Dirofilaria immitis, also known as the heartworm, is a parasite that can cause significant disease in dogs and cats including disease of the lungs and other organs, and possibly heart failure. The best way to deal with heartworm is to prevent it in the first place, as treatment options for dogs can be costly and painful, and are virtually nonexistent for cats.

How do pets get exposed to heartworm?
The adult worms make their home in the large vessels of the heart. The females, after mating, give birth to live young known as microfilaria. These microfilaria then travel through the circulatory system all the way to small blood capillaries, where they can be easily sucked up by the vector and intermediate host, the mosquito. This immature form of the heartworm is not immediately infectious to dogs and cats. They require time to develop and mature, often a few weeks. The mosquito then transfers the more mature larval stage to a new, susceptible host during the time it takes another blood meal. The larva will move from the mosquito’s saliva into the skin of the new host. The parasite then stays in the skin where it continues to develop for three or so months until it can finally migrate into the blood stream and eventually to the large vessels associated with the heart, where it can mate. It takes approximately six months from the time the mosquito drops off the immature larva to when the adult heartworm is in the position to mate.

How can I prevent my pet from getting heartworm?
The main pillar of the prevention of heartworm disease is the REGULAR use of heartworm preventative medication. These include, most commonly, the monthly use of oral or topical medications or in the case of Proheart (available only for dogs), an injection every six months. Most of these medications target the larval stage of the heartworms as they arrive and develop with in the skin and before they migrate to the bloodstream. Speak with your local veterinarian as to what types of preventatives are available, and what would suit your pet. Yearly blood testing for heartworms should also be preformed on dogs to ensure effectiveness of prevention measures. If a lapse in prevention has occurred dogs should be tested immediately, then tested again six months later.

Other ways to prevent heartworm include measures to decrease your pet’s exposure to mosquitoes. Screened in open windows and fans help deter these pesky bugs. Avoid having pets outside for too long during periods of the day when mosquitoes are most active (e.g. twilight periods). Care should be taken to avoid any accumulation of stagnant, standing water around your property. This is the preferred medium in which mosquitoes lay their eggs.

Heartworm exposure is increasing
The state of heartworm disease in the United States is changing and appears to be on the rise. The aftermath of hurricane Katrina saw some 250,000 pets dispersed throughout the country in an attempt to find new homes. Unfortunately MANY of these pets were infected with heartworms. In addition to this information, we also now find that one of the main predators of mosquitoes—bats—are being killed off by a fatal fungal disease known as White-Nose Syndrome. With the predator numbers depleted, more mosquitoes may be available to help spread heartworms through wild and domestic populations. This means that now, more than ever, every cat and dog should be on heartworm preventative when at an appropriate age.

Prevention is much easier than treatment. If your dog is infected, aside from the detrimental effects the heartworms have on the body, treatment can be costly and painful for the dog involved. Cats on the other hand are not the ideal host for these parasites and infection can cause severe, sometimes fatal outcomes. And even if a cat clears the infection, long term inflammation can occur throughout the body causing a variety of maladies, the worst of which affects the lungs causing significant, chronic respiratory disease. So again I must stress that prevention of this disease is ever so important for our furry little friends. Talk with your veterinarian about prevention today! – Peter Olson, DVM

Peter Olson, DVM, is a 2007 graduate of The Ohio State University School of Veterinary Medicine. He and his wife Beth share their home with two dogs, two cats, four turtles and a Russian Tortoise.

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