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.

Office Hours























Find us on the map

Featured Articles

Read about interesting topics

    The Do’s and Don'ts of Pet Summer Safety

    Do you know how to keep your pet safe this summer? ...

    Read More

    The Most Common Vaccinations for Your Cat and Dog

    Do you know what vaccines your cat or dog needs? ...

    Read More

    Preparing for Your Kitten’s Developmental Milestones

    Need to hone in on your kitten knowledge? Check out the milestones your new pet will reach during its first year. ...

    Read More

    What Is Ataxia in Dogs?

    Could balance or gait issues mean your dog has ataxia? ...

    Read More


    If you’re thinking of getting a pet fish, you should know that your veterinarian has a lot of good advice about pet ownership. Fish can be very rewarding as pets, and you just may be surprised about how much fish actually interact with their owners. Here’s more valuable information about choosing ...

    Read More

    Caring for Senior Cats

    Thanks to advancements in veterinary care, today’s cats can live well into their teen years. It is not uncommon for cats to live to be 18 or even older. However, in order for cats to live a long full life, they need proactive veterinary care to stay healthy. As cats age, they are at greater risk for ...

    Read More

    Feline Stomatitis: Treatments

    Cats rarely display their pain, but cats with feline stomatitis are often the exception. If your cat appears to have mouth pain, is reluctant to eat, doesn't want to groom, is drooling, and doesn't want you to open its mouth, it may be suffering from this debilitating, degenerative oral condition, and ...

    Read More

    Feline Leukemia Virus: What You Need to Know

    Feline leukemia (FeLV) is a virus that weakens your cat's immune system. Unfortunately, when the immune system does not function properly, your cat may be more likely to develop other diseases, such as cancer and blood disorders. How Cats Contract Feline Leukemia Cats get feline leukemia from other cats. ...

    Read More

    Family Cats and Pregnant Women: Take Measures to Prevent Toxoplasmosis Infection

    Nothing must spoil the joys of becoming a new parent. Not even your pets. But family cats with normal, every day habits can pose a risk to expectant women. Women's immune systems can be disturbed by a parasite carried in fecal matter. If you're the primary caretaker of your family's feline friend it ...

    Read More

    Create an Environment Your Cat Will Love

    The Journal of Feline Medicine and Surgery confirms that feline emotional wellbeing, behavior and physical health are a result of how comfortable they are in their environment. Understanding how our cats interact with their environment can help us create a space for owners and cats to mutually thrive ...

    Read More

Newsletter Sign Up