When not controlled, water hyacinth will cover lakes and ponds entirely; this dramatically affects water flow and blocks sunlight from reaching native aquatic plants which often die. The decay processes depletes dissolved oxygen in the water, often killing fish. The plants also create a prime habitat for mosquitos, the classic vectors of disease, and a species of snail known to host a parasitic flatworm which causes schistosomiasis (snail fever). Directly blamed for starving subsistence farmers in Papua New Guinea, water hyacinth remains a major problem where effective control programs are not in place. Water hyacinth is often problematic in man-made ponds if uncontrolled, but can also provide a food source for goldfish, keep water clean and help to provide oxygen.