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 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. 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.
