An estimation of the entomological inoculation rate for Ifakara: a semi‐urban area in a region of intense malaria transmission in Tanzania

Abstract

SummaryAn entomological study on vectors of malaria and their relative contribution to Plasmodium falciparum transmission in the semi-urban area of Ifakara, south-eastern Tanzania, was conducted. A total of 32 houses were randomly sampled from the area and light trap catches (LTC) performed in one room in each house every 2 weeks for 1 year. A total of 147 448 mosquitoes were caught from 789 LTC; 26 134 Anopheles gambiae s.l., 615 A. funestus, 718 other anophelines and 119 981 culicines. More than 60% of the total A. gambiae s.l. were found in five (0.6%) LTCs, with a maximum of 5889 caught in a single trap. Of 505 A. gambiae s.l. speciated by polymerase chain reaction, 91.5% were found to be A. arabiensis. Plasmodium falciparum sporozoite enzyme-linked immunosorbent assay tests were performed on 10 108 anopheles mosquitoes and 39 (0.38%) were positive. Entomological inoculation rate (EIR) estimates were generated using a standard method and an alternative method that allows the calculation of confidence intervals based on a negative binomial distribution of sporozoite positive mosquitoes. Overall EIR estimates were similar; 31 vs. 29 [95% confidence interval (CI): 19, 44] infectious bites per annum, respectively. The EIR ranged from 4 (95% CI: 1, 17) in the cool season to 108 (95% CI: 69, 170) in the wet season and from 54 (95% CI: 30, 97) in the east of the town to 15 (95% CI: 8, 30) in the town centre. These estimates show large variations over short distances in time and space. They are all markedly lower than those reported from nearby rural areas and for other parts of Tanzania.

A prospective study of Plasmodium falciparum multiplicity of infection and morbidity in Tanzanian children

“…It is now apparent that, owing to ongoing malaria control activities and urbanization, there has been a sustained decrease in transmission in this area in the last decade . A recent study reported an EIR of as low as 29 (95% CI 19-44) bites per annum (Drakeley et al, 2003). As a consequence of the lower transmission rate our study had less power than expected and even the oldest children in our study had experienced much less P. falciparum exposure than those children elsewhere in whom multiple infections appeared to be protective (Al Yaman et al, 1997a;Beck et al, 1997;Contamin et al, 1996;Farnert et al, 1999;Robert et al, 1996).…”

“…A decade ago, the average entomological inoculation rate (EIR) recorded in nearby rural areas was more than 300 infectious bites per person per year (Smith et al, 1993). However, recent field surveys have found much lower levels of malaria transmission, in particular in Ifakara town where an annual EIR of 29 infective bites per person has been estimated (Drakeley et al, 2003). Ifakara town is the site of the St Francis Designated District Hospital (SFDDH), which provides primary health care facilities for the population of the town, as well as acting as a referral hospital for the whole district.…”

Malaria entomological inoculation rates in gold mining areas of Southern Venezuela

“…An. marajoara was the most abundant species in the study area although its man-biting rate was only 1.27 bites/person/night (Table), which is lower than the biting rates (5.2-31.1) previously reported for this species in Western Venezuela (Rubio- (Drakeley et al 2003); c: mean caught per night by 2 human baits + 3 CDC light traps + 3 UV light traps (number of nights = 121). Three light traps = 1 human bait (Moreno et al 2002); d: biting rate x sporozoite rate x 365; e: other species, includes Anopheles braziliensis, Anopheles oswaldoi and Anopheles triannulatus; EIR: entomological inoculation rates.…”

“…To compare the observed differences in sporozoite rates between anopheline species, the confidence intervals were calculated according to the limits of expectations of binomial distributions (Drakeley et al 2003). The program StatXact ® (version 8: Cytel Inc 2007) was used for data analysis.…”

A quantitative analysis of transmission efficiency versus intensity for malaria

“…Estimates of the EIR differ in predictable ways, depending on the methodology and frequency with which human biting rate and sporozoite rate are measured 39,40,[44][45][46][47] . An additional explanation for the variability is that the efficiency of transmission differs among vector species 48 .…”