Heartworm Society Information
"All" information posted
here and more.......
can be found at the American Heartworm
Society 2003 Guidelines
(Their website is founded
by an educational grant provided
by Fort Dodge Animal Health
and sponsored also by:
Heska, IDEXX, Pfizer, Merial
THE AMERICAN HEARTWORM
SOCIETY AIMS TO:
· Further scientific progress in the study of heartworm
· 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
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 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
Microfilariae cannot mature into
adult heartworms without first passing through a mosquito.
How Do You Detect Heartworm
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
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.
|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
|The normal habitat of the adult heartworm
is in the right ventricle and adjacent blood supply of
||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
|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