Heartworm disease is a serious and potentially fatal
condition caused by parasitic worms living in the
arteries of the lungs and occasionally in the right side
of the heart of dogs, cats and other species of
mammals, including wolves, foxes, ferrets, sea lions
and (in rare instances) humans. Heartworms are
classified as nematodes (roundworms) and are filarids,
one of many species of roundworms. Dogs and cats of
any age or breed are susceptible to infection.
Where is Heartworm Disease?
Heartworm disease has been reported in all 50
states. The map below shows particularly
endemic areas based on the number of cases
reported by clinics.
The first published description of heartworm in dogs
in the United States appeared more than 100 years ago
in an issue of "The Western Journal of Medicine and
Surgery."1 Heartworm in cats was first described in the
early 1920's.2, 3
Since then, naturally acquired heartworm infection in
cats and dogs is identified as a worldwide clinical
problem. Despite improved diagnostic methods,
effective preventives and increasing awareness among
veterinary professionals and pet owners, cases of
heartworm infection continue to appear in pets around
How Heartworm Happens: The Life Cycle
First, adult female heartworms release their young,
called microfilariae, into an animal's bloodstream.
Then, mosquitoes become infected with microfilariae
while taking blood meal from the infected animal.
During the next 10 to 14 days, the microfilariae
mature to the infective larval stage within the
mosquito. After that, the mosquito bites another dog,
cat or other susceptible animal, and the infective
larvae enter through the bite wound. It then takes a
little over 6 months for the infective larvae to mature
into adult worms. In dogs, the worms may live for up
to 7 years. Microfilariae cannot mature into adult
heartworms without first passing through a mosquito.
Within the mosquito, the microfilariae mature into the infective larval
stage. When the mosquito then bites another dog, cat, or susceptible
animal, the larvae are deposited on the skin and actively migrate into the
new host. For about 2 months the larvae migrate through the connective
tissue, under the skin, then pass into the animal's venous blood stream
and are quickly transported to the arteries of the lung. It takes a total of
approximately six months for the infective larvae to mature into adult
worms that begin producing offspring, microfilariae. Adult heartworms
can live for five to seven years in the dog.
In the dog, the larvae progress in their development to an adult form of
the worm, and live in the pulmonary vessels, where they continue the life
cycle and cause extensive injury. The period of time when heartworms
are reproductively capable is referred to as patency. In cats, it takes seven
to eight months before adult worms potentially reach patency in the
pulmonary vessels, and this is referred to as transient patency, as
reproductive capability in the cat is usually very short (months)
compared to that of dogs (years). In most cases the cat is not an effective
reservoir host, since microfilaria are produced in less than 20% of the
What Are the Signs of Heartworm Disease?
For both dogs and cats, clinical signs of heartworm disease may not be
recognized in the early stages, as the number of heartworms in an animal
tends to accumulate gradually over a period of months and sometimes
years and after repeated mosquito bites.
Recently infected dogs may exhibit no signs of the disease, while heavily
infected dogs may eventually show clinical signs, including a mild,
persistent cough, reluctance to move or exercise, fatigue after only
moderate exercise, reduced appetite and weight loss.
Cats may exhibit clinical signs that are very non-specific, mimicking
many other feline diseases. Chronic clinical signs include vomiting,
gagging, difficulty or rapid breathing, lethargy and weight loss. Signs
associated with the first stage of heartworm disease, when the
heartworms enter a blood vessel and are carried to the pulmonary
arteries, are often mistaken for feline asthma or allergic bronchitis, when
in fact they are actually due to a syndrome newly defined as Heartworm
Associated Respiratory Disease (HARD).
Heartworm disease may cause a combination of medical problems in the
same dog including dysfunction of the lungs, heart, liver and kidneys.
The disease may have an acute onset but usually begins with barely
detectable signs resulting from a chronic infection and a combination of
physiologic changes. Dogs with a low number of adult worms in the
body that are not exercised strenuously may never have apparent signs of
heartworm disease. However, in most dogs, the heart and lungs are the
major organs affected by heartworms with varying degrees of clinical
Clinical Signs Associated with Canine Heartworm Disease
No abnormal clinical signs observed
Cough, exercise intolerance, abnormal lung sounds
Cough, exercise intolerance, dyspnea (difficulty
breathing), abnormal lung sounds, hepatomegaly
Severe (enlargement of the liver), syncope (temporary loss of
Disease consciousness due to poor blood flow to the brain),
ascites (fluid accumulation in the abdominal cavity),
abnormal heart sounds, death
How Do You Detect Heartworm Disease?
Heartworm infection in apparently healthy animals is
usually detected with blood tests for a heartworm
substance called an "antigen" or microfilariae,
although neither test is consistently positive until
about seven months after infection has occurred.
Heartworm infection may also occasionally be
detected through ultrasound and/or x-ray images of the
heart and lungs, although these tests are usually used
in animals already known to be infected.
Antigen tests detect specific antigens primarily found in adult female
heartworms and are used with much success to detect canine heartworm
infection. Currently, tests are available as in-clinic tests as well as at
many veterinary reference laboratories. Most commercial tests will
accurately detect infections with one or more mature female heartworms
that are at least seven or eight months old, but the tests generally do not
detect infections of less than five months duration.
