Page 223 - Small Animal Internal Medicine, 6th Edition

CHAPTER 10 Pulmonary Hypertension and Heartworm Disease 195

tract. Most are enzyme-linked immunosorbent assays rather than a migratory movement pattern. Nonconcentra-
(ELISAs), although immunochromatographic test methods tion tests include examination of a fresh wet blood smear or
VetBooks.ir are also used. These tests are generally specific and have adjacent to the buffy coat of a spun hematocrit tube.
Concentration tests are done using either a millipore filter
a good sensitivity. Positive results are generally obtained
when at least four (and usually fewer) female worms 7 to
niques lyse the red blood cells and fix any existing microfi-
8 months or older are present. Most HW Ag tests do not or the modified Knott’s centrifugation technique. Both tech-
detect infections less than 5 months old, and male worms lariae. The modified Knott test, which involves formalin
are not detected. A weak positive or ambiguous test result fixation, centrifugation, and staining with methylene blue, is
may be rechecked using a different test kit or repeated after preferred for measuring larval body size and differentiating
a short time with the same type of kit; microfilaria testing D. immitis from nonpathogenic filarial larvae such as Acan-
and thoracic radiographs can also increase or decrease index thocheilonema (formerly Dipetalonema) reconditum (Table
of suspicion for infection. Colorimetric intensity of an Ag 10.1). An occasional false-positive microfilaria test result
test is not a reliable estimate of worm count. A false-positive occurs in animals with microfilariae but no live adult HWs,
Ag test result can usually be traced to a technical error. either due to transplacental transmission of microfilaria to a
False-negative results may occur with a low worm burden, young puppy or due to recent adult female worm die-off after
immature female worms only, male unisex infection, or inac- producing microfilariae.
curate adherence to test kit instructions. False-negative Ag The macrocyclic lactone preventive drugs, administered
tests have also been reported in some dogs due to formation monthly, reduce and eliminate microfilaremia by impairing
of HW antigen-antibody complexes in the blood, effectively the reproductive function of female and possibly also male
preventing free Ag to react with the serologic test. Heating worms. Most dogs become amicrofilaremic by 6 to 8 months
the blood sample test tube to 104° C for 10 minutes before after initiating treatment with these drugs. Preventive drugs
Ag testing can separate Ag-Ab complexes and cause a previ- that kill microfilaria very rapidly may cause life-threatening
ously negative Ag result to turn positive. Heating the sample inflammatory reactions during microfilaria die-off. There-
is only recommended in cases where false-negative testing fore microfilaria testing was historically mandatory when
due to Ag-Ab complexation is suspected (e.g., a dog that is diethylcarbamazine (DEC) was routinely used as a HW pre-
HW Ag-negative but microfilaria-positive). Routine heating ventive and is also currently recommended if milbemycin is
of samples before Ag testing is not currently recommended. used (the most potent microfilaricide among the macrocyclic
For these reasons, the American Heartworm Society rec- lactones).
ommends that HW antigen test results be interpreted and
recorded as either “positive” or “no antigen detected” (NAD), Clinical Features
rather than “negative.” There is no specific age or breed predilection for HWD in
Microfilaria identification dogs. Although most affected dogs are between 4 and 8 years
The American Heartworm Society recommends that all
dogs screened with HW Ag testing be concurrently screened TABLE 10.1
for presence of circulating microfilaria. Microfilaria testing
can identify HW Ag-positive patients that are reservoirs of Morphologic Differentiation of Microfilaria
infection, can assess whether high numbers of microfilariae
are present before a monthly preventive drug is adminis- DIROFILARIA ACANTHOCHEILONEMA
tered, and can identify HW-positive dogs who have false SMEAR IMMITIS RECONDITUM
negative Ag testing (due to Ag-Ab complexation).
The vast majority (~90%) of HW-positive dogs that are Fresh Few to large Usually small numbers
numbers
not treated with a monthly preventative have circulating smear Undulate in one Move across field
microfilariae. The remaining so-called occult infections, in place
which there are no circulating microfilariae, can result from Stained Straight body Curved body
an immune response that destroys the microfilariae within smear* Straight tail Posterior extremity
the lung (true occult infection), unisex infection, sterile adult hook (“button hook”
HWs, or the presence of only immature worms (prepatent tail); inconsistent
infection). Low numbers of microfilariae and diurnal varia- finding
tions in the number of circulating microfilariae in peripheral Tapered head Blunt head
blood can also cause false-negative microfilaria test results. >295-325 µm <275-288 µm long
Occult (microfilaria-negative) infections can still cause long
severe disease. >6 µm wide <6 µm wide
Microfilaria concentration tests that use at least 1 mL of
blood are recommended for detecting circulating microfi- *Size criteria given for lysate prepared using the modified Knott’s
test (1 mL blood mixed with 9 mL of 2% formalin, then centrifuged
lariae. The nonconcentration tests are more likely to miss low for 5 minutes; sediment stained with methylene blue); microfilariae
numbers of microfilariae, although they do allow observa- tend to be smaller with lysate of filter tests. Width and morphology
tion of microfilarial motility. Dirofilaria have a stationary are the best discriminating factors.