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(From the American Heart Worm Society Web Page At http://www.heartwormsociety.org/pet-owner-resources/heartworm.html)
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
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 the world.
1 Osborne, TC. Worms found in the Heart and Bloodvessels of a
Dog; Symptoms of Hydrophobia. The Western Journal of Medicine and Surgery,
2 Riley, WA.
Dirofilaria immitis in the heart of a cat. J Parasitol 1922;9:48
3 Travassos, LP. Notas Helminthologicas. Brazil-Med. An. 1921;35
How Heartworm Happens: The Life Cycle
First, adult female heartworms release their young, called
microfilariae, into an animal's bloodstream. Then, mosquitoes become
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
Microfilariae cannot mature into adult heartworms without first passing
through a mosquito.
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
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
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"
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.
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, monthly
topicals and a six-month injectable product available only for dogs. 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.
Heartworms in the heart of a dog
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.
Heartworms in the Pulmonary Artery of a dog
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 dead worms.
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.
This is intended as a short summary. Please refer to the
section information, or
for more detailed information.
Cats versus Dogs
|Susceptibility to infection
||Lower than dogs - 61% to 90% of cats exposed to infective
larvae become infected
||Very high - virtually 100% of dogs exposed to infective
larvae become infected
Longevity of worms
|Number of worms
||Usually less than 6, 1-2 worms most common
||Not uncommon to find more than 30
|Single-sex infections in meso- to high-endemic
- Transient (Lasts about 1 month)
- Seen in less than 20% of naturally infected cats
- Very common (80%-90%)
- Can last years, even after death of adult worms
|Organ with greatest pathology
||Heart and lungs
|Clinical importance of small worm burdens
||Clinical importance depends on the size of the dog, the size of the
worm burden, and exercise level
- None approved
- High risk of complications
- 1 compound approved
- Complications manageable
|Compounds for prevention
||4 approved in US
||Several approved in US