Take action:
Clean litter trays regularly while wearing plastic gloves (pregnant women should not do this job at all, as they are at risk for contracting toxoplasmosis); if the litter tray is dirty, cats will not use it, and will use other places in the house/flat; use clumping litter, as you can scoop out the dirt and leave the rest; place the litter tray far away from the feeding area, otherwise the cats will not use it; if the cat is using other parts of the house, put clumps of cotton wool drenched in citronella oil in these places. Cats hate this smell and will return to the cat litter tray.
Any questions? Ask CyberVet.