The Identification of Infective Filarial Larvae in Mosquitoes: with a Note on the Species Found in “Wild” Mosquitoes on the Kenya Coast

Abstract

1. At least twenty-two species of filarial worms are known to develop in mosquitoes. Filarial larvae of non-human origin must often be present in mosquitoes dissected during filariasis surveys yet the larvae seen are commonly assumed to be of human origin.2. On Pate island off the Kenya coast where bancroftian filariasis is prevalent, several distinct species of infective larvae were found in mosquitoes both in the houses and in the bush. Seven species of filarial worms were found in the local animals and a collection of known infective larvae was made by feeding mosquitoes on these different infections. A study of the material in this “reference collection” showed that many of the larvae in “wild” mosquitoes could be identified with confidence.3. A technique has been developed for the dissection of preserved and stained mosquitoes; this method has many advantages and can be used in routine filariasis surveys.4. The characters which were most useful in differentiating the infective larvae were: the length; the position of the anus; and the shape of the caudal extremity. Eight species of infective larvae were available for study, these are described and illustrated: W. bancrofti, W. malayi, W. pahangi, W. patei, D. corynodes, D. repens, D. immitis and S. equina. The literature dealing with a further fourteen species is reviewed.5. A simple key has been produced to help identify the species of infective larvae found in mosquitoes.6. On the Kenya coast W. bancrofti infective larvae have been found only in C. fatigans, A. gambiae and A. funestus. Infective larvae of animal origin which might easily be mistaken for those of W. bancrofti have been found in Ae. pembaensis, Ae. aegypti, M. uniformis and M. africanus.7. Formulae for transmission indices based on infection rates in insect vectors are of little value unless there is confidence in specific identification.8. A study of the infective larvae in Simulium neavei from an onchocerciasis area in Uganda has shown that not all the larvae are of human origin. A closer examination of the morphology of infective larvae in the vectors of filarial parasites of man in other areas will almost certainly show that the interpretation of infection rates may be complicated by the presence of infections of nonhuman origin.9. Feeding experiments were carried out with Aedes pembaensis and Ac. aegypti on a donkey infected with Setaria equina; both of these mosquitoes proved to be suitable vectors of this filarial worm.

The prevalence of naturally acquired multiple infections of Wuchereria bancrofti and human malarias in anophelines

“…The remaining mosquito material from landing catches was placed into a 1-5 ml vol. Eppendorf microcentrifuge tube filled with 70% ethanol, and later stained in Mayer's acid haemalum and dissected for the presence of W. bancrofti larvae by the fourth member of the team (Nelson, 1959). Mosquitoes from resting catches were dissected immediately by the fourth member of the team and examined at 400 x magnification for filarial larvae.…”

Biodiversity of filariae (Nematoda: Filariata), parasites of birds in Uzbekistan

“…For the collection of insects, we used routine entomological methods (Agrinsky, 1962). The dissection of insects and identification of larvae were done using the methods described by Nelson (1959Nelson ( , 1960 and Nelson and Pester (1962). The recorded nematode larvae were fixed in 1.5%-2% formalin.…”

Microepidemiology, the key to the control of parasitic infections

“…Further histological study of the bowel and liver revealed that 31% of the baboons were infected (MILLER, 1959(MILLER, , 1960STRONG et al, 1961). This was a very exciting discovery and it led to much more extensive surveys for schistosomiasis in baboons and other animals throughout East Africa (NELSON, 1960;. (Fig.…”

“…pembaensis for our experiments; it was such a good vector of so many different species of filarial parasites we were able to make a collection of a variety of infective larvae for comparison with larvae from wild-caught mosquitoes. This collection, together with specimens from many other species of mosquitoes and infective larvae from Simulium and other insect vectors formed the basis of a World Helath Organization (WHO) reference centre and we received requests for identification from many parts of the world (NELSON, 1959;NELSON & PESTER, 1962). It soon became obvious that many of the infective larvae in mosquitoes and Simulium, not only from East Africa but from many other countries, were of animal origin and that the mathematical 6 models being developed by 'macroepidemiologists' to study the dynamics of transmission and to monitor filariasis and onchocerciasis control programmes were valueless unless there was confidence in-the identification of infective larvae (Fig.…”