FACTORS INFLUENCING THE SELECTION OF ANTIMALARIAL  DRUGS ENVIRONMENTAL  FACTORS Duration of stay Travellers planning to spend a long time in a malaria area should  continue to take antimalarial drugs as well as precautionary measures against mosquito  bites. Although it has been documented that people living in malaria areas develop a  natural resistance to malaria, this assumption does not apply to travellers. * Since extended use of chloroquine can cause retinal damage, 6 monthly  ophthalmologic checkups are recommended. * Mefloquine should not be used for longer than 1 year. * Doxycycline should not be used for longer than 3 months. People planning to move to a country which is known to have a malaria  risk should take antimalarial drugs and measures against mosquito bites as described  above. After arriving in the country the local health authorities can be approached to  advise them on the risk of malaria in the specific area where they are staying. The risk  may vary depending on factors such as the altitude, presence of water providing breeding  places for vector mosquitoes, the incidence of malaria in the local population etc.  Precautionary measures may be limited to measures against mosquito bites and the use of  antimalarial drugs may be restricted to the rainy season. Type of accommodation The risk of contracting malaria may vary according to the type of  accommodation and the environment. The risk is less in built-up areas in well-structured  buildings than in undeveloped areas in tent accommodation. Time of year In southern Africa malaria transmission is seasonal, although in  certain areas such as Mozambique and the Zambezi valley in Zimbabwe (amongst others),  there is a risk of contracting the disease throughout the year. During dry seasons, the  number of mosquitoes may be less, often rendering precautionary measures against mosquito  bites sufficient and antimalarial drugs unnecessary. However, irrespective of the season,  visitors should watch for any signs and symptoms of malaria from one week to several  months after returning from the area.  The guidelines are endorsed by the Medical Association of South  Africa. Compiled by the Department of Health in  collaboration with the Subcommittee for Chemoprophylaxis and Therapy of the National  Malaria Advisory Group. October 1996