Four rare white rhinoceroses have died in recent weeks at an Australian zoo after displaying mysterious neurological problems such as stumbling. The Taronga Western Plains Zoo near the New South Wales state city of Dubbo said in a statement Wednesday that it had begun a veterinary investigation to pinpoint the cause of the deaths and is working with rhino specialists in Africa and North America. Three other rhinos survived, zoo spokesman Mark Williams said. Williams declined to answer further questions. In a May 3, 2005 photo a White Rhinoceros walks through the grasses at Lake Nakuru National Park in Kenya. Four rare white rhinoceroses have died in recent weeks at an Australian zoo after displaying mysterious neurological problems such as stumbling, officials said Wednesday, March 21, 2012. (AP Photo/Mary Schwalm, File) Close The zoo's general manager, Matt Fuller, said bacterial infections, snake venom, toxins and many types of viruses have been ruled out as possible causes. Fuller said in a statement that no other species at the zoo had been affected by the illness and the surviving rhinos, which have been placed in quarantine, are healthy. They had shown no sign of illness. Fuller said results of a microscopic examination of tissue from the rhino carcasses may be available next week, but he added that it could take several weeks to get results from virology cultures. According to the World Wildlife Fund, white rhinos are classified as near endangered with an estimated 20,000 in the wild in Africa. The zoo's rhinos are from the southern white subspecies found in South Africa and Kenya.