Adult Trichinella spiralis were maintained in vitro using defined media and the material excreted/secreted (ES) during this time examined for proteolytic enzyme (proteinase) activity using an azocasein assay and gelatin-substrate gels. Several discrete proteinases in the size range 14–100 kDa were observed with optimal activity at pH 7·5. The use of a class-differentiating panel of proteinase inhibitors indicated that serine proteinases were predominant although some inhibition was evident in the presence of cysteine and metalloproteinase inhibitors. Of a panel of potential natural protein substrates tested, ES proteinases only degraded fibrinogen and plasminogen and degradation was, in part, susceptible to the action of serine, cysteine and aspartyl proteinase inhibitors. In addition, antibody harvested from immune but not normal mice inhibited ES proteinase activity, an observation of relevance to the immunobiology of Trichinosis.

A method of interrupting the life-cycle of the human blood fluke Schistosoma by increasing the proportion of genetically insusceptible intermediate host snails in natural populations was first proposed nearly 25 years ago. The method assumes that insusceptible snails will be at a selective advantage over susceptible snails when the schistosome parasite is present, and therefore natural selection will act to increase the proportion of alleles for insusceptibility. A major objection to the proposed technique is ‘If insusceptible snails are at a selective advantage, then why are they not predominant in natural populations that transmit disease?’ One explanation of this paradox is that insusceptibility may be associated with a disadvantageous character or a physiological defect. This study tests this hypothesis by measuring the relative reproductive success of susceptible and insusceptible snails under controlled conditions. Results indicate that insusceptible (unsuitable) snails are negatively affected in the presence of either susceptible snails or schistosome parasites. Furthermore, in the presence of both susceptible snails and schistosome parasites, insusceptible snails are selectively disadvantaged compared to susceptible snails. These results obtained under laboratory-controlled conditions suggest a plausible answer as to why insusceptible snails are not predominant in natural populations that transmit disease.

The relationship between faecal nematode egg count and the number of adult Telordasagia circumcincta was examined in 508 naturally infected lambs at 6–7 months of age. The relationship was found to be convex and was empirically described by a gamma-type function of the form y = anb e−cn, where n is the number of adult nematodes present and y is the number of nematode eggs per gramme (epg) of faeces. This equation predicted that the peak expected egg count (277 epg) would occur at a worm burden of 2167 adult worms. The absence of a linear relationship between egg counts and worm numbers is consistent with the existence of severe density-dependent constraints on the fecundity of T. circumcincta.

Properties of ecological networks facilitate the understanding of interaction patterns in host-parasite systems as well as the importance of each species in the interaction structure of a community. The present study evaluates the network structure, functional role of all species and patterns of parasite co-occurrence in a host-parasite network to determine the organization level of a host-parasite system consisting of 170 taxa of gastrointestinal metazoans of 39 marine fish species on the coast of Brazil. The network proved to be nested and modular, with a low degree of connectance. Host-parasite interactions were influenced by host phylogeny. Randomness in parasite co-occurrence was observed in most modules and component communities, although species segregation patterns were also observed. The low degree of connectance in the network may be the cause of properties such as nestedness and modularity, which indicate the presence of a high number of peripheral species. Segregation patterns among parasite species in modules underscore the role of host specificity. Knowledge of ecological networks allows detection of keystone species for the maintenance of biodiversity and the conduction of further studies on the stability of networks in relation to frequent environmental changes.

Parasitic bronchitis is a serious disease of cattle and is caused by the nematode, Dictyocaulus viviparus. For over 30 years, a radiation-attenuated larval vaccine has been used for prevention of this disease. This vaccine has been used with considerable success in the UK and parts of Western Europe, however, it has several disadvantages. It has a short shelf- life and the vaccine has to be produced annually necessitating the use of donor calves. Following vaccination, calves must receive further boosting from natural challenge to maintain protective immunity. Sales of the irradiated larval vaccine have decreased dramatically since the 1970s. This is thought to be due to increased reliance of farmers on anthelmintic programmes to control lungworm infection. It is possible that, under certain circumstances, these programmes do not allow sufficient parasite exposure to stimulate protective immunity to further Dictyocaulus challenge. This is borne out by the recent documented increase in the number of outbreaks of parasitic bronchitis in the UK. A stable vaccine against D. viviparus that is capable of stimulating a more prolonged immunity would be beneficial. Recent research has been directed at identification and isolation of components thought to be involved in parasite survival in the host and examination of their potential as vaccine candidates. One of these components is acetylcholinesterase (AChE), an enzyme secreted by adult worms. This review describes the development of the secreted AChE as a vaccine candidate, as well as documenting recent developments in the immunodiagnosis of D. viviparus.

