Prevalence of circulating filarial antigens in Standard 1 pupils from 10 rural primary schools in Tanga District during surveys 1 and 7–9. Bar = prevalence in all 10 schools combined at the pre-MDA survey in 2004. Thick line = prevalence for all 10 schools combined during surveys 7–9. Thin stippled lines = prevalence in school clusters according to their location to the south (Kirare, Mapojoni and Marungu schools; no. 1–3), north (Kiomoni and Mafuriko schools; no. 4–5), close west (Maweni and Kange schools; no. 6–7) and more distant west (Pongwe, Kigandini and Ziwani schools; no. 8–10) of Tanga city during survey 7–9. Vertical stippled lines indicate rounds of MDA.