Shelter dogs absolutely have a higher percentage of heartworm in the general population. No question. I am talking about normal dogs that are healthy, well-fed, and cared for. And, I should have noted in my earlier post about prevention, chemical-free.
Let's take a look at the likelihood of heartworm infestation occuring. There are over 3,000 varieties of mosquitos. Of these, approximately 70 species have been identified as capable of transmitting heartworms. East of the Mississippi, where heartworm cases are most prevalent, only 16 of the 70 species are known to exist. In Florida where the climate is conducive to year-round mosquitoes, only 11 of these 16 species are found in abundance. For a mosquito to transmit heartworm it must first feed on an infected dog - a task in and of itself given the widespread use of heartworm preventive. Then it takes 30 days at the right temperatures before the ingested microfilariae develop into 3rd stage infective larvae, which is the stage at which it is transmittable to the dog. Guess what the average lifespan is of a female mosquito (the only one that feeds on blood meals) is? Three to four WEEKS! So the timing has to be absolutely perfect for the complete cycle of heartworm transmission to occur. When a dog is fed upon by a heartworm carrying mosquito, it takes about 70-90 days before the worms reach young adulthood and migrate to the heart. For unknown reasons, the percentage of infective third-stage larvae that reach maturity vary in different breeds of dogs. It is only after adult worms mate and start to discharge tiny motile microfilariae that circulate in the blood that disease becomes apparent. Microfilariae appear in the blood about 200 days after infection. It is important to note that heartworm tests are based on the detection of microfilariae (heartworms can also be detected by xrays). These larvae MUST be picked back up by another mosquito and re-injected into another host to mature into adults.
This is one reason yearly testing is essential - if the dog is infected in August, testing in March or April will reveal the infection before it is too extreme. If you're in a higher-risk climate, or if you decide not to use preventive, twice-yearly testing may be indicated.
Also, Despite the high use of heartworm preventative drugs, the rate of heartworm across the US is the same as it was in 1982 according to C.J. Puotenin in The Encyclopedia of Natural Pet Care.
The information that dogs can combat heartworms on their own comes from Martin Goldstein, DVM and Don Hamilton, DVM - and the many people - breeders - on the mailing lists I'm on who have not given heartworm preventives for DECADES and have NEVER had a case of heartworm in their dogs. At least one of these breeders is in South Carolina, where they have climates that are very hospitable to heartworm infestation. Also from Richard Pitcairn, DVM, who notes that wild animals such as coyotes don't die from heartworm, but develop a mild infestation, then become immune.
Yearly or twice-yearly testing will detect the worms before they are a problem, and homeopathic treatment is effective and safe (far safer than the arsenic treatment allopathic vets use). IMO, the chances of my dogs contracting and then succumbing to heartworm are far more slim than the chances of them having and adverse reaction to the heartworm preventives. Again per C.J. Puotenin, the American Veterinary Medical Association has reported that 65% of the adverse drug reactions it monitors and nearly half of all drug-related deaths are caused by heartworm
prevention medicine. Heartworm is curable - major kidney damage from the preventive is not.
(he is all of these to me)lovicon 
