Activist investor Bill Ackman's Pershing Square Capital Management said in a filing with the Securities and Exchange Commission that it purchased an approximate 8.5% stake in animal health company Zoetis for about $1.5 billion. Zoetis, which develops animal health medicines and vaccines sold in more than 120 countries, was spun off from Pfizer in early 2013. Reporting revenue of $4.6 billion last year, it is the world's largest animal health company. Zoetis acknowledged it received a call from Ackman indicating that Pershing Square made an investment in the company. It said it is preparing to share more details about the company's business model and growth strategies at its Investor Day on November 18.