HEARTWORMS, WHAT ARE THEY?
Heartworms, otherwise known as Dirofilaria immitis, are nasty internal parasites that reside in an animal's heart and pulmonary (lung) vessels. These critters can cause irreversible damage to your pet's heart, lungs and other organ systems and lead to death.
Dogs are the natural hosts for these parasites. However, there are several atypical hosts including cats, coyotes, wolves, foxes and even humans. Being the natural host means that the heartworms can reside for a long period of time and reproduce at an astonishing rate in that animal. Dogs have been known to harbor hundreds of worms in their heart and lungs.
An atypical host can still be infected with the heartworms, but they usually do not reside there as long and do not reproduce at such a high rate. However, these atypical hosts are a great source of transmission. The immature worms living in these hosts can also cause significant damage to the animal's organ systems and can produce respiratory disease, otherwise known as HARD (heartworm associated respiratory disease).
HOW ARE HEARTWORMS TRANSMITTED?
The transmission of heartworm can be explained with their life cycle. The first step of the life cycle starts with a mosquito taking a blood meal from an infected animal. This can be a natural or atypical host. The mosquito ingests a microfilariae (L1 - larvae 1), which then develops for an additional 10-30 days in the mosquito's gut (L2) before traveling to their mouths (L3). This infective larvae enters a dog or cat's body with an infected mosquito bite by sitting on the skin and easing its way into the skin (L4), migrating to the animal's heart and nearby vasculature. This is where they mate and start reproducing more microfilaria in a 6-7 month time frame.
HOW MUCH RISK IS MY PET IN FOR HEARTWORM DISEASE?
As you can gather from this map...heartworm disease is prevalent in most of the United States. Heartworm preventatives also de-worms animals for Roundworms, Hookworm +/- Whipworms (pending which preventative you choose). These nasty parasites can be found all over the United States as well, and can be infective through the winter months. It is always better safe than sorry.
The map above shows the incidences that have been reported. Keep in mind that not every incidence is reported and some areas may be better at reporting them than others. Every year we see an increase in Heartworm disease and also see this map's red spreading. Just look at the map below comparing heartworm incidence from 2010-2013...this is only a difference of 3 years.
WHEN SHOULD YOUR ANIMAL BE TESTED FOR HEARTWORM DISEASE?
I recommend testing at 6-7 months of age, again in 6 months, and then yearly. This is best, since heartworm disease can take 6 months to show up positive. I also recommend retesting if your animal is off heartworm prevention, and again 6 months later. For cats, I recommend testing prior to putting on prevention.
Heartworm disease is progressive and can become very serious. If we catch the disease early, then treatment will be much more successful and have a lot less negative side effects for your pet. Heartworm prevention is 98-99% effective, so, although unlikely, your pet can still get the disease. However, if you have proof of purchasing the prevention through a reputable pharmacy, and that your animal has remained on the prevention every month, then the company should refund you for the treatment.
CLINICAL SIGNS OF HEARTWORM DISEASE
Dogs
The most common presentation for dogs that test positive is a healthy dog coming in for their annual examination. This is because the worms have to develop and start causing damage before you will start seeing the symptoms of this disease.
The most common clinical signs of heartworm disease in dogs are coughing, exercise intolerance, weight loss and decreased appetite. You will start noticing other signs of heart disease as heartworm disease progresses such as obtunded abdomen and labored breathing. Dogs can also develop Caval Syndrome. This is a life threatening form of heartworm disease that is caused by a sudden block of the blood leading to the heart. You will notice collapse, labored breathing, dark urine and pale gums. This is an emergency and requires an aggressive surgery to correct.
Cats
Most of the clinical signs in cats are very similar to dogs - coughing, asthma attack-like breathing, weight loss and decreased appetite. They may vomit as well, have difficulty walking and may even faint. Unlike dogs, which are commonly asymptomatic, the most common presentation for heartworm disease in cats is sudden collapse and/or death. This is why prevention in cats is so important.
WHAT HAPPENS IF MY PET HAS HEARTWORM DISEASE?
Dogs
When a dog first tests positive for heartworm disease, we will recommend re-testing and confirming that it is positive before starting the harsh treatment. We will also recommend testing for baby heartworms (microfilaria) and take chest x-rays to visualize the heart and lungs. This allows us to know how far the disease has progressed, and develop the correct treatment plan for your pet.
One of the most important aspects of treatment is exercise restriction. Allowing your dog to be active during treatment increases the risk of dislodging the worms from the heart/lungs and clogging other parts of the venous system. This can lead to sudden death. Strict exercise restriction is needed for 3 months (the length of treatment).
Treatment lasts for about 3 months. It consists of oral steroids (pending the test results mentioned above), oral antibiotics and intramuscular injections that cause soreness and pain at the injection site. I will send home pain medication, as well, for my patients.
Once treatment is finished, your dog will need to be tested to make sure the heartworms are indeed gone, tested 6 months later, and then yearly as the general recommendation.
The treatment can cost thousands of dollars, depending on how severe the case, and can do significant damage on your dog’s organ systems. This is why we stress heartworm prevention so strongly.
Cats
Remember that cats are the atypical host for heartworms. This means that some infections can resolve on their own. However, it can still affect and damage the cat's heart and lungs. Because the lung burden is so low in the cat, it can be more difficult to detect. This is why we recommend testing for antibodies, antigens, and we may even have to recommend an x-ray, further lab work, or an ultrasound.
Unfortunately there is no medication that is safe to use in cats that is specific for treating this parasite. The goal for feline treatment is to stabilize and keep them feeling good with anti-inflammatories and antibiotics, as needed. Your cat may, at times, require hospitalization. We always recommend starting on prevention as well, as there is a possibility that this can help decrease the worm’s strength.
IS HEARTWORM PREVENTION REALLY IMPORTANT?
