Very high DDT-resistant population of Anopheles pharoensis Theobald (Diptera: Culicidae) from Gorgora, northern Ethiopia.

Standard WHO insecticide bioassay tests were carried out in Gorgora, northern Ethiopia to evaluate the susceptibility status of Anopheles pharoensis Theobald for the insecticides DDT, malathion, permethrin and deltamethrin. The mortality and when appropriate knockdown effect of the insecticides were observed. The results indicated that this species was resistant to DDT. A high mortality was obtained after exposure to permethrin and deltamethrin but below 97 % which is the limit for susceptibility according to WHO. A prolonged knockdown time was noted for DDT and the two pyrethroids. An. phoaroensis was found to be susceptible to malathion.

The isolation of Leishmania tropica and L. aethiopica from Phlebotomus (Paraphlebotomus) species (Diptera: Psychodidae) in the Awash Valley, northeastern Ethiopia.

In a survey of Leishmania infections in phlebotomine sandflies in a highly suspected focus of leishmaniasis in the Awash Valley (northeastern Ethiopia) between January 1994 and August 1997, a total of 3307 females of 11 Phlebotomus species (P. orientalis, P. fantalensis, P. saevus, P. sergenti, P. gemetchi, P. alexandri, P. bergeroti, P. duboscqi, P. arabicus, P. martini, and P. rodhaini) were dissected. Promastigotes were detected in 17 females of three species (11 P. saevus, 4 P. sergenti and 2 P. arabicus). Of these, only two P. saevus (one from Upper Awash and one from Middle Awash) and three P. sergenti (from Upper Awash) positives were successfully isolated in culture and were typed by isoenzyme analysis. Four isolates (two each from P. saevus and P. sergenti) were identified as new zymodemes (Z) of L. tropica and one isolate from P. sergenti was typed as a new zymodeme of L. aethiopica. This is the first finding of natural infections of P. saevus and P. arabicus and the first evidence for the former to be a vector of L. tropica. This is also the first time P. sergenti has been implicated in L. tropica transmission in Ethiopia; the isolation of L. aethiopica from a Paraphlebotomus species (P. sergenti) is also a new record. The possible presence of human cutaneous leishmaniasis (L. tropica and L. aethiopica), and wild reservoir host(s) of the parasites, especially rock hyrax (Procavia capensis) in the Upper and Middle Awash Valley remain to be determined.

Mapping the potential distribution of Phlebotomus martini and P. orientalis (Diptera: Psychodidae), vectors of kala-azar in East Africa by use of geographic information systems.

The distribution of two principal vectors of kala-azar in East Africa, Phlebotomus martini and Phlebotomus orientalis were analysed using geographic information system (GIS) based on (1) earth observing satellite sensor data: Normalized Difference Vegetation Index (NDVI) and midday Land Surface Temperature (LST) derived from advanced very high resolution radiometer (AVHRR) of the global land 1km project of United States Geological Survey (USGS), (2) agroclimatic data from the FAO Crop Production System Zone (CPSZ) of the Intergovernmental Authority on Development (IGAD) sub-region, and (3) the FAO 1998 soils digital map for the IGAD sub-region. The aim was to produce a predictive risk model for the two vectors. Data used for the analysis were based on presence and absence of the two species from previous survey collections in the region (mainly Ethiopia, Kenya and Somalia). Annual, wet season and dry season models were constructed. Although all models resulted in more than 85% positive predictive values for both species, the best fit for the distribution of P. martini was the dry season composite (NDVI 0.07-0.38 and LST 22-33 degrees C) with a predictive value of 93.8%, and the best fit for P. orientalis was the wet season composite (NDVI -0.01 to 0.34 and LST 23-34 degrees C) with a predictive value of 96.3%. The two seasonal composites models derived from satellite data were largely similar with best fit models developed based on the CPSZ climate data: average altitude (12-1900m), average annual mean temperature (15-30 degrees C), annual rainfall (274-1212mm), average annual potential evapotranspiration (1264-1938mm) and readily available soil moisture (62-113mm) for P. martini; and average altitude (200-2200m), annual rainfall (180-1050mm), annual mean temperature (16-36 degrees C) and readily available soil moisture (67-108mm) for P. orientalis. Logistic regression analysis indicated LST dry season composite of the satellite data, average altitude, mean annual temperature and readily available soil moisture of the CPSZ data as the best ecological determinants for P. martini while LST annual composite was the only important ecological determinant for P. orientalis. Spearman's rank correlation revealed several factors to be important determinants for the distribution of the two vectors. None of the soil types analysed appeared to be important determinant for the two species in East Africa, unlike in Sudan where P. orientalis is mainly associated with eutric vertisol (black cotton clay soil).

