Canine Distemper

Canine distemper virus is a paramyxovirus closely related to the viruses of measles and rinderpest. The fragile, enveloped, single-stranded RNA virus is relatively unstable outside the host. Infection is transmitted mainly via aerosol droplet secretions from infected animals. Some infected dogs may shed virus particles for several months.

The virus initially replicates in the lymphatic tissue of the respiratory tract. A cell-associated viremia results in infection of all lymphatic tissues, which is followed by infection of respiratory, GI, and urogenital epithelium, as well as the CNS and optic nerves. Disease follows virus replication in these tissues. The extent of viremia and of virus transmission to various tissues is moderated by the extent of specific humoral immunity in the host during the viremic period.

A transient fever usually occurs 3–6 days after infection with canine distemper virus, and there may be leukopenia (characterized by lymphopenia) at this time; these clinical signs may go unnoticed or may be accompanied by anorexia. The fever subsides for several days before a second fever occurs, which may be accompanied by serous nasal discharge, mucopurulent ocular discharge, lethargy, and anorexia. GI and respiratory signs, typically complicated by secondary bacterial infections, may follow; rarely, pustular dermatitis may be observed. Encephalomyelitis may occur in association with these signs, follow the systemic disease, or occur in the absence of systemic signs. Dogs surviving the acute phase may have hyperkeratosis of the footpads and epithelium of the nasal planum, as well as enamel hypoplasia in incompletely erupted teeth.

Overall, a longer course of illness is associated with the presence of neurologic signs; however, there is no way to anticipate whether an infected dog will develop neurologic manifestations.

Typical neurologic signs include:

• localized involuntary muscle twitching (myoclonus, chorea, flexor spasm, hyperkinesia)

• seizures, including salivation and chewing movements of the jaw (chewing-gum fits)

Other neurologic signs include:

• circling

• head tilt

• nystagmus

• paresis to paralysis

• seizures ranging in type from focal to generalized

Emerging viral strains may be associated with greater neurotropism. Increased morbidity and mortality from neurologic complications has been observed.1

A dog may exhibit any or all of these multisystemic clinical signs during the course of the disease. Infection may be mild and inapparent or lead to severe disease with most of the described clinical signs. The course of the systemic disease may be as short as 10 days; however, the onset of neurologic signs may be delayed several weeks or months as a result of chronic progressive demyelination within the CNS.

Clinicopathologic findings are nonspecific and include lymphopenia, with the possible finding of viral inclusion bodies in circulating leukocytes very early in the course of the disease. Thoracic radiographs may reveal an interstitial pattern typical of viral pneumonia.

Chronic distemper encephalitis (old dog encephalitis, or ODE), a condition often marked by ataxia, compulsive movements such as head pressing or continual pacing, and incoordinated hypermetria, may be observed in fully vaccinated adult dogs without a history suggestive of systemic canine distemper infection. Although canine distemper antigen has been detected in the brains of some dogs with ODE by fluorescent antibody staining or genetic methods, dogs with ODE are not infectious, and replication-competent virus has not been isolated. The disease is due to an inflammatory reaction associated with persistent canine distemper virus infection in the CNS; however, the mechanisms that trigger this syndrome are unknown.

• Preliminary diagnosis: clinical evaluation

• Confirmation: reverse transcriptase PCR (RT-PCR) and antibody detection tests

Canine distemper should be considered in the diagnosis of any febrile condition in dogs with multisystemic clinical signs involving the respiratory, GI, and/or neurologic systems. Unvaccinated dogs, or dogs of unknown vaccination status, should be particularly suspected. An appropriate index of suspicion based on clinical signs is a necessary first step. Widely available RT-PCR and antibody detection tests (ELISA, immunofluorescence assay [IFA]) are used to confirm the presence of virus or an immune response against it in appropriately selected clinical cases.

Clinical signs in the respiratory and GI systems may be modified by, or confused with, concurrent parasitism and numerous viral or bacterial infections. Characteristic neurologic signs sometimes do not appear until late in the disease.

Distemper in dogs can be confused with other systemic infections, such as:

• parvovirus infection

• infectious canine tracheobronchitis

• infectious canine hepatitis

Intoxicants such as lead or organophosphates can cause simultaneous GI and neurologic signs.

