Page 1513 - Small Animal Internal Medicine, 6th Edition

CHAPTER 96
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Polysystemic Viral

Diseases

Many viruses infect dogs and cats. Several, including canine replicates in epithelial tissues and may result in clinical signs
distemper virus (CDV), some feline coronaviruses, feline of disease. Dogs with good cell-mediated responses and
immunodeficiency virus (FIV), and feline leukemia virus virus-neutralizing antibody titers by postinfection day 14
(FeLV) can cause systemic signs of disease. See other chap- clear the virus from most tissues and may not be clinically
ters for discussions of viral diseases primarily associated with affected. Most infected dogs develop CNS infection, but
disease of specific organ systems. clinical signs of CNS disease occur only in dogs with low or
no antibody response. Acute demyelination results from
restrictive infection of oligodendrogliocytes and subsequent
CANINE DISTEMPER VIRUS necrosis; chronic demyelination is caused by immune-
mediated mechanisms, including antimyelin antibodies and
Etiology and Epidemiology CDV immune complex formation and removal.
CDV induces disease predominantly in terrestrial carni-
vores, but many other species, including seals, ferrets, skunks, Clinical Features
badgers, porpoises, and exotic Felidae, have been infected by Many clinically affected dogs are unvaccinated, failed to
either CDV or related morbilliviruses. Virulence of the CDV receive colostrum from an immune bitch, were inappropri-
strains varies by genetic lineage. Strains of CDV occurring ately vaccinated, or are immunosuppressed and also have a
in dogs in North America now vary genetically from isolates history of exposure to infected animals. Owners generally
evaluated in the 1900s, and there are now antigenic variants present affected dogs for evaluation of depression, malaise,
that may affect current vaccine efficacy (Anis et al., 2018; oculonasal discharge, cough, vomiting, diarrhea, or CNS
Kapil et al., 2008). In addition, more than one strain of CDV signs. Dogs with poor immune responses generally have the
can be present in the same area (Wostenberg et al., 2018). most severe signs and progress rapidly to life-threatening
The virus replicates in lymphoid, nervous, and epithelial disease. Some partially immune dogs have only mild respi-
tissues, and is shed in respiratory exudates, feces, saliva, ratory disease, presumptively diagnosed as canine infec-
urine, and conjunctival exudates for up to 60 to 90 days after tious respiratory disease complex. Tonsillar enlargement,
natural infection. After inhalation, the virus is engulfed by fever, and mucopurulent ocular discharge are common
macrophages and within 24 hours is carried by the lymphat- physical examination findings. Increased bronchial sounds,
ics to tonsillar, pharyngeal, and bronchial lymph nodes, crackles, and wheezes are usually auscultated in dogs with
where replication occurs. Central nervous system (CNS) and bronchopneumonia.
epithelial tissues are infected approximately 8 to 9 days after Hyperesthesia, seizures, cerebellar or vestibular disease,
initial infection. paresis, and chorea myoclonus are common CNS signs that
The degree of clinical illness and the tissues involved vary generally develop within 21 days of recovery from systemic
depending on the strain of the virus and the immune status disease (Table 96.1). CNS disease is generally progressive and
of the host. Nonimmune dogs of any age are susceptible, but carries a poor prognosis; it can develop in some dogs that
the disease is most common in puppies between 3 and 6 never had systemic signs of disease recognized. Old dog
months of age. Massive replication of the virus in the epithe- encephalitis is a chronic, progressive panencephalitis in dogs
lial cells of the respiratory tract, gastrointestinal system, and older than 6 years thought to be attributable to CDV infec-
genitourinary system occurs in dogs with poor immune tion in which microglial proliferation and neuronal degen-
responses by postinfection days 9 to 14; these dogs usually eration in the cerebral cortex result in depression, circling,
die from polysystemic disease. In dogs with moderate head pressing, and visual deficits (see Chapter 64 for more
immune responses by postinfection days 9 to 14, the virus information on CNS distemper).

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