False Paragonimus Eggs in Sputum Cytology.

Due to passage through the oral cavity, a sputum sample may be contaminated by saliva or other elements, such as food particles. The presence of different types of contaminants in sputum smears, including pollen grains, fern spores, diatoms and others, which mimic parasitic eggs, has been occasionally documented in the medical literature.[1]

A 52-year-old male, heavy smoker (two packs a day) arrived at the emergency room with chest pain, a productive cough, and intermittent episodes of haemoptysis over the last 3 months. His body temperature was normal and wheezing was evident on auscultation. The rest of the clinical exam was unremarkable. Chest x-ray, sputum cytology, blood test, and mycobacterial culture were initially carried out. During the course of the exam, the patient reported traveling to Southeast Asia 1 year prior.

The chest x-ray led to the initial diagnosis of bronchiectasis, which was later supported by a thoracic CT, showing numerous dilated and thick-walled airways filled with mucus in many zones of both lungs.

The sputum smear, stained using the Papanicolaou method, showed the presence of two wide structures [Figure 1a], that were ovoid in shape and measured approximately 85 × 50 μm, with a greenish/bluish hue. One of the ends was rounded but the other seemed flattened (black arrow), similar to an opening (operculum-like). Both structures were delineated by a thin, transparent capsule. Some striations were observed under the surface.

Figure 1

(a) Sputum cytology. Starch granules (Papanicolaou stain, ×400). (b): Starch granules under polarized light. (c) Sputum cytology. P. westermani egg (Papanicolaou stain, ×400). (d) P. westermani egg under polarized light

In addition, some areas of the sputum smear showed squamous metaplasia and many Globet cells. No atypical/malignant cells were observed.

Among the different diagnostic possibilities, the presence of contaminants was taken into account, but because of the patient's clinical records, a Paragonimus pulmonary infection with the presence of worm eggs in the sputum smear was not ruled out. Nevertheless, examination of these structures under polarized light, showed a typical Maltese cross appearance [Figure 1b], which is characteristic of starch granules, such as those in the talc grains from medical glove powder.[2]

On the other hand, the results of the blood test were within normal ranges, including the eosinophil count, and the mycobacteria culture was negative.

In view of these findings, the final diagnosis was bronchiectasis in a heavy smoker patient, and a pulmonary parasitic infection due to the trematode Paragonimus westermani was ruled out.

Nevertheless, it is necessary to highlight key morphological features in order to differentiate between true P. westermani eggs and starch granules.

The eggs of P. westermani are golden brown, oval in shape with a flat operculum at one end (black arrow), approximately 45-70 × 80-120 μm in size, and have a thick birefringent shell [Figure 1c]. Under polarized light the shell is only noticed [Figure 1d].

Starch granules observed in sputum smears, and stained by the Papanicolaou method, are ovoid in shape, vary greatly in size, and have a thin cell wall. They may appear as translucent/refractive structures or with different colours. Under polarized light, a Maltese cross appearance is very characteristic.

Pulmonary paragonimiasis is a food-borne zoonotic disease caused by a trematode of the genus Paragonimus. About 48 species of Paragonimus have been described. Nevertheless, only 16 of these can infect humans, P. westermani being the most common. They can cause pulmonary and pleural inflammation following the ingestion of raw freshwater crabs or crayfish infected with metacercariae, the infective stage of the worm.[3] This infection is commonly observed in Asia, in several countries such as China, Japan, Korea, Taiwan, the Philippines, Indonesia, and India. It has been also reported in South America and Africa.

Although this lung infection is rare in developed countries, some cases have been reported after the consumption of raw crayfish from rivers.[4]

The principal manifestations of pulmonary paragonimiasis are persistent cough, fever, and haemoptysis. This infection may also be misinterpreted as lung cancer or pulmonary tuberculosis.[56]

In the life cycle of this trematode, the eggs are not yet mature when laid, therefore no embryonic structures are observable inside. Unembryonated eggs may be expelled with the sputum or may be swallowed and eliminated through the faeces.

Therefore, we think that this finding should be taken into account during microscopic observation, as a correct diagnosis is important. It will prevent the patient from going through more aggressive procedures, such as a biopsy, unnecessary therapeutics, and complicated diagnostic analysis.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.