VetInfo Digest                   April 2003

Table of Contents:

Heartworm Disease in Dogs and Cats

New Products

Using the VetInfo Subscriber Site

This month's note:

We have a lot of trouble convincing our clients that doing nothing is sometimes the best approach to an illness, disorder or orthopedic problem. Most people who come to us seem to expect that we will be able to provide a medication, a surgical therapy or some other form of treatment for any condition their pet may have. Unfortunately, there are lots of times when that isn't true.

When we do not have an effective therapy for a problem is it better to do nothing or to provide a therapy we know to be ineffective?

Clients seem to feel cheated, or perhaps upset with themselves, when we tell them that the best approach to their pet's problem is to do nothing. I think that some of our clients ask themselves why they bothered to come to the office if that was all we were going to say. The answer lies in understanding what you are paying for at the veterinary office. It is worth having someone familiar with pet health care examine a pet who is lame, is itchy, has urinary tract inflammation or other problems, to make sure that there isn't a good therapy and that the disorder isn't likely to become much worse if ignored. Finding that you can afford to wait and see what happens should be viewed as a wise use of your money, not as a waste of it and the time it took for the examination.

 

Heartworm disease

Heartworm disease is one of the most complex diseases that affects dogs and cats. It is a very different disease in the cat than in the dog, making it even more confusing to many people. Heartworm disease is complex enough that many veterinarians have misconceptions about the disease and there are some strong disagreements among veterinarians over the best ways to care for infected pets. Heartworms occur in the United States, Japan, Australia, Italy, Mexico, Canada, Caribbean islands, South America, portions of Africa and southern European locations other than Italy.

Heartworms got their name because of where they choose to live. Their preference is to live in the right chambers of the heart and the pulmonary arteries within a few inches of the heart. Adult heartworms are fairly long, with female heartworms attaining lengths of up to 14 inches and male heartworms topping out at 5 to 7 inches. They are very thin, though. Most dogs with heartworms probably have less than 15 to 20 worms and most cats have less than 3. There can be very large numbers of worms in some individuals. I have performed necropsy (post-mortem) examinations on several dogs who had over 150 heartworms in their hearts and pulmonary arteries. Heartworms live approximately 3 to 5 years in dogs and 18 months to 2 years in cats.

When heartworms reproduce they have produce live babies (microfilaria) that are microscopic in size. It is not unusual to see 5 to 10 microfilaria in a drop of blood in infected dogs, in our practice area. Considering that there are about 20 drops of blood in a milliliter of blood and a 50 lb. dog probably averages around 2000 milliliters of blood in its body, you can see that there can be a huge number of microfilaria in the body -- greater than 200,000 in many dogs. Obviously, it would be really bad for a dog if this many microscopic microfilaria grew into adult worms. This is probably why microfilaria never grow into adult heartworms unless they are first picked up by a mosquito. If the microfilaria stay in the blood stream they will die in approximately two years, staying microscopic the whole time.

Mosquitos are responsible for spreading heartworm disease from one dog to another. The mosquito sucks up a small amount of blood from the dog. If this blood contains a microfilaria, it is activated during its stay in the mosquito. This process takes some time and requires specific environmental temperatures to occur. This part of the transmission process for mosquitoes is poorly understood by many veterinarians and most pet owners. After a microfilaria is activated in the mosquito, it is called an infective larva.

If the environmental temperature is below 47 degrees Fahrenheit (8 degrees Celsius), microfilaria can not develop into infective larvae. It takes approximately 8 days for microfilaria to develop into infective larvae in the mosquito when the temperature is consistently greater than 86 degrees Fahrenheit (30 degrees Celsius. An average temperature of about 64 degrees Fahrenheit it takes much longer, approximately 29 days, for the activation to occur. This is important for several reasons but primarily because the average mosquito only lives 20 to 30 days "in the wild". With these temperature and time figures in mind, it is possible to come up with a rough estimate of how long the heartworm transmission season is for most areas of the country.

As might be expected, heartworms can be transmitted all year in the southern portions of Florida and Texas and in Hawaii. Portions of Alaska have no heartworm transmission period at all and even in the portions of the state in which heartworms occur the transmission is limited to two to three months of the year. For most areas of the United States, heartworms can only be transmitted after April and the transmission period ends by November. However, the newer heartworm prevention medications kill heartworms for the month preceding their administration, so this means that the medications must be given from May until December.