1. T. raiae and Bodo caudatus are investigated using Feulgen's nucleal reaction in addition to the usual nuclear stains. The nature and value of this method is discussed.

2. In T. raiae the parabasal body or kinetonucleus is found to consist of chromatin as shown by the nucleal reaction. An achromatic element in its composition is suggested in the division by the desmose between the two halves being achromatic.

The blepharoplast is an achromatic structure. It seems probable that it persists in the aflagellar phase of the trypanosome. In T. raiae the division of the blepharoplast precedes but does not apparently orientate the division of the parabasal body. The division of the parabasal body does not present a mitotic figure.

3. The nucleus in T. raiae consists of an achromatic karyosome body with some chromatin surrounding it, there is also chromatin arranged upon the inner surface of the nuclear membrane.

4. At division the karyosome apparently draws out into an achromatic spindle figure, the chromatin collects in the prophase into a loose granular mass and then becomes arranged at the equator. The mass divides into two portions; it is not possible to distinguish chromosomes with sufficient certainty. The nuclei are finally reconstructed from the achromatic ends of the spindle and the chromatin which has travelled to either pole.

5. In Bodo caudatus the parabasal body (kinetonucleus) is composed of chromatin and of an achromatic substance, the blepharoplasts or basal granules are achromatic bodies which play an orientating part in the division of the parabasal body: this division does not show an equatorial plate stage. The nucleus is made up of an achromatic nucleolus-karyosome surrounded by a hollow sphere of chromatin; there is a very definite division process which takes place apparently within the nuclear space. There is no equatorial plate formed and the achromatic figure which is derived directly from the karyosome is a division column which inserts itself into the already dividing chromatin and is not of the spindle fibre type found in the mitosis of the metazoan cell and of many protozoa.

Data concerning the life cycle of the various species of the genus Trichuris are somewhat controversial. The present paper is devoted to the study of T. muris, a common parasite of the caecum of wild house-mice and rats. The possibility of transfer of T. muris in the laboratory mice would enable the author to solve various contradictory points.

Of the 39 most common helminth parasites of man listed by Peters & Gilles (1977), 30 may be detected and often diagnosed by finding their eggs or larvae in the faeces of infected people. The preferred habitat of many of these helminths is the alimentary tract itself and the continual flow of material through the food tube provides a simple, passive and reliable means of transport and eventual exit from the host for the reproductive stages of a worm. Helminth eggs are generally less than 150 μm in diameter and although individually comprise a small material investment by the adult worm in the propagation of the species, the chances of infecting a host are so slim (though this may be a teleological inference) that adult worms are often highly fecund: a female Ascaris lumbricoides is estimated to release daily into the intestine approximately 200,000 eggs (WHO, 1967). Yet numbers should not always be equated with great substance or bulk – an equally important determinant of the chances of finding an egg in faeces: assuming a spherical diameter of 60 μm and a specific gravity of 1·2 (Sawitz, 1942) then a female Ascaris is estimated to produce only about 25 mg of eggs each day, which is 0·01% of a moderate daily output of 250 g of faeces.

The trematode and cestode flatworms include numerous parasitic forms of major medical and economic importance. A better knowledge of the neuromuscular physiology of these animals could lead to development of new control measures against these parasites. Since these animals are near the stem from which all other animals have evolved, better knowledge of these animals could also yield valuable information about the early evolution of nerve and muscle systems in the animal kingdom. This review focuses on what is known about the characteristics of the somatic muscle in these animals. The anatomy of the muscles is described along with a review of current information about their electrophysiology, including descriptions of the ion channels present. Also included is a summary of recently acquired data concerning the nature of serotonin, peptide, acetylcholine and glutamate receptors on the membranes of the muscles.

The DNA content of Plasmodium falciparum gametocytes during intra-erythrocytic development and during gametogenesis was established by cytophotometric methods. Intra-erythrocytic micro- and macrogametocytes (Stage I-Stage VB) contain about twice the amount of DNA of haploid sporozoites and ringstages, indicating that DNA is synthesized during transformation of ringforms into Stage I gametocytes. Microgametocytes, after activation at pH 8, rapidly duplicate their genome several times, while the DNA content of macrogametocytes remains constant during gametogenesis.

The cuticle of the third-stage larva of Nippostrongylus brasiliensis consists of seven layers: an outer triple-layered membrane, a double-layered outer cortex, an inner cortex, a matrix layer, a striated layer and two fibril layers. In each ‘annule’ two fibres run transversely around the nematode and lie between the inner cortex and the matrix layer. There is no basement lamella.

