Canine Distemper Virus
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 canine distemper virus 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 diarrhea. It is not uncommon for dogs to have some but not all signs associated with this disease.
Since the inapparent 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 titers 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. X-rays may show signs of pneumonia that are typical for viral infection but not definite for distemper virus alone. In many instances the course of the disease finally provides the diagnosis as initial signs of a generalized illness change to neurologic signs over time.
At present there is no specific treatment to kill the distemper virus. General supportive care and control of neurologic signs such as seizures may result in a reasonable recovery from this infection. This may be a long term project, though.
Prevention of infection is the best way to deal with canine distemper. Adequate vaccination of puppies is necessary. Veterinarians tend to begin vaccinations for this disease at approximately 6 weeks of age and continue until 12 or even 16 weeks of age at 3 to 4 week intervals. The vaccine is repeated due to interference with vaccination from antibodies passed to puppies in the mother's milk. These antibodies prevent the vaccine from working in about 75% of puppies at six weeks of age, about 25% of puppies at nine weeks of age and only a very few puppies at twelve weeks of age. The first vaccination is therefore an attempt to treat the 25% of puppies who are susceptible and the follow-up vaccinations are given to eventually provide protection to almost all puppies who receive vaccination.
Some strains of distemper vaccine provide nearly lifelong immunity after the initial series and one-year booster while other strains provide a shorter duration of immunity. Some puppies develop signs of canine distemper following vaccination even though they do not appear to have the disease. In these puppies encephalitis occurs. This can be fatal, although most puppies probably recover. Canine distemper virus is shed in all body secretions from infected animals. Dogs may spread the virus for several weeks during the illness and subsequent recovery period. The virus is not especially stable in the environment, probably lasting no more than a few weeks. It is susceptible to disinfectants, especially the quaternary ammonium compounds such as Roccal (TM).
The incidence of canine distemper infections is much lower than in the past. Good vaccination practices are almost certainly a big part of the reduction in cases of distemper. It is still present all over the world and continued vigilance on the part of veterinarians and dog owners is necessary to prevent a resurgence of this deadly illness.