It is extremely difficult to get older veterinarians to believe the information in the paragraph above this one. It is hard for me to convince many of my clients that this is the case, because the symptoms of heartworm disease occur months after infection and it often seems to my clients that their dog was infected during the winter if they stop the medication during the winter. However, it is becoming increasingly clear that heartworm transmission is limited to 7 months or less in the great majority of the United States.

Some veterinarians believe that it is best to encourage clients to give preventative medications all year in order to maintain the habit of medication administration and because the medications generally control roundworms and often other parasites, as well. I think that this is a decision that clients might want to make based on their own behavior patterns. I do notice that a small number of my clients consistently forget to restart heartworm prevention after taking any time off from it -- but most are good about starting again in May after stopping in December. Be honest with yourself about your habits and act accordingly.

It takes about 5.5 months for infective larvae to grow into adult heartworms in a dog. All of the current testing procedures can only detect adult heartworms. This means that heartworm testing prior to 6 months of age is of no value. In addition, it makes sense to take into account the timing of a puppy's birth when figuring out when to do the first heartworm testing. A puppy born in January isn't likely to be exposed to heartworm disease before May of its first year of life unless it lives in the deep South. This means that testing this puppy before it is eleven months old makes very little sense.

The interval between heartworm testing continues to be a major area of controversy between veterinarians and their clients, as well. For a very long time the only heartworm medications available had to be given daily. In addition, these heartworm medications could cause great harm if they were administered to a dog who had heartworm disease. So it was traditional to test dogs for heartworms on a yearly basis, even when the pills were being given year round and especially when they were being given seasonally. The newer heartworm medications work better and are reasonably safe to administer even to a dog who has heartworm disease. Now the focus of heartworm testing has shifted a little, from protection of the individual dog from the risk of heartworm medications to detecting failures in the medication administration and to monitor for resistance to the medication developing in heartworms. For these reasons, it is now considered reasonable to heartworm test on a semiannual basis (every two to three years) by most heartworm disease experts and the American Heartworm Society guidelines.

Prescription laws may dictate a different testing schedule, depending on their interpretation. Heartworm medications are prescription medications, which means that their use is regulated. The veterinarian prescribing these medications needs to be sure the patient is actually alive and well within a reasonable time frame of the prescription (a year is usually considered reasonable) and is required to make an objective evaluation of the possibility that the pet may not have received the medication. If you didn't buy heartworm pills last year, for instance, your vet would be obligated to check for the disease prior to filling a new prescription for the medication. Some vets really believe it is best to check for heartworm disease yearly. Others are comfortable with longer intervals. We test every two years at the present time. This is a compromise interval that we choose in order to minimize the risk of missed tablets but not make it too difficult for our clients to comply with the need for heartworm testing. We try not to dispense heartworm medications for patients we have not seen in more than 12 months, as well. It is surprising how often we discover clients buying heartworm prevention for pets we have not seen when we insist on seeing the dog we have in the record. When they can't produce that pet, they usually tell us they were just going ahead and giving the pills to "the new dog". In order to prescribe medications there has to be a legitimate doctor/client/patient relationship. The legal definition of what constitutes that relationship is often subject to interpretation. In general, though, meeting the recommendations of the American Heartworm Society for testing interval and having an examination record for the pet within a year should satisfy prescription recommendations.

Testing for heartworm disease involves drawing blood and looking for microfilaria or testing for antigen (proteins) of heartworms using a test kit or by sending blood to a reference laboratory. The antigen testing procedure can only detect female heartworms old enough to reproduce. All male heartworm infections and very small numbers of female worms (between 1 and 3) may be missed. This is acceptable from a clinical point of view since worm burdens this low probably don't require therapy. A bigger problem is interpretation of positive test results in areas of the country in which heartworms are not very common.

If the percentage of dogs who will develop heartworms in a particular location during their lifetime is over 20%, a positive test indicates that the dog really does have heartworms about 95% of the time, even if no symptoms of the disease are present. If the percentage of dogs in an area who will develop heartworm disease during their lifetime if not on preventative medications is around 1%, then a positive heartworm test result is just as likely to be wrong as to be right - approximately 50% of the test results will be incorrect. This is a problem known as "predictive value" for testing procedures. It is probably best to repeat heartworm tests, using a different testing procedure, whenever there is a positive test in a patient with no symptoms, especially when the history of medication administration, age, or other circumstances makes it unlikely that heartworms are present.

