Southland traditionally has a mild, warm wet Autumn that can last well into May (and in recent years into June), and these weather conditions are ideal for the development and survival of parasite larvae, fungal and bacterial infections. In Southland, we traditionally see an increase in colitis (severe acute diarrhoea) in the autumn months. The exact cause is often not determined but may be due to a bacterial colitis (e.g. salmonella) or due to parasitism. This year with a feed pinch during the summer months followed by warm autumn rain, sets up the ideal scenario for larval cyathostomaisis to develop. The development of severe diarrhoea secondary to the emergence of encysted cyathostome larvae from the wall of the large colon and caecum in horses results in a common pattern observed following periods of reduced pasture grazing height such as that seen during a summer drought or during a cold frosty winter. The damage caused to the intestinal wall results in profuse watery diarrhoea and the development of severe endotoxaemia that requires intensive treatment and can be fatal. Preventive measures include monitoring your horses' worm burden by regular faecal egg counts pre-emptive deworming with an anthelmintic targeted at encysted larvae. The main anthelmintics that have adequate efficacy against encysted larvae are Moxidectin (Ultramox or Equest Plus) or administration of a ‘Panacur Bomb’ which involves a five-day course of Fenbendazole (Panacur) followed by Ivermectin. In severely affected horses with repeatable high worm counts, the course of Panacur/Ivermectin may need to be repeated at two weekly intervals. Grazing management including twice-weekly picking up of manure and cross grazing with other animal species (e.g. sheep or cattle) are recommended. Although commonly carried out, harrowing paddocks in the autumn and spring followed by grazing within 6 weeks is not recommended as a means of controlling infective larvae as the warm moist environment allows larvae to survive and become infective.