The hypodermis is a thin layer between the muscles and the cuticle, but expands to form the dorsal, ventral and lateral cords. The nerves lie between the plasma membrane of the hypodermis and the basement membrane or between the plasma membrane of the hypodermis and the sarcolemma of the muscles. The muscle cells are typical of those described previously for nematodes. The ‘myofibrils’ are apparently similar to those of Ascaris.

An excretory canal is present in each lateral cord and is enclosed by the basement membrane but is not embedded in the hypodermal tissue. Numerous small vesicles appear to move across the wall of the excretory canal and open into the central lumen.

I am grateful to Dr P. Tate, Dr R. W. Horne and Dr K. A. Wright for helpful discussions, to Professor C. P. Read and Dr A. Enders for the use of facilities at Rice University, Houston, Texas and to Professor J. D. Boyd for permission to use the electron microscope in the Department of Anatomy. Thanks are also due to Mr A. J. Page for technical assistance.

Various authors have recorded the seasonal occurrence of different species of nematode parasites in sheep. Some of these (Ross & Gordon, 1936; Gordon, 1950; Taylor, 1934) have been mainly interested in the epidemiology of outbreaks of parasitic disease, but others have recorded the incidence of different species in flocks in which no outbreak has occurred (Gordon, 1948, 1950; Morgan & Oldham, 1934; Morgan & Sloan, 1947; Morgan, Parnell & Rayski, 1950; Parnell, 1954; Parnell, Rayski, Dunn & Mackenzie, 1954; Stewart & Douglas, 1938; Tetley, 1935, 1941, 1949; Wilson, Morgan, Parnell & Rayski, 1953).

Recrudescing Plasmodium falciparum parasitaemia is attributed to the switching of PfEMP1, a variant antigen family encoded by the var gene repertoire, and the host's immune response. We have developed a mathematical model which incorporates var gene switching, and variant specific, non-variant specific and non-specific immunity. By conducting a sensitivity analysis of the model we have defined the parameter limits which produce chronic and recrudescing infections. We explore 3 switching mechanisms: ordered, random and uncoupled switching. We show that if var genes switch on and off independently at variable rates through the repertoire a chronic clinical infection is predicted. The fastest switching-on rate that produces a chronic infection is 0·03% per generation. The model predicts that non-variant specific immunity plays an important role in reducing disease severity. This work illustrates the complex relationship between the malaria parasite and its host and shows that var gene switching at rates substantially slower than 2% are essential for parasite survival.

There is unequivocal evidence that parasites influence the immune activity of their hosts, and many of the classical examples of this are drawn from assessment of helminth infections of their mammalian hosts. Thus, helminth infections can impact on the induction or course of other diseases that the host might be subjected to. Epidemiological studies demonstrate that world regions with high rates of helminth infections consistently have reduced incidences of autoimmune and other allergic/inflammatory-type conditions. Here I review and assess the possible ways by which helminth infections can block or modulate concomitant disease processes. There is much to be learned from careful analysis of immuno-regulation in helminth-infected rodents and from an understanding of the immune status of acutely and chronically infected humans. The ultimate reward from this type of investigation will likely be a more comprehensive knowledge of immunity, novel ways to intervene in the immune response to alleviate autoimmune and allergic diseases (growing concerns in economically developed areas), and perhaps the development of helminth therapy for patients suffering from specific inflammatory, autoimmune or allergic disorders.

Neospora caninum causes economical impact in cattle-raising farms since it is implicated as the major cause of bovine abortions. Although infection by the parasite has been widely described in mammals, the role of birds in its life-cycle is still obscure. Therefore, this work aimed to evaluate the infection by N. caninum in different chicken models. Experimental infections were conducted in 7-day-old chicks, laying hens and embryonated eggs, where samples were analysed for parasite burden, IgG antibodies and lesions promoted. Chickens demonstrated an asymptomatic infection, although with seroconversion and systemic replication of the parasite. In laying hens, no signs of vertical transmission were observed. However, embryonated eggs inoculated by the allantoic cavity route demonstrated susceptibility to infection, with mortality rates around 50% independent of the inoculum dose. Additionally, dogs became infected after ingestion of different amounts of inoculated eggs, producing either oocysts or specific IgG antibodies. The results herein presented demonstrate that chickens may be intermediate hosts of N. caninum and that embryonated eggs could be a useful model to study the parasite's biology.

When measured 56 days postinfection the length, wet weight and dry-weight of Hymenolepis diminuta were all found to decrease with increasing number of cysticercoids given up to 20. The mean position of the worms in 10, 12 and 20 worm infections is significantly posterior to that of 1, 2 and 5 worm infections and the worms are attached over a wider area of the intestine.

