Interceptor prevents heartworm in dogs, and will also control roundworms, hookworms, and whipworms.  It comes in a chewable form that is not as well taken as the HeartGard chewable, but is small enough to be 'shoved' down if your dog does not take it readily.  It has a proven "track record" and good safety record.  In cases where chronic or recurrent whipworm infections are a problem (common in dogs that are kept in pens or kennels with dirt floor) this is the preventive of choice. 

The cost is $2.50 to $6.00 per monthly dose.