By Jocelynn Jacobs DVM, CVC

Mosquitoes and Heartworm Prevention.

If you have lived in Michigan all your life like I have, you know that mosquitoes are a pesky critter we deal with about 9 months out of the year. And some of us (like ME!) are like a mosquito magnet… I can be outside with friends and not one mosquito is on them because they are all feasting on me!

Mosquitoes love our dogs just about as much as they love us humans. Unfortunately, however, mosquitoes can transmit some pretty nasty parasites to our dogs such as heartworms. Mosquitoes that are infested with heartworm larva will transmit heartworms into our dogs through their needle-like mouths when they are engorging on blood. These larvae then mature and find their way into our dogs’ hearts where they can get up to 18-20 inches long! They eventually clog up the heart valves and arteries and cause our dogs to develop heart failure and die.

As dog owners, most of us prevent heartworm disease by giving our dogs’ a heartworm prevention medication which causes the larvae injected by the mosquitoes to die. EVERYONE who lives in ALL parts of Michigan need to give their dog preventative or their dog will be at high risk for acquiring this disease.

About 20 years ago we only had 1 option for heartworm prevention – it was a daily medication called DEC. If an owner missed even 1 day of medication, the dog was at risk for getting heartworms because this medication only killed the L3 larva of heartworms (there are a total of 5 larval stages of heartworms: L1-L5).

In the mid-1990’s a monthly medication for heartworm prevention was introduced. The benefit was we didn’t have to remember every day to give the dog its medication. On the downside, we had to remember once a month and sometimes the months fly by when we get busy! A study done years ago determined the most common months for owners to forget to give their monthly heartworm medication was during the summer when they were busy with graduations, family vacations and kids being home for summer break. Guess what months have the highest concentration of mosquitoes? Yep, the summer months, thus the highest risk time is the time when most people will forget to give their monthly prevention.

Monthly medication works by killing off the L3 and L4 larva in the skin and body of the dog on the day it is given so the larva did not grow into the L5 (adult) worm.

In 2002 a new medication was introduced called Proheart which is an injectable heartworm prevention that lasts 6 months! Whew! Finally, we had a medication that a veterinarian could inject directly into the dog that would last 6 months and there was no need to “remember” daily or monthly! Proheart worked in a very unique way too so that the dog got a continual “blanket” of protection from those nasty mosquitoes every day for those 6 months. It kills both L3 and L4 larva every day during those 6 months so there is continual daily protection for our dogs.

Proheart is also extremely safe for dogs, humans and other animals. The active ingredient in it called Moxidectin is used in humans in 3rd world countries for intestinal parasite infections. It is so safe it can be given to women who are pregnant and nursing without any side effects to the mom or baby. Thus Proheart can be given to dogs that are pregnant and nursing as well.

Dogs need to be at least 6 months old to get their first Proheart injection…not because of safety issues but because puppies grow really fast the first 6 months of their lives and can double, triple and even quadruple their weight until they are 6 months of age! How will the veterinarian know how much to dose the dog for based on weight if they double or triple their weight in those first 6 months? They won’t, so it is best to start the medication at 6 months of age when the dog’s weight start to plateau.


Not every veterinarian has Proheart at their veterinary clinic, but the majority of our clients (including me and my staff) use and recommend this prevention at our veterinary hospital. It is a safe, effective and convenient way to know our dogs are protected from the dangers of those darn mosquitoes!

Stop in our clinic or call us at 989-695-2200 during office for more information or to purchase heartworm prevention medication.