Morphology of the Microfilaria of Brugia pahangi and of the Larval Stages in the Mosquito

Abstract

The detailed morphology of the microfilariae of B. pahangi following several methods of preparation is described. Possible features which will allow its rapid differentiation from the microfilariae of B. and of B. patei are the size and proportions of the cephalic space. Among eleven species of Louisiana mosquitoes tested, Anopheles quadrimaculatus, A. crucians, and Psorophora confinnis were susceptible to infection with B. pahangi. Although infective larvae were recovered from all three species, encapsulation of some larvae was seen in A. quadrimaculatus and P. confinnis. As the chemical nature of the encapsulating material is not known, it is suggested that the term chitinization reaction be dropped. In the development of B. pahangi in the mosquito, the first molt occurs between 4 and 5 days and the second molt at about 8 days, after which the larvae are immediately infective. During the first 3 days of development, microfilarial structures are rearranged and the nerve ring is obliterated. Cell multiplication during this period is minimal; cell growth is marked and larvae become much shorter. The developmental period just preceding the first molt is distinguished by rapid cell multiplication, lengthening of the body, and differentiation of internal structures. The cephalic space of the microfilaria does not form the buccal cavity or any other structure of the infective larva but is shed undeveloped at the first molt. It is suggested that the supernumerary nuclei of the terminal caudal appendage of the microfilaria may be the retained polar bodies. The tail of the microfilaria containing the supernumerary chromatin dots is retained throughout the first and second stages of development. If not dislodged by dissection procedures, its presence affords a reliable distinguishing feature for separating the firstand second-stage larvae of B. pahangi from those of other genera, but not of congeneric species. Infective larvae of B. pahangi could not be distinguished from those of B. malayi. The sex of individual larvae can be identified in late second stage by the position of the genital primordium which is at or just behind the esophago-intestinal junction in males, and at the midesophageal level in females. In light of this and other findings, it is suggested that the R1 cell does not form the genital primordium but develops into the intestine, and that the R2-4 cells are incorporated into the rectum. Brugia pahangi (Buckley and Edeson, 1956) Buckley, 1958, is a filarial parasite of the malayi group principally found in carnivores, but also found in a wide range of other types of hosts. Edeson et al. (1960a) were the first to demonstrate that this worm would develop to the adult stage and produce microfilariae in man. An earlier attempt to infect humans (Buckley, 1958a, b) failed to produce a patent Received for publication 12 January 1962. *A portion from a dissertation submitted to the Graduate School of Tulane University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This investigation was conducted under the sponsorship of the Commission on Parasitic Diseases, Armed Forces Epidemiological Board, and supported in part by the Office of the Surgeon General, Department of the Army. t Present address: Department of Tropical Health, Schools of Public Health and Medicine, American University of Beirut, Beirut, Lebanon. infection (helminthologically) but did result in severe symptoms of disease strikingly similar to those of eosinophilic lung (tropical eosinophilia). The body length and certain other features of the microfilariae of B. pahangi have been described and compared with those of semiperiodic B. by Laing et al. (1960). These authors and Edeson et al. (1960b) found that Armigeres obturbans is an efficient laboratory vector for B. pahangi. Among the other mosquitoes tested Mansonia (Mansonioides) annulatus and Anopheles barbirostris proved to be the best hosts. M. annulatus and M. longipalpus apparently are the natural vectors in Malaya. Published descriptions of the larvae include length and breadth measurements, anal ratios of the various stages, and characteristics of the terminal papillae of the infective stage (Edeson et al., 1960b; Nelson, 1959). The primary purpose of the present study