Abstract
This paper attempts to pinpoint the most original morphological anatomical features of the biology of filariae per se and those which are or could be important for triggering regulatory processes in the arthropod vector and uncontrolled pathogenic processes in the vertebrate hosts. The following stages are considered: the motile egg or newly-hatched larva, the microfilaria, in the lymphatic or blood vessels of its vertebrate host; the larva, its migrations and its intrasyncitial development in the hematophagous arthropod subverted as vector; its transfer to the vertebrate host, migratory properties through the lymphatic system, maturation, mating and, finally, egg laying in the tissues they reach. This synthesis is based on parasite morphological features and their functional interpretation, histological features in the different niches the filariae reach, and on quantitative analyses of filarial development at its different phases, as well as on the rare and valuable observations of living parasites in situ. Data have been drawn from various species of Onchocercidae from amphibians, reptiles, birds and mammals. These comparative analyses have revealed the major constraints to which the filariae, including those parasitizing humans, have been subjected during their evolution from their ancestors, the oviparous and heteroxenic spirurids. Emphasis is placed on mechanical events: resistance of the microfilariae to the currents in the blood or lymph vessels, regulatory processes induced in the vector mesenteron by the movements of the ingested microfilariae, transient disruption by the microfilarial cephalic hook of the vectors' tissues and cell membranes during microfilarial translocation, attachment of males to females during mating by means of 'non-slip' systems, etc. Like other nematodes, filariae are equipped with sensory organs and a locomotor system, composed of the muscles and of the original osmoregulatory-excretory cell. Any change in one of these elements will result in the destruction of the filaria, at some stage of its development. In the vertebrate host, the intravascular stages will no longer be able to resist being carried passively towards the organs of destruction such as the lymph nodes or the lungs.
Highlights
The study of human filariae is limited for obvious ethical reasons
The study of each species provides a fragment of information and these pieces, when assembled as in palaeontology, allow outlining the main features of the biology of filariae, which can be extended to the parasites of humans
Filariae, which are confined inside the host tissues, have adapted to transmission by haematophagous arthropods by laying motile vermiform eggs or very young larvae in diapause, the microfilariae (Fig. 1), which are able to reach the peripheral cutaneous blood or lymphatic vessels and become available to the arthropods
Summary
The study of human filariae is limited for obvious ethical reasons. The proportion of inoculated larvae that develop, their route of migration, pairing of the sexes, egg laying and migration of the microfilariae, to name only a few processes, remain unknown. In L. sigmodontis, the microfilarial densities are exceptionally high, reaching 1500 per μl of blood (about 0.06% of the bodyweight of the rodent); the vector, the macronyssid mite Ornithonyssus bacoti, ingests only a small amount of blood due to its small size and, in addition, 80% of the microfilariae arrive dead in the digestive caeca because they are torn by the mite's pharyngeal teeth [62] In another example, a filaria parasitic in birds, the adult worms die after one season of egg laying and the maintenance of the species is ensured by the microfilariae which are transmitted to the nestlings by the mallophagean vector [63]
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