 Ross
River Virus - D/J 05/06
Ground breaking new research is
paving the way to understanding and addressing the transmission of Ross
River virus into the equine population.
by Dr J.H. Stewart
BVSc BSc PhD MRCVS Dip BEP AAIM
Equine Veterinarian and Consultant Nutritionist
The most common mosquito-borne disease in Australia, the Ross River virus
is not only contracted by approximately 5000 people each year but also
affects a number of animal species - including horses. While in humans,
the infection generally results in a flu-like period of illness often
coupled with joint pain and fatigue, many horses that are exposed to the
disease show no outward clinical signs, yet raised antibody levels (indicating
the presence of an infection) can usually be detected in their blood.
In other equine cases, the Ross River virus can cause lethargy, poor performance,
stiffness, joint disease and nervous system disruption.
Transmission
Although the life cycle of the Ross River virus is predominantly maintained
by the interaction of mosquitoes with mammals such as the Western Grey
kangaroo, some marsupials, wild rodents and possibly fruit bats, horses
and humans tend to become involved as hosts to the disease when certain
favourable environmental conditions exist. Immediately following heavy
rains and/or a period of flooding is the perfect time for mosquito colonies
to flourish, therefore the risk of being bitten by an infected insect
is far greater than it would be in the same area during a drier period.
There is also a theory that mosquito eggs can exist in the environment
for a long time once laid, only hatching with the onset of the right weather
conditions, which may explain the rapid appearance of cases of Ross River
disease after heavy rains.
Once the virus has been transmitted to the host animal via a mosquito
bite, the incubation period for the disease is generally from 5-15 days
- although can be as short as three days or as long as three weeks. The
majority of horses don’t exhibit any physical symptoms during this
time, although some may develop a mild fever or show vague signs of stiffness.
Following the incubation period, a small percentage of infected equines
develop acute joint tenderness with minor restriction of movement while,
in even rarer cases, obvious swelling can be seen. The pattern of symptoms
shown by these horses tends to be symmetrical, meaning that it affects
both sides of the body equally.
Detection and Treatment
If Ross River disease is suspected, the presence of a viral infection
can be detected by measuring the antibody levels in a horse’s blood
- with the two main ones of interest being IgM (which indicates recent
infection) and IgG (which may remain elevated for years after exposure).
As an important part of the equine defence mechanism against disease,
antibodies enable the body to ‘read’ special features on the
outer cell wall of invading organisms - such as the Ross River virus -
and to ‘recognise’ them as foreign matter. In response, a specific
group of cells (known as T-cells) are mobilised to clear the infection
and prevent it from invading the horse’s system any further.
The Ross River virus belongs to a select group known as super-antigen
diseases, as do others such as West Nile virus, bird flu and influenza.
The name arises from the fact that, during the process of establishing
itself in the host animal, these viruses produce what are called ‘super-antigens’.
These are a type of ‘alien’ protein that binds directly to the
surface of the T-cells in the horse, with the end result being a massive
release of toxins. Known as cytokines, these toxins cause inflammation
of many organ systems and, if high levels exist, can lead to shock and
death.
Traditionally, treatment for equine cases of Ross River disease has revolved
mainly around anti-inflammatories and rest, but recently, research has
uncovered a specific enzyme that is opening up exciting new treatment
possibilities. This enzyme, found naturally in plant material, acts to
prevent the virus particles from attaching themselves to the horse’s
cells - therefore ultimately stopping the process of toxin release in
its tracks. To date, laboratory trials have confirmed up to 80% effectiveness
rate of blocking the cell infection process when a feed supplement containing
a set concentration of the enzyme is given. After being consumed, the
supplement is absorbed into the bloodstream - giving it a potential role
in preventing not only Ross River disease, but other viral infections
as well.
Protection = Prevention
Particularly for those horses that are kept near high risk environmental
areas - such as seasonal wetlands, salt marshes and coastal areas - it
is important to be aware of how the virus can be contracted and take steps
to curb its transmission. Recommended mosquito control measures include
adjusting paddock turnout regimes to avoid the high insect activity times
of dawn and dusk; applying insect repellent containing diethyl toluamide
(DEET) or picaridin; installing screening material in stable areas and
facilitating drainage of any places where water tend to collect. There
are also a wide range of rugs, neck rugs/hoods, fly boots, face masks
and even belly covers available on the market that can help in limiting
the amount of a horse’s skin that is exposed to a potential mosquito
bite.
