Canine Distemper Virus

Canine Distemper Virus (CDV) infection can cause canine distemper, a common infectious disease in pet dogs. The first virus strain was isolated in 1905 by French veterinarian Henri Carre. The virus genome is single-stranded negative-sense RNA, belonging to the family Paramyxoviridae, genus Morbillivirus. Canine distemper virus not only infects canids but also tends to cross species barriers, spreading among various animals.

Initially, canine distemper virus only infected canids and mustelids. Since the 1980s, there have been reports of CDV infections in ferrets, tigers, lions, seals, dolphins, pandas, and other animals. It is now confirmed that most families within the order Carnivora can be infected by this virus, such as wild canids (wolves, foxes, etc.), procyonids (raccoons, coatis, etc.), small pandas, bears (black bears, giant pandas, etc.), viverrids (civets, genets), hyenids (spotted hyenas), and large felids. CDV has also caused large-scale deaths in some pinniped wild animals (seals, etc.), with terrestrial carnivores like dogs and wolves possibly serving as sources of infection. Canine distemper is currently one of the most harmful diseases to the pet industry, fur animal farming, and wildlife conservation. In recent years, cases of CDV infection in non-human primates in China have also drawn significant attention from laboratory animal professionals.

Canine distemper is mainly transmitted through direct contact and is characterized by high infectivity, high morbidity, and high mortality. CDV virus can be abundantly present in various secretions or excretions of infected animals, including ocular and nasal discharges, saliva, blood, urine, and feces. Although CDV is unstable in the environment and usually survives only a few hours to a few days at most, special attention should be paid to the fact that infected animals can still serve as sources of infection for 1-2 weeks after recovery. Some reports indicate that a few animals may shed the virus for 60-90 days post-recovery. Therefore, animals should be re-examined to confirm they are no longer shedding the virus before being housed with healthy animals after clinical symptoms disappear.

Canine distemper virus is a pantropic virus that can invade multiple tissues. Symptoms vary greatly among infected animals; some may only show seroconversion without obvious clinical signs. This variability may be related to viral strain virulence, animal age, immune status, and environmental conditions. After infection, animals undergo an incubation period: 3-6 days if infected by the same species, and 30-90 days if from a different species. Affected animals mainly show fever, rash, poor appetite, respiratory distress, pneumonia, hyperkeratosis of the footpads, ocular and nasal purulent discharge, and some may develop neurological symptoms. Hyperkeratosis of the footpads is a characteristic lesion of CDV infection but is not diagnostic alone. CDV infection also causes immunosuppression, leading to secondary infections and more diverse and complex symptoms. If animals show similar clinical signs, they should be isolated and treated while contacting a professional diagnostic laboratory for pathogen confirmation.

Reports of natural CDV infection in non-human primates first appeared in 1989. Yoshikawa et al. reported natural CDV infection in Japanese macaques, with 22 wild-caught animals falling ill. Later, CDV antibodies were detected in 44 of 700 Japanese macaques, and the virus was isolated. In 2011, Qiu et al. reported a CDV outbreak in a monkey farm in China in 2006, involving about 10,000 animals, with morbidity of 25%-60% and mortality of 5%-30%. Subsequently, in 2008, 20 newly purchased rhesus monkeys at an animal center in Beijing fell ill during quarantine, with a 60% mortality rate (12/20). The isolated CDV belonged to the prevalent Chinese strain, commonly found in Shandong, Xinjiang, Heilongjiang, and Taiwan. The same year, a batch of crab-eating macaques imported from China to Japan also experienced CDV infection during quarantine, with a mortality rate of about 10.6% (46/432). Sequencing of viruses isolated from sick monkeys confirmed high similarity to the Beijing rhesus monkey CDV strain. This indicates the formation of an infection chain and confirms CDV's sustained infectivity in monkey populations. At the end of 2019, our company detected CDV nucleic acid and IgM antibodies and successfully isolated the virus from experimental monkeys showing fever, rash, and pneumonia (data unpublished). After more than a decade since the last outbreak, CDV reappeared, warranting high attention from experimental monkey production and usage units to prevent further epidemics.

Once an animal is found to be ill, it should be immediately isolated to prevent transmission to others. Vaccination is the most economical and effective means of prevention. However, canine distemper vaccines do not provide permanent protection, usually lasting 1-2 years, so regular vaccination is recommended as needed. For immunization of experimental monkeys, besides attenuated canine distemper vaccines, several studies have confirmed that attenuated measles vaccines offer some cross-protection against canine distemper. It should be noted that animals showing clinical signs should not be vaccinated, as this may worsen the condition. Additionally, experimental monkey breeding facilities should not keep dogs and must implement measures to prevent stray dogs, wild dogs, and wild animals from entering.

References

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