Distemper

Canine distemper is caused by a paramyxovirus, which is related to measles. It is a highly contagious systemic viral disease in dogs that is seen worldwide. The virus affects canids (dogs, wolves and foxes) as well as some other species, such as mink, skunks, ferrets and raccoons. 

The virus is relatively unstable in the environment, but is found in respiratory secretions, as well as fecal material and urine from infected animals. Contact with these infected materials can result in infection of the naïve animal. Some infected dogs may shed the canine distemper virus for several months.

The virus first grows in the lymphoid tissue of the respiratory tract. This viremia then passes to all lymph tissue, and to the epithelium of the respiratory, urinary and gastrointestinal tracts. The virus will also infect the central nervous system (brain and spinal cord) and the optic nerves. Replication of the virus in these tissues results in disease.

Three to six days following infection, dogs usually experience a transient fever, as well as leucopenia. Sometimes, infected dogs may also be anorexic and listless at this stage. These first clinical signs may go unnoticed in many cases.  This primary fever subsides for several days, and is then followed by a second fever that typically remains for over seven days. Other signs that may occur during this time include watery discharge from the nasal cavity and congestion and discharge of pus from the eyes. The owner may first notice the dog squinting and coughing. Diarrhea and vomiting are also common. This systemic aspect of the disease may be as short as ten days, but may persist for a longer period of time.

The nasal plane and footpads may experience hyperkeratosis during the course of the disease. Dogs that have hyperkeratosis often show signs of neurological disease. In addition to, or following this systemic disease, the animal may develop an acute encephalomyelitis. Neurological signs include paresis, paralysis or ataxia of the hindlimbs that is often followed by paresis and paralysis of the hindlimbs. The animal may also experience twitching of muscles of the legs and face, as well as convulsions that are characterized by salivation and chewing-motions. The seizures increase in both frequency and severity as time progresses. Affected dogs often suffer involuntary urination and defecation during the seizures. The onset of neurological disease can be delayed for several weeks or months after infection.

The degree of spread of the virus in the body depends on the level of specific humoral immunity in the dog during infection. Infection can be mild and go unnoticed, or, it can lead to severe disease that shows most or all of the aforementioned clinical signs.

Persistant infection with canine distemper virus in the central nervous system may cause an inflammatory disease known as Chronic Distemper Encephalitis, or Old Dog Encephalitis (ODE).  This condition is noted by ataxia, compulsive movements (head-pressing and pacing), as well as incoordinated overreaching with the limbs. It may be seen in adult dogs that have no history of clinical signs related to canine distemper.

Degeneration of the thymus is common in young puppies that have been infected with the canine distemper virus. Secondary bacterial infections, such as pneumonia, enteritis and skin pustules may also be seen.

Sometimes, the signs of disease may be so mild that they go unnoticed. Often, infected dogs can have a slight fever for over two weeks. Should pneumonia or other serious complications develop, recovery will take much longer than in mild cases. Nervous signs often outlast the rest of the dog’s ailments by several weeks.

Puppies that are born to immune bitches aquire a significant degree of natural immunity through their mother’s colostrum. The degree of protection a pup receives from its mother varies directly with the degree of immunity the bitch has. This maternal protection decreases quickly, however, and the pup must be immunized through administration of the canine distemper modified live virus vaccine (MLV). In order for the vaccine to be effective, interference with maternal antibody must be overcome. Therefore, pups are vaccinated at six weeks of age, and again at two to four week intervals until they are sixteen weeks old. In order to prevent sudden breaks of neurologic distemper in stressed or immune-deficient dogs, annual revaccination is recommended in susceptible dogs in  geographical areas with high prevalence of disease (Vaccination Program in Dogs).

Modified live vaccines have the potential to produce illness after injection in some dogs that have deficient immune systems.