John R. Seed

The epidemiology of Ascaris lumbricoides, Trichuris trichiura, and hookworm in children in the Ranomafana rainforest, Madagascar.

An epidemiological study of intestinal nematodes was conducted with 1,292 children, ranging from birth through 11 yr old, living in the Ranomafana rainforest of southeast Madagascar. Fecal examinations revealed prevalences of 78% for Ascaris lumbricoides, 38% for Trichuris trichiura, 16% for hookworm, and 0.4% for Schistosoma mansoni. Infection intensity was measured indirectly by fecal egg counts and directly by A. lumbricoides expulsion following treatment with pyrantel pamoate. The mean A. lumbricoides worm burden for children, 5-11 yr old, was 19.2 (SD 20.4) worms per child, with a median of 13 worms (n = 428). The distributions were overdispersed for all 3 nematodes. The age profiles showed a rapid acquisition of A. lumbricoides during infancy, increasing to 100% prevalence by age 10. After mebendazole anthelmintic treatment and a 12-mo reinfection period, the nematodes had rebounded to pretreatment prevalence and intensity levels. There was evidence for age-dependent predisposition of the children to infection intensity for each of the 3 nematodes. Dual species intensity correlation was consistently strong for A. lumbricoides and T. trichiura. The significantly higher prevalence and intensity of ascariasis in girls were thought to be related to exposure.

ASCARIS LUMBRICOIDES INTENSITY IN RELATION TO ENVIRONMENTAL, SOCIOECONOMIC, AND BEHAVIORAL DETERMINANTS OF EXPOSURE TO INFECTION IN CHILDREN FROM SOUTHEAST MADAGASCAR

Ascaris lumbricoides worm counts were examined as the outcome products of exposure proxy variables. A survey of 663 children, 4-10 yr old, living in southeastern Madagascar revealed prevalences of 93% for A. lumbricoides, 55% for Trichuris trichiura, and 27% for hookworm. Worm expulsions were conducted on 428 of these children; the data revealed an overdispersed distribution of A. lumbricoides, with an arithmetic mean of 19.2 worms per child. A concurrent socioeconomic household survey was conducted by visitation and interview. Exposure to infection was assessed by environmental, demographic, behavioral, and socioeconomic indicators. Ascaris lumbricoides aggregations were associated with gender, housing style, ethnicity, and agricultural factors. The results suggest that exposure and infection are ubiquitous in this child population, and that A. lumbricoides intensity is influenced by gender-related behavioral and environmental factors that contribute to exposure.

Inhibition of hexose and glycerol utilization by 2-deoxy-d-glucose in Trypanosoma gambiense and Trypanosoma rhodesiense

Abstract It was shown that 2-deoxy-d-glucose (D-2-D) inhibited the fermentation of glucose, mannose, fructose, and glycerol by Trypanosoma gambiense . Evidence is presented that shows D-2-D inhibits metabolism at two sites at least within the Embden-Meyerhof-Parnas pathway. The inhibition of both hexose and glycerol utilization by D-2-D was reversed by the addition of glucose; however, inhibition of glycerol utilization was much more sensitive to reversal. A second analogue of glucose, glucosamine, was also capable of inhibiting glycerol utilization but had no apparent effect on the fermentation of glucose. The structural requirements of compounds capable of acting as inhibitors of hexose and glycerol utilization as well as those compounds capable of relieving glycerol inhibition were shown to be highly specific. At present, the exact enzymatic sites inhibited by either D-2-D or glucosamine are unknown. However, the possibility that D-2-D inhibits the transport of substrates across the cell membrane is discussed.

Growth of pleomorphic Trypanosoma brucei rhodesiense in irradiated inbred mice.

It was shown that irradiation (650 rad) of 7 inbred strains of mice did not block the ability of Trypanosoma brucei rhodesiense to transform from the long slender (LS) to the short stumpy (SS) form or alter the plateau in parasitemia. In addition, it was observed that significant differences in parasitemia levels, in the rate of transformation from the LS to the SS form, as well as in the survival times occurred between the irradiated C3HeB/FeJ and several of the other strains. These differences in the nonspecific ability to control parasitemia appeared to be characteristic for each inbred strain of mice. The resistant strains generally had lower parasitemia than the susceptible strains. However, it was also shown that there is not a one-to-one correlation between the innate ability of a mouse strain to control its initial parasitemia, and the strains ability to clear the parasitemia or increase its survival time. It was therefore concluded that the hypothesis which states that the ability of an animal to increase nonspecifically the rate of transformation, and therefore to lower the parasitemia, allowing intact animals to respond immunologically and survive longer is either incorrect or incomplete. The results further show that the ability of mice to clear their initial parasitemia by an antibody response is not necessarily correlated with their survival time. Therefore, this study suggests that factors other than an antibody response and the nonspecific control of parasitemia are important in resistance.

Nature of the trypanocidal factor in human serum.

The chemical nature of the trypanocidal factor in human serum was investigated. The results show that although the trypanocidal factor is contained within the high density lipoprotein (HDL) fraction of human serum, it is apparently not one of the major apolipoproteins of the HDL complex such as apolipoprotein A-I, A-II, or apolipoprotein B. The factor would appear to be a minor component of the HDL fraction whose chemical nature is still uncertain.

