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Let's Get to the Heart of It!
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by Dr. Scott Koenig,
D.V.M
Understanding and
Preventing Heartworms
Here in Louisiana, one of the most prevalent and
life-threatening diseases affecting our dogs and cats, and even ferrets, is
heartworm disease. As the name implies, heartworms are parasitic worms
(scientific name Dirofilaria immitis)
that live in the heart and pulmonary arteries of unprotected dogs, cats and
Ferrets.
Infection is
spread through the feeding of mosquitoes that have previously
fed on other infected animals. Dogs, cats and ferrets that are kept
exclusively outdoors are at highest risk for becoming infected, but animals
that only go out occasionally and even those kept indoors at all times are
potentially at risk. One mosquito bite is sufficient to infect your pet, so
precautions should be taken with all pets. Heartworm disease is treatable,
but treatment is not without risk and permanent damage can be done to the
heart with even a few worms. Therefore, the primary goal with heartworm
disease is PREVENTION.
LIFE CYCLE: (Click here to view life cycle)
Adult heartworms living in the heart of a dog, cat or ferret
reproduce by releasing small larvae known as microfilaria into the
bloodstream. These larvae cannot become adult heartworms on their own while
inside the same animal. In order to mature into adults that are capable of
infecting another animal, they must first be picked up by a feeding
mosquito. When a mosquito bites an infected animal, it picks up one or more
of the microscopic larvae from the blood. These larvae have to live and
develop within the mosquito for at least 2 weeks in order to become
infective. These infective larvae can then be transferred to another animal
when that mosquito bites and feeds on the unprotected pet. After larvae are
injected into the skin, they move through the body and go through several
molts or stages before entering the heart and becoming adult heartworms. In
rare instances, heartworm larvae can travel to other sites in the body other
than the heart such as the brain or kidney. This process, from first
entering the animal's body to becoming adult heartworms, takes approximately
6 months. If both male and female heartworms are present, reproduction can
occur and microfilaria can be produced and released into the bloodstream.
There they can potentially be picked up by more mosquitoes to continue the
cycle.
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HEARTWORM INFECTION AND DISEASE:
Heartworm infection is the term used whenever an animal has adult heartworms living in the heart and pulmonary arteries. The term heartworm disease is used to describe an animal that is infected with heartworms and is showing symptoms that are related to problems that develop due to the infection. These symptoms are very subtle early on and may only be noticeable to a veterinarian at that time. The most common signs of heartworm disease are coughing, difficulty breathing, weight loss, weakness and occasional vomiting (very common in cats). As the disease worsens, the above signs will become worse and may be accompanied by abdominal swelling, collapse, coughing up blood, nosebleeds and even sudden death. Symptoms are often first noticed following bouts of exercise or excitement but can occur at all times when disease is severe. Left untreated, heartworms continually damage the blood vessel walls and heart valves and lead to pulmonary hypertension (increased blood pressure) which eventually can lead to heart failure. When an animal has so many heartworms that the right side of the heart and pulmonary arteries are filled with them, the animal can develop right-sided heart failure, hemolytic anemia, coagulation problems such as DIC and many die. This particular circumstance is known as Caval Syndrome and can even occur without the animal showing any outward signs of heartworm disease prior to the collapse.
Heartworm infection and disease in cats is becoming a more recognized problem. Cats with heartworms react differently than dogs in several ways. While dogs are often found to have numerous worms in their hearts and pulmonary arteries, cats usually only have a few worms and most only have one or two. Since there are very few worms, there are usually very few or even no microfilaria present in the bloodstream. This makes testing and diagnosis much more difficult. Cats do not show the characteristic signs and symptoms of heartworm disease. Some occasionally have difficulty breathing, others cough, but most will have vomiting or have no symptoms at all. Some cats will die suddenly without any warning. For this reason, heartworm disease has often been overlooked as a potential problem in normal cats. Cats are just as likely to be bitten by an infective mosquito and develop heartworm disease, so prevention is very important.
Ferrets with heartworms
are similar to both dogs and cats in the way they are affected.
Ferrets show most of the clinical signs that are seen in dogs, while
infections tend to be limited to one or two worms like in the cat. Due
to the extremely small heart size of the ferret, even 1 heartworm can be
enough to bring about heart failure and sudden death.