Adult heartworms live inside the blood vessels and ingest nutrients directly from the bloodstream. They do not burrow into the blood vessels or the heart lining. If they did not cause immune reactions in the host (the dog or cat), they would not cause much damage. It is the reaction of the dog's body to the presence of the worms that causes most of the problems. The surrounding blood vessels become inflamed and gradually over time this leads to scarring and a lack of pliability in the vessels. Ultimately this leads to high blood pressure because the blood must be squeezed through a smaller opening. Think about what happens when you compress your garden hose, or partially block the opening of it --- the water pressure increases. When the blood pressure gets high enough that the heart can no longer pump blood effectively, heart failure occurs.

The number of heartworms has an effect on the amount of scarring that is present. The length of time that the heartworms live in the body also has an effect on the amount of scarring that is present. A very small number of worms can live inside a dog for their entire lifetime without causing enough damage to produce heart failure. A moderate number of worms present for a long time may do as much damage as larger number of worms do in a smaller amount of time. This combination of time and the number of worms present is hard to evaluate. There are no easy ways to determine exactly how many heartworms are present. The number of microfilaria in the blood doesn't correlate at all well with the number of adult heartworms. In fact, in many of the worst cases of heartworm disease there are no microfilaria in the bloodstream at all, because the dog's immune system has learned to kill them or to inhibit reproduction.

A dog with clinical symptoms of heartworm disease, including tiring readily, coughing, visible fluid in the lungs on X-rays, chronic weight loss, abdominal distention, fainting or coughing up blood should be treated for heartworms using melarsomine, unless he or she has so much damage already present that heartworm treatment just doesn't seem possible. A dog with a positive heartworm test but no symptoms should probably be tested with a different type of test prior to making any decisions. If the follow-up testing is also positive then an evaluation of the patient for treatment should take place and treatment tailored to the individual patient's needs.

Prior to the development of melarsomine (Immiticide Rx) for heartworm treatment, we didn't take pretreatment X-rays on a routine basis. We did take them when there was any reason to suspect that another problem might be present, or might be contributing to the disease. Our thinking was that the prognostic information gained wasn't worth much, since it was still necessary to treat for heartworms in a symptomatic patient. Melarsomine can be used in two different protocols which vary depending on the seriousness of the heartworm damage at the time of treatment. X-rays can play a valuable role in helping to determine which treatment option to use. In the past we shied away from X-rays when cost was a factor, feeling that the prognostic information gained wasn't valuable enough. Now we sometimes find ourselves taking X-rays to try to save money on the treatment when it seems possible to use the two dose rather than the three dose treatment schedule for melarsomine.

Blood work can help rule out kidney or liver damage from the heartworms but we usually go ahead and treat for the worms even when the blood chemistry values are high, since heartworms can cause problems in either organ system. As long as there is an understanding that heartworm treatment might have to progress even if the findings are bad, there isn't any strong reason not to do the lab work. I do think it is a bad situation when increased liver enzyme values make a vet abandon treatment. There is just too much chance these values are due to the heartworms to give up on treatment.

If the patient is likely to have a low worm burden based on clinical history, such as a dog that has been on the monthly prevention all but two months of one heartworm season or a very young dog, then it may be reasonable not to treat for the adult worms. In this case, the patient should be given a monthly heartworm medication and reevaluated for symptoms of heartworm disease on a regular basis. If no symptoms ever develop then treatment is not necessary. Most commonly, dogs suspected of having a low number of heartworms are put on heartworm medications containing ivermectin as it has been shown to weaken young adult worms over time and is less likely to cause reactions when microfilaria are present. It is important to emphasize the "young heartworms" aspect of using ivermectin. It is estimated that it will significantly reduce the number of heartworms present after a year if the heartworms are less than 8 months old at the time ivermectin administration starts. It has been shown that ivermectin has is little to no effect on fully mature heartworms (McCall, 2003). Choosing not to use adulticidal heartworm medications should be a very carefully made decision, based on the probable age of the majority of heartworms present, the likely number of worms present and the willingness of the veterinary client to allow monitoring of the dog's health status while waiting for the heartworms to die. Adult heartworms live as long as five years in dogs, so there is a great deal of time for them to cause damage when they are left untreated.

