Myxomatosis.
Cause of Myxomatosis
Myxomatosis is caused by
one of several strains of a myxoma virus of the poxvirus family. It grows best
in the skin of rabbits. Virulence of the different strains ranges from a
mortality incidence of 99% in European rabbits to less than 30%. These strains
have varying degrees of virulence with the California strain being one of the
most devastating. California strains have been known to cause over 99%
mortality. The neuromyxoma and Nottingham strains of Europe have both been
associated with substantially less mortality and hence less virulence. The
number and severity of outbreaks varies over time: the myxomatosis virus is
notorious for its ability to mutate from year to year, and the background
immunity in the wild rabbit population also varies.
Myxomatosis in Australia.
Myxomatosis was first field
tested in Australia in 1938 and successfully released in 1950 (Fenner and
Ratcliffe) and resulted in a estimated 99% mortality rate. In the first two
years it reduced the rabbit population from 600 million to less than 100
million.
In Australia Myxomatosis is
used as a control measure for feral rabbits and so new strains of the virus are
continually being released to combat natural immunity and so the risk of your
rabbit getting Myxomatosis is always high – especially in rural areas or
anywhere near natural bushland.
Transmission of the Virus.
Myxomatosis is spread by
blood sucking insects. A major insect parasite that transmits the disease in
this country is the rabbit flea that is frequently found on wild rabbits
although is less common on pet rabbits. Also mosquitoes are a major insect
vector of myxomatosis. The Myxomatosis virus can remain alive in the blood of
fleas for many months.
As the mosquito or flea
bites the rabbit a small amount of the live virus is placed in the skin of the
rabbit as the insect sucks blood. Within a few days the virus is transmitted to
a local Lymph node and then passes into the blood of the rabbit that enables it
to be moved around to several sites.
Incidentally, myxomatosis
is not easily spread by simple contact from one rabbit to another. For instance
if a myxomatosis-infected rabbit is placed in the same hutch as a healthy rabbit
and neither animal is parasitised by fleas or mosquitoes, then the disease is
virtually never transmitted by contact.
The incubation period
varies slightly from one animal to another but can be as short as five days and
as long as fourteen days (incubation period is the time from the point of
introduction of the virus into the animal to the first time that clinical signs
of illness are seen).
Signs and Symptoms
The virus mainly multiples
in the skin around the eyes, the nose, the face, the soft skin inside the ears
and also the skin around the anus and genitals of the rabbit.
After only a few hours - early swelling around the eyes and the ears are already starting to "lop".
Tiny swellings inside the ear. Ear hot to touch.
Some swelling and redness around the vent.
All these symptoms became noticeable within only a few hours
After only a few hours - early swelling around the eyes and the ears are already starting to "lop".
The very first signs we can
see are puffy, fluid swellings around the head and face. 'Sleepy eyes' are a
classic sign along with swollen lips, tiny swellings on the inside of the ear
and puffy swellings around the anus and genitalia. Within a day or so, these
swellings can become so severe as to cause blindness and there may be some
distortion around the face, mouth ears and nose. For this reason feeding and
drinking is often difficult. The common cause of death is a secondary lung
infection, which often occurs around day 8 after the initial incubation of the
disease.
If full-blown myxi then
develops, the rabbit will be a pitiful sight. Severe conjunctivitis accompanied
by swelling of the head and genital region, plus lumps on the body. In pet
rabbits, the disease often progresses more slowly and death is not so rapid
because of the care that the owner gives the rabbit.
Survival
This also varies. Some
animals may survive for weeks or months after infection but, in general, if an
infection is severe in a susceptible rabbit, death occurs within 12 days.
Not all affected rabbits
die. Although recovery is rare in the wild (probably only 5--10% of wild rabbits
eventually recover from myxomatosis) recovery is more common in pet rabbits
because of excellent nursing. If care is taken with feeding, making sure that
water is available and medical care to combat pneumonia is given, then recovery
rates in pet animals may be as high as 40-50% depending on the severity of the
disease.
