The truncated negative binomial distribution is used to demonstrate water mite-induced mortality in a field population of the mosquito host Anopheles crucians. Consistent changes in the parameters of this distribution and in the number of hosts expected in higher mite load classes as truncation is applied to successively lower mite load classes suggest that many heavily parasitized individuals were missing from field samples. These changes are not observed in distributions derived from an association between parasitic larvae of another water mite, Hydrachna conjecta, and the insect Sigara ornata, which is apparently immune to this parasite. Although the method discloses parasite-induced mortality that is non-linearly related to parasite load, it fails to reveal linear parasite-induced mortality in a hypothetical example.

A delayed gamma distribution satisfactorily described the distribution of times to hatching of Ostertagia circumcincta eggs incubated in 0·1% saline at temperatures between 6 and 20 °C. Below 6 °C hatching of eggs was extremely variable. The relationship between rates of development and temperature within the range 10 to 20 °C was more closely described by a non-linear function than a linear one. The non-linear function was incorporated into a temperature summation equation which satisfactorily predicted the hatching of eggs incubated under conditions of alternating temperatures.

The development of the free-living stages of Ostertagia ostertagi incubated at constant temperatures from 5 to 27 °C and in dung pats in the field was observed. The death rates were adequately described by a single exponential function and the distribution of times for development by a delayed gamma density function. The times of appearance of selected development stages such as maximum numbers of embryonated eggs and half the maximum number of infective larvae, were highly dependent on temperature. The relationships between these times and temperature were adequately described by the Arrhenius equation. The parameters of this equation and mean hourly temperatures recorded in dung pats in the field were used to predict development times in the field. A comparison of predicted and observed times showed that an initial delay accounted for a large proportion of the time required for development in dung pats, particularly the embryonation of eggs. This delay was attributed to lack of aeration associated with high moisture content of the dung pat.

Eggs of Oesophagostomum dentatum in faeces deposited on grass plots from May to October developed into infective larvae. Warm wet weather facilitated development but weather conditions which caused the faeces to dry out rapidly resulted in a heavy mortality of eggs and pre-infective larvae. During the winter no development took place and the eggs died. In the early spring and late autumn a few eggs survived and developed into 1st-stage larvae but they died without developing further. In the laboratory no development took place at 4 °C but at temperatures ranging from 10 to 25 °C development into infective larvae was completed, the rate increasing as the temperature rose. Out-of-doors infective larvae survived in faeces and on herbage for 1 year under a wide range of climatic conditions including the exceptionally cold winter of 1978–79. In the laboratory, infective larvae suspended in water survived even longer at temperatures ranging from 4 to 27 °C. They were rapidly killed by continuous freezing and by desiccation when the relative humidity was less than 90%. The relationship between climatic conditions and the development and survival of the free-living stages is discussed.

Eimeria meleagrimitis oocysts were stored at 4, 22, 32 or 41·5 °C for up to 1 year. Decreases in amylopectin levels (measured as glucose) and viability (measured as oocyst production and mortality in turkeys) of the oocysts were generally related to the length of time in storage and the storage temperature. Oocysts assayed immediately after harvest contained 58·29 ± 0·75 μg of glucose/106 oocysts. When the oocysts were stored at 4 °C for 162 days, the amylopectin level decreased to 65% of the original level. In oocysts stored at 22, 32 and 41·5 °C, amylopectin declined to approximately 20% within 162, 76, and 41 days, respectively. Oocysts stored at 4 °C for 1 year produced more oocysts in turkeys than the original fresh isolate, but caused no mortality. Oocyst production from oocysts stored at 22 and 32°C decreased gradually until, after 9 and 7 months respectively, no patent infections were produced. Oocyst production from oocysts stored at 41·5 °C was markedly reduced within 1 month and was not detected after 4 months.

Sporozoites of Eimeria necatrix inoculated into the allantoic cavities of embryonating eggs completed their life-cycle in the chorio-allantoic membranes and produced fully viable oocysts. Subsequently, as a result of repeated passage in this host, an egg-adapted line was developed and was found to be markedly less pathogenic for chickens than the nonpassaged parent strain. Antigens capable of inducing protective immunity against challenge with low doses of the parent strain were retained, however, indicating a possible use for this egg-adapted (attenuated) line of E. necatrix in the immunization of comercially reared chickens.

