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Heartworm by James C. Frank,DVM

Although there is a growing trend to administer heartworm preventive medications 12 months of the year, Lakeside Animal Hospital continues to prescribe these medications for just 6 months out of the year – June 1 to Nov.1. This recommendation is based on the scientific evidence available from studies on the heartworm life cycle, mosquito life cycles, and weather temperature patterns, not on sales promotions by the drug industry.

Heartworm is an internal parasite of dogs and coyotes, and to a lesser extent wolves, foxes, and even occasional cats. Mosquitoes are required to transmit this parasite from one host to the next. Obviously if mosquitoes pick up this disease from a dog, the ‘baby worms” are microscopic while floating in the bloodstream. When mosquitoes feed on the dog for blood, they can suck up these microscopic heartworm forms. After further development in the mosquito – only when temperatures are above 57 degrees for an extended time – these intermediate heartworm forms are passed on to another dog. An important fact is that the development in that mosquito requires 10 days to occur if the air temperature is 80 degrees 24 hours a day for those 10 days or at 70 degrees 24 hours a day for 18 days. In the reality of daily/nightly temperature fluctuations, assuming every day it got up to 77 degrees after going down to 57 at night, it would take 23 days for the heartworm larva in the mosquito to reach a stage that could be transmitted to a dog. If the daily high was 67 and the night’s low was 47, it would take more like 45 days. This worm form once transmitted to the dog, lays dormant for 45 days before finishing its growth to a 6-8” adult heartworm in the chambers of the heart of that next dog.

The life cycle of mosquitoes involves eggs that hatch when warm, moist conditions develop. The larval mosquito feeds in the water, eventually pupates (as butterflies and moths do) and ultimately hatches into an adult mosquito. In order to produce eggs for the next generation, the mosquitoes must mate and the female mosquito must feed on blood in order to produce her eggs. This cycle of course occurs in warmer weather, adults surviving a month or two only. To survive into the next year, some mosquitoes overwinter as eggs (the adults dying off) or in other species they overwinter/hibernate as adult females. The interesting thing is the last egg hatches in the late summer/early fall of these mosquitoes responds to the shortening days by not feeding or reproducing when they hatch. The mosquitoes mate, the males die, but the females feed on sugary plant sources and find sheltered locations to hibernate the winter. In spring/summer, she resumes her 6-8 month life cycle by feeding on a blood source to begin laying eggs for the next generation.

Those advocating 12 month Heartgard usage, are misleadingly saying that because there are as there have always been, live (hibernating) mosquitoes in the winter, dogs are at risk of being bitten at any moment the temperature warms up. If it actually got warm enough, long enough for a mosquito to risk break hibernation, and she fed on a dog’s blood, she would not have any heartworm larva in her to transmit because she has never fed before on blood. If she picks up heartworms in that first blood feeding, it would require months of warm temperatures during the winter/spring to allow the heartworm to become infective in her.

In real life studies in Louisiana and Georgia, dogs kept outside all year round develop at a rate of 93% in any given year. However, if they are kept outdoors from Dec.1 to Apr.1 in these deep south climates, the rate of infection was 0% ! This was repeated a second winter and a third winter and each time they still came out with 0 % heartworm infection rates for a 4 month stretch of winter. In Louisiana and Georgia, the heartworm season is thus something less than 8 months long. Using temperature extrapolations, it is felt that the 6 month heartworm transmission line is at a level of the Virginia/North Carolina border. For Wisconsin, realistically it appears to be at most 4 months.

In analysis of over 100,000 mosquitoes throughout the year in Louisiana, northern Florida, and central Florida, the transmissible larval heartworm could not be found in mosquitoes capture from Dec.1 to Apr.1, totally consistent with what was seen in the dog not picking up heartworm during this season. The only exceptions were a few mosquitoes captured near Tampa, Florida, hardly a hibernating type climate.

Factoring in the way Heartgard works – by killing those larval forms that have been injected into the dog by the mosquitoes, as they lay dormant for 45 days after infection – by using Heartgard from June 1st to Nov.1st, prevention of any infection from April 15th to November 1st is accomplished. When presented with this scientific information, the representatives and veterinarians in the drug companies have no rebuttals and they won’t present this information in place of the winter mosquito scare. The only major concern we have in all of this, is that it is routinely found in studies that only 40-50% of dog owners who intend to give heartworm preventive pills actually remember to give all of the treatments. This is where the problem lies, not in hibernation.