The Virus

    viruses

A schematic representation of virus particles.


















Tiger

Tazmanian Tiger

Canine distemper virus has been thought to be responsible for the extinction of the Tazmanian tiger.

Paramyxoviridae



Canine Distemper is a highly contagious and often fatal, multisystemic viral disease commonly found worldwide. It is characterized by respiratory, lymphatic, gastrointestinal, urogenital, and neuologic symptoms affecting many different species.
 

Canine Distemper Virus (CDV) is a single-stranded, negative-sense RNA virus of the family paramyxovirus and is a close relative of the human measles virus. It is an enveloped virus covered with glycoportein spikes which are responsible for its haemagglutination, neuraminidases and hemolytic properties.  These properties cause multisystemic symptoms which characterize the disease. The envelope surrounding the virus makes it generally senstive to heat, drying, lipid solvents and many disinfectants. It is not able to survive for long outside the host.
The virus initially replicates in the upper respiratory tract, tonsils and bronchial lymph nodes. A cell/macrophage-associated viremia follows and in the absence of an adequate immune response, infection of general lymphoid, gastrointestinal, respiratory, and CNS systems may result.  Virus replication can also damage immune cells resulting in immunosuppression further exacerbating the disease with the risk of secondary bacterial infection.



                 Virusshmirus


            The rinderpest virus, a close relative of the distemper virus



Transmission and Hosts


The spread of the virus is usually airborne. Infected dogs spread the virus by coughing infected respiratory secretions, although there are many other mechanisms of viral shedding, including via urine.

Once a dog has picked up the virus, its body’s macrophages move in to pick up the virus particles. This is an immune response of the body; the intention is that macrophages will engulf the virus and then the cell will be marked for destruction by immune system enzymes. However canine distemper virus is very good at evading this immune response, instead hi-jacking the macrophages as a method of transmission throughout the body. The virus is then able to spread and infect multiple systems in the host.

The first stop is the lymph nodes of the lung; the virus has usually penetrated these cells by twenty-four hours after infection. The virus particles then move onto the spleen, stomach, small intestine and liver. As these organs are infected by the sixth day of infection, fever starts to develop.

At day eight or nine, the immune system of the dog is continuing to mount a response. If the dog is properly capable of defeating the virus, the virus will start to be cleared, and an infected dog could be clear of virus particles by two week after initial infection. However, if the dog is unable to properly fight off the virus, complications can arise.

When the virus infects epithelial cells, a high degree of virus shedding can occur, which can increase the spread to other susceptible animals. The virus may also make its way to the brain, which results in the neurological manifestations characteristic of this disease. The large variety in the degree of symptoms is due to the differences in immune system response by different hosts.

Sometimes the virus will find areas in the body to hide. If this occurs in the skin, callousing can appear. If it is in the brain, seizures will result. These signs can continue after the virus was thought to be purged from the host.

Fortunately, the virus does not survive long in the external environment. As it is an enveloped virus, it is easily destroyed with mild disinfectants.

There are many hosts for the virus. It infects all breeds of the domestic dogs, as well as other members of the Canidae family, including the fox, wolf, dingo and jackal. Other hosts include weasels, ferrets, racoons, badgers, otters, skunks, seals and many species of large cats. Canine distemper virus has been thought to be responsible for the extinction and near-extinction of the Tazmanian tiger and the black-footed ferret, respectively.