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Name: Heartworm Disease, Canine
Short Description
Dirofilaria immitis
Affected Animals: Dogs and cats. Dogs that live outside have an increased risk for
developing a heartworm infection. Felines get this infection less
commonly than canines.
Overview: A parasite that can infect the heart and lungs,
heartworms pose a very serious threat to both indoor and outdoor
dogs. Adult heartworms cause disease because they live in the right
side of the heart and pulmonary arteries where they obstruct the
flow of blood through the heart and to the rest of the body. Because
heartworms can cause serious, fatal diseases, prevention and
treatment of infected dogs are critical.
Transmitted to dogs by mosquitoes that have fed off an animal that
has heartworm disease, the Dirofilaria immitis larvae migrate
through the body tissues until they enter the vascular system.
Within the bloodstream, they circulate and travel to the right side
of the heart and pulmonary arteries. Here, they mature into the
adult stage. The complete maturation of the heartworms, from larvae
to adulthood, takes five to seven months. Dirofilaria immitis
parasites can live for up to five years. Adult heartworms can grow
to be as long as 14 inches and up to 100 or more can be living at
one time.
Treatment is usually through medication and often is effective, but
dogs with serious complications of the heart and lungs due to
heartworm have a much more guarded prognosis for recovery.
Clinical Signs: Coughing from the lung damage, jaundice from
liver damage, and weakness from damage to other organs may develop.
Clinical signs include coughing, exercise intolerance, weakness,
difficulty breathing, and fainting following exercise, if the dog is
able to exercise at all. Eventually, congestive heart failure can
develop, resulting in the backup of fluid within the abdomen and
under the skin of the rear legs. In severe cases, sudden death can
occur following exercise.
Symptoms:
See clinical signs.
Description: A serious threat to both indoor and outdoor dogs,
heartworm is a disease caused by a worm called Dirofilaria immitis.
The parasite is transmitted to dogs by mosquitoes carrying immature
heartworms that they get from biting animals infected by the
parasite. The immature worms circulate in the bloodstream and then
migrate to the right side of the heart and pulmonary arteries where
they mature and can live as adults for up to five years. Adult
heartworms can grow to be as long as 14 inches and up to 100 or more
worms can be living at one time. Heartworms cause serious and
sometimes fatal diseases, so prevention and treatment of infected
dogs is critical.
Diagnosis:
Diagnosis of heartworms is done by examining
under the microscope a sample of the dog's blood for heartworm
offspring, called microfilaria. Another blood test looks for the
adults by detecting antigens made by the female adult heartworm.
However, this antigen test can be falsely negative if there are too
few heartworms and thus not enough antigen is
Another useful diagnostic test, a chest x-ray, determines the
severity of the illness and will show changes that reflect heartworm
disease such as heart enlargement, pulmonary artery enlargement, as
well any lung changes. A urinalysis may reveal any damage that has
been done to the kidneys. Blood work can indicate if the dog is
anemic from the disease, and it will help determine the overall
health of the animal.
Prognosis: The prognosis for animals that have mild or no
clinical signs of disease is good. The prognosis in animals with
severe infection and severe clinical signs is guarded because the
risk of complications is great.
Transmission or Cause: Dogs get heartworms from a mosquito that
transmits the infective larvae from another animal with the disease;
the mosquito obtains this microfilaria through biting. While in the
mosquito, the heartworm larvae mature into the infective stage of
their lifecycle. Next, the mosquito bites another dog, and in the
process, passes on the infective heartworm larvae to that dog. In
the newly infected dog, the heartworm larvae will continue to
develop over the course of five to seven months. The larvae will
gain access to the vascular system, develop into adult heartworms,
and migrate to the heart and pulmonary arteries to live. It is here
that the female heartworms produce many microfilaria, which
circulate in the dog's bloodstream and can be picked up by another
mosquito.
Treatment:
Treatment of heartworms is still somewhat risky,
but it is much safer today than it was years ago, when treatment
involved the use of a medication containing arsenic which had many
severe side effects, including liver failure and death. Now there is
a much safer medicine, melarsomine dihydrochloride, which kills
adult and immature heartworms and does not have as many side effects
as the previous medicine. However, there is still some risk
involved: lethargy, lack of appetite, saliva, increased heart rate,
and retching can occur with this new drug.
Before determining the treatment plan, the veterinarian first will
perform a variety of tests to determine the severity of the disease,
including x-rays, bloodwork, and a urinalysis. Sometimes, an
electrocardiogram and an echocardiogram are recommended to assess
more accurately the condition of the heart.
Generally, the melarsomine dihydrochloride will be administered as
two deep muscle injections, given 24 hours apart. The dog is kept
hospitalized and monitored closely for any reactions. If the disease
is staged as severe, however, the veterinarian will alter the
treatment plan to minimize the potential side effects. Three to four
weeks after the injections are given, a medication will be
administered to kill the circulating microfilaria. The dog must rest
during this entire period to prevent adverse effects from occurring.
No running, jumping, or excessive stair climbing should be allowed.
Owners should walk their dogs slowly on a leash.
Prevention: Prescription heartworm preventive medications
that are properly administered are nearly 100 percent effective in
preventing heartworm disease. Many preventives also will fight
against gastrointestinal parasite infections.
