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What is Canine Distemper? 

Canine Distemper is a contagious and fatal respiratory viral disease caused by the Canine Distemper Virus (CDV), related to the human measles virus. Recognised as a significant global threat to domesticated dogs, it is also considered a threat to a wide range of wildlife across the world (CABI, n.d.; AVMA, n.d.). Canine Distemper is a systemic disease, meaning that the virus affects the whole body, including the digestive, respiratory, cardiac, and nervous system, and more (Loots et al., 2017). 

In the UK, Canine Distemper is well controlled thanks to effective vaccinations in the domesticated dog populations, however outbreaks can still potentially occur in unvaccinated dog populations (PDSA, n.d.), particularly from dogs that have travelled from other countries where vaccination programmes and quarantine shelters are not as robust (Willi et al, 2015). This makes rigorous screening and timely quarantine essential. A lot of research has also highlighted the ever increasing threat of CDV cross infection between domestic dogs to other wildlife species through documented outbreaks that have occurred across the world.

How does Canine Distemper spread?

Canine Distemper is spread through contact with infected aerosol or respiratory secretions – this can mean passing from infected dogs or other animals by coughing or sneezing But it can also be spread by sharing of food and water bowls, toys, and equipment which may have become contaminated by an infected animal (Carvalho, 2012; CABI, n.d.).

Dogs and other hosts have also been found to be asymptomatic carriers of disease, and even dogs that have recovered from the virus can continue to shed canine distemper virus for several months (Durque-Valencia, 2019).

Who is at risk?

All dogs can be at risk of catching Canine Distemper, young puppies or dogs that are not up to date with their vaccinations are at greatest risk (Tupler, 2020). Although domesticated dogs are considered the largest reservoir of CDV; considering how the virus is able to infecta wide range of hosts, these wildlife species are also considered at risk if they come into contact with unvaccinated dogs or contaminated objects (Bieneke, 2015).

See the below section on Outbreaks and Threats to Wildlife for more information.

Prevention

Timely vaccination is considered the best prevention method against CDV. The World Small Animal Veterinary Association (WSAVA) recommends that CDV should be part of the core vaccination for dogs, i.e. vaccinations against globally distributed life-threatening diseases (WSAVA, 2016). This is echoed by the UK Govts Veterinary Medical Directorate (VMD), emphasizing continued importance of core vaccinations for dogs. Although incidences of diseases may seem low in, for example, the UK, the risk of outbreak will always remain with increasing world-wide travel and movement of people as well as animals (Veterinary Medicines Directorate, 2018).

As the CDV can be spread by contaminated water and food bowls, as well as pet equipment and toys, it is also recommended that contaminated objects are frequently and thoroughly cleaned under any risk of contamination by infected animals (Tupler, 2020).

The symptoms of Distemper

The CDV enters the respiratory system, and from there will eventually spread throughout the body. Common symptoms of Canine Distemper include (Carvalho, 2012; Beineke, 2015; PDSA, n.d.):

  • Yellow/Green discharge on their eyes
  • High temperature
  • Nasal discharge
  • Low energy (lethargy) or depression
  • Loss of appetite
  • Diarrhoea & Vomiting

In some cases, more severe symptoms could emerge as CDV starts to affect the host’s nervous system, including:

  • Thickened paw pads and nose (Hence Distemper is sometimes referred to as Hard Pad Disease)
  • Tremors
  • Paralysis
  • Fits and seizures
  • Death

Due to the different and non-specific symptoms that present for CDV, the best method for diagnosis is through PCR blood tests. These are also used to monitor the recovery period and confirm whether the host is free from infection.

For more info on the pathology of CDV, see the sections on ‘Pathology of Distemper’ below.

What is the treatment?

There is no treatment that can cure CDV due to the lack of specific antiviral drugs, hence treatment is mainly to alleviate symptoms and provide supportive therapy (Loots, 2017). In some incidences, if irreversible damage has occurred during infection, dogs may require ongoing, lasting support (PDSA, n.d.).

Infected dogs can still shed virus even after recovering from symptoms so it is important to keep dogs quarantined until they are tested PCR-negative. This is to prevent the spread of infection to other animals in the same household as well as to other wildlife. Toys and equipment should not be mixed, or should be thoroughly disinfected and cleaned between use (Willi et al, 2015)

More information on CDV

Pathology

The CDV initially enters its host through the respiratory system where it will enter the lymphatic system; targeting and replicating inside white blood cells. Through the lymphatic system, the virus will travel all around the body (Carvalho, 2012; Beineke, 2015). This is the first phase of CDV infection. From anywhere between 1-4 weeks after infections, the host may start to display a range of clinical symptoms depending on how strong their immune response is. As CDV attacks white blood cells, this will impair the body’s immune response making the host also susceptible to other bacterial infections. If the body is unable to fight off the infection, the virus will continue to spread throughout the body, causing damage to other bodily systems and organs. In more severe cases, the virus will start to attack the nervous system leading to symptoms such as tremors, seizures, and paralysis (Lempp, 2014; Durque-Valencia, 2019). CDV has a larger fatality in younger puppies as they are more susceptible to viral disease and less able to fight off infections (Tupler, 2020).