If the patient has no overt symptoms of heartworm disease but X-rays show enlargement of pulmonary blood vessels, perhaps minor fluid accumulation or visible changes in the shape and size of the heart, it is probably best to treat for the heartworm disease using melarsomine (U.S.) or thiacetarsamide (areas in which this is the only available therapy). This would also be true if the history is suggestive of the possibility of a large heartworm burden, such as a dog kept on a chain or in a pen where it is hard to get away from mosquitos. Dogs in these situations often have very high heartworm numbers because the mosquitos who bite them don't leave. They just hang around for more easy meals and reinfect the dogs with their own heartworms over and over again.

In most cases we prefer to error on the side of treating for heartworm disease. We treat with melarsomine (Immiticide Rx). This medication is much better than the older treatment, thiacetarsamide (Caparsolate Rx). Melarsomine can be used according to two treatment schedules, based on the severity of the disease at the time the therapy is started.

For dogs with minor symptoms of heartworm disease it is acceptable to give melarsomine two days in a row. Dogs should be restricted from exercise to the greatest degree possible for six to eight weeks following treatment. They should be kept in a cool environment, with temperatures not exceeding 75 degrees Fahrenheit (24 degrees Celsius), if possible.

For dogs with moderate to severe symptoms of heartworm disease the treatment should be done with three injections. The first injection is given on the first day of treatment. The dog is restricted from exercise and kept in a cool environment for 4 weeks and then two additional injections of melarsomine are given 24 hours apart, just as in the other treatment protocol. Then the patient should follow the same post-injection directions as in the previous paragraph. Many veterinarians are more comfortable following this approach to treatment for most patients because there seem to be fewer severe complications. It is critically important to understand that the less active a patient being treated for heartworm is, the better their chance for surviving the treatment. Overall, treatment with melarsomine has about a 2% chance of causing death in the immediate post-treatment period and thiacetarsamide causes death between 8 to 20% of the time in the immediate post-treatment period. Taking all precautions to limit the potential for death during a heartworm treatment makes a great deal of sense.

In the past an effort was made to kill the microfilaria that were still in circulation after killing the adult heartworms. At the present time there is no approved medication for this use, but all of the monthly heartworm prevention medications will kill microfilaria over time. Milbemycin oxime (Interceptor Rx) is a little more likely to cause reactions when microfilaria are present than the other monthly medications but even this risk is pretty low. After treating for adult heartworms we recommend starting monthly prevention medications but do not make any other special effort to kill the microfilaria.

Heartworms are occasionally associated with damage to other organs or cause problems in a different manner than the usual pulmonary and heart related symptoms.

The reaction of the body to heartworms can produce substances that are damaging to the kidneys. A small percentage of dogs with heartworm disease will develop kidney failure as a result of the infection. We have seen this complication in at least one dog who did not have other discernible symptoms of heartworm infection. The potential for this problem is another reason to seriously question trying to use the "slow" methods of heartworm treatment and waiting for adult worms to die.

Heartworms are sometimes found in cysts in the skin or become visible in the interior of the eye. These worms can be removed surgically and doing so seems to resolve any local problems they have caused.

When heartworm numbers are very high and occasionally for other reasons, heartworms can slip through the valve (tricuspid) that separates the right ventricle from the right atrium. The heartworms can then occupy space in the right atrium or the large vein coming from the liver to the heart (vena cava) and sometimes into the liver veins, as well. When this happens there is usually a very rapid deterioration in the dog's health and extreme jaundice is not unusual. This condition is best treated by surgical removal of the worms from these areas, followed by a conventional heartworm treatment when the dog has stabilized. Obviously, removal of heartworms from inside the right atrium and the vena cava presents a large surgical risk but since this is a "do or die" procedure that risk is justified.

Heartworms is a preventable disease. There are currently a number of heartworm medications on the market. None can really claim to be the best in all situations and all are effective for most pets. All of the monthly heartworm preventatives currently available, and the injectable 6 month preventative can kill infective larvae until they are 50 days of age and can kill many of the larvae until they are 70 days of age. This produces a large safety margin for the use of these medications, since it is apparent that even skipping a month provides only a small window of opportunity for the heartworm larvae to survive. If it becomes apparent that a monthly heartworm preventative tablet has been missed, it is best to immediately give the medication and then to be as careful as possible to give it on schedule for several months. It is not necessary to wait until it is possible to do a heartworm examination before starting the pills again.

