Heartworm Disease In Your Dog and Cat

Heartworm disease has now been detected in all 50 states of the country and its threat is constant in many areas, including Maryland. It has now been observed in not only dogs, but cats as well. Heartworm disease has various clinical signs and left untreated can often lead to congestive heart failure and death. Transmission of heartworm disease requires only one mosquito as the intermediate host. While outdoor pets may potentially be exposed to mosquitoes more often, indoor pets are far from being risk free. Female mosquitoes harbor the heartworm larva and are very small. They can easily enter a house through small cracks and openings around windows and doors, just as other small insects can. Traditionally, heartworm infection was thought to be a canine disease. However, it is now being recognized in the feline population with a greater frequency. The topic of feline heartworm disease will be discussed in more detail at the end of this article.

The life cycle of a heartworm involves many stages. The mosquito serves as the intermediate host for the larval form, referred to as microfilariae. When a mosquito bites a pet the larval form enters the wound and then begins a process of migration. Starting in the subcutaneous and muscle tissues larva migrate to the abdomen and then the thorax. They then penetrate blood vessels and reach the heart. Adult heartworms are found in the right side of the heart and the arteries going to the lungs. They can decrease blood supply to other organs, primarily the lungs, liver and kidneys. Clinical signs typically begin with a soft cough and lead to lethargy, fainting, weight loss and labored breathing. Left untreated these signs progress to congestive heart failure and eventually death.

Testing is performed through a blood test that detects the adult heartworm antigen. Dogs that are tested heartworm negative should be placed on a monthly heartworm preventive (Heartgard or Interceptor.) Both of these medications if used properly are safe and easily administered . They also may treat or prevent different types of gastrointestinal parasites. It is very important that dogs be tested on a yearly basis, especially if they have been off of prevention for any period of time. Maple Springs Veterinary Hospital recommends not only yearly heartworm testing, but that owners also give there pets heartworm prevention once a month, every month. We do not advise taking pets off heartworm prevention for the “colder” months due to the unpredictable and often very warm “winter” weather that our climate in Maryland is susceptible to. Dogs that are tested positive for heartworm disease should not be placed on prevention, as this may cause them to have a reaction to the preventive. Heartworm positive dogs should be evaluated by further bloodwork and chest radiographs to determine if they are good candidates for treatment. Treatment and a cure are possible, but the prognosis is much better if the disease is diagnosed prior to the onset of clinical signs and demonstrates why the cliché of “prevention beating the cure” is so very true. The most common and current treatment is an injectable product, Immiticide (melarsomine, dihydrochloride.) that kills the adult heartworms that are present in the dog. It involves giving an intramuscular injection on two consecutive days in the lower back area above the hips. While this medication is effective and safer than previous therapies, it can cause side effects, as well as being potentially painful for a dog to experience. Repeat treatments may be needed in some dogs with heavy worm burdens. After a dog is tested negative for the adult heartworm antigen a further oral treatment is needed to kill the remaining microfilariae in the bloodstream. This will prevent mosquitoes from obtaining the heartworm larva and repeating the life cycle. While heartworm positive dogs may be treated successfully and go on to live a normal life if diagnosed soon enough, the above treatment description once again demonstrates why “prevention beats the cure.”

While feline patients are infected with adult heartworms less frequently than dogs, they are still susceptible to this terrible disease. Even indoor cats are at risk for heartworm disease. The ability of veterinarians to diagnose feline heartworm disease has improved significantly over the years. Diagnostic laboratories have significantly increased the reliability of available tests. Cats that are heartworm positive may experience coughing, labored breathing, vomiting, lethargy and loss of appetite. However, they may not demonstrate any clinical signs, but experience sudden collapse and even death. Once a cat is diagnosed with heartworm disease, treatment is usually not performed since it may cause severe complications. Once again, “prevention beats the cure” and prevention is now available and safe for our feline patients with Feline Heartgard. While still not offered by some hospitals, our policy at Maple Springs Veterinary Hospital is that all outdoor cats and even indoor cats should be on Feline Heartgard. It is important to not give your cat a canine form of heartworm prevention as the dosages and medications are very different. Also, unlike our canine patients, cats do not have to be tested prior to going on prevention. However, we have tested numerous cats at our hospital, including indoor cats, that have been positive for the feline heartworm antibody. While these patients have fortunately not gone on to develop adult heartworm infections (antigen positive), this demonstrates that they have at least been exposed to the disease. Therefore, we have started advise all cat owners to give their cats Feline Heartgard. The last thing that we would want to see is one of our feline patients contract a disease that could have been prevented and may be very difficult to treat.

While heartworm disease has been around for decades, our knowledge about it as a profession continues to change and grow rapidly. One reality that will always hold true is that with yearly testing and monthly prevention we can keep our dogs and cats free from this debilitating and potentially fatal disease.

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