Heartworm
is a parasitic roundworm (Dirofilaria
immitis) that is spread from host to host
through the bites of mosquitoes. The heartworm
is a type of filaria, a small thread-like worm.
The definitive host is the dog but it can also
infect cats, wolves, coyotes, foxes and other
animals, such as ferrets, sea lions and even,
under very rare circumstances, humans. The
parasite is commonly called "heartworm" because
the adult reproductive stage of its life cycle
resides primarily in the right ventricle of its
host where it can live for many years. Heartworm
infection may result in serious disease for the
host.
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History of the disease
The first published evidence of disease in the
dog in the United States was in The Western
Journal of Medicine and Surgery in 1847. Feline
heartworm was first described in the 1920s. |
Distribution
Although at one time confined to the southern
United States, heartworm has now spread to
nearly all locations where its vector, the
mosquito, is found. Transmission of the parasite
occurs in all of the United States (except
Alaska) and the warmer regions of Canada. The
highest infection rates are found within 150
miles of the coast from Texas to New Jersey, and
along the Mississippi River and its major
tributaries. It has also been found in South
America, southern Europe, Southeast
Asia, the Middle East, Australia, and
Japan. |
Course of infection
Microfilaria (as seen at 400X
magnification)Heartworms go through several life
stages before they become adults infecting the
heart of the host animal. The worms require the
mosquito as an intermediate stage in order to
complete their life cycle. Development in the
mosquito is temperature dependent, requiring
approximately two weeks of temperature at or
above 27 °C (80 °F). Below a threshold
temperature of 14 °C (57 °F), development cannot
occur, and the cycle will be halted.[8] As a
result, transmission is limited to warm months,
and duration of the transmission season varies
geographically. The period between the initial
infection when the dog is bitten by a mosquito
and the maturation of the worms into adults
living in the heart takes 6 to 7 months in dogs
and is known as the "prepatent period". After
infection, the third stage larval heartworms
(L3) deposited by the mosquito grow for a week
or two and molt to the fourth larval stage (L4)
under the skin at the site of the mosquito bite.
Then they migrate to the muscles of the chest
and abdomen and 45 to 60 days after infection,
molt to the fifth stage (L5, immature adult).
Between 75 and 120 days after infection these
immature heartworms then enter the bloodstream
and are carried through the heart to reside in
the pulmonary artery. Over the next 3 to 4
months they increase greatly in size. The female
adult worms are about 30 cm in length, and males
are about 23 cm with a coiled tail.[9] By
approximately 6.5 to 7 months after infection,
the adult worms have mated and the females begin
giving birth to live young, called microfilariae.
The microfilariae circulate in the bloodstream
for as long as two years, waiting for the next
stage in their life cycle in the gut of a
bloodsucking mosquito. When ingested by a
mosquito, the microfilariae undergo a series of
molts to the infective third larval stage and
then migrate to the salivary glands of the
mosquito, where they wait to infect another
host. The incubation period required to reach
the stage where the microfilariae become
transmittable to another host can be as little
as two weeks or as long as six weeks, depending
on the warmth of the climate, and the larval
life cycle ceases entirely if the ambient
temperature drops below 14° Celsius (57°
Fahrenheit).
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Clinical signs of infection
Dogs show no indication of heartworm infection
during the 6 month long prepatent period prior
to the worms' maturation, and current diagnostic
tests for the presence of microfilariae or
antigens cannot detect prepatent infections.
Rarely, migrating heartworm larvae get "lost"
and end up in unusual sites such as the eye,
brain, or an artery in the leg, which results in
unusual symptoms such as blindness, seizures and
lameness. But normally, until the larvae mature
and congregate inside the heart, they produce no
symptoms or signs of illness. Many dogs will
show little or no sign of infection even after
the worms become adults. These animals usually
have only a light infection and live a fairly
sedentary lifestyle. However, active dogs and
those with heavier infections may show the
classic signs of heartworm disease. Early signs
include a cough, especially on exercise and
early exhaustion upon exercise. In the most
advanced cases where many adult worms have built
up in the heart without treatment, signs
progress to severe weight loss, fainting,
coughing up blood and, finally, congestive heart
failure.
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Role of Wolbachia
Wolbachia is an intracellular bacteria that is
an endosymbiont of Dirofilaria immitis. An
endosymbiont is an organism that lives within
the cells of another organism, often giving an
advantage to both. It is thought that all
heartworms are infected with Wolbachia to some
degree. Research indicates that the inflammation
that occurs at the die-off of adult heartworms
or larvae is in part due to the release of
Wolbachia bacteria into the tissues. This may be
particularly significant in cats, in which
disease seems to be more related to larval death
than living adult heartworms (see below).
