Endoparasites of Simulium ornatum MG. in South England, with special reference to larval parasitisation

The thesis reports on investigations during 1967-69 of endoparasitism in Simulium ornatum Mg., with particular reference to the larval stage, from the English rivers Lea (hertfordshire) and Tilling Bourne (Surrey). Methods included record of visual evidence of infections, dissections and smears of fresh material, and histological examinations of all material by light, and, by arrangement with Dr R. G. Bird of the E. M. Laboratory, L. S. H. & T. M., by electron microscopy, in the case of certain microsporidia. In preliminary observations no parasites were observed in either pupal and adult stages of Simulium ornatum; the study is concerned with larval parasites, of which several kinds were found. The Order Microsporidia of the protozoan Class Cnidosporidia was well represented by 3 genera, Thelohania, Plistophora and Nosema of the Family Nosematidae. Pseudocysts from two forms of Thelohania, and one form of Plistophora, examined by electron microscopy, revealed the fine structure of sporogonic stages of the sporont and, of the spore of Thelohania and, less intensively, of Plistophora. Microsporidian spore size has been accepted by several investigators, with some controversy from others, as a major criterion for specific classification. In this connexion, spore measurements carried out for Thelohania infections, from S. ornatum were, by statistical and graphical analysis, recognisable as composing four separate forms (A-D), with a minor form also, form E on the basis of spore shape. All Thelohania infections of larvae from the River Tilling Bourne are of form A, having the smallest spore dimensions. The other four forms, "B", "C", "D" and "E", occurred in Simulium larval populations of the River Lea. In the case of Plistophora another criterion in classification, in addition to spore dimensions, was notable, viz. the number of spores produced by the sporont. Two forms of Plistophora were thus distinguished. Nosema infections were of one kind only. All these forms, found in the simulium ornatum larvae, are new records for microsporidia in the British Isles and, on taxonomical features different from those of other microsporidian parasites described from simuliidae from elsewhere. Histological examination also revealed a light infection of a microsporidia-like organism in the Gut epithelial cells of Simulium ornatum larvae from the Tilling Bourne only. Fungal infections were also common. The pathogenic fungi, Coelomycidium sp. of the Order Chytridiales, occurred [missing page] larvae revealed 3 distinct larvae generations of Simulium ornatum per year, and the occurrence and abundance of the several parasites was noted in respect to these different larval populations. Laboratory culture of "clean" material was desirable for the transmission experiments undertaken for Thelohania, and for Coelomycidium. To obtain such a culture, it was necessary to affect both copulation of Simulium ornatum adults and to induce females to blood-feed. Repeated attempts at artificial insemination proved unsuccessful; blood-feeding was successful with only a low percentage of adult females. Failing to establish a "clean" culture resort was made to the use of apparently uninfected wild-caught larvae for experiments on the transmission of infections, but without success in inducing hatching of Thelohania spores or propagation of Coelomycidium ingested by Simulium larvae. It remains still obscure whether or not, in nature, the Thelohania infections are transmitted transovarially through infected adults emergent from infected larvae; and the mode of transmission of Coelomycidium is also not determined. While the pathogenic parasites found must have controlled larva densities in the two streams, much remains to be studied and understood about their biology, transmission and cultivation before their application in biological control could be envisaged.