A proposed density-dependent model of long slender to short stumpy transformation in the African trypanosomes

A simple arithmetic model is developed that is based upon the assumption: (1) that transformation of replicating long slender Trypanosoma brucei to nonreplicating short stumpy forms is parasite population density dependent; (2) that as the slender population increases there is a change in the external environment that triggers the slender to stumpy transformation; and (3) that stumpy forms of T. brucei do not induce the change in external environment that triggers slender to stumpy transformation or do so to a lesser extent than slender forms, thus preventing the proportion of stumpy forms in a population from reaching 100%. A simulation based on these assumptions shared many features with curves on numbers of long slender, intermediate, and short-stumpy forms of T. brucei during the first parasitemic wave of the 3 T. brucei subspecies in intact and immunosuppressed inbred mice.

Ascaris lumbricoides aggregation in relation to child growth status, delayed cutaneous hypersensitivity, and plant anthelmintic use in Madagascar.

The relationships between Ascaris lumbricoides worm burden, growth status, general delayed cutaneous hypersensitivity (DCH) response, and plant anthelmintic use were investigated in a 12-mo prospective survey of 663 children, 4-10 yr old, living in the Ranomafana rainforest, Madagascar. Initial fecal examinations revealed prevalences of 93% for A. lumbricoides, 55% for Trichuris trichuria, and 27% for hookworm. After anthelmintic treatment and a 12-mo reinfection period, 428 of the children participated in worm expulsion studies using pyrantel pamoate, revealing an overdispersed A. lumbricoides worm population, mean = 19.2 (SD = 20.4). Malnutrition was common with 72% of the children growth stunted, 61% underweight, and 6% wasted. The children were also skin tested to recall antigens for DCH, with 94% reacting. The DCH immune response was significantly decreased in the malnourished children; however, DCH was not reduced in relation to increasing worm intensity. Growth status, growth velocity, and triceps skinfold did not vary significantly in relation to A. lumbricoides worm burden. Traditional plant anthelmintic treatment was effective in significantly reducing worm intensity. This study indicates that, in human communities where the children are predominately stunted, A. lumbricoides does not aggregate in the most malnourished or immunodepressed children.

A REVISED ARITHMETIC MODEL OF LONG SLENDER TO SHORT STUMPY TRANSFORMATION IN THE AFRICAN TRYPANOSOMES

An arithmetic model that closely approximates an African trypanosome infection in immunosuppressed mice is presented. The final model was based on an examination of the following parameters: the rate of long slender to short stumpy transition, the maximum percentage of long slender to short stumpy stages that can be induced, the survival time or half life of the short stumpy stage in vivo, and the rate (%) of long slender to short stumpy stage transition following the peak in transformation. The model is based on the assumption that the long slender to short stumpy transition is parasite population dependent and that in mice the long slender to short stumpy transition only begins when the trypanosome population reaches a density of 1 x 10(7) trypanosomes/ml. The model predicts that the parasitemia during the first several days of an infection is controlled solely by the kinetics of the transition of the dividing long slender stage to the nondividing short stumpy stage. It was not necessary to include in the model the hosts immune response in order to simulate the early growth kinetics of pleomorphic trypanosomes in infected mice.

RELATIONSHIP BETWEEN HUMAN SERUM TRYPANOCIDAL ACTIVITY AND HOST RESISTANCE TO THE AFRICAN TRYPANOSOMES

Results reported here show that humans have various levels of trypanocidal activity in their sera. This difference appeared stable when different samples were taken from the same individuals over time. It was not possible to account for the variability between individuals by obvious differences in health, nutrition, or living habits. In addition, the trypanocidal titers did not vary significantly when stored for various lengths of time at -70 C. To examine the relationship between the titer of trypanocidal activity in a host and the degree of human serum resistance of the challenge trypanosome inoculum, mice (C57BL/6J) were pretreated with various amounts of different human serum and then infected with clones having different degrees of resistance to human serum. It was demonstrated that host susceptibility to an African trypanosome infection depends upon 2 variables: the level of trypanocidal activity in individual human serum and the degree of human serum resistance of individual clones of African trypanosomes. Based upon the animal model presented here, it is hypothesized that this relationship is under selective evolutionary pressure and will influence the susceptibility of animals in endemic areas as well as the transmission of human trypanosomiasis.

CHARACTERIZATION OF HUMAN SERUM-RESISTANT AND SERUM-SENSITIVE CLONES FROM A SINGLE TRYPANOSOMA BRUCEI GAMBIENSE PARENTAL CLONE

A protocol was developed to select clones of Trypanosoma brucei gambiense having different levels of resistance to normal human serum. Human serum-resistant clones were selected from a single parental clone by continuous serum treatment of infected immunosuppressed mice. Human serum-sensitive revertant clones were also obtained by continuous passage of resistant clones in immunosuppressed mice but without human serum pressure. It has been demonstrated that our trypanosome clones express distinct but stable levels of resistance. The variant antigenic type of each clone was characterized serologically and by 1-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis. After selective pressure with human serum, variant antigen-type differences always occurred among clones in which different human serum susceptibilities were found. The work reported here demonstrates that in our T. brucei gambiense immunosuppressed mouse model there is a predictable association between variant antigen type and human serum resistance.