FRANKFURT (Reuters) - German healthcare and chemicals company Merck KGaA flagged that a slight decline in operating earnings was on the cards this year as the coronavirus pandemic could place a significant burden on the global economy.Earnings before interest, taxes, depreciation and amortisation (EBITDA), adjusted for one-off items, would likely be in a range of 4.35 billion euros ($4.70 billion) to 4.85 billion euros, compared with 4.39 billion last year, it said on Thursday.First-quarter..