Heartworms – What you need to know

Edited by:  Diane Robertson

Information By:  The American Heartworm Society

What is heartworm disease?

Heartworm disease is one of the major health problems of dogs in the United States and throughout the temperate and tropical areas of the world.  As well as being found in dogs and other species, it is now being found in cats in ever-increasing numbers.  The disease develops when a pet becomes infected with parasites called Dirofilaria immitis that are transmitted through the bite of an infected mosquito.  Dogs may be infected by a few or up to several hundred heartworms.  Cats are similarly infected although usually by only a few worms.  Heartworm infection often leads to severe lung disease and heart failure and can damage other organs in the body as well.

 Heartworm disease has been found in all 50 states.  A number of recent cases have been reported in El Dorado County and local veterinarians are trying to get the word out to have your pets tested and placed on a preventative.

How is heartworm disease spread from one pet to another?

It is only spread my mosquitoes.  Adult female heartworms living in an infected dog or other host release their young, called microfilaria, into the bloodstream.  Mosquitoes become infected by the microfilaria while taking a blood meal from these infected animals.  During the next 10 to 14 days, microfilarias mature to the infective larval stage within the mosquito.  When the mosquito then bites another dog, cat or susceptible animal, the infective larvae exit the mosquito’s mouth and are deposited onto the surface of the animal’s skin.  The infective larvae can then actively enter the new host through the fresh bite wound.

 What are the physical signs of heartworm infection?

Dogs:  Heartworms may accumulate gradually over years, or quickly when conditions allow exposure to high numbers of mosquitoes carrying infective heartworm larvae.  Clinical signs of disease may not be easily recognized in pets that have been recently infected or in those with low numbers of heartworms, as they may not yet exhibit outward signs of disease.  However, pets heavily infected with heartworms or those with chronic disease often show prominent clinical signs.

 In dogs, 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 commonly recognized by an accumulation of fluid in the abdomen giving the pet the appearance of a “swollen belly.”  Dogs infected with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life threatening form of cardiovascular collapse called “caval syndrome.”  Signs of caval syndrome include a sudden onset of labored breathing, pale gums and dark bloody or “coffee-colored” urine.  Without prompt surgical removal of the heartworm blockage, few pets survive.

 Cats:  Signs of heartworm disease in cats can be very subtle and misleading.  Signs may include coughing, asthma-like attacks, intermittent vomiting, lack of appetite, or weight loss.  Occasionally ataxia (difficulty walking), seizures, fluid accumulation in the abdomen and syncope (fainting) have been reported as well.  Unfortunately, the first sign in some cases is sudden collapse of the cat, or sudden death.

 Are tests available to detect heartworm disease?

Yes, for both dogs and cats.  Your pet must test negative before being placed on a heartworm preventative medication.  Heartworm tests may be combined with other tests and typically run between $25 and $50.  Check with your local vet or vaccination clinic.

Dogs:  Annual testing for dogs is recommended for several important reasons.  First, many of us do not take our own medications as directed let alone medicate our pets.  We’re busy; we forget; we miss a dose here and there.  Second, even if you never miss a dose there is nothing to prevent your dog from eating some grass and vomiting up the medication you just gave.  Your pet would be without protection for an entire month.  Third, if your pet accidentally became infected with heartworms, your veterinarian needs to detect it as soon as possible before irreversible heart and lung damage occur.  Early detection and treatment are always best.  Finally, some heartworm tests now come combined with tests that monitor other significant diseases (Lyme disease, Ehrlichiosis and Anaplasmosis).  With annual testing, you know your pet is heartworm free and not infected with these serious tick-borne diseases, some of which could affect family members.  The tests are quick and accurate and make sure your pet is free from infection.  Annual testing provides peace of mind in knowing that your pet is free of heartworms, and should your pet be infected, it assures you of early diagnosis.

 CatsHeartworm infection in cats is easily overlooked and harder to detect than in dogs.  The preferred method for screening cats includes the use of both an antigen and an antibody test.  Your veterinarian may also use X-rays or ultrasound to look for heartworm infection.  Your cat should be tested prior to starting a preventive, and annually thereafter.  Remember, since there is no approved treatment for heartworm infection in cats, prevention is critical.

What is the treatment for heartworm disease?

Dogs:  If a dog is infected with heartworms, the treatment needs to kill the adult and immature worms.  Currently, only one product is approved by the FDA for this purpose (Immiticide®- melarsomine hydrochloride).  It is given by deep injection into muscle.  A series of injections are given, either over a 24-hour period or two treatment periods, one month apart.  While treatment may be administered on an outpatient basis, hospitalization for the procedure is often recommended.  Other medications may be given at the time of treatment depending on the stage of heartworm disease.

Cats:  There is no FDA approved treatment for heartworm infection in cats.  Prevention is critical.  Indoor cats have less exposure to mosquitos so you should consider keeping your cats exclusively indoors.

For the full American Heartworm Society FAQ, go to http://www.heartwormsociety.org/pet-owner-resources/faqs.html#q13

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