A supervised land cover classification of a western Kenya lowland endemic for human malaria: associations of land cover with larval Anopheles habitats.

BACKGROUND: A supervised land cover classification was developed from very high resolution IKONOS satellite data and extensive ground truth sampling of a ca. 10 sq km malaria-endemic lowland in western Kenya. The classification was then applied to an investigation of distribution of larval Anopheles habitats. The hypothesis was that the distribution and abundance of aquatic habitats of larvae of various species of mosquitoes in the genus Anopheles is associated with identifiable landscape features. RESULTS AND DISCUSSION: The classification resulted in 7 distinguishable land cover types, each with a distinguishable vegetation pattern, was highly accurate (89%, Kappa statistic = 0.86), and had a low rate of omission and commission errors. A total of 1,198 habitats and 19,776 Anopheles larvae of 9 species were quantified in samples from a rainy season, and 184 habitats and 582 larvae from a dry season. Anopheles gambiae s.l. was the dominant species complex (51% of total) and A. arabiensis the dominant species. Agricultural land covers (mature maize fields, newly cultivated fields, and pastured grasslands) were positively associated with presence of larval habitats, and were located relatively close to stream channels; whilst nonagricultural land covers (short shrubs, medium shrubs, tall shrubs, and bare soil around residences) were negatively associated with presence of larval habitats and were more distant from stream channels. Number of larval habitats declined exponentially with distance from streams. IKONOS imagery was not useful in direct detection of larval habitats because they were small and turbid (resembling bare soil), but was useful in localization of them through statistical associations with specific land covers. CONCLUSION: A supervised classification of land cover types in rural, lowland, western Kenya revealed a largely human-modified and fragmented landscape consisting of agricultural and domestic land uses. Within it, larval habitats of Anopheles vectors of human malaria were associated with certain land cover types, of largely agricultural origin, and close to streams. Knowledge of these associations can inform malaria control to gather information on potential larval habitats more efficiently than by field survey and can do so over large areas.

Designation of chemicals in terms of the locomotor responses they elicit from insects: an update of Dethier et al. (1960).

A scheme updating that of Dethier et al. (1960) (J. Econ. Entomol. 53: 134-136) for chemicals influencing insect locomotor behavior is introduced. Attractant, repellent, and arrestant retain their previous definitions. However, attractants or repellents are now recognized to operate both by kinetic and tactic mechanisms. Locomotor initiator is a new term for stimuli that activate normal levels of kinetic locomotion. Locomotor stimulant is reserved for activation of abnormally high kinetic locomotion, like that arising upon sublethal exposure to certain insecticides. The new terms engagent and disengagent apply to chemicals that, by their effects on locomotion, increase or decrease interaction with the source of stimulation, respectively. With these clarifications, insect behavioral terms unique to medical entomology but contradicting Dethier et al.'s classical scheme can be reconciled with the vocabulary of formal behavioral science.

Visceral leishmaniasis and malaria prevalence in West Pokot District, Kenya.

The prevalence of visceral leishmaniasis and malaria in the human population of West Pokot district of Kenya was studied in 1986. A total of 2139 people was proportionately screened for the two diseases according to four age categories (0-4, 5-14, 15-44 and greater than 45 years). Diagnostic methods included the enzyme linked immunosorbent assay (ELISA) and Leishmanin skin test for visceral leishmaniasis, and parasitological examination for malaria. The epidemiological value of the spleen rate was evaluated in relation to visceral leishmaniasis and malaria endemicity. A general decline of infection rates with altitude was observed for both diseases. Visceral leishmaniasis was less prevalent than malaria, with less than 2% active cases in any age group and had the same distribution in both sexes. Malaria infection rate was highest in the younger age groups, declining from 21.5% in the 0-4 year old age group to 5.5% in people more than 45 years old. Malaria affected significantly more males than females. The spleen rate was inappropriate for epidemiological survey of either malaria or visceral leishmaniasis due to an overlap in the distribution of the two diseases.

Choice of resting sites by Anopheles gambiae (Diptera: Culici) in Mwea Rice Irrigation Scheme, Kirinyaga District, Kenya.

Investigations on Anopheles gambiae were carried out in Mwea Rice Irrigation Scheme, Kenya, to determine their preference for resting on various colours of fabrics and bare walls inside rural houses. The inside wall surface of each house was divided into upper, middle and lower sections and the section further partitioned into subsections measuring 0.6 x 0.3m. Two sets of experiments were conducted: one in which the walls were partly fitted with one colour of fabric at a time (double-choice situation), while in the other the walls were fitted with a combination of six colours (red, black, yellow, white, green and blue) of fabric at once, (multiple-choice situation). The number of mosquitoes resting on each of the pieces of cloth pinned to the walls and the uncovered subsections of the wall were recorded for each section of the wall after 24 hours. The results showed resting preference for cloth covered parts of the walls. The white colour was found to best attract mosquitoes followed by colours red, yellow, black, blue and green. The hanging strips in the middle of the houses had fewer mosquitoes resting on them than those on the wall.

Cutaneous leishmaniasis in Kenya: Sergentomyia garnhami (Diptera Psychodidae), a possible vector of Leishmania major in Kitui District: a new focus of the disease.

Investigations on phlebotomine sandflies in Tseikuru, Kitui District, Kenya were carried out to determine the vectors of Leishmania major. Sandflies were trapped from animal burrows over a period of one year using castor oil-smeared sticky traps. Nineteen phlebotomine sandfly species were identified and their prevalence in this habitat determined. Dissections on the parous sandflies resulted in the isolation of Leishmania parasites from Phlebotomus martini and Sergentomyia garnhami. The isolated parasites were cultured in NNN medium and were consequently mass cultured in RPMI medium for biochemical characterisation. The parasites from S. garnhami were typed as Leishmania major.

Description and performance of an updraft trap for sandflies.

An updraft trap for sampling sandflies in the field was developed and tested in the Marigat area of Baringo District, Kenya. The main components of the trap were a 12 cm long plastic drain pipe, 9 V d.c. electric motor, aluminium fan and a perspex sandfly collection cage. Comparisons of the updraft trap with a CDC and a 1 x 1 m polythene sheet coated with castor oil (sticky trap) showed the former was more consistent than the other traps in sampling sandflies from animal burrows. A total of 1241 sandflies belonging to eleven species were collected. Sergentomyia bedfordi, Phlebotomus martini, S. antennatus and P. duboscqi were the commonest species comprising 32.5%, 27.0%, 21.1% and 11.3% of the total catch respectively. However, the relative proportion of the different species varied according to the type of trap used. The CDC trap apparently attracted the known vectors of leishmaniases in the area, P. martini and P. duboscqi. Significantly more male than female P. martini and S. antennatus were collected by the various traps. A significantly higher proportion of female S. bedfordi was captured by the updraft and sticky traps while equal numbers of male and female P. duboscqi were collected in all traps.