Adolescent girl presenting with solitary pulmonary nodule. (Pathologic Quiz Case).

A 15-year-old white girl was found to have a solitary pulmonary nodule by a chest radiograph performed following blunt trauma to the chest. The young woman was a resident of eastern Texas and had visited Kansas and Oklahoma but had never traveled outside the United States. She had a history of contact with both dogs and cats; however, she did not recall being bitten or scratched. She admitted to cigarette smoking (1 pack per day for 3 years), smoking crack cocaine, and being sexually active. Family history was noncontributory. There was no history of exposure to tuberculosis. The patient reported a history of subjective fevers without chills and unintentional weight loss of 8 to 10 pounds (4.1 kg) over the previous 2 to 3 weeks.

On physical examination, body temperature was within normal limits. Decreased breath sounds and crackles were auscultated over the right lower lobe. The results of the remainder of the physical examination were unremarkable. A purified tuberculin derivative test was nonreactive. Hematologic and serum chemistry studies were within normal limits. A thorascopic excisional biopsy of the nodule was performed.

The specimen received in surgical pathology consisted of a wedge resection of lung containing a firm, well-circumscribed nodule measuring 1.5 cm in widest dimension and located 0.3 cm from the pleural surface. On cut section, the nodule appeared granular and tan-yellow in color There was softening of the central portion of the nodule. The surrounding lung was unremarkable. Microscopic examination of the nodule revealed an oval-shaped, well-circumscribed pulmonary infarct (Figure 1, A) containing several sections of thrombosed pulmonary artery best illustrated by Movat stain (Figure 1, C). Within a few sections of artery were the structures illustrated in Figure 1, B and C.

[FIGURE 1 OMITTED]

What is your diagnosis?

Pathologic Diagnosis: Dirofilaria-Induced Pulmonary Infarct

Hematoxylin-eosin-stained and Movat-stained sections demonstrated a rounded infarct composed of coagulative necrosis that was well demarcated from the surrounding intact lung parenchyma by a zone of epithelioid histiocytes and fibrous connective tissue. Focal calcifications were seen within both the infarct and the fibrous connective tissue, and some lymphoid aggregates were present at the intersection between the normal lung and the lesion (Figure 1, A). Within a few sections of the pulmonary artery, a nematode typical of Dirofiliaria immitis can be seen in the cross section in Figure 1, B. The worm is necrotic, and the internal structures are not readily visible; however, the characteristic homogeneous cuticle that lacks external ridges, the longitudinal muscle layer just internal to the cuticle, and the presence of internal cuticular ridges (Figure 1, C, arrow) can be identified.

Dashiell (1) reported the first documented pulmonary infarction associated with Dirofilaria in 1961, and Goodman and Gore (2) reported the first report of pulmonary infarct associated with human dirofilariasis in 1964. Humans are an aberrant host for D immitis. In the lung, worms are typically immature, and microfilaria production has not been reported in humans. (3) Typically, the parasite becomes entrapped in small branches of a pulmonary artery, and an asymptomatic infarct follows. The infection becomes apparent incidentally, after a chest radiograph has been performed for other reasons. Most cases are asymptomatic. (3) Resection is usually performed in order to exclude neoplasia. (4)

The immature worms found in human pulmonary dirofilariasis are characterized by diameters ranging from 100 to 350 [micro]m, presence of a thick laminated cuticle, inwardly projecting lateral cuticular chords, and abundant somatic muscle. (3) The outer cuticle of D immitis is smooth; however, other species, such as Dirofilaria tenuis, have external cuticular ridges. (3,4) Often, the worm is highly degenerated, and internal structures, such as the gastrointestinal and genital tubes, may not be readily discernible.

The predominant definitive host for D immitis is a canine, but other mammals, such as cats, foxes, and muskrats, may be infected. (3) The adult worms are typically found within the right ventricle of the heart, and microfilariae circulate within the bloodstream. The vector intermediate host is one of a variety of mosquitoes, including, but not limited to, Aedes, Culex, Anopheles, and Culicidae species. (5) The vector acquires the circulating microfilaria during ingestion of a blood meal from the definitive host. Infective larvae develop in the mosquito salivary glands within about 2 weeks and are injected into the host when the vectors feed. (4)

Diagnosis of pulmonary dirofilariasis has rarely been made prior to surgical resection. (4) Image-guided needle biopsies have not usually provided diagnostic material. (4) Diagnosis is most commonly made after surgical resection. The differential diagnosis of the case presented here was confounded by the patient's young age (previously reported dirofilarial infarcts were exclusively in adults) and the history of a high-risk lifestyle and weight loss, information that proved extraneous.

References

(1.) Dashiell GF. A case of dirofilariasis involving the lung. Am J Trop Med Hyg. 1961;10:37-38.

(2.) Goodman ML, Gore I. Pulmonary infarct secondary to Dirofilaria larva. Arch Intern Med. 1964;113:702-705.

(3.) Dayal Y, Neafie RC. Human pulmonary dirofilariasis: a case report and review of the literature. Am Rev Respir Dis. 1975;112:437-443.

(4.) Ro JY, Tsakalakis PJ, White VA, et al. Pulmonary dirofilariasis: the great imitator of primary or metastatic lung tumor. A clinicopathologic analysis of seven cases and a review of the literature. Hum Pathol. 1989;20:69-76.

(5.) Bailey TS, Sohrabi A, Roberts SS. Pulmonary coin lesions caused by Dirofilaria immitis. J Surg Oncol. 1990;44:268-272.

Accepted for publication June 26, 2001.

From the Department of Pathology, University of Texas Medical Branch, Galveston, Tex.

Reprints not available from the author.