Myxomatosis is a contagious viral disease that is endemic in many parts of Germany.
Myxomatosis is a rabbit disease caused by myxoma virus. The disease is widespread in Germany, where the endemic areas are spreading ever further. Depending on the weather conditions and associated numbers of mosquitoes (which are among the carriers of the virus), and on the incidence of myxomatosis in the wild rabbit population (a natural reservoir of infection), there can sometimes be real epidemic years.
A very high risk of infection exists in the following areas:
Morbidity and mortality rates can reach 100 % in unprotected populations.
This depends on the virulence of the virus and the dose of infection involved. Such high losses can destroy many years of breeding work and may even lead to the total loss of the breeding population. Preventive vaccination is the only effective way to protect against this scenario.
However, in order to build up reliable immunity to myxomatosis and prevent dangerous endemic and epidemic infections, the vaccination coverage of the rabbit population must be higher than 70 %.
Only uniform vaccination coverage can protect rabbits during outbreaks and is therefore an aim to be pursued.
Myxoma virus is transmitted by insects, direct contact and inanimate vectors.
Rabbit myxomatosis virus belongs to the genus Leporipoxvirus. It is an enveloped DNA virus. There are several strains with different virulence which are grouped into five grades according to their survival rate in days. Moderately virulent strains (grade III) are transmitted more effectively than virulent or highly attenuated strains.
Grade I (<13 days' survival time) are highly virulent, with mortality approaching 100 %; grade V (>50 days' survival time) are highly attenuated, with approximately 10 % mortality.
Mosquitoes play the primary role in virus transmission.
The virus is transmitted primarily by biting and blood-sucking insects including mosquitoes (belonging to the genera Anopheles, Aedes and Culex), mites, ticks and the rabbit flea. Mosquitoes especially are responsible for spreading the disease over long distances. However, the virus can also be transmitted by direct contact as rabbits touch and sniff each other. The virus enters the body through skin lesions, the mucosa of the upper respiratory tract, the genitals, and the eyes. Animals can also become infected through contact with inanimate vectors contaminated with the virus, such as grass cuttings, hutch equipment, etc.
Natural reservoirs of the virus are wild rabbits and latently infected animals, which show virus persistence. These include rabbits which have survived an infection.
Animals which have survived an infection must be removed from the population; otherwise, they represent a permanent source of infection for healthy rabbits.
The clinical picture is usually fairly typical. The first symptoms appear 4-10 days after infection.
The virus enters the body through skin lesions, the mucosa of the upper respiratory tract, the genitals, and the eyes. However, it can also be transmitted orally.
The first symptoms appear after a 4-10 day incubation period. Initially, the virus attacks the lymph nodes close to the entry sites. From here, it spreads throughout the body via the bloodstream and the lymphatic system.
There are two different forms of mxyomatosis:
However, both forms may be expressed concurrently.
The oedematous form is almost always fatal, while mixed forms and the nodular form are usually less severe.
Characteristic symptoms of the edematous form are edema and myxomas; skin lesions are typical of the nodular form.
The oedematous form is much more common and usually takes an acute and more aggressive course. Virus replication leads to inflammatory infiltrations, swelling of the skin due to fluid retention (oedema) and firm lumps on the face and around the anus and genitals. Swelling particularly affects the eyelids and the oral mucosa in the mucocutaneous transition area. Sensitivity to light and difficulty in swallowing are also observed. Food intake drops sharply and animals lose weight quickly. The immune system is weakened.
Anogenital tract swellings are a pathognomonic sign.
The skin and mucosal damage caused by the virus often leads to bacterial infections, especially of the respiratory tract. This leads to eye and nasal discharge with pus and, as the disease progresses, to pneumonia accompanied by breathing difficulties. In this case, the bacterium Pasteurella multocida is the agent most commonly isolated from the lungs. After around 1-2 weeks of illness, animals die in a weakened and emaciated condition.
The nodular form is generally less severe. Affected rabbits display firm, lumpy cutaneous and subcutaneous lesions which ulcerate, necrotise and heal. At first, depending on the severity of secondary infections, the general state of health is often only slightly to moderately impaired. Animals usually retain their appetite, and swallowing and breathing difficulties are less evident than in the oedematous form.
At the peak of an epidemic, the peracute form may occasionally be observed. In this case the disease lasts for only a few days, such that clinical and pathological symptoms are often not expressed.
Rabbit with symptoms of the edematous form
Rabbit with symptoms of the nodular form
The clinical picture leads to a tentative diagnosis which can be confirmed by antigen and/or virus detection.
In the predominantly oedematous form, the clinical picture is generally clear, especially in the absence of vaccine protection. If there is any doubt, however, antigen detection can be carried out by special laboratory test.
Serum antibodies can be demonstrated by neutralisation test 14-21 days after infection.
Depending on the progression of the disease, post mortem findings include cutaneous and subcutaneous lesions, and swelling of the eyelids, base of the ear and anogenital region. Together with blepharoconjunctivitis accompanied by pus, these are the most common macroscopic lesions. If there is pulmonary involvement, non-specific pulmonary lesions or catarrhal and purulent bronchopneumonia may also be observed.
Histologically, specific 'myxoma' cells are pathognomonic for myxomatosis. Eosinophilic nuclei can often be detected in these cells' cytoplasm.
The IDT Myxo Map can help to identify endemic areas.
The risk of infection is particularly high in areas which have already had myxomatosis outbreaks in the current or previous year, as these areas still have large numbers of infected mosquitoes or reservoir animals. It is therefore vital to look at the spread of myxomatosis not only in the current year but over several years.
Only in this way can we get a sense of the pressure of infection that breeding populations and domestic rabbits face from infected mosquitoes or reservoir animals.
To this end, IDT Biologika Animal Health has introduced disease monitoring for breeders and vets, and produces a regularly updated Myxo Map. The map shows the reports received in the current year. However, maps for previous years are also available.
Therapy is often futile and the prognosis is poor. Only mild cases should be treated.
In the case of the oedematous form, therapy is usually futile unless the progression of the disease is mild or the infection is a mixed form. In the latter case, and in the nodular form of the disease, an attempt at therapy may be successful. However, it is only possible to treat the existing symptoms.
A decisive factor in any attempt at therapy is the animal's condition on initial presentation. If the rabbit is already having serious difficulty in swallowing or is suffering from shortness of breath, euthanasia is recommended on animal welfare grounds.
Supportive therapy consists of:
It is also important to make sure that the rabbit is eating enough, to stop its immune system becoming any weaker. If necessary, the rabbit may have to be syringe-fed and pampered.
Only rabbits which have received basic immunisation and six-monthly boosters are reliably protected against myxomatosis.
Due to the epidemic nature of the disease and its usually poor prognosis, all rabbits should be vaccinated regularly against myxomatosis, whatever the form of husbandry. To reduce the pressure of infection, high vaccination coverage is important, not least because the virus can overwinter in mosquitoes and because wild rabbits are the natural disease reservoir.
Under practical conditions, vaccination to protect domestic rabbit populations should be scheduled for as early in the year as possible. This is to enable immunity to develop before the arrival of warmer weather and the associated spread of blood-sucking insects carrying the virus.
It is also vital to provide good protection against biting insects by fitting insect nets to doors and windows. Animals should have no contact with wild rabbits and should not be fed plants from grassland inhabited by wild rabbits.