Heartworm in the Okanagan

Parasites are typically thought of as nasty creatures and heartworm is no exception. Fortunately, we do not have to deal with heartworm infections very often at all in the Okanagan at any Okanagan Veterinary hospital. To date, no dogs that have remained in the boundaries of the Central Okangan have been diagnosed with this disease. About two years ago a Yorkshire Terrier from Vernon was infected after traveling to Ontario for dog shows. In the last 3-4 years, there have been about five cases diagnosed in Kelowna, all in dogs that were brought in from outside the country. The bulk of the cases in Western Canada are in Winnipeg, with only a small handful in all of B.C. each year.
Exactly what is heartworm disease? It is a serious condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart primarily in dogs and rarely in cats. The highest infection rates for heartworm have always been in the southeastern states within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries.

Coinciding with mosquito season, heartworm disease is spread by mosquitoes that become infected with heartworm larvae while taking a blood meal from an infected dog. The larvae mature into the infective larval stage within the mosquito. Under certain environmental conditions, when the mosquito bites another dog, it then passes the larvae into the animal’s blood stream through the bite wound, resulting in heartworm infection. It then takes a little over six months for the infective larvae to mature into adult worms that can live for five to seven years in the dog.

A few years ago, the American Heartworm Society’s web site published the following information: “Laboratory studies indicate that development and maturation requires the equivalent of a steady 24-hour daily temperature in excess of 64 F (18 C) for approximately one month. Intermittent diurnal declines in temperature below the developmental threshold of 57 F (14 C) for only a few hours retard maturation, even when the average daily temperature supports continued development.” As a result, heartworm disease is not only geographically limited, but also seasonally limited.
Since that time, they have removed the 64F (18C) reference, and the current information available from the same web site is thus: “As these vectors expand their territory the number of unprotected animals infected will continue to increase. A pivotal prerequisite for heartworm transmission is a climate that provides adequate temperature and humidity to support a viable mosquito population, and sustains sufficient heat to allow maturation of ingested microfilariae to infective, third-stage larvae (L3) within this intermediate host. It has been shown under laboratory conditions in three mosquito species that maturation of larvae within mosquitoes ceases at temperatures below 57ºF (14ºC) and similar activity is expected in other mosquitoes capable of transmitting heartworms. Heartworm transmission does decrease in winter months but micro-environments commonly present in urban areas virtually ensure that the risk of heartworm transmission never reaches zero. Some species of mosquitoes overwinter as adults. While heartworm larval development in mosquitoes may cease in cool temperatures, development quickly resumes with subsequent warming.”

In other words, Heartworm is still as unlikely then as now here in the Okanagan. Brrrrr! It’s cold out there! Plus, it would seem we simply don’t have the right humidity for it to get a foothold.

When it comes to prevention of heartworm, I do not counsel my clients to use the available drugs, mainly because I feel that heartworm does not pose a big enough threat in the Central Okanagan to warrant placing your dog on drugs for 6 months of the year. For the few clients I have that travel into the States or who live in the southern Okanagan areas, I advise them to use only two pills, one in mid-August and the second in late September, or to start the pills 45 days after the temperatures reach those indicated above. Although this is off-label usage, data indicates the medication covers at least the previous 45 days and the temperatures do not usually warm up here consistently until into July. The minimum amount of chemicals you can put in your dog, the better.
As far as the actual dose required to prevent heartworm, several Heartworm Prevention Dose Titration Studies sponsored by Ciba-Geigy Animal Health, the manufacturer of Interceptor, were conducted to determine the minimum effective prevention dose. These studies established the minimal effective dose of this common heartworm prevention drug at 0.1 mg/kg, and for hookworm control at 0.5 mg/kg. To support a dual claim for heartworm prevention and hookworm control, the finished pharmaceutical dosage forms (tablets) were formulated to provide a target dose of 0.5 mg/kg body weight.

If hookworm were of a major concern here in the Okanagan for our Okanagan Veterinary hospital, we might need this size of dose for our dogs. But it isn’t very common at all; therefore, we can get away with a much smaller dose of Interceptor than we thought, in fact 20% of the suggested package dose. Based upon data from studies sponsored by Ciba-Geigy, it is concluded the minimum effective dose for heartworm prevention lies between 0.05 and 0.1 mg/kg body weight. This means you can give your 100 pound Rottie a dose equivalent to a 15 pound Poodle and still protect him from getting heartworm.

I do not recommend that any dogs living in Canada get the injectable heartworm preventative medication call Proheart. This provides a slow release drug into the body over a period of 6 months. The problem is that nowhere in Canada do we have 6 months of “heartworm” weather. A sister station to CBS TV (WBZ TV) did an investigative story March 1, 2004 on this drug. Team investigation found that in the past two and a half years, the FDA has received more than 4,000 reports of dogs getting sick after getting a shot of ProHeart 6. And more than 400 dogs have died nationwide. “A cause for concern” in their words.

Short of keeping your dog indoors during “mosquito hours”, you can also use mosquito repellents such as feeding garlic regularly or using lemon skin tonics. Some herbal products can also be used for a routine summer parasite cleanse that may also aid in preventing heartworm infections, such as black walnut and artemisia, but no studies have been conducted on their effectiveness.

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