A police dog has been euthanased and eight others are sick with a rare illness identified as megaesophagus, Victoria Police have said.

Inspector Jon Woodyatt said they first noticed problems with a couple of dogs in December.

"It affects the oesophagus, which is the tube that comes from the mouth to the dogs' stomach," he said.

"It causes the oesophagus to distend and lose its muscular tone so the dog has difficulty swallowing.

"It can regurgitate food, vomit, in severe cases it can [breathe in] the food, so that can lead to bouts of pneumonia."

He said the dogs can begin to grow weak because they are not getting enough nutrition.

Police use German shepherds and labradors for explosives and drug detection, operational support, responding to incidents, and for tracking and searching.

Cause of illness still a mystery

Inspector Woodyatt said both breeds had been affected, and they were still investigating what had made them sick.

"We don't know what the cause is … it can either be congenital or the cause is just not known," he said.

"It's not contagious. You can't catch it from having two dogs in the same kennel.

"We're confident that no further dogs would be affected."

Inspector Woodyatt could not say whether any more of the sick animals would have to be euthanased.

He said two of the dogs were severely ill, with the others were less affected.

Inspector Woodyard said the dogs were still working, with their handlers keeping an eye on their condition.

"This is having a significant impact on the dog squad," he said.

"Our handlers are allocated dogs when they first get there, the dog works with the handler every day and the dogs live at home with the handler."