The second phase is the invasion of epithelial cells which leads to shedding of replicated virus particles and transmission to other hosts through aerosol (air droplets) and fomite (solid surfaces, such as food bowls and toys) transmission (Durque-Valencia, 2019)

Outbreaks and threats to wildlife

CDV is mainly known as an infectious disease of domestic dogs, and indeed domesticated dogs are the largest reservoir for disease. However, it has been increasingly reported as a worldwide multi-host pathogen – causing outbreaks of disease around the world and mortality to potentially already threatened wildlife species (Loots, 2017). Incidences, outbreaks, and prevalences of disease have been noted in species such as foxes, wolves, coyotes, hyenas, etc. Outside of the family of canis genus, vulnerable species include: ferrets, raccoons, pigs, skunks, red pandas, elephants and bears to name a few. It has also been found in marine animals such as seals, dolphins and porpoises, and outbreaks have also been seen in non-human primates: macaques, rhesus monkeys causing measles-like symptoms (Durque-Valencia, 2019, Beineke, 2015, Loots, 2017, Tupler, 2020). Although the clinical manifestations of the disease show some similarity amongst the different hosts, the rate and extent of morbidity and mortality can vary (Beineke, 2015). This is significant for wildlife conservation when there are high rates of mortality in particular species – e.g. ethiopian wolf with mortality rate of 68%, 45% in free-living racoons in the US (Duque-Valencia, 2019)

This wide spread of prevalence is thought to result from the spillover of CDV from interactions between different domesticated hosts with wildlife species and interactions between different wildlife species as well as spillback into domestic hosts – this is inevitably further compounded by some incidences of the disease being asymptomatic. Other factors such as virulence of different strains, population density, herd immunity are also thought to affect transmissibility between the different hosts (Beineke, 2015). More needs to be done to understand the transmission of CDV across different host populations. However you could consider the range of hosts exposed to the domesticated dogs alone – such as ferrets skunks out on walks, as well as rodents and foxes in cities, and captive wildlife in zoos and wildlife centres.

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References:

AVMA. (n.d.). Canine distemper. American Veterinary Medical Association. Retrieved 20 May 2021, from https://www.avma.org/resources/pet-owners/petcare/canine-distemper

Beineke, A., Baumgärtner, W., & Wohlsein, P. (2015). Cross-species transmission of canine distemper virus—an update. One Health, 1, 49-59.

CABI. (n.d.). Canine Distemper. CABI. Retrieved 20 May 2021, from https://www.cabi.org/isc/datasheet/121101

Carvalho, O. V., Botelho, C. V., Ferreira, C. G. T., Scherer, P. O., Soares-Martins, J. A. P., Almeida, M. R., & Silva Júnior, A. (2012). Immunopathogenic and neurological mechanisms of canine distemper virus. Advances in virology, 2012.

Duque-Valencia, J., Sarute, N., Olarte-Castillo, X. A., & Ruíz-Sáenz, J. (2019). Evolution and interspecies transmission of canine distemper virus—An outlook of the diverse evolutionary landscapes of a multi-host virus. Viruses, 11(7), 582.

Lempp, C., Spitzbarth, I., Puff, C., Cana, A., Kegler, K., Techangamsuwan, S., … & Seehusen, F. (2014). New aspects of the pathogenesis of canine distemper leukoencephalitis. Viruses, 6(7), 2571-2601.

Loots, A. K., Mitchell, E., Dalton, D. L., Kotzé, A., & Venter, E. H. (2017). Advances in canine distemper virus pathogenesis research: a wildlife perspective. Journal of general virology, 98(3), 311-321.

PDSA. (n.d.). Distemper in dogs. PDSA. Retrieved 14 June 2021, from https://www.pdsa.org.uk/taking-care-of-your-pet/pet-health-hub/conditions/distemper-in-dogs#contents-link-1

Tupler, T. D. (2020, October 29). Distemper in Dogs. PetMD. https://www.petmd.com/dog/conditions/respiratory/c_dg_canine_distemper

Veterinary Medicines Directorate. (2018, October 18). Vaccination of dogs. GOV.UK. https://www.gov.uk/government/publications/vaccination-of-dogs

W.S.A.V.A. (2020, October 3). Vaccination Guidelines. WSAVA. https://wsava.org/global-guidelines/vaccination-guidelines/Willi, B., Spiri, A. M., Meli, M. L., Grimm, F., Beatrice, L., Riond, B., … & Hofmann-Lehmann, R. (2015). Clinical and molecular investigation of a canine distemper outbreak and vector-borne infections in a group of rescue dogs imported from Hungary to Switzerland. BMC veterinary research, 11(1), 1-15.

Canine Distemper
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