Canine distemper

What it is

Distemper is caused by canine morbillivirus (canine distemper virus, CDV), an enveloped RNA virus of the family Paramyxoviridae, related to human measles virus. It is labile in the environment and is transmitted mainly by aerosols and respiratory secretions from infected dogs, which can shed the virus for weeks. After respiratory infection, the virus replicates in lymphoid tissue (tonsils and bronchial lymph nodes) using the SLAM (CD150) receptor on lymphocytes and macrophages, spreads by viraemia and causes profound immunosuppression, with lymphopenia. It then infects, through the nectin-4 receptor, the epithelia of the respiratory, digestive and urinary tracts, the skin and the central nervous system. The outcome depends on the immune response: dogs that mount it early can clear the virus without signs, while those that do not develop multisystemic disease.

It is not a zoonosis. It has a very wide host range: besides the dog it affects foxes, wolves, raccoons, ferrets and other mustelids, large felids and seals, and wildlife reservoirs and unvaccinated dogs keep the virus circulating.

Who it affects

Mainly unvaccinated puppies of about 3 to 6 months of age, when maternal antibodies wane, although unprotected dogs of any age can be affected. The risk is higher in shelters, breeding kennels, pet shops and populations of stray or unvaccinated dogs. Ferrets are highly susceptible and the disease is almost always fatal in them.

Anatomy involved

• Brain: the virus mainly affects the white matter of the cerebellum, brainstem and spinal cord, with a demyelinating leukoencephalomyelitis; in puppies it can also damage the grey matter. Old dog encephalitis is a rare chronic form of the cerebral cortex in adult dogs, with viral persistence. Histology shows eosinophilic intranuclear and intracytoplasmic inclusion bodies in neurons and glial cells.
• Lungs: viral interstitial pneumonia, often complicated by secondary bacterial bronchopneumonia because of immunosuppression.
• Small intestine: infection of the intestinal epithelium with catarrhal enteritis, vomiting and diarrhoea, which worsen dehydration.
• Eyes: conjunctivitis with serous and later mucopurulent ocular discharge; infection of the lacrimal gland can cause keratoconjunctivitis sicca. There may also be chorioretinitis, which leaves hyperreflective retinal scars, and optic neuritis with sudden blindness.
• Skin: nasodigital hyperkeratosis ("hard pad disease"), with thickening and hardening of the nasal planum and footpads; puppies may show pustular dermatitis on the abdomen.

Clinical signs

The acute phase begins with fever, lethargy, anorexia and serous oculonasal discharge that becomes mucopurulent. Cough, dyspnoea, vomiting and diarrhoea follow. Neurological signs appear at the same time or weeks later, even in dogs that had no obvious systemic signs: myoclonus (involuntary rhythmic contractions of a muscle group, which can persist for life), seizures (sometimes with chewing movements), ataxia, paresis, vestibular and cerebellar signs and hyperaesthesia. Dogs infected before the permanent teeth erupt can be left with enamel hypoplasia. Neurological disease is usually progressive and is the main cause of death or euthanasia.

Diagnosis

• Clinical suspicion: an unvaccinated puppy with respiratory or gastrointestinal signs followed by neurological signs, myoclonus or footpad hyperkeratosis.
• RT-PCR: the test of choice. Conjunctival, nasal or oropharyngeal swabs, whole blood, urine or cerebrospinal fluid are tested; combining several samples increases sensitivity. Modified-live vaccines can give positive results for some weeks after vaccination (vaccine interference); quantitative RT-PCR or sequencing helps distinguish vaccine virus from field virus.
• Immunofluorescence on conjunctival or blood smears: specific but insensitive.
• Serology: IgM indicates recent infection or vaccination; antibodies in cerebrospinal fluid support the neurological form if the sample is not contaminated with blood.
• Complete blood count: lymphopenia is common in the acute phase.
• Post mortem: histopathology with inclusion bodies and immunohistochemistry.

Treatment and prevention

There is no specific antiviral; treatment is supportive: isolation, fluids, nutrition, cleaning of discharges, antibiotics for secondary bacterial bronchopneumonia and anticonvulsants. The prognosis is guarded when there are neurological signs, and myoclonus usually persists in dogs that survive.

Vaccination is the basis of prevention: the distemper vaccine is a core vaccine for all dogs worldwide. Modified-live or recombinant vaccines are used; the puppy series should end at 16 weeks of age or later, because maternal antibodies interfere with the response. Shelters vaccinate on intake and isolate sick dogs. Keeping vaccination coverage high in dogs also reduces the risk to threatened wildlife.

Key studies

• WSAVA vaccination guidelines (Squires et al., 2024): the international reference for vaccination of dogs and cats. They include distemper among the core vaccines that all dogs should receive, wherever they live, together with canine adenovirus and canine parvovirus.
• AAHA canine vaccination guidelines (Ellis et al., 2022): the North American guideline. It classifies vaccines as core and non-core according to each patient's risk, gives a vaccination and revaccination schedule, explains how to interpret antibody titres, including maternally derived ones, and includes specific recommendations for shelters and outbreaks.
• Costa et al. (2019), meta-analysis of 53 cross-sectional studies with 11,527 dogs. Pooled positivity was 33% with molecular tests and 46% with serology. Unvaccinated dogs (OR 2.92) and free-ranging dogs (OR 1.44) were at higher risk, and complete vaccination was associated with much lower risk (OR 0.18). Dogs under 12 months had lower positivity (OR 0.36), which contrasts with the idea that it is a disease of puppies; one possible explanation is that serology accumulates exposure with age. The studies are observational and highly heterogeneous.
• Martinez-Gutierrez and Ruiz-Saenz (2016), systematic review of 217 articles on the virus in hosts other than the dog. It has been described in carnivores of 12 families and also in rodents, primates, artiodactyls and proboscideans; non-domestic carnivores account for 87.5% of records. It has been found in 43 countries and on every continent except Australasia. It is based on published cases, with the bias that implies.
• Angwenyi et al. (2025), systematic review of distemper in African wildlife. There were outbreaks in six countries, with mortality of 30% to 94%, and 7 of the 11 affected species are threatened. Of 23 studies that examined whether the domestic dog is the reservoir, 14 (61%) concluded that it is, 4 that it is not and 5 could not tell, and none confirmed transmission from dogs to wildlife. The widespread idea that the dog is always the source is only partly supported.