Transmission patterns and the epidemiology of hookworm infection

BACKGROUND

This paper presents a suite of models of hookworm transmission dynamics which vary the mixing patterns and rates of contamination and infection between children and adults. In this context mixing refers to the degree of epidemiological communication between children and adults, for example, whether adults are likely to get infected from infective material passed by children.

METHODS

Three models are described which represent random mixing, no mixing and restricted mixing respectively. Child, adult and population targeted chemotherapy programmes are examined and compared between these models. Data from a hookworm control programme in Zimbabwe were analysed with respect to their fit to the various models.

RESULTS

The analysis suggests that some mixing does occur and that in this study location, the sites where adults deposit faeces are more likely to lead to subsequent contamination than the sites children use.

CONCLUSIONS

Mixing patterns may have a profound effect on transmission dynamics and should be considered in relation to design of control programmes.

Water level regulation and control of schistosomiasis transmission: a case study in Oyan Reservoir, Ogun State, Nigeria

The effect of different water discharge patterns from the artificial Oyan Reservoir in Ogun State, Nigeria, on water level fluctuations and on schistosomiasis transmission was investigated between August 1990 and March 1993 to determine the impact of water level regulation on schistosomiasis transmission and control. The results show that transmission was greatly influenced by the pattern of water discharge during the hot dry season (January-April). A high discharge during this period of no rainfall, high temperatures, and intense sunshine stimulated rapid water level fluctuations and lake draw-down, which led to significant reductions in all indices of schistosomiasis transmission, i.e. snail density, snail infection rates, human water contact patterns, and incidence of infection. Although these results support continuous water discharges from the reservoir during the hot dry season, this may run counter to current water management policies. Further investigation is therefore required to harmonize the potential benefits in this type of discharge pattern with the objectives of efficient water management in artificial reservoirs.

Patterns of infection and transmission of human schistosomiasis mansoni and schistosomiasis haematobium in White Nile Province, Sudan

The overall prevalences of infection with Schistosoma mansoni, S. haematobium or both species among 6122 children from 27 schools in the former White Nile Province of Sudan (now divided into the Kosti, El Getaina, El Gebelein and El Duem Provinces) were 10.1%, 21.4% and 4.5%, respectively. Geometric mean egg counts in the pure S. mansoni and pure S. haematobium infections were 97.7 eggs/g faeces and 12.9 eggs/10 ml urine, respectively. There were marked differences in levels of endemicity between provinces and also between schools within each province. School-specific prevalences and intensities of infection were positively correlated, both for S. mansoni and for S. haematobium. Prevalences and intensities of infection were generally similar in male and female subjects and in the different age groups into which they were split (i.e. 5-9, 10-14 and 15-19 years). Community-based surveys of all age groups in two villages showed typical patterns of infections with S. mansoni and S. haematobium, with indices peaking in those aged 10-19 years, both in males and females. Observations on snail-related aspects of transmission revealed that transmission of both S. mansoni and S. haematobium was highly focal and also seasonal, taking place during the hot, dry and post-rainy seasons.

Urban schistosomiasis in Lusaka, Zambia: a preliminary study

A preliminary survey was conducted in Lusaka, Zambia to determine the extent of schistosomiasis transmission in the city. Stool and urine samples were collected from 240 children from seven schools selected to give a cross-sectional representation of the various socio-economic neighbourhoods of the city. In addition to the parasitological examination, students were questioned with respect to their knowledge of the disease and other epidemiological factors. In addition, water bodies near the schools were examined for the presence of snail hosts. Results of the survey suggest that schistosomiasis is actively transmitted within Greater Lusaka. An overall prevalence of 20.9% for S. haematobium was observed, but only one case of S. mansoni was detected. Prevalence rates for S. haematobium in individual schools ranged from 7.5 to 37.5%.

Household epidemiology of Cryptosporidium parvum infection in an urban community in northeast Brazil

OBJECTIVE

To examine the transmission of Cryptosporidium infection in households with an identified person with cryptosporidiosis.

DESIGN

Prospective cohort study.

SETTING

An urban slum in Fortaleza, Brazil.

PARTICIPANTS

Thirty-one households with a child less than 3 years of age (index case) who was positive for Cryptosporidium parvum using acid-fast and auramine-stained stool smears.

MEASUREMENTS

Three stool samples (at 0, 2, and 6 weeks after identification of the index case) and two serum samples (0 and 6 weeks) were collected from each family member in households with an index case of Cryptosporidium infection.

RESULTS

Forty-five percent of index cases of Cryptosporidium infection were associated with persistent (> 14 days) diarrhea. Secondary cases of Cryptosporidium infection were identified either by stool examination or seroconversion in 18 (58%) of 31 households involving 30 persons, yielding an overall transmission rate of 19%. Of the 202 persons in this study with at least one serum sample available for analysis, 191 (94.6%) had evidence of antibodies (either IgM or IgG) to Cryptosporidium.

CONCLUSIONS

Cryptosporidium parvum is highly transmissible and infective in the family setting, with transmission rates similar to other highly infectious enteric pathogens such as Shigella species. These data are cause for added concern because of the rapidly increasing rate of seropositivity for human immunodeficiency virus.

