Canine Herpesvirus Infection

Etiology and Epidemiology

Canine herpesvirus (CHV-1) is an enveloped virus that belongs to the family Herpesviridae (Figure 16-1). It has been reported from the United States, Canada, Australia, Japan, England, and Germany. CHV-1 was first recognized in the mid-1960s in association with a fatal disease in puppies.² The virus is commonly blamed for acute neonatal puppy death or failure to thrive, sometimes termed the “fading puppy syndrome.” When confirmed as a cause of disease, untreated CHV-1 infection in neonates can cause high (up to 100%) mortality among littermates. The virus is temperature sensitive and prefers to replicate at temperatures less than 37°C. It is not stable in the environment and is readily inactivated by disinfectants. As with other herpesviral infections, recovery from disease is associated with lifelong latent infection of the neural ganglia, with periodic reactivation of shedding in association with stress or immunosuppression, such as that which results from overcrowding and pregnancy. CHV-1 infection has not been reported in cats.

FIGURE 16-1 Structure of a herpesvirus. Herpesviruses are large, enveloped DNA viruses. The DNA is contained within an icosahedral capsid.

Clinical Features

Signs and Their Pathogenesis

Transmission of CHV-1 can occur subsequent to close contact with infectious vaginal fluids during whelping or with vulvar or oronasal secretions in the postpartum period. Exposure of a naïve bitch to CHV-1 during the last 3 weeks of gestation results either in late-term abortion of a litter or neonatal death within the first few weeks of life, because inadequate periparturient maternal antibodies exist to allow passive immunity to be acquired by the neonates. Puppies born to a naïve bitch may also become infected as a result of contact with other dogs that shed the organism. The incubation period of CHV-1 disease is 6 to 10 days. The virus replicates in the epithelial cells of the oronasal and pharyngeal mucosa, the genital tract, and the regional lymphatics. Replication and spread of the virus is facilitated by the presence of a low body temperature (<38°C or 100°F), which is normal for canine neonates.

Older (>3 to 5 weeks of age) puppies that are exposed to CHV-1 may develop subclinical infection, or the course of disease is less severe, as a result of their ability to mount a febrile response. Latent infection also may develop. Concerns have been raised about latency and the possibility of late development of neurologic signs.³

Clinical signs are more likely to occur in animals that are hypothermic or immunosuppressed. Signs in the neonate are not specific and include incessant vocalization, anorexia (with poor weight gain), dyspnea, abdominal pain, incoordination, diarrhea, serous to hemorrhagic nasal discharge, and petechiation of the mucous membranes. The mortality rate in litters infected in utero or during birth can approach 100%, with deaths occurring during the first few days to a week of life. Exposed, surviving older neonates may develop a late onset of central nervous signs including blindness, ataxia, and deafness; apparent complete recovery has also been reported.

The recently infected brood bitch generally shows no clinical signs. Healthy adult dogs of either gender can develop mild upper respiratory signs (sneezing, serous oculonasal discharge, keratitis) for a few days but are otherwise usually clinically unaffected.4–6 Additional information on respiratory and ocular disease caused by CHV-1 can be found in Chapter 17.

Diagnosis

Antemortem diagnosis of CHV-1 infection in neonates can be challenging. Necropsy of a deceased littermate is usually required.

Virus Isolation and Molecular Diagnosis Using the Polymerase Chain Reaction

Tissues obtained at necropsy can be submitted for virus isolation or molecular diagnosis using the PCR. Diagnosis by virus isolation takes days. CHV-1–specific PCR assays can also confirm infection, in a more timely fashion. Clinicians should confirm the availability and turnaround time of commercially available diagnostic tests when attempting to investigate a possible outbreak of CHV-1 infection in puppies.7–9

Pathologic Findings

Gross Pathologic Findings

Gross changes in the kidneys at necropsy of pups infected with CHV-1 include multifocal petechial to ecchymotic subcapsular hemorrhages (Figure 16-2). The pleural surfaces may be mottled pink and red due to coagulation necrosis (Figure 16-2, B). Multifocal random acute necrosis of other organs, including the liver, pancreas, intestine and adrenal glands, may also be apparent. Lesions may resemble those of bacterial sepsis.