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HEARTWORM TESTING
Antigen Tests: Antigen is a protein which causes the body to mount an immune response by producing antibodies. Antibodies that come into contact with the corresponding antigen will attach to it in order to try to destroy it and remove it from the body. Antigen tests for heartworms work by using antibodies against heartworm antigen and mixing them with a blood sample from the animal being tested. If heartworms are present in the animal, antigen will be present in the blood sample. The antibodies attach to any antigen and produce a positive test. Larger numbers of worms will result in larger amounts of antigen in the blood and will produce a stronger positive result on these tests. Though not perfect, it can give us an idea of the number of heartworms present. This is a very sensitive test and can even detect infections where there are only a few worms present. Most antigen tests look for antigen released by adult female worms, so infections involving only male adults could produce a negative result. This is not usually the case, however. Since the test detects adult heartworm antigen, animals that have been exposed within the last 5-6 months in which the heartworm larvae have not yet made it into the heart and become mature adult worms will be negative on testing even though they have heartworms. For this reason, young animals less than 6 months of age are not tested for heartworms.
Microfilaria Tests: When adult heartworms reproduce, they release tiny larvae into the bloodstream called microfilaria. Some tests for heartworms involve checking for the presence of these microfilaria in the blood. This can easily be done by a direct blood smear. A drop of blood is placed on a slide and examined under a microscope. Larger samples of blood can be spun down with a centrifuge to increase the chances of finding microfilaria. These tests are not ideal for testing for heartworms for several reasons. If it is a recent infection, the adult heartworms may not have produced larvae yet or may have only produced them in small numbers. If only a few microfilaria are present in the bloodstream, the small blood sample taken may not contain any larvae. Also, if most of the heartworms are of one gender (male or female), few if any microfilaria may be produced. Heartworm infections in cats and ferrets involve very few worms and usually do not yield microfilaria in detectable numbers, so these tests are not useful in cats and ferrets. Another case where microfilaria tests would not be sufficient is a dog that missed several doses of heartworm preventative but is currently back on it regularly. Monthly preventative does not kill adult heartworms, but it does kill larvae injected by a mosquito within the past 2-3 months as well as microfilaria. If doses of preventative are missed, some larvae can slip by and develop into adult heartworms. At this point, no further infection can occur and any microfilaria produced by the adult heartworms will be killed. These dogs would actually have heartworms and come up negative by the various microfilaria tests.
Antibody
Tests:
Similar to antigen tests, these heartworm tests look for immune products
found in the bloodstream. While antigen tests look for antigen released by
live adult worms, antibody tests look for antibodies produced by the animal
against heartworm antigen. These tests are of questionable use in the
diagnosis of heartworm infection, because there is no way to differentiate
between prior exposure and current infection. These tests can be useful
when there are only 1 or 2 heartworms and antigen levels are relatively low
or when there are only male heartworms present. Most cat heartworm testing
involves antibody tests due to the fact that cats usually only get 1 or 2
worms in an infection and antigen levels are low and microfilaria are rarely
seen.
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The best, easiest, safest and most economical approach is PREVENTION. While heartworm disease is treatable, the treatment does carry some amount of risk with it. These risks are assessed prior to treatment and any necessary pretreatments are done to minimize potential problems. Even so, post-treatment prognosis depends mainly on the severity of the infection. When heartworms die, they break apart and travel through the bloodstream to the lungs where they can block blood flow and cause serious breathing difficulties and death. This is an unavoidable side effect to treatment known as pulmonary thromboembolism, but it can be controlled and effects minimized with proper supportive care and monitoring. Heartworm treatment involves 2 main steps. The first step is the adulticide treatment that is a staged killing of the adult heartworms. The second step is the microfilaricide treatment, which kills any microfilaria present in the bloodstream.
