Heartworm is a
parasitic roundworm (Dirofilaria immitis) that is spread from host to
host through the bites of mosquitoes. The heartworm affects dogs,
wolves, coyotes, foxes, and some other animals, such as ferrets, sea
lions, and even humans. The parasitic worm is called a "heartworm"
because the parasite, in the final reproductive stage of its life cycle,
resides in the heart of
its host where it can stay for many years, until it kills its host
congestive failure of the heart.
History of the disease
Heartworm was first discovered in dogs over a century ago and documented in
cats by the 1920s. Since then, diagnostic tests and treatments for heartworm as
well as preventative measures have been developed. Heartworm
infestation may be extremely serious for the infected host; infected dogs that
go untreated can die and even treated dogs must go through a long period of
uncomfortable treatment (sometimes requiring surgery) to kill the worms and
remove them from the body. The best defense against heartworm is the use of
prophylactic treatment given regularly during the mosquito season.
A course of heartworm prevention begins with a
blood test to see if the parasite
is present. If the dog is parasite free, a prophylactic medication can be used
to prevent heartworm infection. A positive test result, on the other hand,
usually requires treatment to eradicate the worms.
Heartworm in North America
Although at one time confined to more southern
climates, heartworm has now spread to nearly all climates where its vector, the
mosquito, occurs (a vector is an intermediate host for the developing parasite,
that spreads the disease from host to host). Transmission of the parasite from
dog to dog occurs in all of the United States (except Alaska) and the warmer
regions of Canada. The highest infection rates in North America occur in dogs
within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries. However, high rates of
infections are found in any area with large mosquito populations.
Heartworms go through several
life stages before they become adults infesting the heart of the host animal.
The worms require the mosquito as an intermediate stage in order to complete
their life cycle and so at least two animal hosts other than the mosquito are
required for the heartworm to reproduce. A mosquitoe ingests heartworm larvae,
called microfilariae, from an infected host. The mosquitoe then transfers the
larvae to another uninfected host when next it feeds. The microfilariae then go
through several changes to reach adult form, eventually traveling to the right
side of the heart to reproduce. Reproduction results in the dispersal of microfilariae into the bloodstream
where ingestion by another feeding mosquito spreads the microfilariae to another
At this stage, the host dog will likely be
asymptomatic. Once the infestation reaches a certain concentration in the lungs,
the now adult worms migrate from the pulmonary artery to the right side of the
heart and begin to reproduce in earnest, filling the blood with microfilariae.
At this point the host will begin to show symptoms of infestation. These
symptoms can manifest earlier or increase in severity depending on the activity
level of the animals as infestation reduces cardio-pulmonary
capacity. Very active animals may experience symptoms at lower heartworm
concentrations and have more severe symptoms than less active animals.
Heartworms can reach up to 12 inches in length and can remain in the host's
heart for several years.
Course of infestation
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 some 6.5
to 7 months in dogs and is known as the prepatent period.
Heartworms bear live young, known as microfilariae, producing thousands of
them every day. The microfilariae then 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, or third, larval stage and then migrate to the head of the
mosquito, where they wait to infect another host. These changes can occur in as
little as two weeks and as long as six weeks, depending on the warmth of the
climate, and generally cease entirely at ambient temperatures below 14° Celsius
After infection, the third stage larval heartworms deposited by the mosquito
grow for a week or two and molt to the fourth larval stage under the skin at the
site of the mosquito bite. Then they migrate to the
muscles of the chest and abdomen and, some 45 to 60 days after infection, molt
to the fifth stage (immature adult). Between 75 and 120 days after infection
these immature heartworms then enter the bloodstream and are carried through the
heart to lodge in the arteries of the lungs. Over the next 3 to 4 months they
increase greatly in size, growing backwards until they fill the right atrium and
of the heart. By approximately 6.5 to 7 months after infection the adult worms
have mated and the females begin producing microfilariae.
Symptoms of infestation
Dogs show no indication of heartworm infestation 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.
Many dogs will show little or no sign of infection even after the heartworms
have matured. To some degree these dogs may be described as seeming to age
slightly faster than normal as the worms slowly damage the lungs,
kidneys and liver. These animals usually have a light infection and live a
fairly sedentary lifestyle. However, active dogs and those with heavier
infections will quickly show the classic symptoms of heartworm disease. Early
symptoms include a cough, especially on exercise, and early exhaustion upon
exercise. More advanced cases progress to severe weight loss, fainting, coughing up blood, and, finally, congestive heart failure.
Heartworm is present on every continent except
where the mosquito vector is noticeably absent. The presence of heartworm in a
geographic region is dependent on the following factors:
susceptibility of host population
stability of the
population stability of vector species
proper climate conditions
Dogs are considered the definitive susceptible host for the parasite.
Untreated dogs also provide a stable disease reservoir. (Cats, on the other
hand, are considered a resistant host and a poor disease reservoir. However,
cats are more difficult to treat and so prevention is even more critical with
felines.) Mosquitoes of several different species are the vectors. Development
of the microfilariae in the mosquito ideally requires a temperature at or above
80° Fahrenheit for about two weeks. No larval development takes place in the
mosquito below 57° F.
