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God helse og ernæring er viktige forutsetninger for god utdanning. Selv om dødeligheten hos barn i skolealder er lavere enn hos yngre barn, er ernæringsmessige mangler og infeksjonssykdommer fortsatt de hyppigste dødsårsakene. Mange skolebarn er fortsatt underernærte, har tarmparasitter og hudproblemer. Skolehelsetjenesten i landet som Etiopia er svak. Anemi og kronisk underernæring sees ofte hos samme barn, og deler felles risikofaktorer, men få har undersøkt dette i Etiopia. Selv om det er dokumentert en høy forekomst av innvollsormer, er det få studier om intensitet av innvollsormer, noe som er avgjørende for å forstå hvor syke barn blir. Mange skolebarn har skabb eller soppinfeksjoner i huden. Mange barn er også affisert av en «jordloppe»-infeksjon (tunga penetrans). Hovedmålsetningen med avhandlingen var å undersøke hyppig forekommende helseproblemer blant skolebarn i alderen sju til fjorten år i Sør-Etiopia. De spesifikke formålene med avhandlingen var 1/ å undersøke samtidig forekomst av anemi og kronisk underernæring 2/ å kartlegge forekomst og intensitet an innvollsormer blant skolebarn, og 3/ å analysere risikofaktorer for hyppig forekommende hudproblemer som soppinfeksjoner, skabb og jordloppe infeksjoner. Studien ble utført i Wonago området i Gedeo i Sør-Etiopia. Utvalget bestod av 864 skolebarn som ble tilfeldig utvalgt fra 2384 skolebarn i tre skoler på landsbygda. Vi samlet sosiodemografiske data, og målte ernæringsstatus, hemoglobin, forekomst av innvollsormer og hudinfeksjoner. Til å analysere data bruke vi også multivariate, multilevel, mixed-effect, regresjonsmodeller for å forstå hvorledes risikofaktorer på individ, familie, skoleklasse og skolenivå kan forklare resultatene. Dataene i artikkel om hudinfeksjoner ble også analysert med Bayesiansk statistikk. Fattigdom og hygieniske faktorer som hyppighet av handvask, bruk av sko, og tilgang på vaskefasiliteter på skolene forklarte noen av risikofaktorene for flere av infeksjonssykdommene. Studien viser at anemi og kronisk underernæring er et moderat stort helseproblem. Selv om det har vært noen helseprogrammer, er forekomsten av innvollsormer og hudproblemer høy. Risikofaktorer for disse helseproblemet er relatert til faktorer på skolene, i heimene og hos det enkelte skolebarn. Fattigdom og hygieniske faktorer som hyppighet av handvask, bruk av sko, og tilgang på vaskefasiliteter på skolene kan forklare noen av risikofaktorene for flere av infeksjonssykdommene. Avhandlingen, som er en av de første i Etiopia om skolehelse, anbefaler at intervensjoner må omfatte forebyggende og behandlende tiltak både på individ, heim og skolenivå. Background: Even though the global enrolment of children in schools is increasing, school-aged children in Ethiopia are at risk of various health problems, including malnutrition, micronutrient deficiencies, infections with soil-transmitted helminths (STHs), and skin problems. Anemia and stunting are interlinked health problems and share common risk factors, but few studies have investigated their coexistence in Ethiopia. Although there are many school-aged children, few studies have been conducted on the nutritional status of school-aged children in rural southern Ethiopia. In addition, previous studies on helminth infections have focused mainly on urban northern Ethiopia, although southern Ethiopia is a region where access to clean water and sanitation facilities is poor at schools. Furthermore, despite improvements in reporting the prevalence of helminth infections, there are few studies on the intensity of helminth infections in rural southern Ethiopia. The risk factors for intensity of helminth infection were not reported in several previous studies, and the interpretation of helminth data was mainly done according to the categorical outcome (i.e., the presence or absence of helminth infection). Thus, the use of a count model considering helminth egg concentration as an outcome variable is rarely reported. In addition, helminth egg data are usually overly saturated with zeros, and the interpretation of such data requires specific statistical models. However, there have been no studies on helminth egg counts among schoolchildren in Ethiopia that account for excessive zeros and overdispersion. More than half of school-aged children in southern Ethiopia are affected by scabies and tungiasis. Tinea infections are also common skin diseases among school-aged children in Ethiopia, especially tinea capitis, with approximately one-third of these children having experienced this infection at some point. Despite these facts, no compiled reports of such health conditions have been published. Moreover, there have been no studies that have investigated whether a Bayesian model could provide further evidence of these health conditions. Therefore, we applied both frequentist and Bayesian approaches to identify risk factors for scabies, tungiasis, and tinea infections. In our data, individuals were clustered within the same classroom, and classrooms within the same school. Thus, unlike previous studies, we applied a multilevel analysis to account for clustering effects. General objective: The overall objective of this study was to investigate some of the prevalent health problems among schoolchildren aged 7–14 years in rural southern Ethiopia. Specific objectives: 1) To assess the prevalence and risk factors