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Heartworm Diagnostic Tests and Treatment Options
by Tate Allen DVM

From the VetMed Forum:

1) Is the microfilaria test reliable for detecting microfilaria, that is for dogs not receiving heartworm preventives?
how accurate is it?

2) At what number of worms, how many approximately is considered burdensome for a dog to live with?

3) Does the age of a dog affect the treatment Immiticide's success?
supposing a dog is quite fit and healthy but in his senior yrs.

4) What is the newest best kind of antigen test out in the market?

5) Supposing a dog with a slight 'acute' case of infection say, with only approximately 10 to 20 worms in his system, would this affect or be more favorable towards a successful Immiticide treatment with a risk of say nil to very slight?

6) My understanding is that Immiticide treatment does not damage kidney or livers and a dog can be as well as it was before the treatment, I read this from a website somewhere. The only risk is pulmonary embolism. However, it is necessary that the kidneys and liver are okay before treatment so that they can metabolize and breakdown the arsenic in the drug. (Although I read of some cases where dogs with liver and kidneys not at the peak of condition, made it through cause Immiticide is much milder and not hazardous) is this correct?

- CAROLINE1110

Tate Allen DVM responds:

1. There are three tests used in detection of microfilariae (MF) of Dirofilaria immitis. A fresh drop of blood examined under the scope is the least accurate as a screening test. Concentration test techniques (Knott’s test, filter concentration tests) are a better screening test. The answer to your question was about reliability of these tests, especially for non-Heartworm preventative treated dogs is that they are reliable, within the limitations of the tests. That sound a bit confusing. If you get a “positive” test result, then the test has been both sensitive and specific. (False positives can occur as a result of contaminated filter holders or persistent MF after the death of adult heartworms). However, a negative result does not say there is no infection. It specifically says no evidence of HW infection was seen and that’s all it means.

Another kicker is the part about Heartworm preventative medication. It depends upon which type the dog has been on. If a “monthly” type has been used, the test for MF will be negative in most cases. Your question about the accuracy of test is not easily answered, at least in short format. I think to say that the tests are accurate within the bounds and limitations of the techniques is correct.

2. Worm numbers and being a burden to the dog question: Actually this question does not have a real answer because the presence of the worms is not “Heartworm Disease” (HWD). The pathophysiology of HWD is basically divided into 5 areas.

I. Pulmonary vascular disease.
II. Cardiac disease.
III. Pulmonary parenchymal disease.
IV. Caval (or portalcaval) syndrome
V. Glomerulonephritis

The first 4 are all a result of the presence of adult HWs while the 5th is a result of the MF damage and collecting of immune complexes. The simple answer is that (usually) the more adult worms the more severe the signs will be and the faster they can develop.

3. Does age of dog affect success of treatment with Immiticide?. Simply put, NO. The drug’s effects are on the parasite, not the host. About the only down side I have seen with it is the pain that usually develops at the injection sites.

4. “Newest best antigen test”? Once again you have asked a question that can not be answered simply. Part of answer will depend on your meaning of “best test”. I assume that you are referring to sensitivity and specificity. To me it would include those but will also include other factors: How many tests are being done? When are they done? How much technician time is needed for test? How difficult is the test to do? Repeatability?? Simply put, I think the majority of the ELISA tests used are pretty much equal.

5. Number of worms and success of treatment: I am afraid that this question really is not a clear one. ‘Acute’ means sudden onset (opposed to one that has been there a while -> chronic). ‘Slight’ to me means minimal. However, 10 to 20 worms, is not a ‘light’ load of parasites. To speak plainly, the outcome of the treatment will be based on the pretreatment patient evaluation. Chest x-rays, selective labs (urinalysis, and RBC count are the recommended labs to be done). A thorough physical goes without mentioning. Ancillary tests such as ECG, echocardiograms and angiography may be useful. The patients are divided into classes (I,II,III). It is this process that will be best predictor of success and potential problem that could occur.

6) Immiticide is a more efficacious drug than Caparsolate (thiacetarsamide) when considering killing adult worms (especially, females). Also the risk of liver and/or kidney problems are much less with it.

What I have said above is based on texts, reports and clinical trials.

The following is based on my personal experience. Since Immiticide being introduced, it is the only drug I have used to treat HW infected dogs. It was noted that the dogs initially had what appeared to be a bit more difficulty in first week or two post treatment. By this I mean they were listless, occasionally ran fevers, decreased appetite, coughing and in general felt bad. As a result of these observations, I routinely tell clients that the divided treatment protocol is the preferred way to go if safety is the main factor. Unfortunately, the money involved is often a factor and I have treated several dogs in single treatment protocol. I have had no deaths in these patients although a few have sacred me a bit. So, I may be being overly cautious but if I am to make a mistake, I would rather it be on the more conservative side.

Tate Allen, DVM

Related Reading
View entire discussion thread - "A few questions on heartworm"
Veterinary Q & A - Heartworm Disease
Heartworm Links and Resources

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