Heartworms can be detected by
blood test. The filtration test finds microfilariae in the blood; the occult
tests (antigen and antibody) are used to detect adult worms. Many veterinarians prefer to do both tests, since the absence of microfilariae in
the blood does not necessarily mean that there are no adult worms in the heart.
Both tests are done with a single blood draw, preferably in the early spring
before daily temperatures warm above 57° F.
X-ray radiographs and, to a lesser extent, ultrasound
can also detect the presence of adult heartworms in the heart and lungs. X-rays
also can provide a good estimate of the amount of lung damage caused by the
presence of heartworms.
If either a blood test or the onset of symptoms betray the presence of
heartworms, treatment is indicated. Treatment is highly efficacious if the
disease is diagnosed early in the disease process. Before the worms can be
treated, however, the dog must be evaluated for good heart, liver, and kidney
function to ensure the animal can survive the treatment. Any insufficiencies in
these organs must be dealt with first, before treatment, as the eradication
process can be taxing on organ function. Usually the adult worms are killed with
an arsenic-based compound. The currently recommended compound, Melarsomine
dihydrochloride, is marketed under the brand name Immiticide. It has a greater
efficacy and fewer side effects than previous formulation (Thiacetarsamide
sodium, sold as Caparsolate) which makes it a safer alternative for dogs with late-stage
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. Aspirin can be used during this time to help
prevent further arterial damage and thromboembolism.
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 come back 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.
Long term monthly administration of
ivermectin (but apparently not moxidectin, milbemycin or selamectin) year round
for at least three years at the dose normally used for heartworm prevention (see
removes most adult heartworms from most dogs. However, this is not the treatment
of choice for removal of adult heartworms for two reasons. First, not all dogs
are completely cleared of heartworms by this treatment. More importantly, adult
heartworms do not begin to die until some 18 months of treatment have elapsed,
which is not acceptable under most circumstances. This treatment is normally
reserved for dogs that are not likely to tolerate treatment with the harsher,
but more effective, melarsomine or instances where the owner cannot afford the
more expensive melarsomine treatment.
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.
Prevention of heartworm infection can be obtained through a number of
veterinary drugs. Most popular are
ivermectin (sold under the brand name Heartgard), milbemycin (Interceptor) and
moxidectin (ProHeart) administered as pills or chewable tablets. These drugs are
given monthly during the local mosquito season. Moxidectin is also available in
a six-month sustained release injection administered by veterinarians, but the
injectable form of Moxidectin was taken off the market in the United States due
to safety concerns. Selamectin (Revolution), on the other hand, is a topical
preventive that is likewise administered monthly. Some of these drugs also kill
other parasites, including intestinal worms. In addition, Selamectin controls
ticks, and mites.
Preventative drugs are highly effective, and when regularly administered will
protect more than 99 percent of dogs from infection. Most failures of protection
result from irregular and infrequent administration of the drug. However, the
monthly preventives all have a reasonable margin for error in their
administration such that if a single month's dose is accidentally missed,
adequate protection is usually provided so long as the next two monthly doses
are administered on schedule.
Cats may be treated with ivermectin (Heartgard for Cats), or the
topical selamectin (Revolution for Cats).
Monthly heartworm prevention should be administered beginning within a month
of the onset of the local mosquito season and continued for a month after the
cessation of local mosquito activity. In warm climates, such as the warm
temperate climate along the immediate Gulf coast of the United States and in
tropical and subtropical regions, heartworm prevention must be administered year
round. Some authorities recommend year round administration even in colder
climates on the theory that mosquito activity may occur during the occasional
unseasonable warm spell, but others argue that computer models indicate
heartworm transmission is highly unlikely under such circumstances.
Human health considerations
The dog heartworm is a negligible public health risk, because it is unusual
for humans to become infected. Additionally, human infections usually are of
little or no consequence, although rarely an infected human may show signs of
respiratory disease. In most cases, however, the heartworm dies shortly after
arriving in the human lung, and a nodule, known as a
forms around the dead worm as it is being killed and absorbed. If an infected
person happens to have a chest X-ray at that time, the granuloma may resemble lung
cancer on the X-ray and require a
biopsy to rule
out the life threatening condition. This may well be the most significant
medical consequence of human infection by the dog heartworm.
At one time it was thought that the dog heartworm infected the human eye,
with most cases reported from the southeastern United States. However, these
cases are now known to be caused by a closely-related parasite of
raccoons, Dirofilaria tenuis. Several hundred cases of subcutaneous (under the
skin) infections in humans have been reported in Europe, but these are almost
always caused by another closely-related parasite, Dirofilaria repens, rather than the dog heartworm.
Resources and external links
American Heartworm Society Founded in 1974, the American Heartworm
Society is internationally recognized as the definitive authority with
respect to heartworm disease in dogs and cats.
American Heartworm Society Information for Pet Owners:
American Heartworm Society Information for Veterinarians. (Every three years
the American Heartworm Society holds an international symposium. Following each
triennial symposium, the Society updates its guidelines for veterinarians to
incorporate any new discoveries. The following guidelines are widely considered
"best practices" and should be closely followed by veterinarians in dealing with