The Ins and Outs of Heartworm Medications for Cats


How Do Heartworm Preventives Work in Cats?


By Jennifer Kvamme, DVM


As a cat owner, keeping your cat free of heartworms is an important job. It’s your responsibility to have your cat 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 medications prevent heartworm disease in your cat?

You may be surprised to learn that heartworm preventives do not stop the actual infection from occurring in your cat. If an infected mosquito happens to bite your cat, he/she may still be infected with the larvae. The medications available today work to kill off the larval heartworms that made it into the cat’s body during the past month. The worms are killed at certain stages of development, before they can become adult heartworms. However, preventives will not kill adult heartworms.


There are many choices for heartworm preventive medication available, from topical products to chewable oral medications; most come in both dog and cat versions. Some medications require a monthly administration.


Monthly preventive medications do not actually stay in your cat’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 from your veterinarian to be able to purchase heartworm medication online? And why will your veterinarian not give you the heartworm medications without first testing your cat for heartworm infection?

The reason for this is that your veterinarian wants to make sure your cat doesn’t have an active infection of heartworms before giving a preventive medication. Animals with active infections can have a severe, possibly life-threatening, reaction to the dying, circulating microfilariae (adult heartworm offspring) if given these medications. The reaction can send the body into a shock-like state and it can be fatal.

Additionally, there are several other reasons your veterinarian requires a yearly test for heartworms before giving you a prescription for heartworm medication. You may have happened to miss a dose, or your cat may have spit the medication out or vomited it up, leaving your cat unprotected for a period of time that you were unaware of. If for any reason your cat did become infected, 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 cat is infected, the disease may progress to the point of no return for your cat. It’s better to know in advance so treatment can be started before the damage becomes too severe. Combination tests for heartworm now also help your veterinarian check your cat for other diseases, such as those that may be transmitted by ticks, like Lyme disease and ehrlichiosis. These tests are run in the veterinarian’s office and require only a small blood sample from your cat.

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