Fortunately, once a horse has been infected with Ross River virus, its
immunity against further attacks of the disease is thought to be life-long.
It’s a comforting thought, however, that in the not-too-distant future,
it seems that we may well be able to prevent the discomfort and debilitation
it can cause in horses from developing in the first place.
Ross River - Facts and Fallacies
Major outbreaks of Ross River virus in the human population have been
recorded in Victoria and South Australia (1993 and 1997), New South Wales
(1996 and 1997), Western Australia (1991/1992 and 1995/1996), and Queensland
(1996). Queensland had the largest number of cases of Ross River virus
infection between1998 to 2000. One could expect that, although figures
on the equine infection rate during these times are not available, they
could well mirror these results.
Western Australia has received particular attention in recent times in
relation to both Ross River disease and Murray River Encephalitis, with
the Public Health Agency of Canada monitoring the increase in their activity
in the South West, South Coastal and Metropolitan regions during 2004.
If a human or animal is bitten by an infected mosquito, it doesn’t
necessarily mean that they will contract Ross River disease, as the infection
rate is less than one in three people. Ross River virus also can’t
be passed from person to person, from horse to horse or between horses
and humans.
Infection by the Ross River virus is not fatal, but a debilitating form
of arthritis that it is associated with is thought to contribute to a
shortened life span. Other equine diseases that may be mistaken for Ross
River include botulism - sometimes contracted after eating chaff containing
bits of minced rodent - and Equine Herpesvirus 1, which produces central
nervous system signs including swaying of the hind limbs and dribbling
of urine.
Scientific Aspect
Once it has entered a horse’s body via a mosquito bite, a virus such
as Ross River is like a little ‘space invader’ and uses one
of its two ‘arms’ - called the haemagglutinin - to grab onto
an equine cell. Once firmly attached, the other arm - called the neuraminidase
- drills a hole in the cell wall and the virus then passes through this
opening. This invasion, and subsequently the ‘taking-over’ of
the horse’s normal cell ‘machinery’ allows the Ross River
virus to rapidly multiply and release large numbers of ‘alien’
cells within its body.
The enzyme that has now been identified as capable of blocking the Ross
River infection process acts by ‘cutting off’ the haemagglutinins
and neuraminidases, effectively rendering the virus helpless - a bit like
a convoy of army trucks with their wheels removed.
Once these ‘arms’ have been detached from the virus, it is unable
to anchor itself to a horse’s cells. In addition, the same enzyme
also renders the virus incapable of stimulating an inflammatory response.
In effect, the action of this amazing enzyme causes the whole ‘military
operation’ to fall apart as, not only does the planned viral invasion
fail, but the horse’s body also doesn’t have to divert its own
resources to defend and protect itself.
 About
the Author
Dr Jennifer H Stewart’s PhD may have been on the subject of foals
but her interest and involvement is with all things equestrian. She is
a consulting nutritionist, official racetrack veterinarian and worked
at the Sydney Olympics. As well, Jennie has been involved in collaborative
research with the University of Sydney, major studs and vet practices
and is currently working on ground breaking reseach that involves all
disciplines. Dr Stewart can be contacted on drjhs@bigpond.net.au
Reader's Story - Undiagnosed Ross
River
When Pam Beetham’s gelding, Thomas (left), developed intermittent
swollen glands and swelling in his legs in the middle of 2002, little
did she know that the eventual outcome would be a diagnosis of Ross River
disease.
Initially treated with anti-histamines and an anti inflammatory due to
an insect bite (at this stage unknown) and an allergy was suspected as
the cause of his symptoms. Thomas then started to cough - mainly when
being worked for what, at the time, was a promising dressage career at
Novice level. The coughing became progressively worse until it reached
the stage when the Warmblood x Thoroughbred was unable to be ridden and
even started coughing when at rest in his paddock.
