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Dogs and Mosquitoes: Understanding Canine Heartworm Disease

If you’ve ever forgotten your insect repellant on a summer evening, then you know how miserable mosquitoes can make you feel.  In addition to the painful bite and the itchy welt that develops later, mosquitoes are a major vector for disease.  Humans, horses and dogs are all susceptible to diseases spread by mosquitoes.

One of these diseases is Canine Heartworm Disease.  Dogs are the natural host for heartworms, and the mosquito is essential to the spread of this parasite.

Let’s consider the lifecycle of the heartworm.

While dogs are the natural host for heartworms, other species can harbor the parasite and contribute to the spread of the disease.  Adult heartworms living in coyotes, foxes, wolves or domestic dogs produce microscopic baby worms called microfilaria that circulate in the bloodstream of the infected animal.  When a mosquito takes a blood meal from this animal, it picks up these baby worms, which then develop further into an infective form in about two weeks.  The mosquito can then pass along those infective worms to another host.

Having found a new host, the larvae develop into adult heartworms in about six months.  The adult worms live in the heart, lungs and associated blood vessels and can grow to be 14 inches long each.  Left untreated, the adult worms continue to produce offspring.  Dogs with severe heartworm disease can have hundreds of worms in their bodies.

The early stages of heartworm infection show no signs.  Clinical signs start to show when the infection worsens and are seen earlier in very active dogs, dogs with underlying disease and those with heavy infections.  Signs include a mild but persistent cough, fatigue after moderate activity, decreased appetite and weight loss.  Dogs that are suffering from heartworm infection are also capable of passing the microfilaria on to mosquitoes.

All dogs should have a yearly blood test.  This combination test, which also screens for tick-borne disease, is done in-house and is complete in ten minutes.  Dogs that test positive should have an additional diagnostic test done to confirm the test result.  Dogs that truly have heartworms can be treated.  Treatment is generally successful, and the protocol will depend on how healthy your dog is and if there is underlying disease.

Heartworm has been diagnosed in all fifty states.  While treatment for this parasite is available, prevention is easier and less expensive.  Depending on your dog’s needs, heartworm prevention is available as a monthly oral tablet or as a six month injectable.

All dogs are at risk for heartworms.  The mosquitoes that transmit the parasite have been found in apartments 21 stories above ground.  Given the ease with which this insect finds it’s way into buildings, indoor dogs are still at risk.

Can cats get heartworm disease?  Yes, but cats are an atypical host and the disease manifests very differently in cats.  Most worms in cats don’t survive to the adult stage and cats that do harbor adult heartworms usually have only one to three worms.  Cats can still develop respiratory problems from the immature form of the parasite, and the medication used to treat heartworm disease in dogs cannot be used on cats.  Most cats affected by heartworm don’t show any symptoms.  If you have a cat that goes outside, consider using a topical preventive.  Revolution, which is applied monthly, does protect cats from heartworms.

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