Currently, there is no worldwide treaty governing the protection of animal welfare.
In 1978 UNESCO was presented the Universal Declaration of Animal rights. More recently animal welfare organizations (such as the World Society for the Protection of Animals and the ASPCA) are leading a campaign to gather 10 million signatures to present to the United Nations along with the text of a "Universal Declaration for the Welfare of Animals (UDWA)" There have been two animal welfare conferences which have produced two different text of the UDWA(Manila 2003) and the UDWA(Costa Rica 2005). They are still in the process of promoting the adoption of UDWA.
In addition to UDWA , another proposed text is the "International Convention for the Protection of Animals" [from Animal Legal & Historical Center]. For further discussion, see "Movement Toward an International Convention for the Protection of Animals - The Further Adventures of Four Rabbits" in Animal Welfare and the Law INTL KD3424.A75 A53 1989.
The European Union has regional treaties on certain issues relating to animal welfare for a further discussion please see the section below on E.U. treaties