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→Laboratory Tests
In both dogs and cats, routine haematology, biochemistry and urinalysis should be performed. Most parameters are usually within normal limits, but an anaemia can often be seen. Eosinophilia and basophilia are also common, and when seen together suggests occult dirofilariasis or allergic lung disease <sup>1, 3</sup>. Eosinophilia peaks as L5 enter the pulmonary arteries and subsequently varies. An inflammatory leukogram is also possible<sup>3</sup>. Hyperglobulinemia due to antigenic stimulation is an inconsistent finding<sup>1, 3</sup>. Right-sided heart failure or immune-complex glomerulonephritis can lead to hypoalbuminaemia and, very occasionally, nephrotic syndrome<sup>1</sup>. Because of this, it is possible for urinalysis to reveal proteiunuria<sup>1, 3</sup>. Haemoglobinaemia and haemoglobinuria are associated with caval syndrome<sup>3</sup>.
In both dogs and cats, routine haematology, biochemistry and urinalysis should be performed. Most parameters are usually within normal limits, but an anaemia can often be seen. Eosinophilia and basophilia are also common, and when seen together suggests occult dirofilariasis or allergic lung disease <sup>1, 3</sup>. Eosinophilia peaks as L5 enter the pulmonary arteries and subsequently varies. An inflammatory leukogram is also possible<sup>3</sup>. Hyperglobulinemia due to antigenic stimulation is an inconsistent finding<sup>1, 3</sup>. Right-sided heart failure or immune-complex glomerulonephritis can lead to hypoalbuminaemia and, very occasionally, nephrotic syndrome<sup>1</sup>. Because of this, it is possible for urinalysis to reveal proteiunuria<sup>1, 3</sup>. Haemoglobinaemia and haemoglobinuria are associated with caval syndrome<sup>3</sup>.
Direct microscopic examination
If present, microfilariae can be easily identified on direct
microscopic examination of a drop of fresh blood
because they can vigorously move the surrounding red
blood cells. However, although this method offers an
easy and inexpensive diagnosis, it is not sufficiently sensitive,
especially when there is a low concentration of
microfilariae in the bloodstream. Filtration methods (eg,
Difil-test; EVSCO Pharmaceuticals, Buena, New Jersey,USA) and the modified Knott's test (haemolysis, centrifugation
and staining with methylene blue) are more
sensitive and allow morphological examination of the
microfilariae. In cats, given the absence or short-term
presence of circulating microfilariae, these tests have
little practical value.
Antigen tests
ELISA antigen tests detect specific circulating proteins
released by the reproductive tract of mature female
worms. These are available as either in-house or laboratory
tests and their sensitivity and specificity approach
100 per cent (see top table on page 354). The antigen
levels become undetectable eight to 12 weeks after adulticidal
therapy and this should be taken into account
when re-screening for heartworm disease or evaluating
the response to treatment. Small worm burdens, the
presence of immature females or male-only infections
are common causes of low antigen titres and false
negative results, especially in cats, where these circumstances
occur more frequently. In dogs, specific agglutination and immunochromatography techniques are also
available. An antigen test performed too soon after infection
(first six to seven months) may yield false negative
results because of the low antigen levels in the circulating
blood.
Antibody tests
Antibody tests are currently available for routine screening
of feline heartworm infection, either as in-house or
laboratory tests (see table below). The presence of antibodies
provides information about previous exposure,
but not necessarily current infection. Consequently, antibody
tests are more useful to help rule out infection
rather than confirm it. These tests are no longer used in
dogs given their low specificity and the availability of
highly reliable antigen tests.
PCR-based tests
Polymerase chain reaction (PCR)-based tests offer a very
sensitive and specific diagnostic tool for routine identification
of mature and immature adult worms, especially
in unconventional hosts. However, at present, these tests
are not widely available.
he antigen detection test is the preferred diagnostic method for asymptomatic dogs or when seeking verification of a suspected HW infection. This is the most sensitive diagnostic method available to veterinary practitioners. Even in areas where the prevalence of HW infection is high, ~20% of infected dogs may not be microfilaremic. Also, monthly macrolide prophylaxis induces embryo stasis in female dirofilariae.
he antigen detection test is the preferred diagnostic method for asymptomatic dogs or when seeking verification of a suspected HW infection. This is the most sensitive diagnostic method available to veterinary practitioners. Even in areas where the prevalence of HW infection is high, ~20% of infected dogs may not be microfilaremic. Also, monthly macrolide prophylaxis induces embryo stasis in female dirofilariae.