Filaria 
The life histories of the various species of human filaria are 
similar except for the specific arthropod host and the location of the 
adults within the human body. These blood nematodes differ from the 
intestinal species in that the diagnostic stage is a prelarval form 
called a microfilaria and there is no external environment period. 
Most of the structural details have been included in the drawings of 
the microfilariae but other stages are more diagrammatic. Within the 
group, the diagnostic and infective stages have been drawn to scale, 
and to a lesser degree, the adults. 
Like other helminths, filariae do not multiply in man. Furthermore, 
passage through the arthropod host is necessary for transmission of 
the infection. For example, microfilariae in transfused blood will 
circulate in the peripheral blood of the recipient but are unable to 
cause infection and will die within a relatively short time. 
Within the human host, the worms mature slowly requiring several 
months to a year before diagnostic stages (microfilariae) can be 
demonstrated. The location of adults varies with the species (see 
table below) but microfilariae are found in the peripheral blood in 
all species except Onchocerca volvulus where they are present in the 
cutaneous tissues. The appearance of microfilariae in the blood is 
periodic in certain species (Wuchereria bancrofti, Brugia malayi, and 
Loa loa) and non-periodic in others ( Acanthocheilonema perstans , 
Mansonella ozzardi , and the South Pacific strain of W. bancrofti ). 
The reasons for this periodicity are not clearly understood. 
The arthropod host may be a species of mosquito (IT. bancrofti 
and B . malayi), of flies (0. volvulus and L. loa), or biting midges 
C4. perstans and M . ozzardi ). There is no multiplication of the 
filariae within the vector as occurs with the blood protozoa. The in- 
gested microfilariae penetrate the stomach wall of the insect after 
losing the sheath if one is present and develop to the infective third 
stage larvae in the thoracic muscles. The infective larvae then mi- 
grate to the proboscis, and when the insect bites again, they actively 
move down the proboscis to the skin surface and probably enter the 
human host through the bite wound. The development within the insect 
is influenced by such extrinsic factors as temperature and humidity. 
The average developmental time and some of the chief vectors for 
each species are included in the table below. 
While several species of arthropods may be involved as interme- 
diate hosts for the filariae, man is the usual definitive host in most 
instances. A. perstans has been reported in the gorilla, L. loa in the 
baboon (although their role as reservoir host is yet to be proved) 
and, recently, B. malayi has been recovered from cats and monkeys. 
Of the six species, A. perstans and M. ozzardi are the only ones 
usually considered non-pathogenic, although the possible pathoge- 
nicity of A. perstans has recently been suggested. 
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