Heartworm disease

Heartworm Disease

Spring is finally upon us, so it is again time to refresh our understanding of Heartworm Disease (HWD).  As the name implies, HWD affects the heart.  It is caused by an infection with a parasite called Dirofilaria immitis, which is passed between animals by mosquitoes. 

What happens is mosquitoes will feed on an infected animal (typically a dog, but also coyotes, foxes, and cats); where they will pick up younger stages of the parasite called microfilaria.  Over the course of 2 weeks, these microfilaria will grow into young larval stages of the parasite, then the mosquito will feed on your pet and transfer these young stages into your pet.  During the next 6-7 months, these larva travel throughout the body growing into adult worms, which ultimately reside in the right side of the heart (pulmonary artery).  Here, the adult worms will mate and produce more microfilaria, completing the life cycle.

The worms in the heart cause problems by blocking the flow of blood through the vessels and by causing inflammation in the lungs when the body tries to clear the infection.  The infection can range from just a couple worms, which may not cause any clinical signs, to hundreds of worms, which prevent proper blood flow causing weakness, coughing, and difficulty breathing.

The best way to combat the disease is to prevent the infection in the first place.  There are two main ways of doing this: limiting exposure to mosquitoes and using heartworm prevention medications. 
Most species of mosquitoes are most active during the dusk hours, so limiting your dog from going outside during these times can help.  Also, there are topical products that can be applied to your dog’s skin that help repel mosquitoes (Vectra, Advantix), as well as environmental treatments and clean up (prevent standing water) that can be performed.

There are also many HWD prevention products.  The range from oral medications (Heartgard, Trifexis) to topical medications (Revolution, Advantage Multi) to injectable medications (ProHeart).  Most of the products are administered or applied monthly and work by killing the larval stage of the worm before it can grow up into an adult.  Although there have been reports of lack of efficiency in many products, by-and-large these products are very effective.  It is important to follow the label accordingly when giving these medications (with or without food, no bath before applying topical meds, etc) in order to make them the most effective. 

It is also recommended to test your dog for HWD yearly.  There are many bench-side tests that can be performed on a blood sample to check whether your pet is heartworm free.  Even if your pet is on continual heartworm prevention, it is recommended to screen them for infections since, even though rare, there have been reports of lack of effectiveness.  Some companies will help pay for heartworm treatment if your pet becomes infected while using their product.

If your pet does somehow become infected with heartworm disease, there is still a treatment available to clear the infection.  Most veterinarians will recommended treating with an injectable medication called Melarsomine.  A series of 3 injections are typically performed over the course of a month.  There can be serious complications associated with treatment, so it is important to prevent the disease if possible.