The isolation of polysomes in bulk from bloodstream forms of Trypanosoma brucei is described. The polysomes are active in in vitro protein synthesis in the presence or absence of initiation inhibitors. Nascent variant surface antigen (VSA) has been detected on these polysomes using purified radio-iodinated antibody. EDTA-induced ribosomal sub-units and their large rRNA's are characterized. The 26S rRNA is nicked to produce 2 molecules which are both smaller than the 195 rRNA of the small sub-unit which is larger than that found in the majority of eukaryotic small sub-units.

The surface morphology of schistosomula extracted from the skin, lungs and hepatic portal system (hps) of mice was investigated from Days 0 to 18 post-infection. Skin schistosomula and newly arrived schistosomula from the hps were of similar dimensions but were morphologically distinct. Lung schistosomula were considerably elongated with an estimated 53% increase in surface area compared to skin schistosomula. The pitted tegument of lung schistosomula was formed into ridges and troughs. These were compressed together in contracted individuals recovered from the hps on Day 10. The annular ridges were cross-linked by longitudinal septae which possibly prevent further elongation of the body. A regression of the spines between the mouth and the ventral sucker was observed in Day 2 skin schistosomula. In lung schistosomula only the spines at the anterior and posterior of the body remained. New spines were formed after the schistosomula reached the hps. It is suggested that the spines in the mid-region of the body are selectively disassembled and that their loss facilitates migration along the lumina of capillaries with the residual spines acting as anterior and posterior anchors. The mouth opening was enlarged in schistosomula from the hps recovered from Day 10 onwards. Skin schistosomula lost the cercarial apical tegumentary ridges between 24 and 48 h after penetration but a spineless protrusible area remained. After arrival in the hps this area became integrated into the anterior surface as the oral sucker developed around the sub-terminal mouth. The cercarial ciliated papillae were lost on penetration. The migrating schistosomulum had few visible sensory papillae but following arrival in the hps new papillae were observed.

The reproductive duct of the male blood fluke Schistosoma mansoni is relatively short and undulating, extending from the most posterior testis to the beginning of the gynecophoric canal. The reproductive duct may be divided into 3 main regions which differ in ultrastructure; the vas efferens/deferens, the seminal vesicle and the cirrus tube. The epithelium of the vas efferens/deferens consists of a single layer of elongate cells with abundant convoluted lamellae on their apical surfaces. The wall of the seminal vesicle and the cirrus tube are tegumentary in nature. The basic similarities in structure of the surface tegument, the seminal vesicle and the cirrus tube suggest a common embryological origin. The presence of complex lamellae on the luminal surface suggests that the vas efferens and deferens have secretory and possibly nutritive functions apart from temporarily storing developing germ cells. The primary function of the seminal vesicle appears to be that of sperm storage; while the cirrus tube with its complex infolded wall is extensible for insemination. The presence of abundant concentrically distributed sensory papillae around the male genital pore may assist in the pairing of males and females.

Lambs given 60000 third-stage larvae of Nematodirus battus were killed on days 16, 20, 22, 24 and 32 of the infection and the duodenal mucosa was examined by scanning electron microscopy (SEM). On day 16 of the infection the villi were long and finger-like and, although goblet cells were visible, the surface of the villi was not extensively folded, as is the surface of villi of uninfected control animals. By day 20 of the infection the villi had been reduced to flattened, plate-like structures or had formed low, irregular-shaped ridges. On this day of the infection the nematodes appeared to be enclosed by mucus-like material. As the infection progressed the intestinal damage was repaired, so that, by day 24 of the infection, the surface of the intestine was covered by short, smooth, finger-like projections and, by day 32 of the infection, the morphology of the intestine was somewhat similar to that of the control lamb. The possible role of villus atrophy and of mucus in the rejection of N. battus from the intestine is discussed.