 

Heartworm Disease of Cats

Cats can develop heartworm disease but the rate of infection is usually much lower for cats in a given area than it is for dogs. It is estimated that cats develop heartworm infections at about 10% the rate that dogs do in similar surroundings. For this reason, heartworm prevention medications are not used as frequently in cats as they are in dogs.

Heartworm disease is much more difficult to treat in cats than it is in dogs. The death rate from heartworms in cats is estimated to be about 50% and the death rate from attempting to treat cats is also estimated to be between 30 and 50%. This makes treatment an unattractive option. This is perhaps the biggest reason to use heartworm prevention medications even though the risk of infection is low.

The heartworm transmission season for cats may or may not be the same as for dogs in a given area. One of the mosquitoes most likely to feed on cats and to transmit heartworms to cats prefers an indoor environment. Since temperature is often artificially controlled indoors, it is possible that the heartworm transmission period is a little longer for cats than for dogs, although I know of no research to support this supposition. At the present time we make the assumption in our practice that the transmission period will be roughly equal to that for dogs in our area and use prevention medications accordingly.

There are three medications approved for heartworm prevention in cats. Ivermectin (Heartgard Plus Rx) is a chewable tablet given once a month. Milbemycin oxime (Interceptor Rx) comes in tablet form. Selamectin (Revolution Rx) is a topical therapy that prevents heartworms and kills fleas when used monthly. These medications also kill roundworms, lessening the public health impact of these parasites.

We have noticed within our practice a much stronger reluctance among veterinary clients to use heartworm prevention medications in cats. This certainly doesn't apply to all of our clients, but it is an overall trend. To some extent this seems to be a cost issue. We try to encourage cat owners to use heartworm prevention medications but we also try to prioritize care when money is an issue. Weighing the low risk of the heartworms in cats against the high threat of death when cats are infected with them is a tough issue to think about from a cost analysis standpoint. We look at the situation in this manner: if the money for the heartworm prevention will come out of the cat's overall care then we would rather put the money into vaccination, dental care and other more common problems. However, if the client wants to provide good protection against as many possible problems as they can, heartworm medication definitely makes sense.

Cats develop clinical signs much earlier in heartworm infections than dogs do and their reactions tend to be worse, including the possibility of sudden death before clinical signs sufficient to worry the cat owner even occur. In cats, in particular, heartworm disease is probably misnamed. The reaction to the heartworms in the lungs and its blood vessels is probably the major cause of illness associated with heartworms in cats. The first symptoms are respiratory in nature in most cats, with coughing, difficulty breathing and increased respiratory rate often occurring even before heartworms reach physical maturity. Many cats will also have intermittent vomiting as a clinical sign. Unfortunately, the number of illnesses that cause intermittent vomiting in cats is high enough to make this sign difficult to relate specifically to heartworm disease.

Heartworm testing in cats is problematic. Cats tend to have very small numbers of heartworms even when they have severe clinical signs. Only about 20% of cats with heartworm disease have circulating microfilaria and their numbers tend to be smaller than in dogs. In many cases cats have too few heartworms to cause a positive reaction on an antigen test (the type that tests directly for heartworms). Antibody tests are available for cats but they only test for exposure, since many cats are able to kill the heartworm larvae and are not infected but do have antibodies that result from the exposure. X-rays are useful in detecting heartworm disease in cats due to visible changes in the blood vessels and collection of fluid in the chest that occurs in some cases of heartworm disease. Ultrasound examination is also useful as the worms can be directly visualized in most, but not all, cases. When heartworm disease is suspected in cats it is best to use several of these tests to help confirm the diagnosis.

The diseases most likely to be confused with heartworm disease in cats include asthma, chronic bronchitis, lungworms and cardiomyopathy. In an area in which heartworms are common, diagnosis of asthma should prompt a careful evaluation of the possibility of heartworm disease.

There is no approved treatment for adult heartworms in cats. Reports of attempts to use melarsomine and thiacetarsamide for heartworm therapy in cats show a high death rate associated with treatment. Therefore, most veterinarians opt not to attempt definitive heartworm treatment for cats.

