L

(V.) braziliensis

Central and South America

Adapted from Lainson and Shaw (1987); Grimaldi et ai. (1989); Desjeux (1991); and Pearson and Sousa (1996).

network of several thousand mini-circles and 25-50 maxi-circles. The surface membrane overlays a series of closely arrayed microtubules, much like the fabric of an umbrella over its struts.

Vectors

Blood-sucking, female sand flies (Diptera: Psychodidae) (Lewis and Ward, 1987) are the vectors. Lutzomyia spp. are responsible for

transmitting Leishmania in the Americas and Phlebotomus spp. elsewhere in the world. Sand flies are relatively weak fliers. They reside in vegetation in forested areas, in rodent borrows in arid regions, and in debris around houses in sites where peridomestic transmission is observed.

Life-cycle

The female sand fly probes with its proboscis, lacerating capillaries to form a small pool of blood, from which it feeds. It becomes infected when it consumes blood containing amastigote-infected macrophages. Amastigotes convert to promastigotes in the gut of the sand fly at ambient temperatures. Promastigotes multiply and differentiate through a series of intermediate stages, including procyclic, nectomonad and haptomonad forms, before emerging as infectious metacyclic promastigotes, which measure approximately 10-20 ^ in length and 1.5-3.0 ^

in diameter. A single flagellum, which is often longer than the body, extends from the flagellar pocket at the anterior pole and pulls the parasite forward. Development from amastigotes to metacyclic promastigotes takes 1-2 weeks, depending on the Leishmania sp.

When mature, metacyclic promastigotes migrate to the proboscis of the sand fly. They interfere with its ability to take the next blood meal. Although not thoroughly quantified, it is likely that only a small number of promastigotes are deposited in the skin. The saliva of sand flies contains factor(s) that enhance their infectivity for macrophages by inhibiting the L-arginine-dependent nitric oxide killing mechanism of macrophages (Theodos et al., 1991; Hall and Titus, 1995).

Cultivation

Promastigotes can be grown in vitro in a number of media including NNN (Novy, McNeal,

Fig. 13.2 Transmission electron micrograph of L. (L.) donovani amastigotes within a human monocyte-derived macrophage infected in vitro. The nucleus (N), kinetoplast (K) and flagellum within the flagellar pocket (F) are easily seen. Subpellicular microtubules lie under the outer membrane. Dividing amastigotes are present. From Pearson et al. (1983), by permission of the University of Chicago Press

Fig. 13.2 Transmission electron micrograph of L. (L.) donovani amastigotes within a human monocyte-derived macrophage infected in vitro. The nucleus (N), kinetoplast (K) and flagellum within the flagellar pocket (F) are easily seen. Subpellicular microtubules lie under the outer membrane. Dividing amastigotes are present. From Pearson et al. (1983), by permission of the University of Chicago Press

Nicolle) media, Schneider's insect media and others, to which fetal calf serum is added, at 22-26°C. Cultured promastigotes vary in size and shape; some are stumpy, while others are thin and elongated. As the culture enters stationary phase, the number of infectious, metacyclic promastigotes increases. They have distinct lectin-binding characteristics (Sacks et al., 1985; Sacks and Perkins, 1984). Rosettes or clumps of promastigotes are observed in older cultures as well as in the gut of sand flies. Their significance is uncertain.

are inoculated into the skin (Ridley, 1979; Ridley and Ridley, 1984). Although the precise sequence of events that follows in humans has not been defined, histopathological studies in hamsters inoculated subcutaneously with cultured L. (L.) donovani promastigotes suggest that some are killed by neutrophils, while others are phago-cytized by mononuclear phagocytes, convert to amastigotes within them, and then multiply (Wilson et al., 1987). Additional monocytes are subsequently recruited to the site and become infected.

Was this article helpful?

0 0

Post a comment