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American Heartworm Society Information

 

"All" information posted here and more.......

can be found at the American Heartworm Society 2003 Guidelines

Heart Worm Society

(Their website is founded by an educational grant provided

by Fort Dodge Animal Health and sponsored also by:

Heska, IDEXX, Pfizer, Merial and Novartis)

THE AMERICAN HEARTWORM SOCIETY AIMS TO:
· Further scientific progress in the study of heartworm disease
· Inform the membership of new developments
· Encourage and help promote effective procedures for the diagnosis, treatment and prevention of heartworm disease

Heartworms Page Two

What Is Heartworm Disease?


Canine heartworm disease is a potentially deadly infection, caused by worms (Dirofilaria immitis) that may grow to be 14-inch-long adults. These worms live in the right side of the heart and arteries of the lungs. Dogs of any age and breed are susceptible to infection. Heartworm infection can cause potentially serious damage to these arteries, eventually leading to heart failure, and in severe cases, damage other organs such as the liver and kidneys. In extreme cases, a dog can be infected with several hundred heartworms. Cats are also susceptible to the disease.


Coinciding with mosquito season, heartworm disease is spread by mosquitoes that become infected with microfilariae while taking a blood meal from an infected dog. The microfilariae mature into the infective larval stage within the mosquito. When the mosquito then bites another dog, cat, or susceptible animal, it then passes the larvae into the animal’s blood stream through the bite wound, resulting in heartworm infection. It then takes a little over six months for the infective larvae to mature into adult worms that can live for five to seven years in the dog.

Heartworm Life Cycle


Heartworm infection is spread from animal to animal by mosquitoes. Dogs, cats, ferrets, coyotes, foxes, wolves, sea lions and even humans have all been found to be infected by heartworm.

Heartworms are classified as nematodes (roundworms) and are but one of many species of roundworms.

Adult female heartworms release their young, called microfilariae, into the animal’s bloodstream. Mosquitoes then become infected with microfilariae while taking a blood meal from an infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito.

When the mosquito bites another dog, cat, or other susceptible animal, the infective larvae enter through the bite wound. In dogs, it then takes a little over six months for the infective larvae to mature into adult worms that may live for five to seven years in dogs. In cats, it takes about eight months to mature into adult worms that live from two to three years.

Microfilariae cannot mature into adult heartworms without first passing through a mosquito.


How Do You Detect Heartworm Disease?


Detection of heartworm infection in apparently healthy animals is usually made with blood tests for microfilariae or a heartworm substance called an "antigen," although neither test is consistently positive until about seven months after infection has occurred.
Heartworm infection may also be detected through x-ray and/or ultrasound images of the heart and lungs, although these tests are usually used in animals that are known to be infected.

Microfilaremia, the presence of heartworm offspring in the blood of the host, is relatively common in dogs. However, not all heartworm infections result in such offspring circulating in the blood.

These are known as occult heartworm infections and may be the result of a number of factors such as single sex heartworm infections, host immune responses affecting the presence of circulating offspring (microfilariae) and most significantly, the administration of heartworm preventives.

The onset and severity of disease in the dog is mainly a reflection of the number of adult heartworms present, the age of the infection, and the level of activity of the dog. Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes. Until the number of mature heartworms exceeds 50 in a 25-kg dog (approximately 55 pounds), nearly all of the heartworms reside in the lower caudal pulmonary arteries (the arteries of the lower lung lobes). Higher numbers of heartworms will result in their presence in the right chambers of the heart. In such infections, the most common early pathological changes caused by heartworms are due to inflammatory processes that occur in and around the arteries of the lower portion of the lungs in response to the presence of heartworms.
Later, the heart may enlarge and become weakened due to an increased workload and congestive heart failure may occur. A very active dog (e.g., working dog) is more likely to develop severe disease with a relatively small number of heartworms than an inactive one (e.g., a lap dog or couch potato). In an occasional dog with a large number of heartworms, the worms may not only be in the heart but also the caudal vena cava (large primary vein of the lower body) between the liver and the heart. This syndrome (Vena Cava or Liver Failure Syndrome) is characterized by sudden collapse and even death within two to three days if they are not removed surgically.
Interesting Facts:  
After arrival in the heart of a dog, juvenile worms are beginning to grow very fast and reach sexual maturity in 3 months and continue to grow after sexually mature and started reproducing. The cycle for development of microfilaria to L3, which occurs in mosquitoes is dependent on the temperature. This cycle also takes 14 days or longer to occur.
The normal habitat of the adult heartworm is in the right ventricle and adjacent blood supply of the dog. In 1939 a publication reported finding 116 heartworms in one dog. A total of 70 feet of worms.
D immitis have been documented in the wolfe, coyote, fox, bobcat, jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse, orangutan, giffon, seal lion and man. Heartworms are also found in the liver, trachea, esophagus, stomach, feces, eye, brain, spinal cord and vomitus in dog.
In the dog, the number of circulating microfilaria is independent of the number of adult heartworms. Microfilarias have been reported to survive in circulation two+ years following a transfusion with infected blood.
One infected foxhound dog had a reported concentration of 40,460 microfilariaes per cc of blood. Antigens detected in blood of an infected animal is excreted by female worms which release the microfilaria.
An old female heartworm can measure 31 cm in length (1in=2.54 cm.) Only a female mosquito may transfer a heartworm infection.
The level of heartworm antigens in blood is dependent on the number of worms, the sex of the worms, the age of worms, the weight of the dog and how well the dogs liver clears antigens. In dogs concentrations of microfilarias in the blood may vary over a 24 hour period and are generally higher in the late evening and the early morning. The concentration may also vary seasonally, being higher in the summer then in the winter.
D immitis is a filarial disease and it is estimated that more than 200 million people worldwide are infected with filarial nematodes.

Heartworm Infections Reported 2001

As Heartworm has Spread