Adulticide: Immiticide®, also known as melarsomine dihydrochloride, is the drug used to kill and remove adult heartworms. It is a derivative of arsenic, but it is relatively safe to use in dogs at recommended dosages. Treatment is staged in order to kill the heartworms slowly and help reduce the potential complications of treatment. Strict confinement and/or cage rest is very important to prevent complications as well. Most dogs receive 2 intramuscular injections 24 hours apart. In particular cases where the infection is severe or the dog is of poor health or showing severe complications due to the infection, the treatment is often spread out even further to bring about an even slower killing of the heartworms. This usually involves receiving one injection followed in one month by 2 injections 24 hours apart as described above. Most of the time, treatment will take care of all heartworms present, but in rare cases some worms may remain. Great improvement of clinical signs can still be seen even if treatment does not result in total removal of the heartworms, however. Several months following treatment, an antigen test can be preformed to the effectiveness of the treatment.
Microfilaricide: Medications that are normally used as heartworm preventative are effective in killing microfilaria in the bloodstream. This step of the treatment process is usually done several weeks following the adulticide therapy. Much like with the adulticide treatment, killing of large numbers of microfilaria can lead to problems. Close monitoring and supportive care as well as strict cage rest will help reduce the risks involved with this treatment.
Once a dog has been treated for heartworms and confirmed negative by antigen and microfilaria testing, they can be placed on heartworm preventative to prevent future reinfection.
At this time,
treatment for heartworms is only available for dogs. There is no current
treatment for cats. Ferrets can be treated with Immiticide®, but the
survival rate following treatment is less than 50%.
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The bad news is
that heartworm disease is a very prevalent and very deadly problem that is
exp
ensive to treat and that can affect dogs, cats and ferrets of all breeds
and ages whether they are kept strictly indoors or outdoors. The good news
is that heartworm disease is
100% preventable!
There are many different products on the market that are
effective in preventing heartworms in your pet. The choice of preventative
depends on each individual owner and pet’s lifestyle and environment.
Different veterinary clinics stock different products so check with your
veterinarian to see which ones are available and which one is right for you
and your pet. The products available here at Ratcliff Animal Hospital are
listed at the end for the convenience of clients. IMPORTANT: All dogs need
to have a negative heartworm test before starting any preventative
medication.
HEARTGARD® and HEARTGARD PLUS®: These products are once-a-month chewables that contain ivermectin as the active ingredient. They are both available for dogs and cats. The Heartgard Plus® product has added pyrantel pamoate that is effective in removing and preventing roundworm and hookworm infections. Most dogs like the beefy flavor and will accept their monthly “treat” without any problems. Cats can be finicky and some may not willingly eat the chewable preventative. There is no Heartgard® product currently available for ferrets, but the feline dosage is similar and can be used in this species. Since they do contain added beef for flavoring, dogs that are known to have allergies to beef products or those that are currently on food trials for suspected food allergy should use a different product. Collies and other herding breeds of dogs are very sensitive to ivermectin at high doses. The dosage of ivermectin in the heartworm preventative has been shown to be safe for these breeds, but owners of these dogs may want to consider one of the other products strictly as a precaution. Reminder stickers are provided to place on your calendar so that no doses are accidentally missed. Many people choose to give it on the 1st of each month to make it even easier to remember.
INTERCEPTOR® and SENTINEL®: These products contain the active ingredient milbemycin oxime and are once-a-month chewable tablet available for both cats and dogs. Milbemycin is effective at removing and controlling roundworms and whipworms, and at controlling hookworms, as well as preventing heartworms. Sentinel® contains added lufenuron, which is used to help control flea infestations by stopping fleas from reproducing. Lufenuron does not actually kill adult fleas. Both products come with colorful reminder stickers to place on your calendar to help ensure monthly dosing. These two products are a good alternative for dogs with food allergies where the client prefers the monthly dosing.
PROHEART®6: This product is an injectable heartworm preventative for dogs only and is effective for 6 months. The active ingredient is moxidectin, which is also effective in removing and preventing hookworms, as well. Many owners who have trouble remembering the monthly preventatives prefer the twice-a-year dosing. Injections must be administered by your veterinarian, so the dog must be taken in twice a year. Many clinics recommend biannual visits anyway and Proheart®6 shots can be coordinated with those visits. Dogs must be at least six months of age before starting on this, so alternative choices need to be started until pets are old enough to use this product. Owners with outside dogs that are at high risk for roundworm or whipworm infections may want to consider one of the other products.
