Dog heartworm is a dreaded diagnosis that can mean extreme discomfort for your dog with potentially life threatening complications. Heartworm disease is highly preventable. Thanks to effective monthly preventatives, dogs are suffering from heartworm disease far less frequently and enjoying a high quality of life. However, no preventative measure is 100 percent effective; therefore, dog owners must know the clinical signs of heartworm infection so quick action can be taken when suspected.
Transmission of Dog Heartworm
Heartworm infections are transmitted by mosquitos. Often owners think that heartworms are transmitted by a fecal-oral route but this is not the case. It is important to understand how a dog contracts heartworm. Mosquitos are typically the cause. Mosquito bites can transmit 5 to 10 tiny heartworm larvae into your dog’s skin when bitten. Those worms travel through your dog's body into the bloodstream and eventually develop into adult worms. These adult heartworms can grow up to 12 inches long and settle in the pulmonary arteries on the right side of the heart, causing heart and lung distress. Since mosquitoes are the primary cause of dog heartworm disease, owners should take precautions when taking their dog(s) camping, hiking, or outside to play when mosquitos are present.
Another factor in transmission is climatic factors have increased mosquito ranges. Some mosquitos are also active during daylight hours and not just at dusk. In California, for example, there have been invasive mosquito species identified and reported in the State over the past few years increasing the potential for heartworm transmission in animals.
Signs of Dog Heartworm Disease
Heartworm is a parasite, or “worm,” that lives in the heart. Unfortunately, dogs rarely exhibit symptoms during the early stages of heartworm infection. By the time a dog owner notices the signs, the effects of the heartworm disease have likely advanced and the dog is very uncomfortable. Heartworm disease severely impacts a dog’s health, compromising the heart, lungs, and circulation; therefore, annual screening and monthly preventatives are vital to minimize the chance of heartworm infection in your dog.
Early signs of dog heartworm disease include:
- Difficulty breathing
- Persistent dry or moist cough
- Weight loss
- Nose and/or mouth bleeding
Middle to late-stage symptoms of dog heartworm include:
- Abnormal heart and lung sounds
- Enlarged liver or heart
- Panting at rest
- Exercise intolerance
- Fluid backup into the lungs and abdomen
- Respiratory distress
A dog can go from mild to severe symptoms fairly quickly. As the heartworm infection progresses and worm size or worm burden increase, a slight cough may develop, followed by more moderate to severe symptoms. When a dog is brought in for an exam out of concern, veterinarians can often hear a heart murmur.
The Importance of Early Detection
Early detection is vital since heartworm disease can be fatal if unnoticed or ignored. As time passes and this disease remains untreated, the number of worms will continue to multiply. Early detection and diagnosis are essential to avoid more serious symptoms and severe complications that can lead to death. All dogs should be tested annually for heartworm infection, but a dog should be examined immediately if heartworm disease is suspected. This graphic created by the FDA demonstrates the life cycle of heartworm in dogs.
Treating Dog Heartworm
Treating dog heartworm infection can be complicated. Identifying the stage of the heartworm life cycle is the first priority. A veterinarian will conduct ELISA tests, which test for adult female worms. Once a result is reported and it is positive for heartworm infection, a veterinarian will look at the blood for baby worms, called microphilaria, circulating in the body. Those baby worms are what a mosquito transmits to your dog, or takes up from a blood meal when it bites your dog, and potentially transmitting these worms to another dog. Those two positive indicators confirm heartworm infection. Once those results are reported, then treatment can begin.
There are three phases to dog heartworm treatment:
- A veterinarian will administer medication to prevent further infestation.
- Doxycycline antibiotic begins to kill the bacteria the heartworm is living off of. This is a critical step since worms need to be killed very slowly, as there are often many of them. Because each worm can measure approximately 12 inches long, they cannot be killed too quickly, or the dog may experience complications.
- Eventually, a veterinarian will administer three injections of Immiticide or Melarsomine, into the back muscle to further kill the worms in the body.
Heartworm treatment can take months since it requires multiple injections a certain number of months apart. It is an investment of time and attention at home when the dog must avoid strenuous activity.
Heartworm Prevention
Dog heartworm infection is preventable with oral tablets or injectable medications that prevent the larvae from developing into adult heartworms. While no prevention is 100 percent effective, oral or injected preventatives exponentially decrease the likelihood of contracting heartworm.
Prevention begins as early as eight weeks old at a puppy’s first vaccine appointment. When mosquitoes bite a puppy or adult dog and deposit baby worms into their skin, the worms will migrate through the body over two months and eventually reach the bloodstream. While they are still in the skin, these worms are susceptible to being destroyed by heartworm preventatives, which is why prevention is started early in the puppy stage. The dog may get infected, but the parasite is destroyed before it can further infect the dog and cause damage to the heart, lungs, and circulation.
The American Heartworm Society is a trusted resource for heartworm insight and information.