Myxomatosis
..............Eradication of a Species?
Myxomatosis
Caught in the centre of a soundless field
While hot inexplicable hours go by
What trap is this? Where were its teeth concealed?
You seem to ask.
I make a sharp reply,
Then clean my stick. I'm glad I can't explain
Just in what jaws you were to suppurate
You may have thought things would come right again
If you could only keep quite still and wait.
Philip Larkin
2.1 PATHOGENESIS
In Sylvilagus myxoma viruses produce localised benign fibromas which provide a source of infection for transmission of the virus by arthropod vectors. Following inoculation of Oryctolagus cuniculus with virulent myxoma virus there is a well-defined sequence of appearance of the virus in different organs. It is only found in the inoculation site on the first day, by the second day it is also found in lymph nodes, by the third day in the blood, spleen and liver. On the fourth day the virus can be seen in all tissues. Up until this point the only physical sign of infection is a lump in the skin at the inoculation point, however on the fifth day conjunctival swelling is apparent. Signs of infection are generalised on the sixth day by swellings on the skin and muco-cutaneous junctions, the eyelids, nose and anogenital region. Symptomatology reaches a peak on the eight and ninth days and death usually occurs on the tenth.
The cause of death is obscure because although the virus multiples to reach high concentrations in skin there is little involvement with the adrenals, kidneys, spleen, liver, lung and brain. The swellings cannot really be called tumours as they are due to a large extent to an accumulation of mucinous material as well as some degree of cellular proliferation in the dermis. There is also a proliferation of endothelial cells of the small capillaries and venules, and there is some thought that this may be a causative factor in the death of the host.
2.2 TRANSMISSION
The mechanisms of transmission of myxomatosis are of extreme importance since they demonstrate why it is epidemic or endemic in some countries and has localised outbreaks in others. The reasons for differences is due to the effects of environment on transmission, particularly on the availability of vectors for the disease.
2.2.1 Contact Infection
Myxomatosis is accompanied by a profuse ocular discharge as well as a discharge from the skin lesions, both of which are rich in virus. These discharges allow transmission of the virus by direct contact with scratches on the skin or superficial mucous membranes as occurs during social or sexual interaction.
Transmission via the respiratory tract is also possible if rare. Infection does not occur by feeding and therefore there no faeco-oral transmission.
This method of infection is of primary importance in the warren environment of O. cuniculus where there are many rabbits in a small area.
2.2.2 Arthropod Vectors
Insect vectors form a very important method of transmission. A wide number of mosquitoes, fleas, ticks, mites and lice have been shown to be vectors. The insects can feed on the blood of the infected rabbit or more easily on the exposed area of skin lesions. Further more, unlike infected rabbits which die after about ten days after which the possibility of contact infection is reduced, it has been shown that mosquitoes can carry a virus capable of re-infecting rabbits for up to 6 weeks. Rabbit fleas, particularly Spilopsyllus cuniculi, can act a reservoir of infection for several months after rabbits have deserted a burrow.
Taking into account the range of mosquitoes this allows transmission of the myxoma virus over greater areas than are usually travelled by the rabbits alone. This allows the spread of the virus to take place between colonies of rabbits, and in the case of the fleas, allows rabbits from a different colony to become infected be entering a warren where all the occupants have been killed by myxomatosis some months previously.