The following details have been supplied in order to give you some information should your cat or dog have been diagnosed as having suffered from heartworm disease.
What causes heartworm disease?
Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs & cats in many parts of the world. It is caused by a worm called Dirofilaria immitis.
Heartworms are found in the heart and large adjacent vessels of infected pets. The female worm is 2.3 to 5.5 cm long and 5 mm wide; the male is about half the size of the female. One pet may have as little as 1 or as many as 300 adult worms.
How do dogs get heartworm?
Adult heartworms live in the heart and pulmonary arteries of infected cats & 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 pet; a mosquito is required for some stages of the heartworm life cycle. The microfilaria are therefore not infective (cannot grow to adulthood) in the dog or cat.
As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected pet and ingests the microfilariae 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 pet. The mosquito bites the dog or cat where the haircoat is thinnest. However, having long hair does not prevent a dog or cat from getting heartworms.
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.
How do dogs & cats get infected with them?
The disease is not spread directly from pet to pet. An intermediate host, the mosquito, is required for transmission.
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 pets. The disease is seldom diagnosed in a pet under 1 year of age because the young worms (larvae) take up to 7 months to mature following establishment of infection in a pet.
What do heartworms do to the cat or dog?
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.
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.
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 anaemia.
Severely infected pets may die suddenly during exercise or excitement.
Microfilariae (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 may be deprived of the nutrients and oxygen normally supplied by the blood. The lungs, liver and kidney are primarily affected. Generally however the changes produced by the microfilaria are not particularly significant.
How is heartworm infection diagnosed?
In most cases, diagnosis of heartworm disease can be made by a simple blood test. Further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog or cat can tolerate heartworm treatment. Depending on the case, we will recommend some or all of the following procedures before treatment is started.
Serological test for antigens to adult heartworms:
This is a test performed on a blood sample. It is the most widely used test because it detects antigens (proteins) produced by adult heartworms. It will be positive even if the pet does not have any microfilaria in the blood; this occurs between 10-50% of the time. Pets with less than five adult heartworms may not have enough antigen to turn the test positive, so there may be some false negative results in mild or early infections. Because the antigen detected is produced only by the female worm, a pure population of male heartworms will give a false negative, also. Therefore, there must be at least 5 female worms present for the most common test to be positive.
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. However, the microfilariae are seen in greater numbers in the summer months and in the evening, so these variations must be considered. Approximately 10-50% of pets do not test positive even though they have heartworms because of an acquired immunity to this stage of the heartworm. Also, there is another microfilarial parasite which is fairly common in pets; on the blood smear, these can be hard to distinguish from heartworm microfilariae.
Complete blood counts and blood tests for kidney and liver function may give an indirect indication of the presence of heartworm disease. These tests may be performed on pets diagnosed as heartworm-infected to determine the function of the dog’s organs prior to treatment.
A radiograph of a pet with heartworms will usually show heart enlargement and swelling of the large artery leading to the lungs from the heart. These signs are considered presumptive evidence of heartworm disease. Radiographs may also reveal the condition of the heart, lungs, and vessels. This information allows us to predict an increased possibility of complications related to treatment.
Ultrasonography (Ultrasound scan):
An echocardiogram allows us to see into the heart chambers and visualise the heartworms themselves. Although somewhat expensive, this procedure can diagnose heartworms when other tests fail.
How are dogs treated for heartworms?
There is some risk involved in treating pets with heartworms, although fatalities are rare. New treatments offer marked improvements in safety & can provide a complete cure so that many pets can return to a normal life
How is heartworm prevented?
Prevention is always best when it comes to heartworm. A number of medications exist and it is always best to consult your Veterinarian as to which one is best suited to your pet. One of the easiest & best medications available for dogs is a quick & simple injection which can offer up to 12 months of protection