generally administered in the morning and the dog is monitored until the afternoon for any immediate complications. A short course of an anti-inflammatory medication called prednisone may be prescribed following each injection. A few days of a pain medication may be used to treat soreness at the injection site. STRICT EXERCISE/ACTIVITY RESTRICTION IS CRITICAL DURING ADULTICIDE TREATMENT. A dog should be confined to a crate, small playpen or small room to prevent running or jumping. Stairs should be avoided if possible. The dog should always be walked on a leash when outside, and he or she should only be walked for the purposes of going to the bathroom. These restrictions should start immediately after the first melarsomine injection is given and continue until 6-8 weeks after the last injection. Dogs that are very active may require mild sedation during this period. Although this may seem extreme, activity restriction is likely the single most important factor in limiting complications of adulticide therapy.
The “slow-kill” method is not recommended for otherwise healthy dogs since this protocol may take up to 2 years to eliminate most of the adult worms. In that time period ongoing, potentially irreversible damage is occurring to the pulmonary arteries.
Two months after the last melarsomine injection, a microfilariae test is recommended. If microfilariae are still present, treatment with a microfilaricide followed by another microfilariae test 4 weeks later is recommended.
A recheck heartworm antigen test is recommended 6 months after the final melarsomine injection.