Diagnosis

    Since dirofilariasis infection is often asymptomatic, diagnosis most frequently occurs serendipitously during routine chest x-ray or physical examination.  Upon x-ray, the nodule is often misdiagnosed as a carcinoma.  In such cases, only after surgical removal of the nodule, is the nematode pathogen identified.  Upon removal of the worm, microscopic evaluation of tissue cross-sections and study of macroscopic characteristics of the worm serve as the main methods of dirofilariasis diagnosis.  Another important diagnostic factor is the geographic area where the infection is believed to have been acquired.  Even though dirofilariasis infection is rare, much of the medical community is in agreement that extrapulmonary dirofilariasis should be considered in the differential diagnosis of nodular inflammatory infection, and pulmonary dirofilariasis should be considered in the differential diagnosis of solitary pulmonary nodules.
    Human dirofilariasis infection, especially human pulmonary dirofilariasis, is often a benign and self-limiting condition that does not necessitate immediate treatment.  However, it is often confused with life-threatening diseases, (such as tumors, tuberculosis, fungal infections, and hamartomas), and invasive surgical procedures are generally required in order to arrive at a definitive diagnosis (Muro, et al).  An additional problem is that misdiagnosis occasionally leads to inappropriate and bodily harmful therapeutic interventions (Vakalis, et al).  For example, this may occur if dirofilariasis infection in the breast were mistaken for breast cancer and chemtherapy were prescribed to treat the cancer.

Figure 1: Chest radiograph revealing a solitary, noncalcified, left upper lobe pulmonary nodule.  From the x-ray alone,
it is not apparent that the nodule is caused by dirofilariasis infection.  Image from April, 2001 volume of Chest.

    Another incentive for the development of a diagnostic technique that does not rely on analysis of the actual worm specimen is the fact that diagnosis of specific Dirofilaria species based on worm morphology can be complicated by by tissue degeneration and poor specimen preparation.  For example, the longitudinal ridges of the cuticles of some Dirofilaria can be useful for morphological diagnosis, but they can also be a source of misdiagnosis.  These ridges are well defined in D. repens, D. tenuis, D. ursi, and D. subdermata, but they are absent in D. immitis and poorly developed in D. striata and D. lutrae.  Due to the long incubation periods of many dirofilariasis infections, the dead worm has much time to undergo morphological degeneration within the nodule.  The lysis of the cuticle of the dead worm by eosinophils causes the worm to have a scalloped appearance that can be mistaken for ridges in cross sectioned specimens (Gutierrez).  Thus, if morphological degeneration has occurred, it is easy to confuse one Dirofilaria species for another.
    It would be beneficial to develop noninvasive, reliable diagnostic tests for dirofilariasis infection so that misdiagnosis could be prevented, infected individuals could be informed of the nature of the infection before having to go through the invasive process of having the worm surgically removed, and accurate diagnosis of the specific Dirofilaria species for the purpose of epidemiological studies could consistently and easily be achieved.

Figure 2: Low power view of bulbar conjunctival biopsy of a 72-year-old Australian man,
 showing degenerate pieces of an immature Dirofilaria immitis worm in cross- and
 longitudinal-sections.  From emedicine.com: http://author.emedicine.com/ped/topic599.htm.
 


Diagnostic Methods

Figure 4: Histopathologic D. immitisspecimen obtained by wedge resection.
Image from April, 2001 volume of Chest.
 

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