The Animal Hospital of Lynchburg

1705 Memorial Avenue
Lynchburg, VA 24501


      Heartworm Disease: 

Welcome to The Animal Hospital of Lynchburg's Heartworm Prevention webpage!
March is when we shift our attention to testing and prevention of Heartworm Disease. 


Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs.  It is caused by a worm called Dirofilaria immitisPREVENTION IS KEY....It is always less costly and safer to prevent heartworms than to treat them and the damage they cause !!!  Yearly testing & monthly preventative is the cornerstone of a heartworm-free pet. 


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. As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae (juvenile forms of the worm) during a blood meal. The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouth parts of the mosquito.  The microfilariae are now called infective larvae because at this stage of development, they will grow to adulthood when they enter a dog.    

The microfilariae (young worms) 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.  Adult heartworms are found in the heart and large adjacent vessels of infected dogs.  The female worm is 6 to 14 inches (15 to 36 cm) long and 1/8 inch (5 mm) wide; the male is about half the size of the female.  One dog may have as many as 300 worms.   They survive up to 5 years and, during this time, the female produces millions of young (microfilaria).  Adult worms cause disease by clogging the heart and major blood vessels leading from the heart.  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.  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.  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.  Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds.  In advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation.  There may also be evidence of weight loss, poor condition, and anemia.  Severely infected dogs may die suddenly during exercise or excitement.  


Serological test for antigens to adult heartworms
:  In most cases, the diagnosis of heartworm disease can be made by a blood test run in the veterinary hospital.  Our hospital uses a test that detects antigens (proteins) produced by adult heartworms.  It will be positive even if the dog does not have any microfilaria in the blood; this occurs about 20% of the time.  In addition, our blood test screens for Lyme Disease, Ehrlichia and Anaplasmosis...commonly seen tick-borne diseases in our area.   

Blood test for microfilariae:  A blood sample is examined under the microscope for the presence of microfilariae.  If microfilariae are seen, the test is positive.  The number of microfilariae seen gives us a general indication of the severity of the infection.  

The Animal Hospital of Lynchburg recommends the use of HEARTGARD once monthly, 12 months of the year, to prevent both heartworms as well as multiple intestinal parasites.  The medication is safe for all breeds of dog, all weights and ages as young as 6 weeks. Heartgard is guaranteed to be 100% effective in preventing Heartworms, Roundsworms, and Hookworms in dogs.  If you pet is on Heartgard monthly, 12 months of the year and has its annual heartworm testing, but still comes up positive for any of these parasites, we will cover the entire cost of diagnostics and treatment to remove those parasites.
Be sure to check out these important links:

American Heartworm Society at

Check out Heartgard Plus the most recommend prevention for heartworm disease, parasite control products for dogs and cats at