Heartworms
Heartworms are a serious and sometimes fatal parasite that live in the heart and lungs
of infected pets. Adult heartworms are spaghetti shaped worms that inhabit blood
vessels, lungs, and hearts. Female heartworms produce offspring called microfilariae
into the blood stream of the infected animal. When a mosquito takes a blood meal from
that animal it also takes in some of the microfilariae. These "baby heartworms" develop
inside the mosquito for a short time and then are passed on to other animals as the
mosquito feeds on them. After blood meal mosquitoes inject an anti coagulate into their
host the microfilariae are passed during this exchange. Heartworms are typically found
more in dogs however recent research has found that many more cats may be infected
than previously thought. Prevention is key! Monthly heartworm preventions are available
for both dogs and cats and are very reliable in preventing both heartworm infection and
heartworm disease when given properly.
Microfilariae develop in about six months in dogs and eight months in their feline
counter parts before they become adults capable of releasing their own microfilariae
into the bloodstream. During this time the microfilariae migrate through the dog or cats
blood vessels until they reach the heart and lungs.
There are a few different ways of diagnosing heartworm infection in dogs. The most
reliable way is to run a heartworm test. These tests are run using a small blood sample
that is placed on the test and gives results with in just a few minutes. There are two
forms of this test, one that only tests for heartworm and one that tests for heartworm
along with three of the major tick borne diseases. Another way of detecting infection by
performing a direct smear. This is where you look for the appearance of the microfilariae
in a blood sample when viewed under a microscope. Unfortunately, there is a chance of
missing these parasites with this test due to either immature heartworms not yet
releasing microfilariae along with the amount of microfilariae needed in the bloodstream
to see them this way. A negative result on a direct smear does not mean that the tested
dog is not infected with heartworm. We recommend a heartworm test yearly for pets not
on heartworm prevention and every two years for those that are. It is also important to
test animals that have visited a section of the country where heartworm is more
prevalent such as Florida, Mississippi, Louisiana, and Texas. Due to the lifecycle of
heartworms it is not necessary to test dogs and cats less than six months of age.
Cats are a little different story when it comes to detecting heartworms. Until recently the
only way to test a cat for heartworm was to send a blood sample off to the lab. Last year
a snap test was released for cats that tests for heartworm along with feline leukemia
and FIV. Heartworm can not be detected in a stool sample in either cats or dogs.
If a heartworm infection is left untreated or found later on during the infection it can lead
to what is referred to as heartworm disease. This is when the heartworms begin to
make the infected animal sick. The animals can present with anything from a mild cough
to congestive heart failure. This occurs for a variety of different reasons. The first reason
is due to damage to the pulmonary arteries. These arteries do not function well with
worms living inside of them and the lining of the artery often becomes damaged within
days of the worms arrival into the heart. Cells of the immune system are called in to the
area with the irritation but the worms are to big to be destroyed by these cells. The
resulting inflammation continues to damage the artery, which becomes dilated.
Aneurysms and abnormal clotting can result. This causes blood flow to be shifted to
other areas of the body where there is less resistance for the blow to flow. Fluid begins
to accumulate in the lung around the worm filled arteries and blood being sent to the
lungs can not become properly oxygenated. Areas of lung become consolidated and
these areas are no longer able to participate in oxygenating. During this process
coughing and exercise intolerance become apparent. Nose bleeds can also occur due
to abnormal blood clotting in the lung. Pneumonia can also occur due to the excessive
infiltration of the immune cells in the lung.
Heart failure can be another problem associated with heartworm disease. Since the
worms are plugging arteries with in the lung the heart must work much harder to try to
push blood into this area. This is called pulmonary hypertension this causes the right
side of the heart to increase its ability to work. It may be strong enough but it may not.
Like any muscle when the heart has to work harder it begins to thicken this can cause
problems with the normal electrical impulses that control the heart. The heart may have
disruption to its pumping and filling rhythm and an arrhythmia may result. If the right
side of the heart can not keep up with this pressure fluid can also back up into the chest
cavity and the abdomen. Coughing is also a typical symptom of these changes in the
heart.
Dogs can also develop problems from the chronic immune stimulation that happens
when heartworms go untreated. This causes the immune system to produce lots of
antibodies all the time. These over abundant antibodies can damage delicate tissues in
the eye, kidneys, blood vessels and joints. These antibodies stuck in these areas can
cause pain and tissue damage.
Caval syndrome is an especially disastrous form of heartworm disease. This is happens
when there are so many worms living in the heart that they start to back up into the
large veins that feed the right side of the heart. Often the first symptom is collapse,
shock, and red blood cell destruction. The only way to treat caval syndrome is to open
up the jugular vein and try to remove enough worms to restore blood flow in that area.
Dogs affected with caval syndrome usually do not live beyond 2 days of initial signs of
the disease. In cases of heartworm infection becoming heartworm disease both the
worms and the disease must be addressed for recovery.
Cats infected with heartworm disease in a different story in dogs due to the fact that
heartworms usually reside in the lungs of cats. Cats are not a natural host for
heartworms, which means that rarely do they complete their lifecycle within cats. Very
few larval heartworms survive to be adults in cats. A moderate heartworm infection in
dogs typically consists of 20-35 adult heartworms in cats this number drops to about 6.
