Heartworm Disease

What is Heartworm Disease?

This is a preventable disease.

Heartworm disease is a serious and potentially fatal parasitic disease most commonly seen in dogs. It is caused by Dirofilaria immitus a blood borne parasite. Heartworm disease is found in every state in the US and all over the world. These worms can be 12” long and live in the heart, lungs, and associated blood vessels of affected pets, causing severe lung disease, heart disease, and damage to the other organs in the body. Adult heartworms may live up to 5 years and produce millions of offspring called microfilaria which live in the blood stream. Microfilariae are spread from dog to dog by mosquito bite. Heartworm disease the silent killer….pets can be asymptomatic for years before clinical signs appear. Our mom still sees positive dogs in our hospital every year. The key to this problem is education and PREVENTION!

Life Cycle of the Heartworm: The life cycle of the heartworm is complicated.

The heartworm parasite requires the mosquito as an intermediate host. 70 different species of mosquitos can transmit the heartworm parasite.

  1. Adult heartworms in the infected dog mate and produce microfilariae, which circulate in the dog’s blood stream. The stage 1 microfilaria (L1) cannot infect another dog.
  2. A female mosquito bites the infected dog and ingests a blood meal containing microfilariae. Over the next 2-4 weeks they mature in the mosquito’s gut and molt twice (L1 to L2, then L2 to L3) L3 infective larvae, then travel to the mosquito’s mouth parts.
  3. The mosquito feeds on the next dog where it deposits the L3 infective larvae through the bite.
  4. **This is the point in the life cycle where prevention is effective!!** Prevention is aimed at L3 larval stage.
  5. The L3 infective larvae molt to the next stage L4 under the skin of the new host. The L4 larvae then migrate through the body eventually ending up in the blood stream.
  6. As the larvae migrate through the tissues over the next 50-70 days they molt into juvenile adults
  7. Juvenile adults eventually migrate to the blood vessels in the lungs where they remain and mature into sexually active adults.
  8. **Adult females test positive on the snap Heartworm test.** Testing finds proteins from adult females.
  9. The adult heartworms then breed and release new microfilariae into the blood stream 6-9months after infection by the mosquito.

Have you ever found a mosquito in your house or in your car? Both indoor and outdoor pets are at risk for heartworm disease. Some mosquitos prefer in door environments. Buildings may have microclimates where mosquitos can thrive in the “off season.” Indoor pets can get mosquito bites. Mosquito bites may contain (Ll3) heartworms.

Pets at Risk for Heartworm Disease:

Heartworm is primarily a disease seen in dogs, the definitive host for heartworms, but can also occur in cats, ferrets, wolves, coyotes, foxes, sea lions – and in rare instances – humans.

Dogs. The dog is the natural host for heartworms, which means that heartworms in dogs mature into adults, mate and produce offspring completing their life cycle. Left untreated, dogs have been known to harbor several hundred adult worms in their bodies. Heartworm disease causes lasting damage to the heart, lungs and arteries, and can affect your dog’s health and quality of life long after the parasites are gone. Prevention is the best option and if needed treatment should be done as early as possible in the disease process for positive dogs. Positive dogs can be treated.

Cats. The disease in the cat is very different than heartworm in dogs. The cat is an atypical host for heartworms, and most worms in cats do not survive to the adult stage. Cats with adults typically have only 1-3 worms, and many cats affected by heartworms have NO adults. This means heartworm disease often goes undiagnosed in cats. It is important to understand that even immature heartworms cause real damage in the form of a condition known as Heartworm Associated Respiratory Disease (HARD). Some cats can have an allergic reaction to heartworms as well. Medications used to treat positive heartworm infections in dogs cannot be given to cats, so prevention is the only option for protecting the cat from the effects of heartworm disease. There is no treatment for positive cats.

Heartworm Testing:

Cats. Since cats often do not have adult heartworms, the test generally used for dogs may not be accurate. In cats blood is sent out to a diagnostic lab to determine if the cat has antibodies against heartworms.

  • Test initially and then as recommended by your veterinarian.

Dogs. In the dog we test for adult heartworm proteins called antigens. This is done as a snap test in the clinic. We take a few drops of blood and have the test results in about 15 minutes. (Snap 4Dx tests for Heartworm, Lyme, Ehrlichia, and Anaplasma all in one test at the same time.)

  • First test after 6 months of age and again 6 months later.
  • Test annually for all adults.
  • Heartworm is a serious progressive disease. The earlier disease is detected the better the chances for recovery.
  • This is a silent disease with no early signs, testing is the earliest means to detect the presence of heartworms.
  • Remember dogs do not test positive until they have female adult parasites at least 6 months after they became infected!

American Heartworm Society “Think 12”

  1. Get your pet tested every 12 months for heartworm.
  2. Give your pet heartworm preventive 12 months a year.

Heartworm Prevention for Test Negative Dogs:

Topical Products:

  • available for both cats and dogs
  • must be applied every 4 weeks (28 days)
  • repellant works on premise the mosquito does not ever bite your pet

Oral Tablets:

  • available for both cats and dogs
  • must be administered every 4 weeks (28 days)
  • observe your pet for a while after giving to make sure it does not spit out or vomit the dose of preventive
  • make sure to mark your calendar to give the medication on the correct day

Injectable ProHeart:

  • used for dogs only
  • injection given every 6 months (+/-) 2 week window
  • best treatment convenience: dose lasts 6 months pet parents do not have to worry about dosing every month
  • well tolerated by most dogs

All dogs should remain on prevention all year around. The cost, for the heartworm test and preventive medication per year, is less than 10% of the cost for treating a positive dog and does not have any of the long term health consequences of heartworm disease. Heartworm preventives are among the safest medications used in veterinary medicine. They have been proven safe to use in all breeds including Ivermectin sensitive collies. Heartworm prevention is almost 100% effective when dosed and used properly.

Heartworm disease is a heart breaking and devastating disease that can be prevented. We all take our preventive on schedule and wish we did not see the positive cases happen, we see them every year. My mom says she hopes she never sees another positive test in her career, and I hope she is right. Wishing all the neighborhood dogs stay parasite free and enjoy the upcoming summer.


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