Anne E. Keymer

Gametocyte sex ratios as indirect measures of outcrossing rates in malaria

The frequency of recombination between unlike genotypes is central to understanding the generation of genetic diversity in natural populations of malaria. Here we suggest a way of investigating the problem which could complement conventional biochemical approaches to the population genetics of malaria. Sex allocation theory is one of the most successful areas of evolutionary biology. A well-supported prediction is that progressively less female-biased sex ratios are favoured with more outcrossing; equal numbers of males and females being evolutionarily stable in randomly mating outbred populations. We present a simple game theory model to support the idea that outcrossing rates in malaria will be correlated with the sex ratio of gametocytes in the peripheral blood of vertebrate hosts. Blood films from epidemiological surveys and culture-adapted isolates from Madang Province, Papua New Guinea, were used to estimate average gametocyte sex ratio of Plasmodium falciparum in the area. The geometric mean proportion of males in the population was 0.18 (95% confidence limits: 0.15-0.22). From our model, we estimate that, on average, 36% of zygotes are the result of outcrossing. This estimate assumes that most microgametes released following exflagellation are capable of fertilization. If, on average, fewer than about 70% of microgametes are capable of fertilization (as is the case in at least one other species of Plasmodium), the observed sex ratio would be consistent with between zero and 36% of zygotes being the result of outcrossing. These estimates suggest that there is usually a numerically dominant genotype in the gametocyte population in a blood meal, and that a considerable amount of selfing is occurring in P. falciparum populations in the Madang region, even though it is an area of intense year-round transmission.

Life history covariation in intestinal nematodes of mammals

Using data on 66 species from 18 families and 6 orders, we examine patterns of interspecific covariation in female size, egg size, time from infection to production of infective stages (prepatency period), duration of reproduction (patency period), and fecundity in mammalian intestinal nematodes. Nematode species with shorter prepatency periods are smaller, have lower rates of somatic growth, lower fecundity and shorter reproductive periods; those with longer prepatency periods have the opposite suite of characters. These patterns are very different from that found in interspecific analyses of life history variation in other taxa. This may be a consequence of the energy-rich environment intestinal nematodes exploit, though comparable studies of free-living nematodes or other soft-bodied invertebrate phyla have not yet been done. The advantages of delaying reproduction, with the subsequent increase in fecundity and reproductive lifespan, depend on a number of factors, such as the relative importance of prepatency in the determination of parasite generation times, and adult mortality rates. Contrary to previous claims, nematode egg size is shown to be highly variable (as variable as female size), yet this variation is not associated with

Reinfection with hookworm after chemotherapy in Papua New Guinea

Reinfection with hookworm (Necator americanus) following chemotherapy was studied over 2 years in a rural village in Madang Province, Papua New Guinea. The prevalence of hookworm infection had returned to pre-treatment levels after 2 years, and the geometric mean hookworm burden had returned to 58% of the pre-treatment value. The rate of acquisition of adult worms was independent of host age, and was estimated as a geometric mean of 2.9-3.3 worms/host/year (arithmetic mean 7.9-8.9 worms/host/year). There was significant predisposition to hookworm infection; the strength of this predisposition did not vary significantly between age or sex classes.

Helminth parasite richness among vertebrates

The numbers of intestinal helminth species (parasite richnesS) recorded from each of 488 vertebrate host species are compared using data compiled from the published literature. Associations between parasite richness, sampling effort, host size and host habitat (aquatic versus terrestrial) are assessed using a method designed to control for phylogenetic association. Parasite richness increases with the number of surveys on which each estimate of parasite richness is based (sampling effort). When the effects of sampling effort are controlled for, there remains a strong positive relationship between parasite richness and host body size. There is no tendency for aquatic hosts to harbour more parasite species than terrestrial hosts independently of differences in sampling effort and body size. The results are interpreted in the context of hosts representing habitats for parasite colonization, resource allocation between parasite species, and the age of the major mammalian radiations.

Factors affecting the reliability of the McMaster technique

Factors detracting from the reliability of faecal egg counts based on the McMaster technique include variation in flotation time (interval between loading chamber and counting eggs) and sample dilution (ratio of faecal material to salt solution). We recommend standardization of both these variables as normal laboratory procedure, and propose optima of a 30 minute flotation time and a sample dilution of 15 ml salt solution/g faeces for use of the McMaster technique in the estimation of the fecundity of Heligmosomoides polygyrus (Nematoda).