REVOLUTION®: Selamectin is the active ingredient of this topical product that is applied between the shoulder blades once monthly. It is approved for cats and dogs and is even safe to administer to puppies and kittens 6 weeks of age or older. Selamectin is also effective in killing and preventing fleas, ticks, ear mites and sarcoptic mange mites in cats and dogs and also hookworms and roundworms in cats. This is a popular choice for owners with cats or dogs who will not eat the oral preventatives. This product is also a good choice for food allergy dogs and cats. Revolutionâ is absorbed systemically within hours of application to the skin and is effective even after bathing.
FILARIBITS® AND FILARIBITS PLUS®: These products are chewable tablets that are given daily rather than monthly and contain the active ingredient diethylcarbamazine citrate or DEC. Filaribits Plus® also contains oxibendazole, which prevents hookworms and kills and prevents roundworms and whipworms. These two products are for dogs only and can be used in all ages as well as pregnant or nursing females. DEC is only effective against heartworm larvae that have been injected by mosquitoes within 9-12 days, so consistent daily dosing is very important to ensure adequate protection. If days are missed it is important to let your veterinarian know before restarting these products to avoid any potential problems.
Any of the
above listed products can be purchased from your veterinarian to safely and
effectively prevent heartworm disease in your beloved pets. As an added
preventative measure, any and all efforts to reduce your pets’ exposure to
mosquitoes should be taken. Do not leave pets outside for extended periods
of time particularly during peak mosquito feeding times such as dusk and
nighttime. Mosquitoes breed in standing water, so drain any swampy areas
near your home if possible and avoid allowing water to stand in buckets,
planters, etc. around the house and yard. Since mosquitoes can carry other
diseases that can affect you and your pets, these steps are important to
remember in order to reduce the number of mosquitoes near your home and
reduce chances of contact with them.
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Here are some points to remember when thinking about heartworms and your pet:
1. Heartworm disease is extremely common and dangerous to both cats and dogs.
2. Heartworms are transmitted by mosquitoes.
3. Even animals kept strictly indoors or that go outside only for short time periods can be bitten by infective mosquitoes and get heartworms.
4. Treatment is available but can be risky.
5. The best treatment is PREVENTION and there are numerous product options available for you and your pets at your veterinarian’s office.
Here at Ratcliff Animal Hospital, we take heartworm disease very seriously. We recommend to our clients and their pets to start on some form of heartworm preventative starting at 10 weeks of age and to continue for the lifetime of their pet. We perform yearly heartworm antigen testing on dogs in conjunction with the pet’s annual visit to make sure they are free of infection. Due to the problems involving cats and heartworm disease testing that were discussed above, we do not routinely test cats as part of their annual visit. However, testing is available here in cases where heartworm disease is suspected in cats or should client’s request that their cat be tested. We also test any new or stray dogs over the age of 6 months before starting on heartworm prevention and recommend treatment to those with current infections. When a dog comes to us to be treated for heartworms, a thorough physical exam is performed and the overall health status is assessed prior to any treatment. Any necessary pretreatments are performed and the safest treatment schedule for that particular pet is employed based on the level of infection and age and health of the dog. All of our treatment protocols involve both stages of treatment, adulticide and microfilaricide. We hospitalize heartworm treatment patients for several days to help maintain a quiet and stress-free, comfortable but confined environment to ensure a safer treatment period. Routine follow-up exams are done at no extra charge to make sure that the patient is not having any problems following treatment. Heartworm antigen and microfilaria tests are performed following treatment to make sure that treatment was successful and then the pet is restarted on whatever form of preventative is best for the owner and pet.
The products we offer to our clients and their pets include:
For dogs: Heartgard® Plus, Interceptor® , Sentinel®, Proheart®6, Revolution®, Filaribits Plus®
For cats: Revolution®, Interceptor®
There are not any heartworm preventatives currently available for ferrets. Many veterinarians recommend feline Heartgard® or an oral preparation of its ingredient ivermectin.
While these are the products we currently keep in stock, should any alternative medication be desired by one of our clients, that product can be ordered for them.
We sincerely
hope that this information has answered any questions you might have
regarding heartworms and your pets. If you have any additional questions or
would like to make sure that your dog or cat is heartworm free and protected
against heartworm disease, please contact your veterinarian.
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"Knowledge is the root of all blessings"
SL
