Heartworm Prevention

What is Heartworm?

Heartworm is a parasitic roundworm carried by mosquitoes. It can infect dogs, cats and other animals, though rarely humans. 

Transmission & Life Cycle

Heartworm lifecycle © Intervet Inc., a subsidiary of Merck & Co., Inc. Used by permission.Heartworm lifecycle © Intervet Inc., a subsidiary of Merck & Co., Inc. Used by permission.

  • The mosquito sucks blood from an infected dog, thus ingesting baby worms called microfilaria.
  • Within the mosquito, these microfilaria change into infective larvae.
  • The mosquito then infects a healthy dog when feeding.
  • These larvae develop into adult worms which migrate to the heart and large blood vessels.
  • The adult female worms will start producing more microfilaria and allow the cycle to repeat.

Heartworms in a dog heart. Image courtesy of American Heartworm Society www.heartwormsociety.orgHeartworms in a dog heart. Image courtesy of American Heartworm Society www.heartwormsociety.org

  • The first symptoms of disease include coughing, breathing difficulty,  weakness and exercise intolerance. 
  • As the number of adult heartworms increase, they interfere with the normal functioning of the heart.
  • Consequently, congestive heart failure and death can occur. 

  Due to the presence of already infected dogs and the year-round presence of the disease carrier, the mosquito, the incidence of Heartworm in dogs in Hong Kong is high.


Heartworm is easier to prevent than cure. Heartworm treatment is expensive and can be risky. Heartworm treatment requires hospitalisation and a long period of post treatment care. If left untreated, it will become more severe and eventually lead to death.

There are two options:

Monthly heartworm tablets are dispensed according to body weight.Monthly heartworm tablets are dispensed according to body weight.

1. Monthly Heartworm prevention tablets

  • Ideally, all puppies should commence tablets such as TRI-HEART® from 3 months of age (2nd DHPPiL vaccination) and continue for the rest of their lives. No test is required if prevention is started at this age.
  • They are sold according to dog's body weight. Remember to weigh your dog regularly to ensure correct dosing.
  • These are available by prescription from your veterinary surgeon. Please consult your preferred clinic for more details.

Annual PROHEART® injectionAnnual PROHEART® injection

2. Annual PROHEART® injection

  • This injection is given yearly to dogs over 9 months of age by a veterinary surgeon.
  • It can be given to dogs between 3 to 9 months of age but protection doesn’t last a full year in such cases.
  • Please consult your veterinary surgeon for more details.
Heartworm testing

Under certain circumstances, a blood test might be required prior to starting heartworm preventative treatment.

  • If a dog has not started heartworm prevention by 6 months of age.
  • If you forget to give your dog its monthly heartworm prevention tablets or the PROHEART® injection is overdue, please consult your veterinary surgeon for advice regarding the need for a blood test.

Download Heartworm information sheet