That map is cool, but I do not know if I would be comfortable following it. My dogs get Sentinel all year. That map says I could start in June and end in November. We have crazy weather here and sometimes it gets plenty warm enough for mosquitoes in April or May.
The thing people are missing --
mosquitoes alone do not constitute a HW infection risk.
The TEMPERATURE has to maintain at 80F or higher for at least two weeks (or six weeks at temperatures around 57F) DAY AND NIGHT before the larva inside the mosquito incubate to the stage where they can infect dogs. If, at
any time during that 2 to 6 week period, the temperature dips below 57F even
once, THE LARVA DIE inside the bug. Infection becomes impossible.
For places where the temperatures are closer to 60F for most of the year, take into account that the average lifespan of a wild mosquito is generally one month, and it reduces the chances of infection in that region even more.
The maps are based on the average temperature levels for each region over 30 years,
not their mosquito seasons. Plus, they are
conservative estimates, which means they actually leave
more of a safety buffer around when to dose than is actually reflected by the data.
Check the Heartworm map
from here for Indiana. Overall your state has a very low infection rate.The numbers collected in that map reflect the total HW positives recorded over 7 years of vet clinic surveys. The surveyer is Idexx, one of the leading makers of HW testing kits (so it is not in their interest to record low numbers).
The only drawback I can find about not doing year round prevention is that if for some reaosn your dog does contract heartworms and you start the medicine again, you can severely injure or kill your dog.
Heartworm preventative dosages only kill HW larva in the bloodstream, it doesn't affect adult worms (unless you do it monthly for a year or more, which is actually one accepted strategy for treating mild infections. It is actually normal procedure for
a HW-positive dog to get a dose of regular ivermectin-based medication before starting the full treatment:
VeterinaryPartner.com: said:
The first step in treatment is clearing the migrating immature worms. If we were to jump directly to killing the adult worms first, the adult worms we remove could be readily replaced shortly afterwards by those that were in the process of migration at the time of treatment. By addressing the migrating immature worms first, we minimize the number of adult worms we must kill in the second step. Fewer adult worms dying at once means less risk.
Happily, the microfilariae, L3, and L4 larvae can all be killed by monthly ivermectin-based heartworm preventive products (i.e. Heartgard®, Tri-Heart® etc.). The milbemycin based products (Sentinel® and Interceptor®) will also do the same job but will kill the microfilariae much faster, which can create circulatory shock if there are large numbers of microfilariae dying all at one time. The newer products using selamectin and moxidectin do not clear microfilaria well enough to be used in the treatment of an active infection, so right now the ivermectin based products seem to be the best for this use. The American Heartworm Society recommends 1 to 3 months of a preventive prior to treating the adult worms. How long you choose to wait depends on how urgent the dog’s need is to have the adult worms removed. After all, it is the adult worms that cause heartworm disease, not the immature worms addressed by the preventives.
Where you're correct about that, is that if you use a
milbemycin-based medication (Interceptor or Sentinel) on a heavily infected dog, it can kill too many larva too quickly and cause toxic shock. But that it only in regards to that
one specific chemical.
If killing heartworm larva in the blood was potentially fatal to a dog, you'd be risking it's life every time you gave them a HW pill. HW pills do not
prevent infection, they only kill whatever larva are already present in the blood BEFORE they can become adult worms in the heart (as the above article explains).
The arsenic treatment is actually obsolete -- there are
safer medications now. Also, the strong possibility that using Doxycycline on the dog before regular HW treatment can greatly reduce the risk of fatal complications from the dead worms, means that it is actually safer to treat a HW-positive dog now than it was in the past. Of course, that's if your vet is keeping up on the research and not still using the riskier old-school treatments.
