canine distemper virus

Other Scientific Names

• distemper virus

• Domain: Virus
•     Group: "Positive sense ssRNA viruses"
•         Group: "RNA viruses"
•             Order: Mononegavirales
•                 Family: Paramyxoviridae
•                     Genus: Morbillivirus
•                         Species: canine distemper virus

Canine distemper virus (CDV) is an enveloped, single-stranded RNA virus that has been taxonomically assigned to the order Mononegavirales, family Paramyxoviridae, and genus Morbillivirus.  It is closely related to the viruses that cause measles in man and rinderpest in ungulates. At least eleven distinct genetic lineages of CDV are recognized – see Pathogen Characteristics section.

The origin of the word distemper is from the Middle English distemperen, meaning “to upset the balance of the humors,” which is originally from the Latin dis- and Latin temperare, meaning “to not mix properly.”  The disease, canine distemper, is sometimes called “hardpad disease” in canines due to the hardening of the footpads and nose that can be occur in infected dogs.

Given that the virus can infect a variety of species other than canids, especially within the order Carnivora, some investigators have recently suggested that CDV should be renamed ‘carnivore distemper virus’ (Terio and Craft, 2013).

It has been suspected that canine distemper was imported from Peru into Spain during the 17^th century.  It was well-described in 1746 by Antonio de Ulloa; during the mid-18^th century, it was first reported in Spain, followed by England, Italy (1764) and Russia (1770) (Blancou, 2004).  Edward Jenner published an extensive description of the course and clinical features of the disease in dogs in 1809 (Jenner, 1809). The viral aetiology was demonstrated in 1906 by Henri Carre (Blancou, 2004).  The domestic dog has largely been responsible for spreading the disease to new geographic areas and for introducing canine distemper to previously unexposed wildlife.  As human populations expanded, increased domestic dog contact with wild carnivores exacerbated the risk for disease transmission (Butler et al., 2004; Cleaveland et al., 2000).

Canine distemper virus (CDV) is an enveloped, single-stranded RNA virus within the genus Morbillivirus (family Paramyxoviridae) and is closely related to the viruses of measles and rinderpest.  Similar to other members of the family, CDV virions are pleomorphic, 150-350 nm in diameter, enveloped and covered with large glycoprotein spikes 8-14 nm in length, and contain a “herringbone-shaped” helically symmetrical nucleocapsid, approximately 1 µm in length and 18 nm in diameter (MacLachlan and Dubovi, 2011).  The genome consists of a single linear molecule of negative-sense, single-stranded RNA, ~16 kb in size.  The RNA does not contain a 5’ cap and is not polyadenylated at the 3’ end, but does have functional 5’ and 3’ non-coding elements.  The genome of CDV has 6 genes separated by conserved non-coding sequences that contain termination, polyadenylation, and initiation signals for transcribed mRNAs.  The virion proteins include three nucleocapsid proteins (an RNA-binding protein (N), a phosphoprotein (P), and a large polymerase protein (L)) and three membrane proteins (an unglycosylated matrix protein (M), and two glycosylated envelope proteins – a fusion protein (F) and a hemagglutinin (H), which functions as an attachment protein (MacLachlan and Dubovi, 2011)).  Viral replication occurs in the cell cytoplasm and virions exit the cell by budding from the plasma membrane.  The enveloped virus is sensitive to lipid solvents and most disinfectants and has limited survivability outside the host animal.

At least eleven distinct genetic lineages of CDV are recognized worldwide, based on phylogenetic sequence analysis of the H gene.  These CDV lineages are known as America-1, America-2, Arctic, Asia-1, Asia-2, Asia-3, Asia-4, Europe-1/South America-1, Europe wildlife, Rockborn-like, and Africa-1 (Panzera et al., 2012; Radtanakatikanon et al., 2013; Sarute et al., 2014; Zhao et al., 2010).  The vaccine strains of CDV - Snyder Hill, Onderstepoort, Lederle – are all included in the America-1 lineage.  Despite genetic differences among field strains of CDV, cross-neutralization studies show only minor antigenic differences (MacLachlan and Dubovi, 2011).

McCarthy et al. (2007), studying CDV from a variety of different hosts, found that molecular adaptation at known receptor-binding sites of the haemagglutinin gene was associated with independent instances of the spread of the virus to novel hosts.

While there may be little economic impact of canine distemper in terms of agriculture, there is certainly impact associated with declines in wildlife populations or individuals.  These impacts can be felt most significantly in zoos and wildlife parks around the world, in addition to the increased efforts, and cost of operations, of various wildlife conservation groups.  Additionally, pet owners spend a significant amount of money each year obtaining canine distemper vaccination for their dogs (or other domestic pets susceptible to the disease).

Impact on Biodiversity

Canine distemper virus can have a significant effect on wildlife populations.

The most intensively studied CDV outbreak has been in the African lions (Panthera leo) of the Serengeti National Park, Tanzania.  In 1994, a large epidemic, which had probably spread from domestic dogs (Cleaveland et al., 2000), caused fatalities in ~30% of the population (Roelke-Parker et al., 1996).  By 1994, 85% of the Serengeti lion population had anti-CDV antibodies and the epidemic had spread north to lions in the Maasai Mara National Reserve in Kenya; by 1996, it was estimated that only 2,000 Serengeti lions remained (Roelke-Parker et al., 1996), although the population recovered quickly (Gilbert et al., 2014).  In addition to the lions, other carnivore species, including leopards (Panthera pardus), hyenas (Hyaenidae), bat-eared foxes (Otocyon megalotis), jackals, and domestic dogs were affected during the 1994 CDV outbreak. Extinction of the endangered African wild dog (Lycaon pictus) in the Serengeti coincided with epizootic canine distemper in sympatric domestic dogs (Daszak et al., 2000).

As well as the species mentioned in the previous paragraph, studies cited by McCarthy et al. (2007) report mass mortalities in palm civets (Paguma larvata), red pandas (Ailurus fulgens), raccoons (Procyon lotor) and black-footed ferrets (Mustela nigripes); the last remnant wild population of the latter species (before more recent reintroductions), in Wyoming, USA, was reportedly extirpated by CDV. Timm et al. (2009) conclude that CDV was probably the cause of a precipitous decline in the population of the endangered Island Fox (Urocyon littoralis catalinae) on Santa Catalina island, California, USA (in contrast, elsewhere in California there is no evidence that disease has a significant impact on the endangered San Joaquin Kit Fox, Vulpes macrotis mutica [muticus], despite a high prevalence of antibodies to CDV – US Fish and Wildlife Service, 2010).

Gilbert et al. (2014) modelled the effects of CDV on Amur Tigers (Panthera tigris altaica), in which infection has recently been diagnosed, and concluded that the 50-year risk of extinction in their study area was between 6.3% and 55.8% higher (depending on the scenario used) in the presence of the virus than in its absence.

The Threatened Species table lists some more species considered to be threatened by the virus.

• Proved invasive outside its native range
• Benefits from human association (i.e. it is a human commensal)
• Has high reproductive potential
• Reproduces asexually

• Negatively impacts animal health
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Pathogenic