Microfilariae of Onchocerca volvulus were found in venous blood samples in 17 of 20 patients with moderate to severe ocular onchocerciasis in a Sudan-savanna focus in Cameroon. They were present in the urine in 7 of the 20 patients. The numbers of microfilariae increased during DEC therapy, both in the blood and urine, and they were not completely eliminated after 2 weeks of therapy.

The effects of housing density on endocrine organ weights and on retention of the parasite Microphallus pygmaeus by male and female mice arc described. A reduction in gonadal function with increasing housing density was found in each sex. Male mice showed clear evidence of greater adrenocortical activity at higher densities but this was not as clearly defined in females. Animals of both sexes showed a decline in relative weights of both thymus and spleen at the highest densities. The number of parasites recovered, a standard time interval after introduction by stomach tube, was increased significantly in both males and females housed at higher densities. The results suggest that a progressive increase in population density in both male and female mice stimulates pituitary-adrenocortical activity and that there is concomitant suppression of gonadal function. The change in susceptibility to the parasites may also be a direct or indirect consequence of the change in pituitary-adrenocortical activity, on immunological responses or intestinal physiology, such as an increase in the duration of exposure to adverse pH values.

Using the indirect fluorescent antibody technique, the antibody response of cats to Brugia pahangi infections was studied using whole microfilariae and sectioned adult worms of B. pahangi as antigens. Microfilarial antigen was ineffective in detecting antibody response in cats during the microfilaraemic phase of the infection, although it was possible to detect circulating antibody in cats which became microfilaria negative after repeated and prolonged reinfection and in one cat which after infection failed to show microfilariae in its blood

The structure of the cuticle of third and fourth stage larvae and of adult Brugia pahangi (Nematoda: Filarioidea) was studied by light microscopy and by transmission and scanning electron microscopy. The cuticle of these worms was basically typical of the class Nematoda and consisted of cortical, matrix, and basal layers. Finely spaced annulation grooves were present and their function is discussed.

Descriptions of the cuticular ultrastructure of filarial nematodes are meagre. Studies on the cuticle of adult worms are limited to those of Kagei (1960, 1963) on Setaria cervi and Litomosoides carinii and of McLaren and Hockley (personal communication) who examined L. carinii and Dipetalonema vitea

Neoheterakis chapini n.g., n.sp. (Nematoda: Heterakoidea) is described from the caecum of a Congopeacock Afropavo congensis Chapin, 1936. The diagnostic features of the subfamily Spinaspidoderinae are extended to accomodate the new genus.

Microfilariae of Onchocerca cervicalis were injected subcutaneously into jirds, Meriones unguiculatus. At 72 hours post-inoculation the microfilariae had redistributed and the distribution pattern remained stable for approximately 100 days. Live active microfilariae were recovered from jird skin 168 days post-inoculation.

Fifty nine per cent of the microfilariae recovered at 72 hours were from the skin of the tail and inguinal region. The inguinal region was the preferred site in male hosts and the tail in females, the difference in distribution being attributed to temperature differences in the inguinal region of the hosts. It is suggested that the microfilariae migrate along a thermal gradient rather than at random.

The structure and affinities of the progenctic larva of Echinobothrium reesae Ramadevi, 1969, are described and discussed.

A study of the distribution and population of Lymnaea gedrosiana in relation to the transmission of Ornithobilharzia turkestanicum in various habitats was carried out in Khuzestan.

Populations were higher in canals and swamps than in ponds, with peaks in spring and autumn and low densities in summer and winter.

The reproductive capacity of L. gedrosiana was higher in running water than in stagnant water. The peak reproduction rates occurred in spring and autumn and reproduction continued at a low level in summer but was nil in winter.

Infection with O. turkestanicum was higher in large than in small snails (0.1 %, 0.5% and 1.1% in snails measuring 0.6 mm, 6–10 mm and 10–14 mm respectively). In canals and swamps transmission occurred throughout the year, with a peak in late summer in the former and early summer in the latter. In ponds, most transmission probably occurred in spring and autumn. Drains played a very minor role in transmission.

The laboratory infection of Lymnaea gedrosiana with 1,2,5,10 and 20 miracidia of Ornithobilharzia turkestanicum, resulted in infection rates of 54.5%, 57.1%, 62.5%, 84.0% and 100% respectively, with a prepatent period of 18–21 days.

The numbers of cercariae shed daily by single snails exposed to one or 2 miracidia were significantly lower than from those exposed to 5,10 or 20 miracidia.

Few cercariae were shed during the early patent period, but the number increased with the duration of infection. All infected snails died at the peak of cercarial shedding, and no infected snails survived more than 60 days after exposure to miracidia. The differences between the mean life-span of infected snails and non-infected snails in the control group were statistically significant (P < 0.001).

Distinct morphological differences were found between microfilariae of Onchocerca cervicalis in the uterus of the adult and those in the skin of the horse, suggesting that development occurs during migration from the adult worm to the skin.

Uterine microfilariae can be divided into two types which represent stages of development not embryonic male and female worms.

Skin microfilariae of O. cervicalis were readily distinguishable from those of O. volvulus and O. gutturosa. Morphological comparisons between adult and larval O. cervicalis collected during this project with published descriptions of O. reticulata show that there is insufficient evidence for their specific separation.

The first two papers in this series have dealt with the prevalence of Onchocerca cervicalis in British horses and with the pathology caused by the parasite to the definitive host. The present paper is concerned with the morphology and taxonomy of the parasite, particularly the microfilarial stage.