Insecticide susceptibility level of Anopheles arabiensis in two agrodevelopment localities in eastern Ethiopia.

Anopheles arabiensis strains reared from larvae and pupae collected from two different localities, Metehara and Melka-Worer, eastern Ethiopia, were evaluated against three insecticides. Resistance states of adult females were determined using the WHO test kits under field condition. The insecticides used were WHO discriminating doses of 4% DDT, 0.75% permethrin and 0.1% propoxur. The study revealed that 42.5% of the An. arabiensis population was resistant to DDT in Melka-Worer while only 30% of the species was resistant in Metehara to the same insecticide. In Metehara, 25% of An. arabiensis was also resistant to permethrin while the population was highly susceptible to the insecticide in Melka-Worer. Propoxur, which was not evaluated in Melka-Worer, was highly toxic to An. arabiensis in Metehara. The knocked-down time (KD50) for permethrin was 14.5 and 12.5 minutes in Metehara and Melka-Worer, respectively. The implications of these findings are discussed.

Impact of cattle keeping on human biting rate of anopheline mosquitoes and malaria transmission around Ziway, Ethiopia.

OBJECTIVE: To assess the impact of livestock keeping on the human biting rate (HBR) of anopheline mosquitoes and malaria transmission around Ziway in the middle course of the Ethiopian Rift Valley. DESIGN: As a passive experiment, man landing captures were done in homesteads with mixed dwelling, separate cattle shed and without livestock; and as an active experiment, captures were in experimental tukuls (huts) of cattle, goats, and without livestock. Parasite and spleen rates of children were compared among those residents under variable living conditions mentioned for passive experiment. SUBJECTS: For entomological study, human-baits were used for man-landing captures of mosquitoes. Study subjects for parasitological and clinical studies were children below 10 years old. MAIN OUTCOME MEASURES: Human-biting rate (HBR) of anopheline mosquitoes; and the parasite and spleen rates of the study subjects in different living conditions. RESULTS: In the passive experiment, the mean HBR of Anopheles arabiensis in mixed dwelling, separate cattle shed and without livestock was 8.45, 4.64 and 5.97, respectively. Similarly, the HBR of An. pharoensis was 2.88, 1.79 and 1.61, respectively. In the active experiment, the mean HBR of An. arabiensis in tukuls with cattle, goats, and without livestock was 3.50, 3.38 and 1.43 respectively; while that of An. pharoensis was 0.37, 0.70 and 0.55 respectively. Parasitologically, mean parasite rates of 26.67%, 15.05% and 23.85% were, respectively, recorded from children living under the above conditions stated for passive experiment. Similarly, the mean spleen rates of 50.0%, 26.9%, and 47.37% were recorded, respectively. CONCLUSION: These observations in the present study indicate that the presence of cattle in homesteads tends to increase the man biting rate of An. arabiensis, although differences in the mean HBR of vector mosquitoes were not statistically significant for all groups. In contrast, cattle keeping in separate cattle sheds outside of the human dwellings tends to reduce the man biting rate of An. arabiensis and malaria transmission in the study area.

Leishmaniasis in the middle course of the Ethiopian Rift Valley: I. Clinical and leishmanin skin test surveys.

A clinico-epidemiological study of cutaneous leishmaniasis (CL) and visceral leishmaniasis (VL) was undertaken involving 1,809 residents of ten representative villages from Zeway-Langano, Wajifo-Mirab-Abaya and Blate-Dimtu areas in the middle course of the Ethiopian Rift Valley from November 1994 to June 1996. Community prevalence of positive leishmanin skin test (LST) was very low ranging from 5% in Olge village to 0% in Kello-Langano area. Sera collected from 57 clinical VL suspects originating from the different villages tested negative for anti-leishmanial antibodies. The rate of splenomegaly ranged from 5% in Kello-Langano area to as high as 80% in Korga village. Furthermore, the frequency and size of splenomegaly was related to the reported past and recent history of attack(s) of malaria. The low community prevalence of LST suggests minimal transmission of leishmania infections in spite of the knowledge of the presence of the sandfly vectors of CL and VL in the area. However, with increasing villagization and agricultural development activities, the potential risk for the establishment of VL and/or CL as endemic diseases can not be excluded.