Given this patient’s history, clinical presentation, eosinophilia (as noted in the complete blood count) and the worm (microfilaria) seen in the peripheral smear, the patient’s diagnosis is most likely filariasis. Filariasis is caused by thread-like nematodes that are transmitted by mosquitos. The diagnostic form for most of these nematodes is the microfilarial larval form of the parasite found in the peripheral blood or tissue depending on the worm. Adult worms are found in the lymphatics in filariasis.
The morphology of the microfilaria in the photos provided as part of the clinical presentation is characteristic of Brugia malayi. Clearly evident on the microfilaria seen in the photos is the presence of a well-defined sheath. The presence of a sheath on a suspected filarial worm will lead you to suspect of any one of the four pathogenic filarial parasites to infect humans with sheaths. The four filarial parasites immediately suspected are Brugia malayi, Brugia timori, Wuchereria bancrofti, and Loa loa.
There are four other filarial worms that can be found in humans and they include Mansonella ozzardi, Mansonella perstans, Mansonella streptocerca and Onchocerca volvulus. All of the latter have no sheath and only Onchocerca volvulus is of clinical significance as it is the cause of ocular filariasis or river blindness (name given because it is found in areas with fast moving rivers where -the vector- black flies thrive) and is a major cause of blindness in endemic areas of Africa where it is endemic . Mansonella ozzardi can produce symptoms such as moderate fever, headaches, skin itch, pulmonary symptoms, lymphadenitis etc, while Mansonella streptocerca and Mansonella perstans are many times asymptomatic.
With regards to Loa loa, the filarial worm is one of four sheathed microfilariae produced by the adult worms of clinical significance. But this filarial worm is best known for causing loaisis or African Eye Worm infections characterized by calabar swellings which are non-painful, itchy and usually located around joints. The migration of adult worms under the skin on the bridge of the nose or over the sclera of the eye (thus the name African Eye Worm) are another characteristic of this filarial worm. So based on the clinical presentation Loa loa would not be considered in the diagnosis.
The other three filarial nematodes with sheaths can be strongly suspected however. Brugia malayi, Brugia timori, and Wuchereria bancrofti are the cause of elephantiasis or swelling of extremities including the scrotum in men and breast in women. Some of the symptoms may be more prominent in one filarial worm over the other. For instance, scrotal and breast swelling is more prominent in infections with Wuchereria bancrofti where swelling can be debilitating and lead to fever, with lymphadenitis, lymphangitis, lymphedema, elephantiasis, and hydroceles. Wuchereria bancrofti infections can lead to skin thickening and complications can lead to secondary infections due to the compromising of skin tissue. These secondary infections can be fatal. All of these symptoms of swelling are due to the adult worms overwhelming the lymphatic system leading to the restriction of normal lymph flow. Brugia malayi and Brugia timori symptoms, though similar to Wuchereria bancrofti, are not as likely to progress to the same intensity of a Wuchereria bancrofti infections. Nevertheless there is limb swelling/elephantiasis, and thickening of the skin that can lead to a compromised skin and bacterial/fungal infections. This is especially true in infections that are chronic. Wuchereria bancrofti filarial infections are far the most common at 90%.
Differences in the four sheathed microfilariae consist of the following. The microfilaria of Brugia malayi morphologically have a round anterior and a pointed to slender posterior end. The row of nuclei seen at the end/tail end is noncontinuous and there are two terminal nuclei that are distinctly separated from the other nuclei in the tail. Nuclei within the microfilaria are described as crowded. Wuschereria bancrofti’s tail will contain no nuclei at all. Loa loa on the other hand will have nuclei that are a continuous row in the tail. Brugia malayi will also contain a cephalic space that has a ration of 2:1 which is very much apparent in the photo provided. Brugia timori has a sheath that tends to stain weakly – compared to B. malayi and the cephalic space is longer as well as having a longer single file of nuclei extending down tail.
The microfilariae of Brugia malayi measure 175-230 µm. But size is a difficult criteria to use in helping to distinguish the four as there is an overlapping of sizes and the difficulty in measuring microfilaria that take on a serpentine formation. The one exception of the sheathed microfilaria is Loa loa which tends not to readily take on the serpentine curved formation. Below is a table that lists some of the identifying characterisitics of each microfilaria as well their vectors.
Filarial Worms Identification Chart
Diethylcarbamazine (DEC) is the primary drug of choice in treating Brugia malayi and other filarial species causing lymphatic filariasis. This drug can effectively kill adult worms in the lymphatics and reduce the number of microfilariae in the blood. Other drugs that can be added are Albendazole and Ivermectin. The most interesting of drugs that can be added to the treatment is Doxycycline which is meant to target the bacteria Wolbachia which is crucial to the worms for survival and reproduction. Without the symbiotic presence of Wolbachia the worms can not reproduce.
I hope you enjoyed this clinical case study. Please subscribe to get alerts when posts come up. Soon to follow will be a crossword review of filarial infections so be on the look out!