Tech Talks: Heartworm

Tech Talks by Emily Harbury, LVT

What is heartworm?

Canine Heartworm or, Dirofilaria Immitis, is a worm that grows up to 14 inches long and lives in the heart and pulmonary arteries of an infected dog. Dogs can only acquire this parasite through the bite of an infected mosquito. Mosquitoes readily pick up larval heartworms from infected dogs who have developed second generation heartworms called microfilaria. If the mosquito bites a dog who is in this stage of heartworm development, the mosquito can then transmit the larvae to other dogs. Some geographic areas have severe heartworm problems while others have virtually none.

Adult heartworms produce second generation heartworms called microfilariae; there are five larval stages and are termed L1, L2, L3, L4 and L5.

Heartworms do not lay eggs like other worm parasites; instead they give live birth and the baby worms are called microfilariae. Microfilariae are released into the circulatory system in hope that they will be slurped up by a mosquito taking a blood meal and carried to a new host. Microfilariae may live up to two years within the host dog in whom they were born. If, after this period, a mosquito has not picked them up, they die of old age. Microfilariae may also be transmitted across the placental barrier to unborn puppies if the mother is infected with heartworm. It is important to realize that such puppies will not develop adult heartworms or heartworm disease from these microfilariae; in order for a heartworm to reach adulthood, it must be passed through a mosquito. The mosquito who ingested L1 stage larval will stay with that mosquito until it develops to L2 and finally to L3. How long this process takes depends on the environmental conditions. In general, it takes a few weeks. A minimum environmental temperature of 57 degrees Fahrenheit is required throughout this period. If the temperature drops below 57 degrees, the mosquitoes will die and no heartworms can be transmitted. This is why there are so few cases of heartworm in Washington, Alaska, Montana, Colorado. We have been blessed with a long cold fall and winter.

That being said, there was a case of heartworm in Washington State in 2008 where a dog tested positive for heartworm. Supposedly the dog had lived its entire life in Washington and had never traveled. There is no evidence that there is any increase in new heartworm cases in our state.

There is also no evidence that heartworm is enzootic in Washington, meaning it has not become established as a disease that is consistently present in our animal communities. The most common heartworm occurrence in Washington pets has been diagnosed in those that have relocated to the state from known heartworm enzootic regions. Diagnosis has still been very uncommon and occurred most often on or around military installations and communities with a large mobile population. Over the last three decades there have been a handful of heartworm cases like this in the State – some of them up to incomplete recollection on the part of owners or to owners that have not had the animal its entire life and so missed a bit of its history. The rest? Well, they very well could be the uncommon case that has occurred.

If your dog has traveled through enzootic regions you should get your dog tested for heartworm. The dog’s eventual health, after treatment largely depends on the severity of his infection and how long he has been heartworm positive. The treatment will kill the worms, but the shock of the worms dying quickly could cause your dog to die. The worms can block off circulation. Luckily the testing for adult heartworm is fairly inexpensive; about $40 Treatment for heartworm in a dog who has a large load of worms can be dangerous and requires strict kennel confinement, injections of an immiticide and then monthly preventative medications for 1 year. The dog should be retested every 6 months while receiving the preventative and then annual retesting if the dog continues to live in an endemic region. 

Click the button below for more informatoin from the American Heartworm Society.

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