Egg production by the worms was followed up to day 56 postinfection; the number of eggs produced per worm and even per rat decreased with increasing population density. Thus the best way to get most eggs and to maintain the parasite in the laboratory is to have rats infected with only one tapeworm.

Rats given 1–20 cysticercoids showed a mean recovery of 100–65%, while rats given 40–200 cysticercoids showed a mean recovery ranging from 13 to 2%. In addition to ‘normal’ worms, defined as worms > 10 mm, small, most probably destrobilated, worms were found. In the 50 and 100 cysticercoid infections, worm recoveries were, respectively, 8% ‘normal’, 16% small, and 2% ‘normal’, 5% small. From the significantly lower recovery from heavy infections it is concluded that a deleterious factor is operating during the 8 weeks after the infection.

To determine whether the relative abundance of immature Ixodes dammini (the vector of Lyme disease and human babesiosis) is related to habitat structure, we examined tick burdens on their main host, the white-footed mouse (Peromyscus leucopus), in 4 structurally diverse sites on Great Island, Massachusetts, USA. Vegetation structure at each site was quantified with respect to 25 habitat variables. Principal components analysis was used to reduce this set of habitat variables to seven new and orthogonal variables. Immature tick abundance varied widely among grids. Regression analysis of tick burdens on the habitat principal components showed that larval burdens were related strongly to the density of woody vegetation and negatively to herbaceous vegetation. Nymphal burdens were related negatively to herbaceous vegetation, but the relationship was not as strong as in the case of larvae. An experimental reduction in the abundance of white-tailed deer (Odocoileus virginianus), the main host of adult ticks, substantially reduced tick burdens and altered their relationships to habitat structure. Nymphal burdens were unrelated to habitat structure following deer removal. Manipulating habitat structure may have utility as a control strategy against this important vector.

Egg production by the freshwater gill monogenean Discocotyle sagittata infecting rainbow trout (Oncorhynchus mykiss) was monitored at 5, 13 and 18°C; eggs were incubated at 6, 10, 13, 15, 16, 18, 22 and 26°C and the hatching times determined. Mean per capita output at 13°C ranged from 0·8 eggs/worm/day (e/w/d) to 18·3 e/w/d for 25 hosts (burdens 1–17 worms/host). Average egg production rates for worms in single burdens varied from 3 e/d to 14·6 e/d at 13°C, mean 7·6 e/d (±0·63) (n=8). Egg production was temperature dependent, increasing from a mean of 1·5 e/w/d at 5°C to 12·0 e/w/d at 18°C. Developmental rate was also temperature dependent: larvae began hatching after 84 days at 6°C, 28 days at 13°C, and 20 days at 18°C. Eggs laid within the same 24 h period hatched over 3–7 days at 13–18°C; this range was extended at decreasing temperatures. Egg viability was generally high: between 13 and 18°C a mean of 92% of larvae successfully emerged. However, viability declined at the highest and lowest temperatures (23% hatching at 6°C, 55% at 8°C and 47% at 26°C). The implications for seasonal parasite transmission are discussed.

A gene, CRP136, from the intestinal protozoan parasite Giardia duodenalis, expressed at a high level in a metronidazole-resistant line, encodes a 136 kDa protein with 23 copies of a 40 amino acid repeat. The protein is cysteine-rich and has the typical membrane-spanning region and CXXC amino acid motifs of a family of Giardia cysteine-rich surface proteins (CRSPs). The repeat unit in CRP136, shares 57% homology with the gene encoding the precursor of the sarafotoxins, a group of snake toxins from the burrowing adder known to cause symptoms similar to those of humans acutely infected with Giardia. The sarafotoxins are low molecular weight sulphydryl cross-linked peptides which are proteolytically cleaved from a precursor polyprotein. CRP136 has homology over the entire length of the sarafotoxin precursor, and the repeats are of the same length. Thus CRP136 represents the first evidence for a potential Giardia toxin. The genomic copy number of CRP136 appears to be the same in both the parent and drug-resistant lines and expression of this gene, and at least one other, is associated with a conserved partial duplication, but not amplification, of one chromosome.

Plasmodium mexicanum is a common malarial parasite of the western fence lizard, Sceloporus occidentalis, in northern California, USA. Infected female lizards store substantially less fat during the summer activity season and produce smaller clutches of eggs than do non-infected animals. Stored fat is utilized in the production of eggs; the energy content of the decrement in stored fat is approximately equal to the energy content of the average reduction in number of eggs. Thus, there is ongoing strong selective pressure on the host to evolve appropriate anti-parasite measures.