How Do Heartworm Preventives Work in Dogs?
By Jennifer Kvamme, DVM
Keeping your dog free of heartworms is an important job. It’s your responsibility to have your dog tested yearly for the disease and to renew his/her prescription for heartworm preventive medication. While you may have been doing this for some time now, have you ever wondered just how these preventive medications thwart heartworm disease in your dog? How the treatment for heartworms work?
You may be surprised to learn that heartworm preventives do not stop the actual infection from occurring. If an infected mosquito happens to bite your dog, he/she may still be infected with the larvae. Heartworm medications available today work to kill off the larval heartworms that made it into the dog’s body during the past month. The heartworms in dogs will die at certain stages of development, before they can become adult heartworms. However, heartworm preventives will not kill adult heartworms that are already present.
Some heartworm medications require a monthly administration, while others work even longer (up to six months with an injectible product called moxidectin). There are many choices of heartworm preventive medications available, from topical products to chewable oral medications; most come in both dog and cat versions.
Monthly heartworm preventive medications do not actually stay in your dog’s bloodstream for 30 days. The active ingredients work to kill any larvae that have been in the system for the past 30 days, clearing the body each month. The heartworm medication is only needed once a month because it takes longer than a month for the larvae to develop to a stage where they reach the body tissues.
Prescription Required for Heartworm Medication
So, why do you need a prescription fromyour veterinarian to be able to purchase heartworm preventives online? And why will your veterinarian not give you the heartworm medications without first testing your dog for heartworm infection?
The reason for this is that your veterinarian wants to make sure your dog doesn’t have an active infection of heartworms before giving a heartworm medication. Dogs with heartworms can have a severe, possibly life-threatening reaction to the dying, circulating microfilariae (adult heartworm offspring) if given these heartworm medications.
Additionally, there are several other reasons your veterinarian requires a yearly test for heartworms before giving you a prescription for the heartworm medication. You may have happened to miss a dose, or your dog may have spit the heartworm medication out or vomited it up in the yard, leaving your dog unprotected for a period that you were unaware of. If for any reason the dog became infected with hearworms, treatment to rid the body of the infection must be started as early as possible to prevent permanent heart and lung damage.
If you don’t test for the disease and your dog is infected, the disease will gradually progress to the point of no return for your dog. It’s better to know in advance so treatment can be started before the damage is too severe. Combination tests for heartworms in dogs now also help your veterinarian check for other diseases, like those transmitted by ticks, such as Lyme disease and ehrlichiosis. These tests are run in the veterinarian’s office and require only a small blood sample from your dog.
Keep Heartworm Preventives Going Year-Round
Veterinarians also now commonly recommend that dogs be given heartworm preventive medications all year round. In some parts of the country, where mosquitoes are less active in the winter months, dog owners are in the habit of only treating their dogs for heartworms half the year. Due to unpredictable seasonal temperature changes, the American Heartworm Society is now recommending year-round prevention for animals in every state. This is a good practice to help you remember to always protect your dog from heartworms, no matter what the season.
Some heartworm preventives contain medications that also remove other parasites, such as fleas, mites, ticks, roundworms, hookworms, and whipworms. Depending on which heartworm medication you choose for your dog, he/she may also be protected year-round from these parasites. Ask your veterinarian for help in choosing the best possible heartworm preventive medication for your dog.