Global horse movements and climate change may open the door for Ross River Virus – review

More than 40 mosquito species have been identified as competent vectors for the Ross River Virus. Photo:
ProjectManhattan, CC BY-SA 3.0 via Wikimedia Commons

The possibility of the Ross River Virus emerging as an infectious disease worldwide should be considered seriously, according to the authors of a just-published review.

The mosquito-borne virus, maintained in the wild in a cycle between marsupials and mosquitoes, can infect humans and horses. Indeed, Ross River Virus infection remains the most common human mosquito-borne disease in Australia, with a yearly estimated economic cost of $4.3 billion.

Infection in humans and horses can cause joint inflammation, which can turn into a chronic, long-term debilitating illness.

However, current knowledge around the development of the disease, and the immune system’s response to the virus, remains unclear in humans and horses.

University of Queensland researchers Ka Yuen and Helle Bielefeldt-Ohmann, in their review in the journal Pathogens, warned of a likely shift of the virus dynamic into the temperate areas of Australia, with longer active months of infection.

They cautioned that internationalization and global animal movements raised the potential for the spread of mosquito-borne viruses beyond borders.

The possibility of Ross River Virus emerging as an infectious disease worldwide should be taken seriously, they wrote.

The pair noted that minimal diagnostics are available, especially in veterinary medicine.

“Current diagnostic tests would require weeks before the owner of the horse receives a definitive diagnosis of Ross River Virus.

“This timeframe would not be acceptable for racehorses as the value of the horse would decrease over the time of suspicion, with euthanasia being a very likely consequence.”

The pair said there was also a lack of appropriate surveillance for the disease.

Ross River Virus is closely related to other viruses of veterinary and medical importance, including Chikungunya virus, Barmah Forest virus, Sindbis virus and Getah virus.

The disease it causes in humans is often referred to as Ross River Fever, or epidemic polyarthritis.

Ross River was first isolated from mosquitos in the 1950s, then from a human in the early 1970s and a horse in the late 1970s. It is present in all states of Australia, Papua New Guinea and the Solomon Islands.

It was also isolated in New Caledonia, Fiji, American Samoa, and the Cook Islands during the Pacific regional epidemic between 1979 and 1980.

In humans, it can cause prolonged, debilitating symptoms, although some infected individuals show no symptoms.

In acute cases, patients develop joint pain, most commonly in the knee, ankle and wrists, as well as lethargy or tiredness, and joint stiffness. Muscle pain and skin rashes are common. The disease can be accompanied by a fever and a flu-like illness.

The severity and duration of clinical signs vary from a few weeks to a few months. However, more than half of patients continue to experience symptoms a year later.

To date, no deaths have been reported that have been directly attributed to the virus. However, it is worth noting that uncommon consequences, such as brain swelling, have been reported.

In horses, the authors noted that clinical signs are not well documented, with only a few scientific reports published.

In general, clinical signs are non-specific, ranging from lethargy to neurological problems.

The infection in horses is non-fatal, with most horses recovering several months after initial presentation.

The pair described the transmission cycle as complex, involving multiple hosts and vectors. It is believed the primary reservoir hosts are marsupials, including wallabies and kangaroos.

It has been suggested that in areas where marsupials are uncommon, such as metropolitan or urban areas, brushtail possums and humans are likely to be the main reservoir/spillover host.

Recent outbreaks in northeastern Australia have also suggested a likely human–mosquito–human transmission cycle.

Horses and flying fox bats are also considered as possible reservoir hosts in both areas around towns and cities, and rural areas.

The role of birds in Ross River virus transmission has traditionally been thought to be insignificant, but is somewhat debatable, the authors said.

“The role of horses in the transmission cycle remains unclear.

“Serological and molecular studies suggested that horses are unlikely to be a dead-end host and potentially could be an amplifying host for Ross River Virus during an epidemic.

“The answer to this question is important to the equine industry, especially the racing industry, and people living in areas with a high density of horses, for example, the Hunter Valley region in New South Wales, and outer suburbs of Greater Brisbane in Queensland.

“This will also affect the public health measures and intervention from the state governments to mitigate this most common arbovirus in Australia,” they said.

“In order to maintain and complete the transmission cycle, the competent vector must be able to breed, bite an infected reservoir host, ingest an infectious blood-meal, and bite another susceptible host.

More than 40 mosquito species have been identified as competent vectors for the virus, they noted.

Rainfall seems to be the most important environmental risk factor for outbreaks across Australia. In tropical coastal northeastern Australia, transmission of the virus occurs throughout the year, but the peaks are associated with heavy monsoonal rainfalls.

Prevention is entirely based on general prevention of mosquito bites and management of mosquito breeding sites. There are no approved therapeutics to treat the disease, nor registered vaccines to prevent infection, in either humans or horses.

Treatments, in both human and veterinary medicine, generally relate to easing the effects of symptoms, commonly in combination with non-steroidal anti-inflammatory drugs, and rest.

The authors noted that most parts of the world are currently experiencing the effects of climate change as a result of human activities.

“Generally speaking, average ambient temperature and sea levels are rising, and the frequency of extreme weather events, for instance, heavy destructive rainfall, and prolonged severe drought, are increasing as well.

“This would have implications for mosquito-borne diseases as the vector migrates and adapts to its environmental conditions.”

Climate warming will likely increase the range of the virus in Australia as the mosquito population dynamic shifts to adapt.

“Therefore, temperate regions, such as Victoria, where Ross River Virus circulation status is currently a yearly epidemic, may change to endemic as infection level increases.”

The high-risk season in other areas will likely lengthen. However, case numbers may decline in the hottest tropical regions, depending on how well the mosquito vectors adapt to the upper heat limits.

“Explosive outbreaks may occur in inland Australia that suddenly receives heavy rainfall after a prolonged drought period,” they said.

“The Getah Virus outbreaks in Japanese racehorses in recent years make incursion of exotic alphaviruses into Australia a very real possibility as high-performance Australian racehorses travel around the world to attend competitions, or overseas horses arriving in Australia for competition or breeding purposes.”

They proposed that authorities strengthen mosquito-borne disease monitoring in horses to protect Australia from introducing zoonotic mosquito-borne diseases of both human and veterinary importance.  Horses arriving or returning from overseas should be tested for exotic alphavirus infections during quarantine, they said.

The pair noted that while there are no approved therapeutics or registered vaccines to treat or prevent infection, clinical trials of various potential drugs and vaccines are currently under way.

They noted some major knowledge gaps, around the immune response and progression of the disease in humans and horses; the limited diagnostic options (especially in veterinary medicine); and the lack of appropriate surveillance for endemic and exotic mosquito-borne viruses of human, livestock and veterinary importance.

Yuen, K.Y.; Bielefeldt-Ohmann, H. Ross River Virus Infection: A Cross-Disciplinary Review with a Veterinary Perspective. Pathogens 2021, 10, 357.

The study, published under a Creative Commons License, can be read here.

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