For many years canine distemper virus was the most feared of the viral diseases affecting dogs.

Parvovirus may have surpassed it in this regard but it is still a deadly virus that kills dogs and other members of the canine family. It is also infective to ferrets, mink, weasels and their kin among the Mustelidae family as well as raccoons, pandas and other members of the Procyonidae family. Recently it is believed to be the culprit in the death of a number of African lions. Canine distemper virus is an RNA virus from the morbillivirus family. In humans, measles is caused by a member of this virus family.

Distemper virus is more likely to affect puppies than older dogs. This is probably due immunity acquired through vaccination or exposure to the virus naturally, leading to immunity. It can affect dogs of any age, though. It causes very variable clinical signs which makes ruling it out in a young sick dog a difficult process. In some dogs a transient fever, perhaps accompanied by a lack of appetite or mild depression may be the only signs of onset of distemper. Other dogs are affected by a systemic illness with nasal and ocular discharges, coughing, fever, depression, lack of appetite, vomiting and diarrhoea. It is not uncommon for dogs to have some but not all signs associated with this disease. Since the unapparent infections often go undiagnosed and the severe infections often are present in dogs who die from distemper virus the mortality rate of canine distemper was always thought to be very high. Over time, it was recognized that a number of dogs were surviving the initial infection only to develop neurologic signs from one to a few weeks after infection. Seizures, behavioral changes, walking in circles and other ambulatory problems commonly develop. Many dogs who develop neurologic signs develop rhythmic motions or "tics". These are known as chorea. Sometimes affected dogs appear to be chewing gum due to the steady contractions of the muscles of the head. Dogs that survive both the initial infection and subsequent neurologic disease may go on to develop retinal damage, corneal discoloration or extreme hardness of the skin of the nose or foot pads. Infection with the distemper virus can be hard to diagnose with certainty. It is not uncommon for puppies with suggestive clinical signs to have a recent vaccination for the virus. This makes it hard to judge infection by antibody titres in many instances. There is a latent period from the time the virus enters a dog's body until clinical signs appear of approximately 10 to 14 days which means that puppies already infected may be vaccinated before clinical signs appear. The vaccination is not likely to be effective in preventing the disease when it is given after infection occurs.

Sometimes the virus can be identified in infected tissues using immunofluorescent techniques. This works in the white blood cells several days after infection and in conjunctival (the pink part of the eye area) swabs up to 21 days after infection. Inclusion cysts may be seen in conjunctival swabs as well.

General blood chemistry and blood cell count values are usually pretty uninformative when distemper is present. Cerebrospinal fluid (CSF) taps may indicate antibodies to distemper virus and increased protein.