Transmission of Schistosoma mansoni in Rhino Camp, Uganda

Non-participant observations totalling 204 hours relevant to the transmission of Schistosoma mansoni infection were carried out in Rhino Camp at the shores of Albert Nile in North Uganda. A cross-sectional study of 636 individuals from Rhino Camp revealed a prevalence of S. mansoni infection of 77.8%. Occupational and domestic purposes were the most important reasons for water contact, whereas recreational purposes ranked lower and mainly concerned children. Both sexes were equally active in water contacts. A distinct preference of Nile water was noted despite availability of borehole water in the area. It is concluded that control measures against schistosomiasis have to take into consideration that water contact for recreational purposes might be minimized, whereas it is expected to be extremely difficult to reduce occupational and domestic water contacts.

Dracunculiasis (guinea worm disease) in the Bume (Nyangaton) people of South Omo, Ethiopia

A village-to-village search for active dracunculiasis cases was carried out in an endemic area of the Bume (Nyangaton) tribe of South Omo Region, Ethiopia. A total of 21 cases, of which 6, 5, and 10 had pre-emergent, emergent and complicated Guinea worm disease, respectively, were identified. Twenty-two worms, ranging from 1-3 per patient, were removed mainly from the lower limbs; worm appearance seems to be associated more with the right limb. Adults between the ages of 20-30 years are highly affected and infection appears to be sex-related as 14/21 (66.7%) of the cases are females. Water procured from water-holes drug in dry river beds provides an ideal situation for the transmission of dracunculiasis amongst the tribesmen. The knowledge, attitudes and perceptions of the Bume people towards the disease and the public health significance of dracunculiasis are discussed in relation to the current goal of the national and global Guinea worm eradication programme.

[An epidemiological survey to discover the probable places of infection with sleeping sickness in the Central African Republic]

The sleeping sickness focus at Nola-Bilolo in the forest region was the subject of a survey of 142 patients in order to discover their probable places of infection. Seventy of these patients had been detected by active case-finding during a survey conducted in January-February 1991; the other 12 had been discovered by passive case-finding during 1990. The sample of actively detected patients is significantly younger (average 22 +/- 3 years) than the sample detected passively (30 +/- 3 years). This survey highlights the geographical heterogeneity of transmission in the focus. The places where infection is highest are M'Poyo, which with 18% of cases forms the epicentre of the focus and the main reservoir of infecting flies, followed by Bilolo (9%), Mékara (8%), Modigui-Kouna, Ziendi and Domissili (7%). The patients, 95% of whom belong to the M'Bimou tribe, all engage in agriculture; the main activities are coffee-growing for men and the steeping of casava in the river for women. These two activities determine the main places and times of human-tsetse contact. The information obtained by listing the cases detected passively from medical registers is inadequate for locating the places of transmission. The information gathered during active case-finding campaigns by the mobile teams offers a better approach. The method used here, which is recommended, consists of matching the replies given by the patients about their points of contact with the tsetse flies, not just with their places of residence but also with their successive movements and their places of work. The rigour demanded by this kind of approach can only be ensured by a specific survey. This survey also made it possible to demonstrate substantial movements from village to village, which appear to be characteristic of this focus. The results indicate that poorly targeted vector control is likely to be doomed to failure. The study makes it possible to specify priority areas for vector control by trapping and to avoid a "blind" control strategy whereby the whole focus is swamped with traps. The latter strategy is not financially feasible for many countries of tropical Africa.

The prevalence of Toxoplasma infection among pregnant women in Ibadan, Nigeria

The seroprevalence of toxoplasmosis in pregnant women from the inner area of Ibadan was determined by the dye test. Two hundred and seventy-three of the 352 women (78%) had dye test titres of 1/16 or greater with 165 (47%) having titres of 1/128 or greater. In contrast, 42 of 192 pregnant women (22%) from the Swansea area of the UK were dye test positive with only six (3%) having titres of 1/128. The possibility that reinfection or recrudescence is responsible for maintaining high antibody levels in African women and the consequence of this high level of infection in relation to potential human immune deficiency virus (HIV) infections are discussed. Social and environmental conditions indicate that the source of infection is contact with cat faeces.

Vector-borne disease problems in rapid urbanization: new approaches to vector control

Owing to population growth, poor levels of hygiene, and increasing urban poverty, the urban environment in many developing countries is rapidly deteriorating. Densely packed housing in shanty towns or slums and inadequate drinking-water supplies, garbage collection services, and surface-water drainage systems combine to create favourable habitats for the proliferation of vectors and reservoirs of communicable diseases. As a consequence, vector-borne diseases such as malaria, lymphatic filariasis and dengue are becoming major public health problems associated with rapid urbanization in many tropical countries. The problems in controlling these diseases and eliminating vectors and pests can be resolved by decision-makers and urban planners by moving away from the concept of "blanket" applications of pesticides towards integrated approaches. Sound environmental management practices and community education and participation form the mainstay of some of the most outstanding successes in this area. On the basis of these examples, it is argued that the municipal authorities need to apply a flexible methodology, which must be based on the possibilities of mobilizing community resources, with minimal reliance on routine pesticidal spraying. In this way, vector control becomes a by-product of human development in the city environment. This is now a true challenge.