I've heard that in Southern California, having a dog on heart worm prevention medicine is not necessary?

A dog can get heartworms any were. It dont matter were you go. And b/c its Shi-Tzu (i got 1 too), its more important cuz there more prone to them. Heartworm is one of the most dangerous things a dog canget, it can kill them with in a few days.
Honestly if you want your dog to be safe the heartguard is your best bet, if you dont you could risk the death of your dog.

Would you rather save money and lose you dog?
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heartworm is contracted from mosquito bites. you cannot prevent mosquitos from biting your dog, and once he has heartworm the symptoms are not noticable until the damage is already done. heartworm medicine is cheap, easy to get, and safe for use. there's no reason not to use heartworm prevention! this is the life of your beloved pet we're talking about here! why gamble with chance when you can prevent illness? that's like saying oh i dont want my baby vaccinated against polio, because it's not really nessecary. ummm yes it is, heartworm medication is a standard part of responsible pet ownership.. not an option
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~I live in So Cal. and this year our vet has had us start using heart gaurd. Heart worms have been found in ventura, riverside and san diego counties. I live in Los Angeles County. He said at this point of time since there has been reported cases in all surrounding counties it is just a matter of time. He has been our vet for 13 years and is just now becomming concerned. So I took His advice. He recommened using revuliotion every 6-8 weeks instead of the normal 4 week dose. He said that is what he started on his own personal pets also.

Also mosquitos are the carriers of the heart worm larve. And As a lifetime So Cal resident. I have noticed the last few years more of a mosquito problem than in the past.
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If there are any mosquitoes in your area, he's at risk for contracting heartworms. The monthly preventive is fairly cheap, but It is very expensive and risky to treat once they become infected. Some dopn't survivie the treatment.

Personally, I live in the midwest and keep my dogs on preventive year round–even in the winter. The heartworm preventives also have intestinal dewormers in them, so if they pick up any "nasties" it will take care of them too.

If your dog has tested free of heartworms, you can begin giving him preventive immediately. I still get them checked annually, just in case they spit out the medicine or threw it up and I didn't know, etc. It is important to make sure your dog is heartworm free before giving any preventive; otherwise complications can be fatal.

The difference in the brands is pretty much in what intestinal worms they kill. They are all good for protection against heartworms. My dogs liked Intereceptor, hated Sentinel but LOVE Heartgard! They think it's a treat. Also, as LilBitsMom said, Revolution is an option. It comes in a dropper form like flea repellents and you apply it externally on their back/neck.

Since the cost of most medication is based on weight, you have the advantage of owning a smaller dog so it will cost less.

It's good you're checking into this–alot of people don't think it's necessary until it's too late.
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if you want your dog to die then don't give it the medication. it's always good to be extra sure !
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proud weenie owner - gets the medsss every few months i think?

As another CA resident, I will tell you why there is a higher risk now than there was before. You are obviously aware of all these homes foreclosing right? Well, alot of these homes which have pools are not being atended to. Where do mosquitoes breed? Standing water. In my area up in northern California, we are getting dusted on Tuesday with some kind of mosquito killer because there have been reported cases of west nile virus. All of these unattended pools are causing havoc all over the state as far as mosquito related problems are concerned. Fun! More mosquitoes = higher chance for heartworm. I have never had my dog on the meds before, but I will be getting him tested and on meds in the very near future, just in case.
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Yes use heartguard. when your baby gets infected and has to go thru life threatening treatments that cost 800$ (where I live, SC) and you hear him up at night hacking and coughing because of the damage to his heart and lungs you will wish you had sucked it up and paid the 30 bucks every 6 months for heartguard to keep him healthy!
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I adopted an abandoned pug left on someone's doorstep because he had heartworms, and hes the best dog ever.. unfortunately most expensive too