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Posted on 06-07-2016

Heartworm disease is a serious and potentially fatal disease. It is spread only by mosquitoes and all dogs are at risk. It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body.

The dog is a natural host for heartworms, which means heartworms that live inside the dog mature into adults, mate and produce offspring. If untreated, their numbers can increase, and dogs have been known to harbor several hundred worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect the dog’s health and quality of life long after the parasites are gone. For this reason, prevention is by far the best option, and treatment—when needed—should be administered as early in the course of the disease as possible.

The mosquito plays an essential role in the heartworm life cycle. Adult female heartworms living in an infected dog produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which develop and mature into “infective stage” larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another dog, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats.

And because infected mosquitoes can come inside, both outdoor and indoor pets are at risk. In the early stages of the disease, many dogs show few symptoms or no symptoms at all. The longer the infection persists, the more likely symptoms will develop. Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop sudden blockages of blood flow within the heart leading to a life-threatening form of cardiovascular collapse. This is called caval syndrome, and is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.

Active dogs, dogs heavily infected with heartworms, or those with other health problems often show pronounced clinical signs. The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog or cat is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important. Annual testing is necessary, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working.

Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication—or give it late— it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill—or rub off a topical medication. Heartworm preventives are highly effective, but not 100 percent effective. If you don’t get your dog tested, you won’t know your dog needs treatment. Most important take-home message: it is far better to prevent disease than treat and much less expensive for prevention than treatment! Please contact your veterinarian to discuss the multiple heartworm preventives available at Animal Hospital of Soquel.

Written By: Dr. Lindsay Swirble, DVM

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