Most veterinarians opt to treat the symptoms of heartworm disease with the goal of keeping the cat alive until the heartworms die on their own. Corticosteroids such as prednisone are used to lower the inflammation in the lungs. Bronchodilators seem to help some cats who have symptoms of respiratory distress. If heart failure develops medications such as diuretics, digoxin or oxygen supplementation can be helpful. It is estimated that heartworms will die in most cats within 18 months to 2 years. Treating cats with ivermectin (Heartgard Rx) monthly at the usual preventative dosage may shorten the life span of heartworms and can kill pre adult stages that do cause symptoms in some cats.

It is important to remember several points about heartworm disease in cats. The first is that an indoor lifestyle does not protect cats against heartworm disease as well as most people think it will. The second is that heartworms and asthma are easily confused. If there is difficulty controlling asthma or if heartworms are very prevalent in an area it makes a great deal of sense to look for heartworm infection when asthma is suspected. The last point is that heartworm prevention medications do make sense for cats in areas in which heartworms occur and it is usually possible to administer these easily. Tablets, chewable tablets and topical therapy are all available options and most cats will tolerate at least one of these treatment options.

 

Heartworm Disease Prevention for Traveling Pets

If the area you live in has no heartworm disease but you are going to spend a few weeks in an area in which heartworm disease is prevalent, what should you do about heartworm prevention?  The most important thing to remember is that all of the current preventative medications work by killing heartworm larvae that are up to 45 to 50+ days of age, except ProHeart (tm) injection. Other than this one exception, there is no reason to give heartworm medication until after the first month of exposure occurs. So a dog or cat who is taken to a heartworm endemic area should be given heartworm medication after one month, or after the exposure if it last less than one month. The heartworm prevention medication should be given for at least one additional month, just to ensure a good effect. So a pet who stays in a heartworm endemic area for one week would need a heartworm tablet after going home and then one month later. A pet who stays a month, needs a preventative pill after the first month of exposure, every month until it goes home or heartworm season is over where it is at and then one additional month's treatment. 

 

New Products

Advantix (Rx), from Bayer

Bayer is heavily advertising Advantix (tm) at the current time. This is a product that combines imidocloprid, the active ingredient of Advantage (Rx) with permethrin, which is the active ingredient in many over the counter tick and flea control products, such as Defend (tm), Control (tm) and others.

Based on our clinical experiences with permethrin based products in the past, we do not expect the mosquito repellence to be as good as people seem to be hoping for. Tick repellence is very good for permethrin based products but most of our clients were dissatisfied with the activity of these products after 14 to 21 days. Bayer's studies showed a drop in efficacy in tick repellence to about 85% towards the end of the month.

It is very important to remember not to put Advantix on a cat. Concentrated permethrin products like this can kill cats.

Genesis Topical Spray (tm) from Virbac

For dogs who are too itchy for comfort but not itchy enough to justify oral corticosteroids, Genesis Topical Spray may help. This is a 0.015% triamcinolone spray. We have tried this product in about 10 dogs at the present time and it has been very helpful for about half of them in controlling itching without having to use systemic corticosteroids. This product is absorbed to some degree and so it does cause side effects in some dogs, such as increased drinking and urinating. This is less of a problem with this product than with oral corticosteroids, though.

 

Asking Questions and Using the Subscriber Area

The subscriber area has the back issues of the VetInfo Digest in it and occasionally has notifications of problems with the site or with e-mail. There is an oral drug formulary (dosage schedule) for both dogs and cats. We plan to continue to develop this area over time. To go to the subscriber area directly, use this address: http://www.vetinfo.com/subscriber/subscriber.html. If you do not know your username or password write to mervet@inna.net to have them e-mailed to you.

Dr. Mike is continuing to answer questions for subscribers. Please send them directly to mervet@inna.net and put "subscriber question" in the subject line. We get a great deal of spam and questions may be inadvertently deleted if it is not clear they come from a subscriber. We will continue to state on the site that we are not answering questions at the present time because we can not handle large volumes of questions and doing this keeps the subscriber base within reasonable limits.

 

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The opinions expressed in this newsletter are those of Michael Richards, DVM., author.

Copyright 2003, TierCom, Inc.