Canine Distemper Diagnostics

By Dr. Gordon Andrews
Originally published in the November 2013 issue.

The recent canine distemper outbreak in the animal shelter in Emporia, Kansas has prompted us to briefly review laboratory methods for establishing a definitive diagnosis of distemper virus infection.

The clinical signs of distemper are familiar to most veterinarians and often lead to a tentative clinical diagnosis, but because the clinical signs can mimic other infectious diseases, this review will focus on those laboratory tests that establish a definitive diagnosis. The clinical signs mirror the sequential pathogenesis of viral infection and an understanding of the viral pathogenesis will help determine what samples are most appropriate to select for diagnostic testing.

Exposure to the virus is generally by aerosol contact with epithelium of the upper respiratory tract, with multiplication in tissue macrophages and spread by lymphatics to tonsil, retropharyngeal and bronchial lymph nodes. By 4-6 days post infection, virus replication occurs in lymphoid follicles of the spleen, lamina propria of the stomach and intestines, mesenteric lymph nodes, and Kupfer cells of the liver. At this time there is fever and leukopenia. 8-9 days post infection there is hematogenous lymphocyte associated viremia with spread of virus to epithelial and central nervous system tissues with shedding of virus from all body excretions even in dogs with subclinical infection.

Infection of upper respiratory epithelium results in conjunctivitis and rhinitis with serous to mucopurulent occulo-nasal discharge. A conjunctival or nasal swab placed in viral transport medium is an ideal sample at this time to submit for PCR testing for distemper virus. If viral transport medium swabs are unavailable, a swab moistened with sterile saline and placed in a sealed sterile tube is a good substitute. Whole blood in EDTA is an excellent sample for ante mortem PCR testing in both the acute and chronic forms of the disease. Urine is also a good sample for PCR testing.

Infection of lung epithelium results in an interstitial pneumonia, and secondary bacterial infection is common. The resulting cough can mimic infectious tracheobronchitis (kennel cough). A nasal or tracheal swab can be submitted for the canine respiratory PCR panel, which includes canine distemper virus, Mycoplasma, Bordetella bronchiseptica, canine adenovirus type 2, canine herpesvirus type 1, Influenza A, canine parinfluenza-3, and two strains of canine coronavirus.

Infection of gastrointestinal epithelium results in vomiting and diarrhea. Clinical signs of central nervous system infection are dependent on the region of the CNS involved and can include hyperesthesia, cervical rigidity, seizures, cerebellar and vestibular signs, paraparesis or tetraparesis with sensory ataxia, and myoclonus.

Gross necropsy examination often reveals pneumonia, but this is nonspecific. Some dogs develop digital hyperkeratosis (hard pad), but this is not specific for distemper infection. Dogs that have recovered from distemper may have tooth enamel hypoplasia which is considered specific for prior distemper infection. Histopathologic findings can include lymphoid depletion, interstitial pneumonia, necrosis of ameloblastic epithelium, necrosis of epithelium in the gastrointestinal tract, swelling of transitional epithelium in the renal pelvis and urinary bladder, and necrosis and inflammation in the brain. Viral inclusion bodies can be found in epithelial cells of mucous membranes, stomach, intestines, transitional epithelium of urinary pelvis and urinary bladder, neurons and astrocytes. When inclusions are found in association with histologic lesions in a dog with an appropriate clinical history, they can be considered diagnostic. The presence of inclusions alone however must be interpreted with caution, because distemper inclusion-like bodies have been described in the urinary bladder and brain of normal dogs. Distemper inclusions are not always found however and may only be found late in the disease. Immunohistochemical (IHC) staining of formalin-fixed tissues for distemper virus is a sensitive and specific method of demonstrating viral antigen in tissue sections even before histologic lesions are evident.

When submitting necropsy tissues for a diagnostic workup when distemper is suspected is important to include a complete set of formalin-fixed and fresh tissues even if gross lesions are not present and the patient has no clinical signs referable to a particular organ system. As an example, one dog I examined from the Emporia distemper outbreak had gross and microscopic evidence of chronic pneumonia, but PCR testing of fresh lung and IHC testing of formalin-fixed lung were both negative for distemper. There were no other microscopic lesions consistent with distemper infection in any other tissues except the brain, which did have encephalitis, and IHC staining was positive in the brain in spite of the fact that this dog was not reported to be showing any neurologic signs. Fresh necropsy tissues valuable for PCR testing include lymphoid tissues, lung, kidney, and brain.