Inadequate measures taken for isolation of infected animals entering a country or an area may cause disease to spread from one country to another or from an infected area to uninfected parts of the same country. Inadequate isolation measures may arise from a faulty knowledge of the incubation period for a disease, or insufficient knowledge concerning new and more virulent strains of a well-known disease, such as rabies. Inadequate quarantine measures may occur where there is no quarantine policy for certain diseases.
In the UK, in order to avoid outbreaks of rabies, a six-month quarantine period was normal for imported dogs and cats, until a dog died of rabies 3 months after being released from quarantine. The period was then extended to 9 months and subsequently to 12; but finally reduced again to 6 months, with vaccination on entering quarantine and again one month afterwards.