ANSWERS TO SELF-ASSESSMENT QUESTIONS
What is the vector for transmission of the Loa loa parasite?
a. Mosquitoes
b. Biting midges
c. Black flies
d. Deer flies
Answer: d. The principal vectors for Loa loa filariasis are infected blood-sucking day-biting flies from two species of the genus Chrysops (deer fly), C. silaceus and C. dimidiata, which are found in the tropical rainforest regions of Central and western Africa.
Loa loa exhibits diurnal periodicity. What is the optimum time for blood collection?
a. 10:00 to 14:00
b. Any time
c. 22:00 to 02:00
d. 12:00 to 18:00
Answer: a. Loa loa exhibits diurnal periodicity with microfilaria present in peripheral blood in large numbers during day hours. The optimal time for blood collection is 10:00 to 14:00. Wuchereria bancrofti, Brugia malayi, and Brugia timori exhibit nocturnal periodicity with microfilarial levels that tend to peak at night, and the optimal time for blood collection is 22:00 to 02:00. Mansonella perstans and Mansonella ozzardi do not exhibit periodicity, and specimens can be collected at any time.
Which of the following morphologic features is consistent with Loa loa microfilariae?
a. Less than 200 μm in length
b. Nuclei extend irregularly to the tip of the tail
c. Sheath usually stains bright pink with Giemsa
d. Long headspace
Answer: b. Loa loa microfilariae are large (231 to 250 μm in length) on stained blood films with a short headspace and nuclei that extend irregularly to the tip of the tail. The microfilariae have a sheath that is not always visible, because it does not usually stain bright pink with Giemsa but can be visualized with hematoxylin stains. Of note is that sheaths may be shed if there is a delay in processing the blood specimen.
TAKE-HOME POINTS
Loa loa is a filarial nematode that is endemic in the forested areas of Central and western Africa and is transmitted by the bite of an infected Chrysops fly.
Characteristic clinical manifestations include Calabar swellings and subconjunctival migration of the adult worm. Many individuals are asymptomatic.
The microfilariae circulate in the bloodstream with diurnal periodicity that corresponds to the feeding pattern of the Chrysops fly, which typically bites during the day. Optimal specimens for blood parasite examination are collected between 10 am and 2 pm.
Morphologic identification is based on size, presence and presentation of the sheath, length of headspace, and arrangement of tail nuclei.
Treatment is with diethylcarbamazine (DEC), which is available through the Centers for Disease Control and Prevention Drug Service for patients who meet eligibility criteria in the investigational new drug protocol. Because of the risk for severe adverse events, including fatal encephalopathy or blindness in patients with high microfilarial loads (>8,000 microfilaria/ml) and/or concomitant onchocerciasis, it is important to determine the microfilarial load per milliliter and rule out onchocerciasis using serologic testing, skin snips, or histopathology.
See https://doi.org/10.1128/JCM.01587-20 in this issue for case presentation and discussion.
- Copyright © 2020 American Society for Microbiology.
