I have once asked a VET the same question and he did tell me that he has encountered a few cases of Heartworm infection.
Heartworm prevention will cost you about RM 10.00 per month per dog,a figure that is still afforable. The treatment for Heartworm infection is however costly and difficult. While the infection may not occur to your dog, going on prevention medication is just like buying some insurance for your lovely pets. :)
Both Heartgard and Proheart are common Heartworm prevention medication which cost about RM 60.00 for six tablets.
The cut off age for using these preventive medication on puppy without any heartworm test will be 7 ~ 8th month. This is base on the assumption that your puppy when born will not be infected with Heartworm immediately and these heartworm will take six months to grow into matured adult worm that this medication is not effective against. The best is of course to start before the six month period.
For adult dog, there is a need to perform test to confirm the adsence of the adult heartworm before implement such preventive medication.
For me, Proheart is a better choice. Giving chewable table may risk your dog not taking the full dose if he/she did not take the whole tablet completely. Proheart is a small tablet that can be forced into your pets mouth easily, hence ensuring your pet is taking the complete dose.