Prince Frederick Animal Hospital
60 Stafford Road POBox 310
Prince Frederick, MD 20678
410-535-2590
CANINE HEARTWORM DISEASE
Introduction
Heartworms are parasites which primarily infect dogs. They are transmitted to dogs by the bite of
mosquitoes. A dog infected with heartworms can have 50-100 adult worms up to 14 inches long, living
in their heart. Heartworms can cause kidney and heart damage, pneumonia and even death. The adult
worms remain in the heart and produce baby heartworms known as microfilariae. Microfilariae circulate
throughout the bloodstream, which can be picked up by mosquitoes. An infected mosquito can then
introduce microfilariae into another dog’s blood. Heartworm-infected dogs, therefore, serve as a
reservoir for infection of other dogs in the area.
Symptoms
It is the adult heartworms living and growing in the heart that cause the most problems for the
infected dog. Obstruction of blood flow through the heart can cause coughing, fluid accumulation in the
abdomen, anemia, kidney failure, listlessness, weakness, weight loss and heart failure. Many dogs with
early infections will show no signs at all. The microfilariae circulating through the blood can also cause
adverse reactions if certain “deworming” medications are given inappropriately. Heartworm
preventative medication, given to a dog that has microfilariae, can cause serious complications, so only
give heartworm preventative under supervision of your veterinarian.
Heartworm Testing
Blood tests detect adult female heartworms and circulating microfilariae. Both tests can be run
from the same small sample of blood taken from a dog’s vein. The combination of these two tests
confirms an infection with heartworms.
Treatment
Prince Frederick Animal Hospital follows the guidelines recommended by the American
Heartworm Association for heartworm treatment and prevention.
The elimination of heartworms from infected dogs involves several steps, both at home and in
the hospital. A dog which tests positive for heartworms should have a complete physical examination
and have a fecal checked for other parasites. A dog should be current on vaccinations and have intestinal
parasites treated before being admitted to the hospital and beginning heartworm treatment.
After a dog tests positive for heartworms, your veterinarian will start your dog on monthly
heartworm preventative. This will kill the circulating microfilaria. You may administer the first dose at
home, but you need to monitor for side effects such as weakness, shortness of breath or pale gums.
When you administer the first dose of heartworm preventative, make sure you can monitor your dog for
12 hours afterwards and make sure your veterinarian is open…in case your dog has side effects.
Your veterinarian will also prescribe 30 days of an antibiotic. Antibiotics are believed to lower
the inflammation caused by the heartworms and may slow the heartworm reproduction.
The dog is then hospitalized for 3 days, for testing, treatment and observation. Testing consists of
blood tests, urine tests and chest xrays. These tests grade the heartworm disease from grade 1 (most
mild) to 3 (most severe). Treatment consists of one injection to start the kill of adult heartworms. The
heartworms die slowly and are gradually dissolved by the body over a period of 3-4 weeks. Occasionally
fever and listlessness may develop and require additional medical attention.
Four weeks after the initial hospitalization the dog will be admitted again for 3 days. Treatment
consists of two additional injections to complete the kill of adult heartworms.
Following each hospitalization the dog will be sent home to recover. The owner should check the
dog’s rectal temperature once a day for an additional 4 days, and call us if the temperature exceeds 102.5
F. It is very important that the dog be kept as quiet as possible during the convalescent period!
Vigorous exercise and excitement cause large movements of dead heartworms from the heart to the
lungs causing serious complications and possible death. Call your veterinarian if you notice any
lethargy, shortness of breath, wheezing or coughing.
Finally, the dog is retested 6 months later for adult heartworms and microfilaria. The dog should
stay on monthly heartworm preventative year-round thereafter.
Heartworm Prevention
Preventing heartworm infection is much easier, safer and less costly than heartworm treatment!
Dogs in our area should be given a medication year round to prevent the development of adult
heartworms, even if an infected mosquito bites the dog. Heartworm preventative medications are
available in forms to be dosed on a monthly or daily basis depending on the owner’s needs and
preferences. The recommended products are summarized below:
Heartgard Plus
Chewable
Monthly
Interceptor
Chewable
Monthly
Sentinel
Chewable
Monthly
Revolution
Topical
Monthly
Also prevents hookworms,
and roundworms
Also prevents hookworms,
roundworms and whipworms
Combination of Interceptor
and Program (flea pill)
Also prevents fleas, ear
mites and scabies
All heartworm preventatives are prescription drugs and require that a dog be examined prior to
dispensing the medication. Additionally, all dogs older than six months must have a heartworm test
before being started on preventatives. An accurate body weight is also required to assure that the dog
receives the proper dosage of the medication. Yearly physical examinations including annual heartworm
and fecal checks are recommended to assure the dog remains healthy and parasite free.
Heartworm treatment
Heartworm treatment is divided into 4 phases.
Phase 1: This is the appointment where the dog is tested for heartworms. If the test is positive, a fecal
test should also be done and vaccines updated. The dog is started on heartworm preventative (NOT
interceptor or revolution) and 30 days of antibiotics.
Estimate: professional veterinary visit, heartworm test, fecal test, vaccines, 6 months heartworm
preventative, 30 days doxycycline at 5 mg/kg twice a day=_____________
Phase 2: Hospitalize for 3 days, within 1-2 weeks of positive heartworm test. On day one, the dog gets
blood, urine and xray testing. Then one injection is given to initiate the kill of the adult heartworms.
The most common side effects during the first 3 days are vomiting, diarrhea, muscle aches and fever.
Cost=General Health Profile/Electrolytes/CBC/Urinalysis dipstick and sediment/ 2 xrays/3 days
hospitalization/1 bottle of Immiticide per 44 lb of dog=________________.
Phase 3: Hospitalize for 3 days, four weeks later. On day one and two, an injection is given to complete
the kill of the adult heartworms. The most common side effects during this phase are vomiting, diarrhea,
muscle aches and fever.
Cost=3 days hospitalization/1 bottle of Immiticide per 22 lb dog=__________________.
Phase 4: Six months later, schedule an appointment to have your dog tested for adult and baby
heartworms. If the test is negative, your dog should stay on monthly heartworm preventative yearround.
Cost=Recheck appointment/Knott’s test/Heartworm Occult test/Refill heartworm
preventative=__________________.