Canine distemper is a highly contagious, systemic, viral disease of dogs seen worldwide. Distemper is commonly seen in young puppies between 3 and 6 months of age but can occasionally be found in younger or older pets.Males are found to be more prone the disease than the females. While it is primarily a disease of dogs, it can also be seen in other animals, including ferrets, coyotes, foxes, raccoons and skunks.The disease is highly infectious and frequently a lethal disease in dogs and has a high mortality after the dogs. Canine distemper virus is the etiological agent of the disease. CDV is a member of the genus Morbillivirus, subfamily Paramyxovirinae, family Paramyxoviridae, in the order Mononegavirales. The virus is closely related to the Measles virus in humans and Rinderpest virus of bovine.
CDV is a non-segmented, single-stranded, negative-sense, enveloped RNA virus with a diameter of about 150–300 nm. The genome of CDV (approximately 15.7 Kb) consists of genes for one non-structural protein (C) and six structural proteins: large protein (L), haemagglutinin(H), phosphoprotein (P), nucleocapsid protein (N), fusion protein(F) and matrix protein (M).The non-structural protein (C) is produced by an alternative open reading frame in the P gene.
Mode of Spread
Epidemiological studies of canine distemper have been described in many countries, including India, Denmark, Finland, Brazil and North America.The ability of the virus to jump between carnivores hosts is considered as one of the causes for the success of its worldwide spread. Because of dog abundance and their ability to travel for long distances, as viral reservoirs domestic dogs pose a significant risk to captive and free-ranging wildlife species. Several reports of occurrence in vaccinate dogs are there worldwide. The disease spreads in an aerosol-like manner—through infected droplets of body secretions from the nose, eye or mouthwhile transmission in wild carnivores occurs via urine and droppings or ingestion of infected meat.Following aerosol infection, the virus primarily replicates in lymphatic tissues of the respiratory tract and subsequently reachesvarious organs, including the cells of the lower respiratory and gastrointestinal tracts, the lymphoid organs, the urinary bladder and the central nervous system. It is a multisystemic disease infecting respiratory, gastrointestinal and nervous system. Its epidemiology is complicated by the large number of species susceptible to infection.
Symptoms include fever, cough, coryza and conjunctivitis.When the nervous system is affected, apathy, ataxia, paraplegia, quadriplegia, muscular atrophy, myoclonus, vocalization, tremor, incontinence, night time seizures, coma, dryness of the retina, circling, constant crying, and blindness are observed.
Routine diagnosis of canine distemper virus by immunofluorescence (IF) is not sensitive and can detect canine distemper virus antigens only within 3 weeks after infection, when the virus is still present in the epithelial cells. Due to previous vaccination or subclinical/clinical infection, serological methods like enzyme-linked immunosorbent (ELISA) and seroneutralization (SN) assays, have little diagnostic value.A real-time RT-PCR assayexhibited high specificity and sensitivity was developed for detection and quantitation of canine distemper virus. In dogs with multisystemic signs, the following can be examined by immunofluorescent assay or reverse transcriptase (RT) PCR: smears of conjunctival, tracheal, vaginal, or other epithelium; the buffy coat of the blood; urine sediment; or bone marrow aspirates. Commercially available quantitative RT-PCR can usually distinguish natural infection from vaccinal virus.
There is no specific therapy for this disease. In 1950s the live attenuated vaccines were developed and have successfully prevented CDV infections indomestic dogs.In India only live attenuated vaccines either imported or local made are in use. However, CDV infection and disease in immunized dogs have been observed on several occasions. Herd immunity, the neglecting of vaccinations, leading to poor vaccine coverage, antigenic shift and interference by maternal antibodies areobvious reasons for outbreaks on the population level. Another possible explanation could be the virulence and antigenic characteristics of the strain(s) responsible for the outbreak
Geographically CDV has distinct lineages; America1(vaccine strain) and America-2, Asia1and Asia -2, Europe1/South America1(EU1/SA1) and Arctic clusters. A further seven clusters haverecently been identified: European wildlife (EW), South America (SA2),Rockbornlike(RL, Vaccine D), Africa1,Colombian (South America3,SA3) and another two Asian clusters, Asia3and Asia-4.
Current licensed CDV vaccines are based on strains that have been attenuated by serial passage, either on canine kidney cells (Rockborn strain), hen eggs (original Onderstepoort strain) or chicken fibroblast cultures (Lederle strain). Moreover, CDV vaccines currently used in India are all based on imported strain like Ondersteport and this may be one of the reasons for vaccine failure.
Though highly successful in the past, conventional approaches to RNA virus vaccine development, such as live-attenuation through passaging (forward genetics) or inactivation, may be less efficient for generating good candidates than rational targeted mutagenesis (reverse genetics). Reverse genetics have provided key critical insights into the replication and pathogenesis of RNA viruses and facilitate vaccine development through targeted modifications and directed attenuation.