However, a word of warning - myxomatosis can be a very protracted disease and
affected animals may take weeks or months to recover Even then there may be
severe scaling, scabbing and scarring on the head and body. If an unvaccinated
rabbit catches myxomatosis and develops the full-blown classic form of the
disease, survival is very unusual, even when treated with antibiotics to prevent
secondary bacterial infection. The rabbit can take a fortnight to die and
treatment of this "classic" form of myxomatosis is usually futile. Most affected
pets in this situation are put to sleep to prevent further suffering.
There are two atypical
forms of myxomatosis: one causes pneumonia and a snuffles-like illness; the
other ("Nodular myxomatosis") mainly affects skin and carries a better
prognosis.
Prevention
Since the vaccination
against Myxomatosis is not available in Australia prevention by other means is
vital. Prevention of Myxomatosis is therefore dependent on screening against
mosquitoes and fleas and isolation from possible infected rabbits (feral
rabbits).
Domestic rabbits do not
have any genetically based immunity against myxomatosis. If an unvaccinated pet
rabbit catches myxomatosis, it will almost certainly die. Pet rabbits at
greatest risk are those which live outside, in contact with wild rabbits or
hares, or affected by rabbit fleas - so rabbit owners who also have a dog or cat
that hunts wild rabbits must be particularly careful.
Obviously, isolating pet
rabbits from possible close contact with wild rabbits is sensible. Flea control
is important and may involve positive use of flea control measures such as
sprays, dips and insect repellent strips. Incidentally, there is some evidence
that the domestic cat, which can often be affected with rabbit fleas, may be a
secondary transmitter of the rabbit flea. The virus can survive in overwintering
fleas and mosquitoes sheltering in hay and in houses
Do not forget to control
mosquitoes - it may be possible to use mosquito nets and insect repellent
strips. Any standing or stagnant pools of water where mosquitoes may breed
should be removed. Rabbit cages should be screened by fly wire or shade cloth.
Rabbits should not be allowed to roam free outside at the peak mosquito times of
dawn and dusk. Care should be taken that the bedding of animals is kept dry and
that pet rabbits are not kept in moist conditions that favour mosquito activity.
Please Note: Although there is some treatment
available for rabbits with Myxomatosis, it has been our experience that it is
extremely rare for a bunny to survive Myxomatosis – even with treatment. Please
consider this when contemplating treating your bunny. It may be an expensive,
futile and heartbreaking experience. (WARCI Committee)
Note to WARCI Members. Rules relating to outbreak of
Myxomatosis. (p45)
33.0 OUTBREAKS OF
MYXOMATOSIS AND RCD.
33.1 Any
member of the WARCI in whose rabbitry a case of Myxomatosis or RCD occurs must
report the matter to the WARCI. All members are encouraged to report any
outbreaks of Myxomatosis or RCD.
33.2 Any
member in whose rabbitry a case of Myxomatosis or RCD is confirmed shall be
informed by the Secretary that no rabbits may leave that place until the elapse
of 28 days from the last death. Any rabbits taken from this place during the
time of quarantine to another rabbitry, that rabbitry will be in quarantine
until the elapse of 28 days from the arrival of that rabbit.
Any
rabbitry under quarantine must not advertise or sell rabbits during the period
of quarantine. (Am. 2002)
33.3 Any
member in whose rabbitry a case of Myxomatosis is confirmed, who subsequently
has rabbits recover from having active symptoms of Myxomatosis shall be
informed by the Secretary that their rabbitry is in quarantine and that no
rabbits may leave that place until the elapse of 42 days from the
cessation of active symptoms of rabbit in that rabbitry. Any rabbits taken from
this place during the time of quarantine to another rabbitry, that rabbitry will
be in quarantine until the elapse of 42 days from the arrival of that rabbit.
Any
rabbitry under quarantine must not advertise or sell rabbits during the period
of quarantine. (Am. 2002)
REACTIONS TO
THE VACCINE "CYLAP".
BY KAREN OGDEN.
As if it was not enough that we had to deal with RCD itself,
our relief at having a vaccine readily available was somewhat lessened by the
rabbit’s responses.
A few days following vaccination of our first group of
rabbits, we noticed our REW Cashmere Lop doe seemed to have eaten away a patch
of hair over her shoulders, about 5 cm in diameter, and the skin had become
severely blistered, although she was not in direct sunlight. We applied first
aid, thinking she had been burnt, but our concern grew in leaps and bounds, a
few days later, when several of the vaccinated group (and no others) had similar
lesions with varying degrees of severity.
