THE LIFE CYCLE OF THE HEARTWORM

The fancy name for Heartworm in dogs and cats is Dirofilaria immitis.

The heartworm goes through a total of four molts to mature into an adult worm. The first two occur inside the mosquito and the next two occur inside the final host.

FIRST 2 MOLTS: L1 - L2, L2 - L3
Heartworm disease begins with an infected animal that has circulating microfilaria in its blood. This infected animal is called the source. A mosquito stops by for a meal and inadvertently sucks up a number of circulating microfilaria in the blood. Once inside the mosquito’s body, the microfilaria goes through two molts over 14 days or longer depending on the environment’s temperature. They go through their first two molts and change from an L1 to an L2 and then an L2 to an L3 (third stage of development of the larva).

As an L3, they have migrated into the mosquito’s salivary glands and during the mosquito’s next meal they burrow into the victim through the mosquito's small bite wound.

NEXT 2 MOLTS: L3 - L4, L4 - L5
Once inside an animal (host) where it continues development, the worm takes at least 6 to 7 months to go through the last two molts and to become sexually mature before the infection can be detected by a Heartworm test.

The L3 larva goes through its first molt to the L4 within the first 15 days and as early as 2-5 days after infection. The second molt from the L4 to the L5 occurs within the next 2 months.

The L5 larva is considered a juvenile adult and works its way through the host’s tissues all the way to the heart as early as 70 days after first entering the host. The majority of L5 larvae arrive in the heart by 90 days. They stay here and grow rapidly in length and size. The worm is home and will live here until it dies in 5-7 years.

Sexual maturity is achieved about three months after arrival in the heart. The worms actually continue to grow in size after reaching sexual maturity and the females start to pass microfilaria into the blood. Adult female worms have been known to grow up to 14 inches long and males are generally shorter.

Some pets are infected with numerous worms, all traveling from the site of the mosquito bite to their preferred home in the heart. This mass of twisted and intertwined worms can serve as a significant mechanical blockage to the normal flow of blood. Think of a garden hose. If pieces of debris block the hose, pressure builds up due to the obstruction of the flow of water. This is what happens to the heart and blood vessels when more and more worms congregate within the right ventricle. The smaller the host the fewer worms it takes to cause a problem.

Once the worm becomes an adult it will continue to produce young for many years in the dog, and a shorter time in cats and other animals that are not the usual host for the worm. The adults have been documented to live around 7 years in the dog and 2-3 years in the cat.

This completes the normal life cycle of the Heartworm.

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Stage I
Lowest risk... young healthy dogs with minimal Heartworm disease evident on X-rays and all other tests are normal.

Stage II
Moderately affected dogs... some coughing is noticed, some difficulty breathing, changes are seen on X-rays, and blood work may reveal some kidney and/or liver damage.

Stage III
Severely affected dogs... the patient has weight loss, coughing, difficulty breathing, more damage visible on x-rays, and blood tests shows kidney and/or liver damage.

Stage IV
Vena Cava Syndrome or Caval Syndrome... the dog is collapsing in shock, all of the above abnormalities are more intense and the dog is dying. They are initially treated with surgical jugular removal of some worms if possible. There is no guarantee this treatment will be successful and many patients with Caval Syndrome die in spite of attempts to treat.

TESTING
Heartworms can be detected by blood test. The filtration test finds microfilariae in the blood; the occult test locates adult worms in the heart. Many veterinarians prefer to do both tests as the absence of microfilariae in the blood does not necessarily mean that there are no adult worms in the heart. Both tests are done with a single blood draw, preferably in the early spring before daily temperatures warm above 57 degrees F. Radiographs (X-rays) can also detect the presence of adult heartworms in the heart and lungs.

TREATMENT
If a blood test or the onset of symptoms alert owner and veterinarian to the presence of this devastating parasite, treatment is possible and successful if the disease has not progressed too far. The first step is to evaluate the dog and treat any secondary problems of heart failure or liver or kidney insufficiency so that he can withstand the treatment. The next step is to kill the adult worms with an arsenic compound. Veterinarians now have access to a Immiticide, a new compound that has fewer side effects than the previous drug and is safer for dogs with more severe infestations.

The treatment is administered in two doses each day for two days, followed by several weeks of inactivity to give the dog’s system a chance to absorb the dead worms. Exertion can cause the dead worms to dislodge, travel to the lungs, and cause death.

At least three-to-four weeks after the administration of the drug to kill the adult worms, further treatment to kill the microfilariae is needed. The dog is dosed daily for a week, then the blood test is repeated. If microfilariae are still present, the dose can be increased. Follow-up studies should be done in a year.

Surgical removal of the adult heartworms is possible and may be indicated in advanced cases with heart involvement.

More info: http://www.engelbergkristy.com/heartworm.htm