Answer to Case 804

Answer to the Parasite Case of the Week 804: Brugia sp. microfilaria, favor B. malayi 

As noted by Idzi and others, this is a textbook example of Brugia sp. microfilaria, with beautiful terminal and sub-terminal nuclei and a characteristic long cephalic space. However, Idzi rightly recommends caution in committing to a species since B. malayi and B. timori can have a very similar appearance. He notes that "to decide which one, either geographical information is needed (B. timori is more restricted to the Lesser Sunda Islands of the Indonesian archipelago - from Bali over Timor to the Tanimbar Islands - while B. malayi is more widespread in South-East Asia), or information about the stain for the blood film (both Brugia species have a sheath in fact, but if the smear is stained with Giemsa, the sheath of B. timori is usually not visible).  

While we don't have an exposure history in this case, the stain is Giemsa, which points us to B. malayi. However, I consider the sheath staining to be a soft diagnostic feature, as I've seen this type of faint pink staining with Wuchereria, and not all B. malayi sheaths stain with Giemsa. 

Since the sheath staining is not reliable, we must use the other criteria for differentiating B. malayi and B. timori as follows: 

• B. malayi microfilariae are 175-230 micrometers long in stained blood smears, while B. timori microfilariae are ~310 micrometers. The current case appears to be in the former range, but it is a bit hard to tell from the image.
• B. timori microfilariae have a longer cephalic space and a more single file nuclei in the tail.
• Classically, the sheath of B. malayi microfilariae is deep pink on Giemsa-stained blood films, while the sheath of B. timori does not stain with Giemsa.

Given these features, I think we can make a tentative diagnosis of B. malayi, and try to obtain more clinical history.

Check out Idzi's previous beautiful cases of B. malayi and B. timori on this blog. You can find several more B. malayi cases by searching in the upper left-hand search bar.

The CDC DPDX also has some nice examples of B. malayi and B. timori.

Thanks again to Florida Fan for donating this beautiful case.