The Forestry Commission ordered a series of toxicology tests to be conducted after a number of dogs died following walks in woodland areas around Sherwood Forest in September.
Four dogs are reported to have died from the mystery bug, while a further 14 have been taken ill with similar symptoms.
A Forestry Commission spokesman said it had not applied any chemicals in the area and there had been no recent reports of animals dying or showing signs of the illness.
"Natural England are investigating the incidents and results of the first set of tests carried out to identify chemical poisons, such as strychnine and cyanide, have come back negative. We are now awaiting further test results to clarify the cause of death," said the spokesman.
Warning notices are still in place on routes in to the affected woodlands and owners are advised to keep their dogs under close control or on a lead if they walk in the woodlands.
- Anyone who has visited the area and is concerned their animal has become ill should contact 0800 321 600.
