Heartworm preventatives are not 100% effective. Occasionally a dose gets missed, spit out, vomited, or just not adequately absorbed, resulting in inadequate protection. Heartworm testing is a safeguard against your pet suffering from heartworm disease without your knowledge. Furthermore, giving some types of heartworm preventative to a dog that has heartworm infection may actually result in a shock-like reaction due to a rapid kill of immature heartworms.