Heligmosomoides polygyrus (Nematoda): the influence of dietary protein on the dynamics of repeated infection

The influence of the protein component in the diet of the host on the population dynamics of gastrointestinal helminth infection was studied by using a mouse – H. polygyrus experimental model. Mice fed a 2% (by mass) protein diet ad libitum maintained body weight during the experiment, but gained weight steadily when fed a diet containing 8% (by mass) protein. When repeatedly infected with 5, 10, 20 or 40 larvae every 2 weeks, the mice fed the 2% (by mass) protein diet accumulated adult worms in direct proportion to exposure to the infective stages. Under similar infection régimes, mice fed an 8% (by mass) protein diet acquired a partly effective immunity to reinfection by the nematode. Acquired immunity was principally manifest as a reduction in the survival of adult worms, although a slight increase in the mortality rate and/or the development time of the tissue-dwelling larval phase was observed. Worm fecundity per head was significantly depressed in hosts fed the 8% protein diet. In conclusion, in these experiments it is demonstrated that the nutritional status of the host can influence the population dynamics of helminth infection.

Isotypic variation in antibody responses in a community in Papua New Guinea to larval and adult antigens during infection, and following reinfection, with the hookworm Necator americanus

Summary The natural infection of a community with the hookworm Necator americanus induces a vigorous humoral response to both larval and adult parasite antigens. This response occurs in all five human antibody isotypes, and data are presented to show that, at the population level, isotypes respond differently, following chemotherapy and during reinfection, to changes in antigen stimulation. The differential response probably reflects the fact that the parasite, during the course of its life cycle, presents different amounts of antigens at different anatomical locations. It is suggested that IgG and IgM responses against adult excretory‐secretory (ES) products most accurately reflect the efficacy of chemotherapy, and the load of resident adult infection, while IgG responses against larval somatic antigens reflect continuous exposure to infection. These hypotheses should now be tested, at the level of the individual, in a longitudinal manner using more closely spaced sampling intervals. This repetitive sampling, and the inclusion of a measure of the exposure of the population to infective stages, will allow more definitive conclusions to be made about the role of the immune response in controlling worm burdens.

HOOKWORM (NECATOR AMERICANUS) INFECTION AND STORAGE IRON DEPLETION

The relationship between iron status and the intensity of infection with hookworm was investigated in a rural population on Karkar Island, Mandang Province, Papua New Guinea. There was a significant negative correlation between plasma ferritin level and hookworm burden, which was strongest in males. In contrast, there was no correlation between plasma ferritin and hookworm egg count, and no consistent correlation between haemoglobin level or haematocrit and either measure of hookworm intensity. The results suggest that the role of hookworm in the aetiology of anaemia may be difficult to assess without the accurate measurement of hookworm burden.

Antigenic cross-reactivity between Necator americanus and Ascaris lumbricoides in a community in Papua New Guinea infected predominantly with hookworm

Sero-epidemiological data are presented in which antigenic cross-reactivity between Necator americanus and Ascaris lumbricoides has been investigated in a community in Papua New Guinea infected predominantly with N. americanus. It is our contention that the antigenic cross-reactivity which undoubtedly exists between these species accounted for (i) a peak in antibody levels against N. americanus in 10-13 years old children (driven by infection with A. lumbricoides), and (ii) the maintenance of apparent antibody levels against A. lumbricoides in older age groups (driven by infection with N. americanus in the absence of overt infection with A. lumbricoides). Cross-reactivity was analysed further, and apparently N. americanus-specific epitopes identified, by immunoblotting. These observations could have considerable bearing on the interpretation of data from sero-epidemiological studies which failed to take account of concurrent infection with these parasites.

The influence of protein deficiency on immunity to Heligmosomoides polygyrus (Nematoda) in mice

Summary The influence of dietary protein on the efficiency with which mice could be immunized against infection with the nematode Heligmosomoides polygyrus was investigated. Immunization with irradiated larvae did not protect outbred mice fed synthetic diets containing 2% or 4% protein against a challenge infection, while animals fed a diet containing 8% protein were significantly resistant. In further experiments with high‐responder NIH mice, protein malnutrition was again found to cause a significant depression in immunity. Immunization primed all mice for an intense production of antibody against larval worms in a challenge infection, and although a slightly higher litre of antibody was detected in the plasma of mice fed a 16% compared with a 2% protein diet it seemed unlikely that this was sufficient to account for the reduced resistance of the malnourished mice. The development of eosinophilia in the blood of immunized mice was significantly delayed in malnourished animals following challenge, and it is suggested that a reduction in the number of granulocytes attacking larval worms contributed to the low level of resistance observed in these animals. Protein malnutrition thus markedly suppresses the effectiveness of immunization of mice against an intestinal nematode, and it is suggested that this result may be of general significance with regard to the potential for widespread immunization of people against infections of this type.