Pre-treating heartworm positive animals with an
antibiotic such as doxycycline to remove
Wolbachia may prove to be beneficial, but
further studies are necessary.
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Diagnosis
A blood test to detect the antigens released
from the female reproductive tract is the most
common method of diagnosis. The specificity of
these tests is close to 100 percent and the
sensitivity is more than 90%. False negative
tests can result from low worm counts, immature
infections and all male infections. Heartworm
positive dogs should also be tested for the
presence of microfilariae, usually using a
concentrating filter test. X-rays are used to
evaluate the amount of lung damage caused by the
presence of heartworms.
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Treatment
If an animal is diagnosed with heartworms,
treatment may be indicated. Before the worms can
be treated, however, the dog must be evaluated
for heart, liver, and kidney function to
evaluate the risks of treatment. Usually the
adult worms are killed with an arsenic-based
compound. The currently approved drug in the US,
melarsomine dihydrochloride, is marketed under
the brand name Immiticide. [12] It has a greater
efficiency and fewer side effects than
previously used drug (thiacetarsamide sodium,
sold as Caparsolate) which makes it a safer
alternative for dogs with late-stage infections.
After treatment, the dog must rest (restricted
exercise) for several weeks so as to give its
body sufficient time to absorb the dead worms
without ill effect. Otherwise, when the dog is
under exertion, dead worms may break loose and
travel to the lungs, potentially causing
respiratory failure and death. According to the
American Heartworm Society, use of aspirin in
dogs infected with heartworms is no longer
recommended due to a lack of evidence of
clinical benefit and may be contraindicated. It
had previously been recommended for its effects
on platelet adhesion and reduction of vascular
damage caused by the heartworms. The course of
treatment is not completed until several weeks
later when the microfilariae are dealt with in a
separate course of treatment. Once heartworm
tests are negative, the treatment is considered
a success. Surgical removal of the adult
heartworms is also a treatment that may be
indicated, especially in advanced cases with
substantial heart involvement. [13] Long term
monthly administration of ivermectin year round
at three times the dose normally used for
heartworm prevention (see "Prevention") will
eventually kill adult heartworms. However, this
is not the treatment of choice for removal of
adult heartworms for two reasons. First, this
treatment is not as effective as melarsamine.
More importantly, adult heartworms do not begin
to die until 18 months of treatment have
elapsed, which is not acceptable for dogs with
high-volume infestations. From time to time
various "homeopathic," "natural" or "organic"
products are touted as cures or preventives for
heartworm disease. However, such products have
never been proven effective by rigorous
scientific methods, and the claims should be
viewed with skepticism.
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Prevention
Prevention of heartworm infection can be
obtained through a number of veterinary drugs.
The drugs approved for use in the US are
ivermectin (sold under the brand name Heartgard
and several other generic versions), milbemycin
(Interceptor and Sentinel) and moxidectin (ProHeart)
administered as pills or chewable tablets.
Moxidectin is also available in both a 6-month
and 12-month sustained release injection,
ProHeart 6, ProHeart 12, administered by
veterinarians. The injectable form of moxidectin
was taken off the market in the United States
due to safety concerns [14]. ProHeart 6 remains
on the market in many other countries including
Canada and Japan. Its sister product, ProHeart
12 is used extensively in Australia and Asia as
a 12-month injectable preventive. Topical
treatments are available as well. Advantage
Multi (imidacloprid + moxidectin) Topical
Solution, which utilizes moxidectin for control
and prevention of roundworms, hookworms,
heartworms, whipworms, as well as imidacloprid
to kill adult fleas. Selamectin (Revolution), is
a topical preventive that is likewise
administered monthly and also controls fleas,
ticks, and mites. Preventive drugs are highly
effective and when regularly administered will
protect more than 99 percent of dogs and cats
from heartworm. Most compromises in protection
result from failure to properly administer the
drugs during seasonal transmission periods.[15]
In regions where the temperature is consistently
above 14 °C (57 °F) year round, a continuous
prevention schedule is recommended. It has been
shown that lapses of up to 4 months between
doses of Ivermectin-based products still
provides 95% protection from adult worms. This
'safety net' period is called the 'Reach Back
Effect'.[16] Annual heartworm testing is highly
recommended for pet owners who choose to use
minimal dosing schedules. Heartworm prevention
for cats is available as ivermectin (Heartgard
for Cats), milbemycin (Interceptor), or the
topical selamectin (Revolution for Cats).
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