Most dog owners are aware of heartworm disease, however most are unaware of how dogs actually acquire the disease. This article will help to explain the transmission and progression of heartworm disease in dogs and the need for heartworm preventative.

Heartworm disease is spread from dog to dog through the bite of a mosquito. Heartworms are literally worms that live in the hearts of dogs. In advanced disease, there may actually be hundreds of these long, spaghetti- like worms in the heart! Once this occurs, the dog will likely die of heart failure due to the worms. Advanced heartworm disease takes 3-5 years to develop and this slow often unnoticed progression is the reason yearly heartworm tests and early treatment is recommended.

However, early in the infection, only a few worms are present in the heart. These worms are able to reproduce, creating babies known as microfilaria. These microfilaria circulate in the bloodstream of the affected dog and, if the dog is bitten by a mosquito, are ingested by the mosquito. The microfilaria actually grow and mature inside the mosquito to become what are called third-stage larvae, or L3 for short. The L3 larvae are responsible for causing infection when the mosquito bites another dog.

This maturation process of the heartworm in the mosquito from the microfilaria to the L3 stage is extremely dependent on temperature. Laboratory studies have shown that this development requires a steady 24 hour daily temperature in excess of 64 F for 1 month. Even if the temperature drops below 57 F for just a few hours, development will be delayed. If the daily 24 hour temperature exceeds 80 F, only 10-14 days are required for development to the L3 stage. Knowing this information provides insight into why different locations in the U.S. have differing recommendations for administering heartworm preventative. In New Hampshire, peak transmission occurs in July and August, and under the most favorable conditions the entire heartworm transmission season is less than 4 months. So why would a dog owner in New Hampshire administer heartworm preventative for longer than 4 months (i.e. from June through September)? There are only two viable reasons. Reason one would be that the pet owner enjoys providing income for the large corporate drug companies producing heartworm preventative or, reason two would be if the pet is traveling to the Southeastern U.S. during the winter months. Heartworm preventative needs to be given year round in South Florida….not in New Hampshire!

Once the L3 stage is passed from the mosquito to the dog it continues to develop through another larval stage (L4) and then into an adult. Dogs given monthly heartworm preventative (i.e. Heartguard and Intercepter) during the transmission season will not acquire infection as these drugs are effective against L3, L4, and some young adult worms. The older daily preventative (Filaribits) was only effective against the L3 stage and this is the reason it needed to be given every day. Monthly heartworm preventatives are not stronger or more toxic than daily preventatives; they are just effective against more immature stages of the heartworm. In fact, due to the length of time the immature heartworm spends in each larval stage, monthly heartworm preventatives are actually 100% effective if given every 45 days and 99% effective if given every 60 days.

At the Holistic Veterinary Center, we recommend seasonal heartworm prevention with oral monthly products. We recommend the Heartguard tablets as this product contains no other additional dewormers. If a dog has an allergy to this product, another form can be compounded by a compounding pharmacy. We recommend starting treatment in May/June, continuing every 30-45 days until September/October. Dogs that travel or compete in the Southeastern U.S. during the winter may require year round prevention. An annual heartworm test on the blood is recommended yearly to detect early infection. Geriatric dogs and dogs with serious life threatening disease are given heartworm preventative on a case by case basis.

Holistic Veterinary Center

Holistic Veterinary Center
34 West Street, Concord, NH  03301
Phone: 603-225-9680 • Fax: 603-227-0945