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Heartworm Diagnostic Tests and Treatment Options
the VetMed Forum:
1) Is the microfilaria
test reliable for detecting microfilaria, that is for dogs not receiving
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
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?
Tate Allen DVM
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 (Knotts
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 thats 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
II. Cardiac disease.
III. Pulmonary parenchymal disease.
IV. Caval (or portalcaval) syndrome
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 drugs 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.
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.
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
Q & A - Heartworm Disease
Heartworm Links and Resources
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