Sadly the vets may simply not be that knowledgeable depending on their experiences and education if the products are not routinely used. To suggest not taking preventative measures appalls me.
First, contact these folks. They have always been quick to respond to my questions and I am betting you they have resources to give you better research information and suggestions on whether products have actually been found resistant to flea/mosquitoes. They should also have advice on whether switching to the older daily meds provides more protection: http://www.heartwormsociety.org/
While it is true that fleas may become resistant to some meds, first you would need a long term high usage which, let's be real– Guyana doesn't come to mind. Second, back up flea protection with flea/tick combing as suggested. Before Frontline, I had to flea comb my chow so really, it's not that big of deal
I seriously doubt that heartworms are becoming resistant to meds. And while they didn't say not to, again, if they suggest that, to become resistant, they would need to survive, propagate more resistant offspring, etc. With the massive use of the drug here in the USA, we haven't seen resistance. So at least you can feel sure you can protect them. Additionally, heartguard helps protect against OTHER worms too.
http://www.stanford.edu/class/humbio103/ParaSites2006/Dirofilariasis/Prevention.htm
In South America, success has already been demonstrated with the reduction of heartworm infections across many countries. This can be attributed to:
Better control programs for mosquito populations, resulting in a decrease in vector populations.
Effective chemoprophylaxis for heartworm to prevent increase in number of disease carriers.
Reduction of disease carriers with injectable ivermectin to kill existing microfilariae in dog populations.
Side effect reduction of reproductive capacity of worms from ehrlichiosis control with tetracyclines.
What concerns me is the mosquito issue. Frankly, not much has ever been proven to help a lot. I don't know how other types of mosquito borne diseases are there. You can help with different products, and I have found that neem tree oil spray works marvelous on humans. But here is a rundown for people products. Sadly most are not recommended for dogs due to likelihood of ingesting them.:
http://edis.ifas.ufl.edu/in419
Neem: http://www.clemson.edu/extension/hgic/pests/pesticide/hgic2770.html
Neem is a relatively new and promising botanical insecticide made from extracts of Neem tree seeds. It is used to control a wide variety of insects including leafminers, whiteflies, thrips, caterpillars, aphids, mealybugs, spider mites, scale crawlers, and beetles. Neem is most effective against actively growing immature insects.
Azadirachtin, the active ingredient in neem extracts, has a very low mammalian toxicity. It acts as an insect feeding deterrent and growth regulator. Neem does not produce a quick knockdown and kill, but stops insect feeding. The treated insect usually cannot molt into its next life stage and dies without reproducing.
Many commercial neem products exist, including Azatin XL, Neemix, SouthernAg Triple Action Neem Oil and Safer BioNeem. These products are labeled for use on ornamentals, foliage plants, trees, shrubs and food crops. Many neem products and formulations are also effective as a fungicide against powdery mildew.