In the male reproductive system of Nematodirus battus the spermatogonia and spermatocytes are arranged upon a central rachis and the spermatocytes are characterized by the presence of cup-shaped membranous organelles which are formed in association with Golgi complexes and granular endoplasmic reticulum. Spermatozoa of N. battus in the male nematode have an electron-dense, conical tail-like region which contains the nuclear material and an elongate cytoplasmic region which contains the cell organelles. In the uterus of the female, however, the cytoplasmic region of the spermatozoon has an amoeboid appearance and in the seminal receptacle the cytoplasmic region is dome-shaped and many of the membranous organelles have connexions with the outer membrane of the spermatozoa. Movement was not observed in spermatozoa released from the seminal vesicle of the male worm.

As the infection progresses in lambs, the developing germ cells of male Nematodirus battus become filled with lysosome-like bodies, electron-dense granules and membrane-bound vesicles, and the organization and division of the gametic nucleus appears to be altered. The epithelial cells of the reproductive tract are also damaged and the secretory-type globules produced by the cells of the vas deferens are denatured. These changes are compared with ultrastructural changes observed in other species of nematode affected by the immune response of their hosts.

The adenine nucleotide content and adenylate energy charge of Nematospiroides dubius from laboratory mice and of Trichostrongylus colubriformis from lambs has been measured. Administration of the anthelmintic, levamisole, to infected hosts resulted in only a slight fall in the adenylate energy charge of N. dubius over a 3-h period but there was a greater fall in the adenylate energy charge of T. colubriformis during this period. In neither case did the energy charge fall quickly, nor did it fall to the low levels which would be expected if the levamisole were inhibiting synthesis of ATP. The changes in energy charge of the nematodes which occurred following administration of levamisole to their hosts was of the order which can be satisfactorily explained by changes in the environment of the nematodes, such as reduced oxygen tension. It is concluded that the maintenance of levamisole-induced paralysis of these two species of trichostrongyle in vivo does not rely on the inhibition of fumarate reductase.

The histochemical localization of acid phosphatase in an adult filaria, Setaria sp. obtained from the peritoneal cavity of a cow was closely examined and compared with that of adult nematodes parasitic in the host alimentary canal; special attention was paid to the intestine and body wall of the parasites. Setaria sp. was found to show high acid phosphatase activity in the interchordal hypodermis of the body wall and uterine microfilariae, and similar activity is suspected to occur in the cuticle. The intestine of this nematode exhibited very low, if any, activity. In contrast, nematodes parasitic on the alimentary canal, such as Toxocara cati, T. canis, Physaloptera sp. and Ancylostoma caninum, showed no activity in the body wall and very high activity in the luminal surface of their intestine. The possible function of the abundant acid phosphatase in the body wall of this filaria is discussed.

In this study two aspects of the biology of the cercaria of Himasthla militaris were examined. Firstly, the cercarial emergence from the first intermediate host, Hydrobia stagnorum, was studied. On a 9 h light (9L)/15 h dark (15D) schedule cercariae of Himasthla militaris emerged from naturally infected Hydrobia stagnorum predominantly during darkness. Inversion of the photo-period resulted in an immediate reversal of the emergence pattern. A comparison between the cercarial release patterns under natural conditions and in a constant light/temperature environment showed that an endogenous as well as an exogenous factor is controlling this process. Secondly, the penetration of the cercaria in Nereis diversicolor and the site-finding of the larva within this second intermediate host was investigated. After a short external migration on the body of N. diversicolor the cercaria enters the polychaete via the anus opening, perforates the gut-wall and moves through the coelom to the anterior segments to encyst in the muscle tissue. This internal migration results in a specific distribution pattern of the metacercariae, with more than 50% of the cysts localized in the first 12 segments and a maximum in segment number 9 (13·8%).

Biological control is understood here in the classical sense, which is precisely defined by De Bach (1964) as ‘the action of parasites, predators or pathogens in maintaining another organism's population density at a lower average than would occur in their absence’. This account consists of a survey of the principal causes of disease in nematodes, with a summary of the efforts made to use certain pathogens in practise and a discussion of nematode pathology with reference to destruction of plant pathogenic nematodes.