Heartworm FAQ Sheet

Heartworms are a variety of roundworm with the clinical name dirofilaria immitis. They are spread by mosquitoes.

Dogs can only get heartworm disease through infected mosquitoes. They can't get it from other dogs or other types of animals, from dog feces, or from their mothers while in the womb or through nursing.

The mosquito  bites an animal infected with heartworms.  This mosquito then bites your dog. When the larvae reach stage L4-L5, which takes three to four months, under the right conditions they can travel via your dog's bloodstream to the lungs and heart.

If your dog's immune system doesn't destroy these invaders, they will reach maturity -  the adult stage - in which males can grow to six inches in length and females to 12 inches.



Veterinarians recommend a once monthly medication during the period of time your pet is at risk of contracting the disease, typically June 1 – November 1.


The signs of heartworm infection initially include coughing and exercise intolerance, and can eventually lead to more serious conditions like congestive heart failure, anemia, weakness and collapse, and eventually death.


It is important to know that giving certain heartworm preventatives to a dog already infected with heartworm is dangerous. Therefore, before starting the medication, it is important to have the pet tested for heartworm with a simple blood test. Even if you have used preventative medication in the past, testing is still important as no medication can be considered perfect and poor compliance with medication administration can lead to gaps in protection.

It takes 7 months to show positive on a heartworm test. Once positive, heartworm can only be killed by a series of injections with Melarsomine. Your pet MUST stay on monthly heartworm prevention for a minimum of 6 months until retest date.

Our rescue dogs are tested for heartworm as soon as they get pulled, in the USA.  If infected, they are treated with lmmiticide (usually 2-3 injections depending on what stage of heartworm the dog has). This is followed up with doxycycline (an antibiotic) and prednisone (a steroid).

Approximately 6 months after treatment is completed, your dog should be taken to your veterinarian to have a heartworm antigen test to confirm that all heartworms have been eliminated.

Heartworm Treatment Aftercare Instructions

Your dog has been treated with a medication to kill heartworms. It is important for you

to follow these guidelines for post-treatment care, in order to not place your dog's

health and life at risk.

Your pet may be more tired than normal with a decreased appetite.  The pet may be restless, shifting from side to side to get comfortable. Sit with your pet and keep him resting. You can apply a cool compress to the back muscles where the injection was administered.

The pet may run a small fever post injection, do not give any over the counter medication- please stick to the medications prescribed by a veterinarian. It is sometimes normal to see a

small amount of vomiting or diarrhea after injections.


  • Strict rest for at four (4) weeks, use a crate if necessary
  • Your dog's physical activities must be limited - no rough-housing, no running, no exercise, no back yard play. Short leash walks outside to permit the dog a chance to relieve itself are allowed.
  • Feed your dog a normal diet, but remember that reduced exercise also means the dog will need fewer calories to maintain its weight.
  • Your dog will be sent home with anti-inflammatory medications and antibiotics. Use the medication(s) as directed on the prescription label.

If any of the following occur, call your veterinarian:

  • Severe coughing (Your dog could have some symptoms related to the lungs.  He/she will probably have mild coughing for awhile.)
  • Coughing up blood
  • Distended, fluid-filled abdomen
  • High fever
  • Persistent (for more than a day or two) vomiting or diarrhea
  • Lethargy, listlessness or collapse
  • Loss of appetite