Heartworm - a Year-Round Concern

Florida is a state of transplants; most of the population is from somewhere else. I am a relatively new resident, myself, having moved here in 2007 from Washington State. A number of states - Washington included - don't have a high incidence of heartworm infestations, and so many of the residents there treat their dogs seasonally or not at all. Florida and the southern United States are warm enough to keep this nasty parasite active all year long, so it's important to have a regimen in place for prevention of this insidious killer.

Heartworm disease is present all over the world in various densities except Antarctica. This is a recent development, with increased road and air transport of pets, and the presence of mosquitoes nearly everywhere. Once considered prevalent only in the south, it has spread to all 50 states and is now a major global concern. According to statistics conducted by the Heartworm Society, the highest incidence of heartworm disease occurs within 150 miles of the Gulf and Atlantic coasts and along the Mississippi River, including tributaries. This puts Florida firmly on the heartworm's map of favorite haunts.

The Parasite

The heartworm parasite (Dirofilaria immitis) is transferred to the host by a mosquito bite. After undergoing a number of changes in the dog's bloodstream over a period of 6-7 months, the worms travel to the right side of the heart to reproduce. Once established there, the heartworm takes up space, preventing the heart's proper function by sheer volume and obstruction of the valves. The young (microfilariae) can travel around the dog's circulatory system for up to 3 years before another mosquito comes along and gives them a ride out to the next unprotected dog. This cycle is repeated - often several times - in the unprotected dog's increasingly compromised circulatory system until the heart is working so hard for so little benefit that congestive heart failure becomes inevitable.

The Symptoms

There are few, if any outward symptoms of the heartworm parasite in the first several months after initial infestation. For many affected dogs, heartworm disease is not even detected until the heart is so severely compromised by the large numbers of worms, normal activities, such as chasing a ball or running up stairs, becomes difficult. The worried owner takes the dog to the vet, blood is drawn and the grim discovery is made with a simple snap test. The blood sample is then checked under a microscope to determine the level of infestation. Once that is established, the vet must evaluate the overall health and function of the heart, liver and kidneys to ensure that the animal can tolerate the treatment regimen.

The Cure

The level of treatment at this point of infestation is considered by many to be as bad as (or worse than) the disease. Adult heartworms are killed by a series of injections of Immiticide® (melarsomine dihydrochloride) deep into the lumbar muscle. This drug is a toxin, but is proven less harmful to the dog than earlier arsenic-based formulations. Afterward, the dog must be under strict cage rest for several weeks to help absorb the dead heartworms, allow the heart and lungs to recover and to avoid embolism caused by dead worms dislodging and migrating to the lungs. This treatment is not cheap, ranging anywhere from $300-$1,500, depending on the parasite load, the size of the dog and other factors. There is the added risk that the dog may die as a result, because the animal's system is too compromised by the infestation to tolerate the treatment.

Prevention

From the above description, many would have to agree that the preventative is a far better route, and over the life of the dog, cheaper - both in monetary and emotional cost. There are a number of prescription heartworm preventatives, in specific dosages for the size of the dog. These can run between $5-$20 per dose, depending on the brand, formulation and size of the dog. Your veterinarian can help you decide which is appropriate for your dog.

Many breeders and shelter managers opt for the over-the-counter (OTC) preventative ivermectin, found in farm supply and feed stores. This is a less expensive alternative for prevention. However, this should not be used by the casual pet owner, as under- or over-dosing is possible. Some may attempt to use ivermectin under the mistaken notion that it will treat an active infestation. It is not proven effective - nor is it recommended - for adult heartworms already established in the heart.

Keep in mind that these preventatives are toxins, formulated in small amounts, so as not to injure the dog, yet remain effective in preventing the parasite to take hold. Some breeders are concerned with the regular use of heartworm preventatives, stating that the long-term use could damage reproductive health, specifically the male's sperm count and viability.

With advances in veterinary science, we gain new knowledge almost every day concerning our wonderful dogs. It becomes our responsibility as dog owners to approach all avenues of prevention and treatment armed with all the information we can gather, to help ensure the long-term health and happiness of our companions.