In dogs with multisystemic clinical signs, IFA or RT-PCR can be used to examine:

• smears of conjunctival, tracheal, nasal, vaginal, or other epithelium

• the buffy coat of the blood

• transtracheal wash fluid

• urine

• bone marrow aspirates

Commercially available RT-PCR may not distinguish natural infection from vaccine-derived virus; however, quantitative RT-PCR may overcome this limitation. Antibody titers or ELISA can be performed on CSF and compared with peripheral blood; a relatively higher number of antibodies in the CSF is typical of natural infection versus vaccination. Viral antigen IFA or fluorescent in situ hybridization (FISH) for viral DNA can be performed on biopsies from the footpads or from the haired skin of the dorsal neck.

At necropsy, diagnosis is usually confirmed by histologic evaluation of lesions, IFA, FISH, or a combination of these. Tissue IFA often is negative when the dog is showing only neurologic signs or when circulating antibody is present (or both), requiring that the diagnosis be made by CSF evaluation or RT-PCR analysis.

• Supportive care

As with most diseases due to viruses, supportive care is the mainstay of treatment for canine distemper. Patients require nutrition and hydration support, defense against secondary bacterial infections, protection against the consequences of seizures, and excellent nursing care to optimize the chances of a successful immune response to the virus.

Treatment includes:

• administration of prophylactic broad-spectrum antimicrobials

• provision of balanced electrolyte solutions

• provision of parenteral nutrition

• administration of antipyretics, analgesics, and anticonvulsants

• excellent nursing care

No single treatment of canine distemper is specific or uniformly successful. Experimental in vitro work with antiviral agents shows promise; however, these agents have not yet been widely used. The use of porcine-derived anti-canine-distemper-virus antibodies has shown promise: naturally infected puppies that were administered xenogeneic antibodies as an adjunct to standard supportive care had an increased rate of survival compared to puppies that received only supportive care.1 In addition, for further supportive care, one case report described the use of intramuscular injections of botulinum toxin in one dog to alleviate severe and debilitating myoclonus associated with infection.2

Unfortunately, treatment for acute neurologic manifestations of distemper is frequently unsuccessful. If the neurologic signs are progressive or severe, the owner should be appropriately advised. With prompt, aggressive care, dogs may recover completely from multisystemic clinical signs; in other cases, however, neurologic signs may persist after GI and respiratory signs have resolved. Some dogs with chronic progressive forms of neurologic disease may respond to glucocorticoid therapy.

With the potential increasing virulence of emerging strains and the wide host range of canine distemper virus, widespread vaccination of domestic dogs is essential. Successful immunization of pups with canine distemper modified live virus (MLV) vaccines depends on the lack of interference by maternal antibodies. To overcome this barrier, pups are vaccinated with MLV vaccine when 6 weeks old and at 3- to 4-week intervals until 16 weeks old. Alternatively, measles virus vaccine induces immunity to canine distemper virus in the presence of relatively greater amounts of maternal distemper antibodies. MLV measles vaccine is administered intramuscularly to pups 6–7 weeks old and is followed with at least two more doses of MLV distemper vaccine at 12–16 weeks old.

Many varieties of attenuated distemper vaccine, as well as recombinant vaccine, are available and should be administered according to manufacturers’ directions. MLV vaccines should not be administered in late-pregnancy or early-lactation bitches. MLV vaccines can produce postvaccinal illness in some immunosuppressed dogs. A recombinant canarypox vector vaccine expressing distemper virus proteins is licensed for use in ferrets; the American Association of Zoo Veterinarians recommends extra-label use of this vaccine in many at-risk species held in zoos and parks.

Historically, annual revaccination has been standard because of the breaks in protection that can occur in stressed, diseased, or immunosuppressed dogs, and because vaccines have been labeled for annual administration. Substantial evidence supports the finding that immunity induced by MLV distemper vaccines lasts ≥ 3 years. In many cases, however, a vaccination frequency of less than annual remains an extra-label administration of the vaccine; thus, decisions to revaccinate less often than annually should be considered in light of local prevalence of the disease and other potential risk factors, as well as industry and professional-organization recommendations.

Canine distemper virus is sensitive to lipid solvents such as ether, as well as most disinfectants, including phenols and quaternary ammonium compounds; these substances should be part of the cleaning and disinfection protocols at kennels and veterinary premises.

• American Veterinary Medical Association: Canine Distemper

• PetMD: Distemper in Dogs

• Also see pet health content regarding Canine Distemper (Hardpard Disease).