A zoo in Bulgaria’s capital has been forced to close after several of its animals died of unknown causes over the course of a weekend.

Sofia’s ‘The Zoo’ has been temporarily closed following the deaths of three Indian bison, two yaks and a camel, which stated on 25 August, with the zoo staff unable to explain why.

An outbreak of blue tongue disease – caused by a virus spread by certain types of biting midges – which has killed more than 3,000 animals across the country, was recently reported in Bulgaria, though the zoo’s veterinarians said the dead animals did not display the correct symptoms of the sickness, which affects sheep and cattle.

A number of other animals at the zoo are also reported to be infected with the unknown sickness and it is unknown if the illness can be transferred to humans.

"We ordered checks of the food they were given, the water they drank, the space they lived in," said Yordanka Fandakova, mayor of Sofia, in a statement.

"What worries me most is the condition of some animals which are still unwell. The most important thing is to save the sick animals, so that there is no danger for people."

Blood samples from the dead animals have been sent for testing and results are due by the end of this week (5 September), while food testing suggested there is no danger of spreading contagious and parasitic diseases at the Sofia zoo.