Chapter 13 Canine Distemper
Canine distemper is a severe, highly contagious viral disease of dogs and other carnivores with profound effects on the respiratory tract, gastrointestinal tract, nervous system, and lymphoid tissue. It is seen worldwide.
ETIOLOGY AND EPIDEMIOLOGY
Epidemiology
• Incidence: All ages can be affected; however, the incidence is highest in young dogs (2–6 months of age), especially unvaccinated puppies that are exposed following the loss of passive immunity from maternal colostral antibodies.
Transmission
• The primary route of infection is by inhalation of the virus from aerosolized secretions or from fomites.
• The greatest opportunity for spread occurs where dogs are kept in groups (e.g., pet shops, kennels, animal shelters, and research colonies).
• Viral shedding usually ceases 1 to 2 weeks after recovery; therefore, “carrier state” transmission is not a big problem. Shedding for 60 to 90 days has been reported but is rare.
PATHOGENESIS
Stages of Infection
• Days 2 to 4: Infection spreads to local lymphoid tissues of the tonsils, retropharyngeal lymph nodes, and bronchial lymph nodes.
• Days 4 to 6: Widespread infection of systemic lymphoid tissues involves the liver, spleen, abdominal lymph nodes, and lamina propria of the gastrointestinal tract. This corresponds to a transient fever spike and the onset of lymphopenia caused by viral damage to T and B lymphoid cells.
• Days 8 and 9: The virus is disseminated to epithelial tissues (epitheliotropism) and the central nervous system (CNS) (neurotropism).
Host Immune Response
• If the immune response fails to develop by days 9 to 14, the outcome is rapid, widespread dissemination of the virus to skin; glandular and epithelial organs, such as the respiratory and gastrointestinal tracts; and the CNS, causing acute encephalomyelitis. The results are multisystemic signs, a second fever spike, and a high mortality rate, especially in young puppies.
• If the immune response is sluggish or partial, multisystemic signs are mild or absent and recovery occurs; however, CNS localization can result in chronic demyelinating encephalomyelitis with delayed onset of neurologic signs.
• If the immune response is rapid and effective, the infection is subclinical with complete recovery and elimination of the virus without clinical illness (by day 14 postinfection). It is estimated that more than 50% of CDV infections are subclinical. Delayed-onset CNS disease is occasionally seen in this group.
