Though spring seems very far away right now, warm weather will be back before we know it.  With the change of the seasons comes the inevitable spring and summer nuisance of mosquitoes.  There are numerous diseases spread by mosquitoes but today we are going to go over heartworm disease.  

Heartworms, known as Dirofilaria immitis, are spread by mosquito bites to dogs and even cats.  The dog is the definitive host- meaning that the worms mature into adults, mate, and produce offspring while living inside a dog.  The mosquito is the intermediate host, meaning that the worms live inside a mosquito for a short transition period in order to become infective (able to cause heartworm disease).  The worms are called “heartworms” because the adults live in the heart, lungs, and associated blood vessels of an infected animal.    

Heartworm is most prevalent along the Gulf and Atlantic Coasts as well as the Mississippi River and its tributaries, but it has been reported in all 50 states. 

An infected dog can have the immature offspring called microfilariae in its bloodstream.  When a mosquito bites that dog- the microfilariae transfer into the mosquito where they mature over the next 10-14 days into infective larvae.  Without this stage in the mosquito the microfilariae cannot become infective. When the mosquito then bites another dog, the infective larvae transfer into their new host.  These larvae will mature over 6-7 months into adult heartworms.  

The adults mate and produce the new offspring into the blood stream and the cycle begins again.  

The adult heartworms can live inside the dog for 5-7 years. Males reach about 6 inches in length while the females are about 12 inches in length. The number of adults that typically live in the dog is typically around 15 but can range from 1-250 adult worms. 

The test for heartworm requires 4 drops of blood. It is mixed with a reagent and detects a protein from the female heartworm.  Because this test looks for protein from adult females, it takes 5 months from exposure for a dog to test positive. Dogs that are 6 months of age and older should be tested for heartworms before starting heartworm prevention.  

The preventative will not kill the adult worms and the dog will remain positive until it is sick enough to show symptoms. In addition, giving heartworm preventative to a heartworm positive dog can be harmful or deadly. The immature microfilariae in the dog’s bloodstream can undergo a sudden die off which can cause a shock-like reaction and possibly death.  

There are 4 different classes of heartworm disease described in dogs.  They stage dogs from 1-4 with higher numbers indicating a greater burden of worms and severity of symptoms. 

The symptoms range from a mild cough to congestive heart failure and caval syndrome.  In caval syndrome there are so many worms that they physically block blood flow in the heart.  

This is a life-threatening surgical emergency which must be done at a referral hospital by a boarded specialist. Even with surgery, most pets with caval syndrome die.  

There is treatment for heartworm disease. A drug called Immiticide is given in a series of three injections into the muscles in the back.  As the adult worms die it is quite possible for them to embolize to different areas of the body and produce life threatening consequences. 

Because of the expense and risk associated with heartworm treatment, one can see why it is best to try to prevent versus treat.  There are a variety of heartworm preventatives available from monthly chews to topical spot ons to long acting injectables.  The monthly products work by killing any immature larvae in the pet’s bloodstream that have developed in the previous month.  

We do recommend year-round prevention because, in our region, we often have odd spring like days even in the winter months.  It does not take long to see mosquitoes and other insects during those breaks in the weather.