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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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