Heartworm disease is caused when an infected mosquito bites a dog or cat. Unfortunately, there are no heartworm treatments approved for cats, so palliative care is usually the only option.
Heartworm is treatable in dogs, but it can take anywhere from several weeks to two years (with six to nine months being standard) for treatment to finish.
The heartworm life cycle begins in the mosquito where heartworm larva will be transmitted to its primary host through a bite. The larval form of heartworm (microfilariae) enter your pet’s bloodstream via mosquito bite. Microfilariae reside in the bloodstream for approximately 70 days as they migrate to the heart, lungs, and major arteries. Larvae will mature into adult worms in the cardiopulmonary system where they will reproduce and send more microfilariae into the bloodstream to repeat the process. Microfilaria also have a bacterial symbiont which causes a systemic inflammatory response.
Once infected with heartworm disease, dogs are at risk for irreversible cardiopulmonary damage and are under constant risk of embolism from the presence of microfilaria in the bloodstream. Heartworm infection and presence of microfilaria can be detected by laboratory panel that will identify presence of the heartworm antigen in the blood. This lab test will also have a microscopic evaluation where a blood droplet is inspected for microfilaria.
Treatment for heartworm disease is possible but challenging for dogs and their owners. Treatment for adult heartworms begins with Melarsomine injections into the epaxial muscle. This usually requires the dog to be under some sedation. The procedure requires the epaxial muscle (in the back) to be relaxed during the injection. Once dogs have received their initial Melarsomine dose, they are placed on strict exercise restriction. Dogs are kept in crates with the exception of leashed walks to void for 6-8 weeks. The restriction is in place because dying adult heartworms are a risk for embolism. The treatment is followed by a monthly heartworm preventative which keeps microfilariae from maturing. Doxycycline is typically also prescribed during this time. As microfilariae die off, their bacterial symbionts release into the blood stream usually eliciting a large inflammatory response. Dogs will continue this cycle of Melarsomine, preventative, doxycycline, and heartworm testing until a heartworm antigen and microfilaria test is negative. During this time, the dogs remain on strict exercise restriction. Many dogs need to be on an oral sedative like Acepromazine to ensure they remain calm. Jumping, running, and any rough housing can cause sudden embolism. Depending on the infection, treatment can last anywhere from several weeks to two years. During longer durations, there are pauses in active treatment to allow for the parasites to be killed.
Unfortunately, heartworm is easy to contract and difficult to treat. The best way to protect your pets from heartworm disease is to practice diligence with their monthly heartworm preventative and to keep up with their annual heartworm/tick panels.