While treatment for the cough and allergy symptoms continued, resulting
in a slight improvement, a further problem arose in the form of what looked
like a type of greasy heel but didn’t respond to the standard antibiotic
creams, a change of paddock and attention to keeping his legs dry. As
weeks turned into months, two and a half legs became covered in what looked
like tropical ulcers that were obviously quite painful. Cortisone injections
were the recommended form of veterinary treatment, along with a steroid
cream - the latter proving to be very difficult to apply because Thomas
was so sore. It helped a bit, however, as one and a half of his legs cleared
up but the other, a white hind, just wouldn’t. Pam admits she was
almost in tears every time she had to try to put the steroid cream on,
and says the only touch Thomas seemed to be able to tolerate was from
her Blue Heeler dog, which took to lying on the ground to lick and tease
the scabs off his equine friend’s legs.
Becoming frustrated with the lack of progress being made, Pam approached
her vet at this stage to ask if a blood test or skin scraping could be
done to see what it was they were dealing with - only to be told that
the condition was called vasculitis and that a blood test wouldn’t
show anything useful, as it was already obvious that Thomas had an infection!
A change of vets was made, with the second one saying it was more like
phlebitis, and the treatment (already unsatisfactory) was the same.
Deciding that she wasn’t prepared to simply keep shovelling cortisone
and steroids into Thomas, Pam began to investigate herbal treatments -
first Thuja, which helped a little, then Calendula, which proved to be
quite effective in healing up many of the sores.
Meanwhile, Pam had learnt Bowen Therapy and carried out this treatment
on a weekly basis while continuing with the Calendula topically and Rosehips
internally. As the weather cooled down for winter in 2003, Thomas seemed
a little better and his saddle was even taken out of storage, but this
was short lived.
With the onset of spring, the same symptoms of swollen glands, puffy eyes,
joint swelling, intermittent coughing, low grade colic and breakouts of
sores appeared with a vengeance again. At her wits end by now, Pam tried
comfrey cream while Blue ‘the healer’ dog kept on licking to
help control the scabs. Hair testing saw a change in diet recommended,
which immediately stopped the cough and colic, while herbal tonics seemed
to brighten Thomas’s outlook to some extent.
The breakthrough in finding out exactly what the gelding had been suffering
from all this time came in 2004 when, firstly, Colloidal Silver spray
cleared up the leg sores within a few weeks instead of the usual months.
Soon afterwards, a Homeopathic Vet that Pam had managed to locate organised
a full blood test, right down to trace elements, and specifically asked
for testing for Ross River and Dengue Fever. And there it was in the results,
Ross River! Various feed herbs, additives and homeopathic medicines were
prescribed and Thomas’s treatment regime ‘tweaked’ as he
progressed.
Now only receiving Vitamin C powder, Rosehips and Garlic granules with
his feed every day, and continuing to have Bowen Therapy on a regular
basis, Pam reports that Thomas’s joints are as near to normal as
she can remember them. He has also lost the ‘pudgy’ softness
that was present as part of his symptoms and there has not been an outbreak
of sores on his legs since early 2004. Although he still has slightly
puffy eyes and his glands come up occasionally, they respond well to the
Bowen Therapy and Pam is now able to ride him for 15 - 20 minutes every
second day at the walk and trot.
Of her experience, Pam says she never thought she’d be so happy to
find out that her horse had Ross River Fever yet, after struggling with
all symptoms and problems for years, it was a huge relief to find out
there was a reason for it all;
“I should have insisted that the first vet did the same blood test
so now I’d always recommend to others to go with your instincts and
insist on active investigation to find out the reason a horse is not well
- not just continue to treat the symptoms. While I am disappointed with
the traditional veterinary practitioners who wouldn’t look ‘outside
the square’, thereby making the journey so much longer, I am thrilled
to have found an alternative and that it has helped Thomas to get this
far. It’s been a very frustrating few years that could have been
much shorter if we had a diagnosis earlier, much less expensive and Thomas
need not have suffered as much as he did while I wandered around the medical
then natural remedy wilderness looking for the answers.”
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