7626 Florissant Road

St. Louis, MO 63121 US

(314) 383-4677

Heartworms and Prevention

Heartworm disease is a serious and potentially fatal disease in dogs and cats. It is caused by a worm called Dirofilaria immitis.

Heartworms are found in the heart and large adjacent vessels of infected dogs. The female worm is 6 to 14 inches long and 1/8 inch wide; the male is about half the size of the female. A severely infected dog may have as many as 300 worms. Adult heartworms live in the heart and pulmonary arteries of infected dogs. They have been found in other areas of the body, but this is unusual. They survive up to 5 years and, during this time, the female produces millions of young (microfilaria). These microfilaria live in the bloodstream, mainly in the small blood vessels. The immature heartworms cannot complete the entire life cycle in the dog, a mosquito is required to complete the heartworm life cycle. The microfilaria are therefore not infective (cannot grow to adulthood) in the dog, although they do cause problems, and spread the disease.

As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae during the feeding. The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouth parts of the mosquito. The microfilariae are then called infective larvae and are now able to grow to adulthood when they enter a dog.

When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels, where they grow to maturity in 2 to 3 months and start reproducing, thereby completing the full life cycle.

The disease is not spread directly from dog to dog. An intermediate host, the mosquito, is required for transmission. Spread of the disease therefore coincides with the mosquito season. The number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area. Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. However, the disease is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent, IE- near rivers and in places that have warm winters. It takes a number of years before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in 4 to 8 year old dogs. The disease is seldom diagnosed in a dog under 1 year of age because the young worms (larvae) can take up to 7 months to mature following establishment of infection in a dog. Because visibly signs may not show up until there has been too much damage it is important to test yearly, even when on prevention.

Most dogs infected with heartworms do not show any signs of disease for as long as two years. Unfortunately, by the time signs are seen, the disease is well advanced. The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys from the adult worms and the microfilariae.

The most obvious signs are: a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint.

Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and general weakness because this organ is essential in maintaining a healthy animal. The kidneys may also be affected and allow poisons to accumulate in the body.

Severely infected dogs may die suddenly during exercise or excitement. Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.

Young worms: Microfilariae circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow in these vessels. The body cells being supplied by these vessels are deprived of the nutrients and oxygen normally supplied by the blood. The lungs and liver are primarily affected.


The first step in heartworm prevention is a simple blood test to check for the presence of heartworms. The heartworm prevention available in nearly 100% effective, but there is still a small chance that infection may occur. There is also the possibility that your pet may not swallow the whole dose, or may even spit it out when you are not looking. This makes it important to test yearly to make sure there is no infection, even when on prevention year round.


Once your pet has been found to be free of heartworms, it should be started on heartworm prevention. Puppies should be started on heartworm prevention at 4-6 weeks of age. However, they cannot be tested for heartworms until they are 6 months- testing prior to 6 months of age runs the risk of a false negative due to the incubation period.

Now days most all heartworm prevention is given monthly, in a single dose. Care should be taken to make sure that your pet consumes the complete tablet. It is also important that the pervention be given at monthly (30 day) intervals. If the interval exceeds 30 days the effectiveness is reduced and re-medication is recommended.

Prevention We Offer:

Heartgard Plus- this preventative protects you pet against heartworms, as well as the intestinal parasites roundworm and hookworm. It is available in different dosage strengths for different weights. Each comes in a carton with 6 chewable beef jerky type treats. This is an advantage if you pet is picky about what they eat. It also includes stickers to put on your calender to remind you when the pills are due.

Interceptor Plus This preventative protects against heartworms, as well as the intestinal parasites roundworm, hookworm, whipworm and tapeworm. It is available in dosage strengths for different weights. Each comes in a dispenser card with 6 pills and also includes stickers for you calender.

Advantage Multi-This preventive protects against heartworms, the intestinal parasites hookworm, roundworm, and whipworm, and also kills fleas. It is available in dosage strengths for different weights and is sold in packages containing 6 pills.

Revolution and Sentinel- We no longer stock these two products. In the event that we feel that one of these would be appropriate for your pet we can special order them for you.

Yes. Cats do get heartworms. Mention WEBDEAL 2. One offer per household. This is a limited time offer so call today for details and an appointment.

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