Since the late 1970's and early 1980's, several canine
heartworm antibody tests have been developed and
introduced, but such tests for dogs have been largely
replaced by the more useful antigen tests. This lack of
utility of the antibody tests is due to the fact that these
tests detect the antibody response to exposure to
infection, but not necessarily actual disease. This is
important because not all infections fully mature.
Because heartworm disease is preventable, the AHS recommends that pet
owners take steps now to talk to their veterinarian about how to best
protect their pets from this dangerous disease. Heartworm prevention is
safe, easy and inexpensive. While treatment for heartworm disease in
dogs is possible, it is a complicated and expensive process, taking weeks
for infected animals to recover. There is no effective treatment for
heartworm disease in cats, so it is imperative that disease prevention
measures be taken for cats.
There are a variety of options for preventing heartworm infection in both
dogs and cats, including daily and monthly tablets and chewables and
monthly topicals. All of these methods are extremely effective, and when
administered properly on a timely schedule, heartworm infection can be
completely prevented. These medications interrupt heartworm
development before adult worms reach the lungs and cause disease.
It is your responsibility to faithfully maintain the prevention program you
have selected in consultation with your veterinarian.
Usually, all but the most advanced cases of heartworm disease
can be successfully treated in dogs. Currently, there are no
products in the United States approved for the treatment of
heartworm infection in cats. Cats have proven to be more
resistant hosts to heartworm than dogs, and often appear to be
able to rid themselves of infection spontaneously.
Unfortunately, many cats tend to react severely to the dead
worms as they are being cleared by the body, and this can
result in a shock reaction, a life-threatening situation.
Veterinarians will often attempt to treat an infected cat with
supportive therapy measures to minimize this reaction;
however it is always best to prevent the disease.
Adult heartworms in dogs are killed using a
drug called an adulticide that is injected into the
muscle through a series of treatments. Treatment
may be administered on an outpatient basis, but
hospitalization is usually recommended. When
the dog is sent home, exercise should be limited
to leash walking for the duration of the recovery
period, which can last from one to two months.
This decreases the risk of partial or complete
blockage of blood flow through the lungs by
The primary post-adulticide complication is the
development of severe pulmonary thromboembolism.
Pulmonary thromboembolism results from the
obstruction of blood flow through pulmonary arteries
due to the presence of dead heartworms and lesions in
the arteries and capillaries of the lungs. If heartworm
adulticide treatment is effective, some degree of
pulmonary thromboembolism will occur.
When dead worms are numerous and arterial injury is severe,
widespread obstruction of arteries can occur. Clinical signs
most commonly observed include fever, cough, hemoptysis
(blood in the sputum) and potentially sudden death. It is
extremely important to not allow exercise in any dog being
treated for heartworms. Often dogs with severe infections will
also require the administration of anti-inflammatory doses of
Re-infection during treatment is prevented by
administration of a heartworm preventive. These
preventives may also eliminate microfilariae if
they are present. Dogs in heart failure and those
with caval syndrome require special attention
Immiticide: Usual Dose and Administration
Given intramuscularly (IM) deep in the lumbar
(back) muscle. Do not administer at any
other site. Alternate sides. Usually given
twice 24 hours apart for animals with Class 1
or 2 heartworm disease. May need second
round of treatment 4 months later depending
on repeated test results. Dogs with other
classes of heartworm disease may receive
different doses/timing to decrease risk of
complications. Given in a hospital setting to
allow for the necessary supervision of the
May see pain, swelling, and tenderness at
the injection site or reluctance to move due
to pain at injection site. Firm nodules can
persist indefinitely. May also see coughing,
gagging, depression, lethargy, lack of
appetite, fever, lung congestion, and
vomiting. Less commonly seen are
excessive drooling, panting, diarrhea,
coughing up blood, abnormal heart
rhythms, and death.
Caparsolate: Indications for
compound used to kill
immature (4+ month old)
and adult heartworms
FDA approved for use in dogs for the
removal of adult heartworms. Used
in a hospital setting. Although not
FDA approved for use in cats, it has
been used in cats to treat heartworm
disease. Thiacetarsamide is used in a
Usual Dose and Administration
Given directly into a vein twice a day for 2 days.
Given in a hospital to allow for the necessary
supervision of the patient.
Vomiting is a common reaction. May also see loss
of appetite, depression, liver or kidney damage,
and reaction to the dying heartworms (coughing,
fever, loss of appetite, and lethargy).
Thrombocytopenia (lack of platelets, causing
bleeding/bruising) is seen also. If the medication
gets outside the vein during injection, pain,
swelling, and severe sloughing may be seen as the
skin and underlying tissues die. Less commonly
seen are severe liver damage and severe pulmonary
embolism. Death can occur from reaction to the
medication and/or from reaction to the dying
All natural herbal heartworm
treatment. Containing Garlic,
Black Seed, Licorice, Hawthorn,
Hops, Sorrel, Apricot Pits,
Grapefruit Seed Extract, Alcohol,
Garlic : helps repel parasites
Black Seed : anti parasitic tonic herb
Licorice: Supports lung tissue, immunostimulant
Hawthorn: strengthens heart structures
Hops : Diuretic
Sorrel: cleans the vessels of parasites
Apricot Pits: Natural occuring cyanide
Grapefruit Seed: used for bacterial, viral and