Even with a smaller number of adult heartworms they can wreak havoc due to the
smaller heart and blood vessels of cats. In cats a single worm can be fatal. Heartworms
usually live for about 5 years in dogs they usually only survive for about 2-3 years in
cats, thanks to their strong immune reaction to these parasites. Due to the differences
of canine and feline hosts’ symptoms of heartworm disease is different in cats. Cats
usually show more signs of respiratory problems such as vomiting, coughing, and
respiratory distress that can be mistaken for feline asthma. Cats tend to have the most
distressing effects of heartworm disease at the beginning and end of the infection.
During the first 75-90 days of the infection the presence of these small microfilariae are
very irritating to the tissues of the lungs and disruptive to circulation. This causes an
inflammatory response that can further irritate the tissue and interfere with the cats
ability to breathe. The other problem that cats have is when the heartworm dies. These
dead worms cause extreme inflammation that not only effects the lungs and circulatory
system in cats but can also cause damage to the kidneys, gastrointestinal tract, and the
nervous system as the body tries to clear and digest these worm corpses within the cat.
HEARTWORM INFECTION CAN NOT BE TREATED IN CATS. Cats can not handle
the medication used to kill heartworms in dogs. Instead we try to treat the symptoms
and try to keep the cats comfortable till the heartworms die on their own. This includes
prednisone to try and reduce the irritation in the lungs and allow for easier breathing.
Treatment in dogs is somewhat of an art when it comes to heartworm. The first step
after the positive heartworm test is to take a x-ray to see how much, if any
damage has already occurred in the cardiovascular system.
Evaluation for treatment depends on many factors such as amount of worms the dog is
infected with, size and age of infected dog, previous health problems, severity of heart
disease, and ability for exercise to be limited following treatment. The first step in
treatment is to kill any migrating immature worms. Luckily all stages of worms except for
the adults can easily be treated using monthly heartworm prevention. Adults on the
other hand are much more difficult to clear from the body. The only product on the
market available for the treatment of adult heartworms is called Immiticide it is given in
2-3 doses to kill the adult heartworms.
The patient is giving injections of this medication deep in the muscle of the lower back.
Patients are usually sore after this injection and sent home on pain medication. A
second injection is given 24 hours later on the opposite side of the lower back. Injection
site reactions can occur as anything from severe pain, abscesses, and even the
formation of lumps where the injection has been given.
After treatment with this product dogs must be strictly confined for one month. This
means no walks or running around due to the fact that embolism is somewhat
inevitable. What we try to do in the veterinary community is to minimize embolism
related problems that can be negatively influenced by the increased heart rate and
oxygen demand that exercise places on the body. It is important to watch for signs of
coughing, fever, and nose bleeds after being treated with Immiticide. If any of these
symptoms occur it is very important to contact you veterinarian as soon as possible.
When it comes to heartworms prevention is key. Preventions are available for both dogs
and cats in a few different forms and need to be given monthly to avoid not only
heartworm but several intestinal parasites as well. The way prevention works is by not
only clearing microfilariae but also by killing 2 of the other developing stages of
heartworm. It is very important to know a dog’s heartworm status before putting them on
heartworm prevention for this reason we require a negative heartworm test before
prevention is started. Interceptor is one heartworm prevention available on the market
and the one most used for dogs at Bardstown Veterinary Clinic. Interceptor is a monthly
tablet that prevents heartworm along with the intestinal parasites roundworms,
hookworms, and whipworms. It contains the active ingredient Milbemycin. Milbemycin
has been found to be a very safe and reliable prevention is dogs that are heartworm
negative to begin with.
Another option is called Iverhart Max this is also a monthly tablet that prevents
heartworms but the intestinal parasite coverage is a little different. This product prevents
roundworms, hookworms, and tapeworms for all those dogs who love to hunt for
rodents around the farm. The active ingredient in Iverhart Max is Ivermectin. Ivermectin
is very effective at preventing heartworms when used properly. Unfortunately, this
product is not recommended for herding breed dogs such as collies and shelties due to
a genetic sensitivity to this ingredient. Other than theses particular herding dogs Iverhart
Max is a very safe prevention when used properly.
Along with preventing heartworms our final product also prevents fleas along with
intestinal parasites roundworm, hookworm, and whipworm. This product is called
Advantage Multi and is applied topically once a month. The active ingredient in this
product is called Moxidectin. Moxidectin is a relative of Ivermectin but not the same
medication. Advantage Multi is the heartworm prevention chosen most for cats along
with many dogs kept mostly indoors. Although it is important to note that Advantage
Multi does not prevent ticks so if ticks are a problem in your area a combination of
heartworm and flea/tick prevention may be a better choice for your pet.
It is generally advised to keep your pet on heartworm prevention year round not only for
the heartworm protection but also to keep your pet free of intestinal parasites. Many
people choose to stop heartworm prevention in the winter months when mosquitoes are
not as prevalent. Mosquitoes must have a temperature of 57 degrees to affectivity
transmit heartworm. If you decide to let prevention lapse during the winter months a
heartworm test is required again in the spring before restarting the prevention. If your
pet is on prevention year round we recommend that you have a heartworm test every 2
years to make sure that the product you are using is doing its job properly.
If you still have questions regarding heartworm please feel free to either call the office or
visit one of the following websites.
www.heartwormsociety.org
www.knowheartworms.org
www.petsandparasites.org