Bench Aids for the diagnosis of filarial infections

Overview

The microfilaria

At the light-microscopic level and with the aid of a variety of stains, a microfilaria appears Fig. 1 Typical microfilaria as a primitive organism, serpentine in shape and filled with the nuclei of many cells. Figure 1 is a diagram of a typical microfilaria. In many, but not all, species, the body may be enveloped in a membrane called a sheath. Where a sheath is present it may extend a short or long distance beyond either extremity of the microfilaria. In some species, depending on the stain used, the sheath displays a characteristic staining quality which aids in species identification. The nuclei of the cells that fill the body are usually darkly stained and may be crowded together or dispersed. The anterior extremity is typically devoid of nuclei and is called the cephalic or head space; it may be short or long. Along the body of the microfilaria there are additional spaces and cells that serve as anatomical landmarks. These include the nerve ring (nr), excretory pore , excretory cell , and anal pore. In some species, an amorphous mass called the innerbody and four small cells called the rectal cells can be seen, usually with the aid of special stains. These structures and their positions are sometimes useful for species identification. The shape of the tail and the presence or absence and distribution of nuclei within it are also important in species identification.

Periodidty

Some species of microfilariae circulate in peripheral blood at all hours of the day and night, while others are present only during certain periods. The fluctuation in numbers of microfilariae present in peripheral blood during a 24-hour period is referred to as periodiCity (Fig. 2). Species that are found in the blood during night-time hours but are absent at other times are designated nocturnally periodic (e.g . Wuchereria bancrafti, Brugia malayi); those that are present only during certain daytime hours are designated diurnally periodic (e.g. Loa loa). Microfilariae that are normally present in the blood at all hours but whose density increases significantly during either the night or the day are referred to as subperiodic. Microfilariae that circulate in the blood throughout a 24-hour period without significant changes in their numbers are referred to as nonperiodic or aperiodic (e.g. Mansonella spp.).

The periodicity of a given species or geographical variant is especially useful in determining the best time of day to collect blood samples for examination. To determine microfilarial periodicity in an individual, it is necessary to examine measured quantities of peripheral blood collected at consecutive intervals of 2 or 4 hours over a period of 24-30 hours.