Canine Heartworm Disease Q & A

Heartworm disease is a rapidly growing epidemic within the United States.  Infected dogs can now be found in all 50 states.  This truly is alarming since a) infected dogs serve as a reservoir to infect other dogs and b) heartworm disease can be fatal.


For a disease that is so prevalent in the canine population, there seems to be a lot of misconception and confusion about heartworms.  Here are the answers to some frequently asked questions about heartworm disease in dogs.


Q:   How can my dog get heartworms if he is mainly an indoor pet?


A:    Heartworm disease (or dirofilariasis) occurs when an infected mosquito bites your dog.  Even if your dog stays inside 99% of the time, there is always a chance that a mosquito can enter your home through an open door or window.  For infection to occur, it only takes one bite from one mosquito.


Q:   My dog has a thick hair coat.  How can a mosquito bite through the coat?


A:    Mosquitos can bite your dog on an un-haired area such as the nose, inner ear, or abdomen.  When the mosquito bites your dog, it deposits the infective larvae (L3) onto the surface of your dog’s skin.  The larvae then migrates into the bite wound to begin the disease cycle.


Q:   I only have one dog who never interacts with other dogs.  Does he still need heartworm prevention?


A:    Heartworms are not transmitted directly from dog to dog.  A mosquito bite is the only way a dog can be infected. Mosquitos acquire heartworms by biting an infected animal (host).  In addition to dogs, other hosts include wolves, coyotes, foxes, raccoons, and sea lions.  When a mosquito bites an infected host, it acquires microfilaria (baby heartworms).  These microfilaria go on to develop into the infective stage (L3) within the mosquito.  The time necessary between microfilaria to L3 can be one to four weeks.


Q:   When I adopted my dog I was told he was negative for heartworms.  I have had him on heartworm prevention consistently since then.  However, now, after 8 months, he is testing positive.  How is this possible?


A:    When a dog is bitten by an infected mosquito, it takes approximately six months for the infective larvae (L3) to become an adult heartworm.  Routine heartworm checks test for a protein (antigen) produced only by an adult female worm.  Thus, if your dog was infected with immature heartworms or larvae when you adopted it, the initial test would have been negative.  Administering heartworm prevention kept your dog from acquiring any new infection.  However, the larvae and immature worms previously present in your dog’s body have continued to develop into adults.


Q:   How would I know if my dog has heartworms?


A:    In the early stages of heartworm disease most dogs exhibit few or no symptoms.  The longer the infection is present, the more likely the dog will present with symptoms.  Symptoms include a mild to moderate cough, reluctance to exercise, and decreased appetite.  Advanced symptoms include coughing up blood and a swollen belly (ascites).  The number of worms and activity level of your dog determines the severity of symptoms.  The only sure way to know if your dog has heartworms is to have a blood test(s) performed by your veterinarian.  Two negative tests performed six months apart eliminate the possibility of heartworm disease.  (See #4)


Q:   Is it safe to give heartworm prevention to an infected dog?


A:    Only under the direct supervision of a veterinarian.  If your dog has circulating microfilaria, a preventative could cause an anaphylaxis reaction in your dog as the baby worms begin to die.


Q:   I live in a northern state.  Does my dog need to be on year-round heartworm prevention?


A:    Mosquitos live in areas of temperate, semi-tropical, or tropical climates.  However, mosquito species are constantly changing and adapting to living in cooler climate.  Mosquitos can also live indoors.  For these reasons, the American Heartworm Society recommends year round heartworm prevention for all dogs in all states.  Ask your veterinarian about recommended heartworm dosing protocols in your area.


By:  Susan St. Pierre, DVM

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