Australia = Heartworm: Preventatives for Dogs

Firstly, happy Australia Day! Enjoy whatever celebrations or holidays you are having today and give your pet and extra cuddle - I know I will! Onto more serious topics now, one that should never be ignored by dog owners across mainland Australia.

Heartworm (Dirofilariasis) is a potentially life-threatening disease spread by mosquitoes. Dogs are infected with heartworm when bitten by an infected mosquito. Prevention of heartworm involves a combination of environmental control of mosquitoes, and treatment of your dog to prevent infection developing once bitten by an infected mosquito.

Environmental control of mosquitoes can be quite a challenge. It is known that mosquitoes breed in warm, moist environments, especially those involving stagnant water. Ensuring that stagnant water areas are eliminated can help with mosquito control.

Once bitten by an infected mosquito, microfilariae are transferred into the dog. These microfilariae mature within the dog to become adult heartworms which live in the pulmonary arteries and sometimes in the right chambers of the heart. These adult heartworms interfere with blood flow and cause structural changes in the area with secondary changes to the heart's function, thus minimising the heart's efficiency. Significant pulmonary (lung) disease is also seen.

Heartworm preventatives for dogs focus on killing the microfilariae in the dog's system before they develop into the disease producing adult worms. All dogs in areas with endemic heartworm (most of Australia) should be placed on year round lifelong heartworm preventatives. Similarly any dog travelling from a heartworm free area (i.e. Tasmania which has had only one reported case) into a heartworm affected area should also be placed on heartworm preventative medication. Your veterinarian can advise you of the requirements for your dog.

Dogs not currently on heartworm preventative medication or those newly purchased where the owners are unsure of their heartworm status should be assessed by their veterinarian prior to beginning heartworm preventatives. This is because some medications cannot be given safely to dogs that are already carrying adult heartworms. Your veterinarian can perform blood tests to identify heartworm positive dogs. Treatment of heartworm is possible but prevention is recommended.

The age at which puppies can begin using heartworm preventatives varies depending on the medication chosen. Your veterinarian will be able to advise on when to start. Most medications can be started by 12 weeks of age.

There are numerous heartworm preventatives available for dogs. The choice of medication will depend on an assessment of the efficacy of the product, the breed of dog, the compliance of the dog to medicating, owner convenience and budget and the need for other parasitic control which can be provided by some medications. The use of ivermectin containing medication in collie breed dogs should be discussed with your veterinarian. Collie breed dogs are at risk of having sensitivity to ivermectin which can result in clinical signs associated with central nervous system dysfunction. The manufacturers of Revolution Plus for Cats (which contain ivermectin) state that these products are safe for use in collie breed dogs at recommended doses. For dogs which are difficult to medicate many dosing options are available.

Yearly prevention can be convenient for many owners. Veterinarians give a yearly injection containing the active ingredient Moxidectin under the skin which will prevent heartworm infection from establishing in the treated animal. This injection can be given at the same time as the annual health check and vaccination.

Several products are available which are given or applied to the dog once a month. Many of these products combine ingredients which will also kill fleas, mites, worms and other parasites. These products can exist as tablets/chewables or spot ons.

Grouped according to their active ingredient/s, the tablet/chewable forms include (but are not limited to):

  • Ivermectin containing heartworm preventative tablets/chewables:
  • Moxidectin as the active ingredient - Proheart tablets.
  • Milbemycin Oxime
    • Interceptor Spectrum - also contains praziquantel. This monthly dose helps control heartworm, roundworm, whipworm, hookworm and tapeworm.
    • Sentinel - also contains praziquantel and lufeneron. It's spectrum of activity covers heartworm, roundworm, whipworm, hookworm, tapeworm and helps with flea control by inhibiting the development of flea eggs and larvae.

The monthly spot on treatments are placed on the back of the dog's neck on the skin and continues to prevent heartworm for 1 month. Many of these also help control other internal and external parasites and include the following products:

  • Imidacloprid and Moxidectin - these ingredients are present in Advocate for dogs. This monthly spot on works as a heartworm preventative as well as controlling fleas, roundworm, hookworm, whipworm, lice, sarcoptic mange, demodectic mange and ear mites.
  • Selamectin - contained in Revolution monthly spot on. Revolution is used to prevent heartworm, fleas, ear mites and sarcoptic mange.

As well as providing a convenient method of treatment and in many cases offering coverage against many other internal and external parasites as listed above, the monthly medications often provide a limited reach back effect which provides a "safety net" in the case of a missed dose. Your veterinarian should be consulted to discuss this if any doses are missed.

Daily heartworm preventatives are also available. These tablets must be given every day to provide adequate protection against heartworm disease. Failure to dose the dog daily will result in immediate lapse of the preventative control so strict owner compliance is required when using these medications. The active ingredient, Diethylcarbamazine citrate is present in Dimmitrol daily tablets and syrups.

A heartworm preventative program should be implemented for all dogs living in heartworm endemic areas or those travelling into such areas. Your veterinarian should be consulted to discuss the need for heartworm prevention for your dog and the possible need for heartworm tests to be done prior to beginning medication. Heartworm is a potentially life-threatening disease in dogs which is rapidly spread by mosquitoes. Prevention is essential to stop the heartworm microfilariae from developing into adult heartworms capable of causing heartworm disease.