With the latest deal, Warburg has invested $1.2 billion-$1.3 billion in the Indian market in the last 12-18 months. : MUMBAI : Warburg Pincus has picked up a stake in Chennai-based Appasamy Associates Pvt. Ltd., a manufacturer of ophthalmic equipment and devices, in what is the US-based private equity firm's largest investment in the healthcare sector in India, a statement released by the two firms said.