REVIEW ARTICLE What Do We Know About Risk Factors for Infection in Humans with Toxoplasma gondii and How Can We Prevent Infections? E. Petersen 1 , G. Vesco 2 , S. Villari 2 and W. Buffolano 3 1 Department of Infectious Diseases, Aarhus University Hospital, Denmark 2 Istituto Zooprofilattico Sperimentale della Sicilia, Palermo, Italy 3 Department of Pediatrics, Frederico II Medical School, University of Naples, Naples, Italy Impacts • The main risk factors for human infections are raw and undercooked meat and exposure to the environment, either soil or untreated surface water. • Most risk factor studies do not estimate the attributable fraction for the main risk factors and a large proportion of infections are not explained by the risk factors studied. • Risk factor studies are needed to provide a scientific basis for intervention either through public health assessment or health education. The shortcomings in the studies reviewed here should be addressed in future protocols. Introduction Infections by the protozoan parasite Toxoplasma gondii are widespread in humans and many other species of warm- blooded animals (i.e. mammals and birds). The sexual rep- lication takes place in the intestine of felids, which excrete oocysts in their faeces. The oocysts contaminate the envi- ronment, including soil and water, and may stay viable for months depending on temperature and humidity. The asexual replication takes place in the intermediate host (i.e. mammals, birds and humans), with the production of tissue cysts where the parasite remains viable for life. Based on the T. gondii life cycle, it can be predicted that humans can be infected by consuming meat and meat products infected with live T. gondii tissue cysts and different kinds of contacts with the environment which may be contaminated with sporulated oocysts shed by felids. However, the relative importance of different routes of transmission in different geographical areas globally is not known. Infection in humans is largely subclinical or asymptom- atic, and it is therefore impossible to determine either the time of infection in the majority of cases or the route of transmission (Hill and Dubey, 2002). The best available approach is to interview individuals in case–control stud- ies comparing T. gondii-infected persons with uninfected controls. These studies show that ingestion of contami- nated food is a major route of transmission, but leave up to half of the cases without an explanation for the proba- ble source of infection. Keywords: Toxoplasma gondii; prevention; case–control study; food hygiene; risk factors Correspondence: E. Petersen. Department of Infectious Diseases, Aarhus University Hospital, DK-8200 Aarhus, Denmark. Tel.: +45 89498307; Fax: +45 89498360; E-mail: epf@sks.aaa.dk Received for publication August 8, 2008 doi: 10.1111/j.1863-2378.2009.01278.x Summary Fourteen case–control studies evaluating the importance of different risk factors for infection of humans with Toxoplasma gondii were identified. Surprisingly, up to two-thirds of infections could not be explained by risk factors such as consuming raw or undercooked meat or poor kitchen hygiene. Few studies reported population attributable risk, and when reported, it was low. Protocols including questionnaires used for collecting data, age and socio-economic groups studied, and sample sizes varied among studies. There is a need for care- fully planned, case–control studies to identify sources of infection with T. gondii in humans. New risk factors, such as the manufacture of organic meat, and possible ways to sterilize meat to inactivate T. gondii tissue cysts are discussed. Zoonoses and Public Health 8 ª 2009 Blackwell Verlag GmbH • Zoonoses Public Health. 57 (2010) 8–17