Answer to Case 509

Answer: Dirofilaria sp.; most likely D. repens based on the presentation, geographic exposure, and morphology of the microfilariae in blood.

Idzi and his group recently published this case in EID; you can read about all of the important diagnostic details such as the length of the adult worm, its cuticle, and the morphology of the microfilariae HERE.

Here a just a few important points from this case:

1. Not all round worms removed from the eye are Loa loa. While loiasis is the ocular roundworm infection that most physicians are familiar with, human dirofilariasis is an emerging infection that should be considered whenever a patient presents with a macroscopic worm moving across the cornea. Unlike L. loa which has a limited geographic distribution (West and Central Africa), Dirofilaria repens is widely distributed throughout parts of Africa, Asia, and Europe, including temperate climates. HERE is a great article outlining the distribution of vector-borne diseases in Africa; it has nice maps showing that loiasis is NOT in Senegal where this patient was presumably exposed.

2. The adult worm of Dirofilaria can be easily differentiated from Loa loa by examining its cuticle. Dirofilaria has longitudinal ridges, which are particularly prominent in this case, while Loa loa does not. To me, the longitudinal ridges look like the bark of a tree (awesome photo Idzi!):

In comparison, Loa loa has irregularly-spaced cuticular bosses or 'bumps' that are located along along the long axis of the worm. Here is a good example of these bosses from Parasite Case of the Week 482:

3. Microfilariae are only rarely seen in human Dirofilaria infections - making this case particularly cool! Unlike Loa loa, the microfilariae of D. repens do not have a sheath, and the nuclei do not go all the way to the tip of the tail. Thus the features of the microfilariae can be used to support the diagnosis in this case. 

Thank you for all of the great comments on this challenging case!