THERE is a link between the El Nio weather phenomenon and deadly horse epidemics in South Africa, British scientists said this week. In an article published in the journal Nature on Wednesday, researchers from the Institute of Animal Health said that climate changes resulting from El Nio caused a population explosion of the biting midge, the insect that spreads African horse sickness. They warned that the increasing frequency of El Nio, a warming of the eastern tropical Pacific Ocean that causes havoc with weather systems, could result in an increase in other diseases spread by flying insects, including those that affect humans.