By Jennifer Copley (Last Updated 6 December 2011)
Heartworm is common in tropical, semitropical, and temperate climates throughout the world including North America and southern Europe. Animals can succumb to heartworm at any age.
Heartworm larvae are carried by mosquitoes, which transmit the organisms when they bite an animal. These organisms infest the lungs, heart, and blood vessels, and if the larvae reach the adult stage, they can cause serious health problems.
Many cats rid themselves of the parasites on their own, with fewer than 26% of worms making it to the adult stage in cats, whereas 40-90% reach this stage in dogs. Additionally, heartworms tend to live just 2-3 years in cats, compared to 5-7 in dogs.
In both cats and dogs, the disease can be fatal if not treated. However, cats are more likely to die if the larvae have the opportunity to grow to the adult stage because their blood vessels and hearts are smaller and so the damage caused by the worms is greater.
Symptoms in Cats
Heartworm Associated Respiratory Disease (HARD) is often misdiagnosed as allergic bronchitis or asthma. Symptoms may also mimic other feline diseases, so it tends to be more difficult to diagnose in cats than in dogs. Common heartworm symptoms in cats include:
- Intermittent vomiting
- Loss of appetite and/or weight
- Panting, open-mouthed breathing, or other signs of intermittent breathing difficulties
Symptoms in Dogs
Dogs with heartworm present a different clinical picture. Symptoms, which are usually far more severe in very active dogs, include:
- Abnormal lung sounds
- Intolerance for exercise
- Abnormal heart sounds
- Failure to grow
- Laboured breathing
- Bluish or purplish skin discoloration
- Spitting up blood
- Bleeding nose
- Temporary loss of consciousness
In contrast to active dogs, inactive “couch potato” dogs may show few if any symptoms when they have been infected with heartworms.
Free roaming cats and dogs housed outdoors face a greater risk of heartworm infection, though indoor pets are not free from risk. There are several medications that have been approved by the FDA to prevent larvae from developing into adult heartworms. The American Heartworm Society recommends implementing year-round prevention, particularly in areas where heartworm is common. Preventives for dogs and cats only need to be administered once per month, so it is relatively easy to keep your pets safe.
Late-Stage Treatment for Cats
There are no approved treatments for feline heartworm infection when the heartworms have been left to reach the adult stage. If the disease is affecting the blood vessels and lungs, x-rays should be taken every 6-12 months and a veterinarian may prescribe prednisone. In some cases heartworms can be extracted surgically.
When severe symptoms are present, a cat may require intravenous fluid, oxygen therapy, bronchodilation, antibiotics, cardiovascular medications, and nursing care. Only 25-50% of cats survive adult heartworms.
Late-Stage Treatment for Dogs
For dogs, there are a few different late-stage treatments available. The first is an FDA-approved organic arsenical compound. Another treatment option is Merial’s Immiticide. This treatment is administered via intramuscular injection.
Destroying adult heartworm parasites can lead to serious complications such as pulmonary thromboembolism. Additionally, dogs must be confined and their movement restricted during and for at least one month following treatment to reduce the risk of respiratory problems as a result of the dead worms in their systems. In some cases, a second round of treatment is required. Given the severity of late-stage treatment, preventative treatment is by far the better option.
This article is provided for informational purposes only and is not intended as a substitute for veterinary consultation and care.
- American Heartworm Society. (2011). “Feline Heartworm Disease” and “Canine Heartworm Disease.” AmericanHeartwormSociety.org.
- Merck & Co., Inc., Eds. Cynthia M. Kahn, BA, MA & Scott Line, DVM, PhD, Dipl ACVB. (2007). The Merck/Merial Manual for Pet Health, Home Edition.