for concurrent anemia and stunting (CAS) at the individual, household, and school levels among schoolchildren in rural southern Ethiopia using a multivariate, multilevel, mixed-effect model (Paper I); 2) to assess the prevalence and intensity of helminth infections and identify potential risk factors among schoolchildren in rural southern Ethiopia using a multivariate, multilevel, mixed-effect model and a zero-inflated negative binomial regression model (Paper II); and 3) to investigate the occurrence and risk factors of three parasitic skin diseases (scabies, tungiasis, and tinea) and their risk factors among schoolchildren in rural southern Ethiopia using both frequentist and Bayesian approaches (Paper III). Methods: We conducted this study in the Wonago district of southern Ethiopia. Using a three-stage cluster sampling method, we randomly recruited 4 schools at the first stage, 24 classrooms with 2,384 children at the second stage, and 36 children from each classroom at the third stage, for a total of 864 children aged 7 to 14 years. We recorded weight, height, hemoglobin level, intestinal helminth infections, hygiene practices, dietary habits, household food insecurity, and sociodemographic information and conducted skin examinations. For Papers I and II, a multivariate, multilevel, mixed-effect, logistic regression model was constructed that accounted for random school and classroom effects and identified potential individual-, household-, and school-level risk factors for CAS, as well as any helminth or Trichuris trichiura or Ascaris lumbricoides infection (Papers I and II). A count model with zero-inflated negative binomial (ZINB) regression analysis was performed to estimate potential factors associated with infection intensity using fecal egg counts from T. trichiura and A. lumbricoides infections (Paper II). In Paper III, both frequentist and Bayesian multilevel, mixed-effect, logistic regression models were used to identify potential risk factors for any one of three skin diseases, namely scabies, tungiasis, and tinea, separately. Results: In Paper I, CAS prevalence among schoolchildren was 10.5%, and risk factors for CAS were concentrated at the individual level. The clustering effects, as measured by the intra-cluster correlation coefficient (ICC), were 6.8% at the school level and 19% at the classroom level. Increased age, not washing hands with soap after latrine visits, walking barefoot, T. trichiura infection, and head lice infestation were associated with a high prevalence of CAS. Those using treated drinking water were less likely to have CAS. In Paper II, the prevalence of at least one helminth species was 56%, for while that of T. trichiura and A. lumbricoides was 42.4% and 18.7% among schoolchildren. School and classroom variables for helminth infections explained less than 5% of the variance and thus showed little influence. Being thin, anemic, children with a mother or guardian without formal education, and children in households using open containers for water storage were risk factors for helminths infection. In the ZINB model, older age and unclean fingernails increased the risk of A. lumbricoides infection. Handwashing with soap, deworming, and using water from protected sources reduced the risk of helminth infection. In Paper III, nearly three-quarters of schoolchildren had at least one type of skin disease, with scabies at 5.5% (46/861), tungiasis at 54.4% (468/861), and tinea infections at 39.1% (337/861). In the frequentist model, the cluster effect was insignificant at the school level and 8.8% at the classroom level for each skin problem. In the Bayesian model, the effect was 5.3% at the school level and 16% at the classroom level. For all models, the measured effects were higher than the values in the frequentist model. In the Bayesian model, being a boy, having unclean fingernails, not washing the body and hair weekly with soap, sharing a bed, clothes, or combs, and living in a poor household were risk factors for skin problems, but washing the legs and feet daily with soap was preventative for skin problems. Conclusions: Accounting for school- and classroom-level effects, this thesis identifies CAS as a moderate public health problem, as well as a high burden of helminth infections and skin problems among schoolchildren in southern Ethiopia. The risk of contracting CAS was high with age among those who did not always wash their hands with soap after visiting latrines, walked barefoot, and had T. trichiura infection, so educating children about personal hygiene and providing clean drinking water could reduce the burden of CAS among schoolchildren. Considering clustering at the school and classroom levels and the greater number of zeros in fecal egg counts, we observed associations between helminths and age, thinness, anemia, unclean fingernails, handwashing, deworming treatment, maternal education, household water source, and water storage protection. Therefore, strengthening the school deworming programme could reduce the burden of helminths in schoolchildren. The association between skin problems and most hygiene-related variables requires efforts to improve the personal hygiene of schoolchildren. The wide variation in skin problems across classrooms suggests that there are common risk factors in these locations and that transmission needs to be reduced by downsizing classrooms. Doktorgradsavhandling