Insect-borne viruses in horses: Our sense of security has eroded

 

A female mosquito of the Culicidae family (Culiseta longiareolata). Photo: Alvesgaspar CC BY-SA 3.0 via Wikimedia Commons
A female mosquito of the Culicidae family (Culiseta longiareolata). Photo: Alvesgaspar CC BY-SA 3.0 via Wikimedia Commons

The emergence of variants of West Nile virus in Europe that are highly pathogenic for horses highlights the potential for such insect-borne disease agents to evolve, an authority on equine infectious diseases says.

Dr Peter Timoney, of the Maxwell H.Gluck Equine Research Center at the University of Kentucky, says vector-borne diseases represent a singularly serious threat to the health of humans and domestic livestock.

“Historically, many such diseases were frequently regarded as geographically restricted in their global distribution and not considered a risk to human and animal populations in far-distant countries in other continents or possibly other hemispheres,” Timoney writes in the latest issue of Equine Disease Quarterly.

“Major disease migrations in the last 20 years, however, have undermined that sense of security.

“No longer can the future distribution of specific infectious agents be predicted with confidence.

“This was most recently exemplified by the explosive and unexpected spread of Chikungunya and Zika viruses, both human pathogens, from where they were originally identified in Africa many years ago.

“Concerns are further highlighted by the risk of spreading yellow fever from Angola, Republic of Congo, and Uganda to European Union member states and even further afield to inter-tropical zones in the Americas and Asia.”

Timoney says the most significant group of emerging human and animal diseases are caused by arboviruses – viruses carried by insects – such as West Nile, Chikungunya, and Zika. These single stranded RNA viruses can have high spontaneous mutation rates.

“Arboviruses are transmitted in nature by arthropod vectors,” he says.

“With the exception of African swine fever virus, all arboviruses of medical or veterinary medical importance belong to one of the following four families: Bunyaviridae, Flaviviridae, Reoviridae, and Togaviridae.

“They are maintained in nature by cycling between a host (mammal, bird, reptile, amphibian) that is infected with a particular virus and a vector – mosquito, tick, sandfly, midge – that is a carrier and transmits the virus to other hosts.”

Some of the most important viral diseases of humans were caused by arboviruses, many belonging to the Flaviviridae and Togaviridae families. They include yellow fever, dengue fever, Japanese encephalitis (JE), West Nile encephalitis (WNE), Zika virus infection, Eastern and Western equine encephalomyelitis (EEE and WEE), Venezuelan equine encephalomyelitis (VEE), and Chikungunya virus infection.

Arboviruses are also the cause of a number of highly significant equine diseases, the most important of which are African horse sickness (Reoviridae), VEE, EEE, and WEE (Togaviridae), JE, WNE, and Murray Valley encephalitis (Flaviviridae).

It is evident from the foregoing that many of the listed equine diseases are caused by zoonotic pathogens, he says.

“Of major concern in assessing the health impact of arboviral diseases is the potential of the causal agents to evolve, giving rise to strains of enhanced pathogenicity for humans or animals.

“This is well exemplified by the emergence of variants of West Nile virus (lineage 2) in Europe that are highly pathogenic for horses.

“The same phenomenon has also been observed with respect to human infection with Chikungunya virus and most recently, also, with Zika virus.

“There is mounting evidence that strains of Zika virus have acquired marked neurotropic tendencies, being implicated as a cause of neurologic defects in unborn infants and an increased incidence of Guillain-Barre syndrome in people.

“It is highly likely we will face future threats from the emergence of other arboviruses with epidemic potential,” Timoney says.

“If we are to be successful in preventing such a threat becoming a reality, we need to identify those viruses with the potential for emergence and gain a greater understanding of their biology and epidemiology, complemented by development of more effective vector control strategies, active surveillance, and enhanced ability to diagnose such infections.”

Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.

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