The obvious conclusions were drawn, and, on being invited
with several other members to a Seminar on the disease and vaccine "Cylap",
brought it to the attention of the Cyanamid-Websters State and National
Representatives. They shared our concern and requested Biopsies and scrapings to
rule out infection, incorrect injection methods and parasites. We selected four
of the eight affected rabbits for a scrape and one to a biopsy.
Results of the skin scrapings were clear. To our dismay the
biopsy results showed a definite reaction, not to the oil-base as hoped, but to
the vaccine itself. The results were as follows:
HISTOPATHOLOGY. Lesions of extensive areas of severe acute superficial dermal
necrosis (breakdown of skin layers) and epidermis loss, interspersed with severe
dermal oedema (fluid build up) with mixed inflammatory cells throughout dermis,
especially around vessels. The epidermis - moderately hyperplastic (thickened)
with focal areas of subcorneal pustules, serocellular exudation (serum
production) and ulceration. No evidence of bacterial infection, not typical
reaction of a bacterial pyoderma.
MORPHOLOGICAL DIAGNOSIS. Superficial and deep perivascular dermatitis with
multifocally coalescing areas of dermal necrosis and ulceration. Subcutaneous
region also involved.
COMMENTS.
Most likely an idiosyncratic (a hypersensitive drug reaction) reaction to a
component of the vaccine. The absence of large numbers of oil-containing
macrophages (a type of cell that appears at wounded areas to combat foreign
matter) raises doubt that the reaction is to the oil component of the vaccine.
As Fanciers, we are
predominantly dealing with exhibition stock, and the nature and high proportion
of reactions is not pleasing.(40% of our original group)
The hair loss and
scarring areas of a Black Rex did grow back coloured rather than white, which is
a good sign for exhibitors.
As far as is currently known, the onset of reaction can vary
from within 24 hours to a few weeks. In severity, reactions can range from
slight hair loss (5 cent piece diameter) with no scabrous inflammation, to
multiple reactions around the injection site, bare, red and inflamed skin,
pitted, bruised, and with a thick, crusty scab developing. This appears itchy at
a later stage and causes problems as rabbits then strip them away. One report
was of a Dwarf with approximately one third of its total body area reacting.
First aid consists of anti-inflammatory creams such as
Neotopic-H or Prednoderm, the former being my choice as it is white and does not
harm if ingested in small quantities. Antibiotics may be required in severe
reactions. Unfortunately there is no way to prevent or predict whether a rabbit
will react or not. We have been advised not to revaccinate the REW Cashmere doe
who reacted so immediately and so badly, as her body's response to the vaccine
and virus was so acute. It is known that only 5% to 7% of rabbits require annual
vaccination, so we will take the chance she is one of the 93% with such a
"response risk" and not revaccinate.
Although we can do little to prevent such responses in our
present vaccine strategy, we can all do our part to help further research into
the reaction. As many detailed records and/or photographs of as many reactions
as possible have been requested, in order to accurately quantify and clarify the
types of reactions. A limited number of subsidised needle and surgical biopsies
are also available. The information required includes the number
vaccinated, the number reacted, their breed, sex, colour, age, the severity of
each reaction, the Vet Practise supplying the vaccine and the Batch number (on
the box/bottle or ask your vet). **
Editors Note: This information is no longer required. This is a 1996 article.
With seemingly imminent release of RCD to WA on the horizon,
as much information as possible is needed to follow up on such theories as
breed/line susceptibility (due to our relatively small and isolated gene pool)
and to document severity/numbers of reactions. Considering the percentage of
hair loss reactions in vaccinated animals, an allowance may be needed for
rabbits on the show table, but this is an issue for the new years' Committee.
With everyones co-operation and assistance we shall all benefit.
May I take this opportunity to thank the past Committee for
their hard work, and the new Committee for taking on what I am sure will be a
challenging year. Thanks also to Jim Lombard and Dr Mark Lindsay of Cyanamid-Websters
for their concern and ready assistance, Dr Elizabeth Vickridge for her surgical
efforts and time, and Dr Jenny Mills, Dr Mandy O'Hara and the Pathology team at
Murdoch for their patience and work for me on this matter.
Originally published in the 1996 Year Book.
