Morphology

    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:

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.


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.


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