Merck and Sanofi-Aventis announced today they would combine, and jointly own, their animal pharmaceutical businesses.

The combined business will become the largest in veterinary medicines, with 29 percent market share.

Rival Pfizer has about a 20 percent share of the $19 billion-per-year market.

Sanofi’s Merial unit operates mainly in North American and South America, and makes Frontline flea and tick medicine and Heartgard to combat heartworms. Merck’s Intervet operates in Europe and emerging markets, making vaccines and drugs for livestock.

Combined sales of both veterinary medicine units were $5.3 billion last year.