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 HEALTH FORUMS for Spinone Italiano
 Heartworm Primer
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Author  Topic Next Topic: Subaortic Stenosis (SAS) - Introduction  
J McMaster

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 Posted - 11/22/2007 :  6:14:42 PM Show Profile Add J McMaster to Buddylist
By Cindy Mendonca, LVT
Courtesy of http://www.beaconforhealth.org

Every year we take our Beardies to the vet to be tested for heartworm. Hopefully, the vet tells us that the test is negative, sells us a preventative, and says to use it until after the first hard freeze. How many of us really know what heartworm is and how the preventatives work? What happens if we just skip giving the pills? Do we know why we have to give it even after the mosquitoes have all died off?

Heartworms have a five stage life cycle. I'll start with Larval Stage 1 (L1), although it's a bit of the chicken and egg. You see, you can't have L1 microfilaria in the blood unless you have adult heartworms (L5) infecting the heart and producing offspring. Unlike most parasites that produce eggs, heartworms bear live young. The L1 microfilaria have to be removed from the dog in a blood meal by the mosquito where they undergo two moults (Larval stages 2 and 3). They are only infective in the L3 stage and must be reintroduced into a dog in another blood meal. The L3 stage then migrates through the dog's system while changing to the L4 stage, ending up in the heart where it undergoes L5 or adulthood. It takes about 5-8 months for them to develop into full adulthood, that is, to mate and begin producing more L1 microfilaria.

The microfilaria produced by the adult heartworms in the body can *not* develop to adulthood inside the dog's body without ever having been in the secondary host (the mosquito). If the adult heartworms are not killed, they are still doing damage in the heart. The L1 microfilaria do nothing to *your* dog. But if they aren't destroyed, your dog becomes a "carrier" of heartworm when a mosquito decides to have a quick meal on it and starts the cycle all over again.

If your dog is infected by all male or all female heartworms, there won't be any microfilaria produced, but your dog will still have adult worms damaging its heart. While the Membrane Test, Knott's Test, and microscopic review of a hematocrit or whole blood MAY show evidence of microfilaria, it is hard to pick out the presence of heartworm. The tests involve drops of blood and microfilaria may not be in that portion tested. That's why when you have your animal checked for any type of parasite, the term isn't "negative" for whatever, but rather it is "not seen at this time." The most accurate test at 98% is the antigen test. It does not test specifically for microfilaria. It tests for a uterine protein from the female heartworm and can not detect male heartworms. Please note that all of these tests mean that, if positive, your dog ALREADY has adult heartworms.

It's possible to kill the infective microfilaria before they become adults through the use of heartworm preventatives and the specifics of each preventative should be discussed with your vet. To give you a headstart on the four main products, the following information is provided.

Diethylcarbamazine (Filaribits, Caricide, Nemacide) is an effective preventative that targets the L3 stage. L3 "lives" for three days before moulting into L4. These products, if available, are given daily. Adverse reactions are very rare with DEC unless the animal has an existing case of heartworm. It is very important to have the animal tested prior to beginning this drug. Filaribits are no longer available, but the basic chemical is still available in some markets.

Milbemycin oxime (Interceptor) and Ivermectin (Heartgard) are the other two main preventatives and they are designed to kill the L4 stage. They prevent the microfilaria from surviving to the adult stage which again is the stage that damages and eventually kills the dog through congestive heart failure if not treated. The L4 stage has a life span of *approximately* 45 days before it moults into L5. Why do we give the pill every 30 days then? It is not uncommon for people to forget and it's better to have some overlap time than to miss a cycle. Because there is also a slight risk that a dog with a high number of circulating microfilaria may exhibit a mild shock-like syndrome with either milbemycin or ivermectin, it's important to test before starting the monthly preventatives as well.

Revolution contains selamectin, but the principle to prevention is the same as milbemycin oxime and ivermectin.

If your dog is diagnosed with heartworm, the vet will decide what treatment to give by assigning it to a class. These are:
Class 1: Mild, asymptomatic.
Class 2: Moderate, anemic, heart enlargement (determined by x-rays), cough, intolerant to exercise
Class 3: Severe, Right side of the heart is in failure, constant fatigue and cough
Class 4: Extremely severe, surgery must be done to remove the worms from the heart rather than attempting any type of chemical treatment.

There is currently one approved and proven treatment to kill adult heartworms other than surgery to remove them. This is melarsomine (Immiticide). It is injected into the dog's back, two injections 24 hours apart, on either side of the spine in the case of Classes 1 and 2. With Class 3, one dose is injected and then a month later the two doses are given 24 hours apart. These injections are given between the 3rd and 5th vertebrae of the epaxial lumbar group *deep* into the muscle. Up to 30% of dogs will experience several days or more of pain. Less than 1% will have abcesses at the injection site or anaphylactic shock leading to death. In Class 3 cases where the dogs are hospitalized for treatment because of possible side effects, up to 10-20% may die. Death can occur with Class 1 and 2 if the dead worm breaks into a clot and goes into the lungs. In some cases, the vet may choose to forego treatment as possibly too hard on your dog, but elect to treat symptoms and use a strong dose of microfilaricide to keep it from becoming a "carrier." After that, the vet will likely prescribe a monthly to keep the microfilaria in check, but remember, it does nothing to the adult heartworms still present.

Preventatives kill what's already in the blood. That is, they are "retroactive," not proactive. Many people have the mistaken idea that heartworm kills the infective agent in the next 30 days (or 24 hours in the case of dailies) after giving the pill, so they stop giving the pill as soon as it gets cold -- instead of giving the last one *after* the freeze as directed by their vet. After all, it saves them money for next heartworm season if they already have a dose or two. BUT if there are L4s in the blood since the last dose, the heartworm then has the entire winter to grow before the next antigen test.

Treatment is very expensive and can run over $1000 depending on where you live. The first treatment may not always kill 100% of the adult worms either and a second and even a third treatment may have to be done.

Heartworm is definitely a case where "the prevention is far better than the cure."

 Country: USA  ~  Posts: 5  ~  Member Since: 10/09/2007  ~  Last Visit: 05/10/2012 Alert Moderator 
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