In the natural host, the adult worm varies in size from a few centimeters to 35 centimeters in D. immitis.  The females are generally larger than the males, and females from different species are often indistinguishable.  However, in human infections, mature worms are rarely found.  The actual size of the parasite in human infection ranges from 100 to 600 micrometers in diameter.  Many of the subcutaneous Dirofilaria found in humans are characterized by the presence of morphologically distinct longitudinal ridges on the cuticle, which become visible under a light microscope (Gutierrez).

Morphology of distinct Dirofilaria species:

• In tissue sections from D. immitis, a cross section of the worm ranges from 140 to 300 micrometers in diameter.  The cuticle is smooth and lacks significant longitudinal ridges.  However, attenuated longitudinal ridges, called cuticular markings, can be detected under electron microscopy (Gutierrez).

Figure 1: Cross-section of immature Dirofilaria immitis. The thin smooth cuticle is characteristic
of D immitis. From emedicine.com: http://author.emedicine.com/ped/topic599.htm.

• In D. repens, the worm cross section has a diameter of 220 to 600 micrometers.  Each longitudinal ridge is separated from the others by a distance that is larger than the width of the actual ridge itself (Gutierrez).

Figure 2: Worm of species D. repens removed from its site of infection underneath the bulbar conjunctiva of
a 51-year-old Italian man (scale in cm).  Image from Atlas of Medical Parasitology: http://www.cdfound.to.it/html/dir2.htm.

Figure 3: Hematoxylin-Eosin stained longitudinal section of same D. repens worm as in Figure 2, showing thick
multilayered cuticle and muscle cell layer.  Image from Atlas of Medical Parasitology: http://www.cdfound.to.it/html/dir2.htm.

Figure 4: Hematoxylin-Eosin stained longitudinal section of same D. repens worm as in Figure 2, showing the
organs in the pseudocoelon.  Image from Atlas of Medical Parasitology: http://www.cdfound.to.it/html/dir2.htm.

Figure 5: Transverse section through a mature female Dirofilaria repens removed from the
 superomedial orbital rim of a 67-year-old man.  Characteristic features are the arrangement of the longitudinal
 muscles and the multilayered cuticle, which is expanded in the region of the large lateral chords.
From emedicine.com: http://author.emedicine.com/ped/topic599.htm.

Figure 6: Higher magnification of the same Dirofilaria repens specimen as in Figure 5.  The longitudinal ridges on the
cuticle, 6-7 mm wide, spaced at 11-12 mm intervals, are characteristic of D repens.
From emedicine.com: http://author.emedicine.com/ped/topic599.htm.

• In tissue sections from D. tenuis, the cross section diameter ranges from 150 to 330 micrometers.  The ridges are low and rounded, characterized by a wavy, broken,and branching pattern.  The spaces between the ridges are narrower than the actual ridges themselves (Gutierrez).
• In D. ursi and D. subdermata, the worm cross section is generally less than 200 micrometers in diameter, and the longitudinal ridges are tall, sharply crested, narrow, and separated by a distance that is three to four times the width of the actual ridges themselves (Gutierrez).
• In D. striata and D. lutrae, cuticle longitudinal ridges are irregular and poorly developed, making these two species morphologically indistinguishable from D. immitis (Gutierrez).