Best to vaccinate South African horses against West Nile Virus, say scientists

A micrograph of the West Nile Virus, appearing in yellow.
A micrograph of the West Nile Virus, appearing in yellow. Image: Cynthia Goldsmith, P.E. Rollin, USCDCP, public domain

Horse owners in South Africa are urged to vaccinate their animals against West Nile Virus after researchers examined the circumstances behind a surge in serious cases in 2016-2017.

West Nile virus (WNV) is endemic to South Africa, but became recognized as a significant cause of neurological disease in humans and horses in the country in only the past two decades, as it emerged globally.

Scientists Freude‐Marié Bertram, Peter Thompson and Marietjie Venter, in a study published in the journal Pathogens, described the 54 WNV‐positive cases identified across South Africa during 2016-2017 through passive surveillance in horses with a fever and/or neurological signs at the Centre for Viral Zoonoses at the University of Pretoria.

The trio, who are with the university, compared their findings with 120 randomly selected WNV‐negative controls with the same case definition and during the same time period.

Of the WNV‐positive cases, 52% had a fever, 92% displayed neurological signs, and 39% died.

Cases occurred mostly in unvaccinated horses aged 5 or younger, during late summer and autumn after heavy rain, in the temperate to warm eastern parts of South Africa.

WNV‐positive cases that had only neurological signs without fever were more likely to die.

The odds of WNV infection were associated with late summer, higher altitude, more highly purebred animals, a younger age, and failure to vaccinate against WNV.

“Vaccination is currently the most effective prophylactic measure to reduce WNV morbidity and mortality in horses,” they said.

The authors noted that research performed in 2000-2001 among South African Thoroughbreds on widely spread stud farms found that 11% of yearlings had already seroconverted against WNV, relative to sera collected about 12 months prior.

Among their mothers, 75% had also seroconverted, yet no neurological clinical signs had been reported in any of these horses.

“This is consistent with typical WNV occurrence worldwide, as most of the infected horses do not display overt clinical signs (approximately 80%), although viral encephalitis is seen in up to 90% of the symptomatic cases.”

South Africa reported an average of 10 cases a year from 2008 to 2015. The six WNV‐positive cases in 2016 in the current study were consistent with that incidence rate. However, the 48 cases diagnosed in 2017 represented a marked increase.

The spike, they said, was most likely because of environmental factors that promoted the extensive breeding of WNV vectors (mosquitoes), with temperature, humidity, rainfall and hydrology all playing a part.

The authors noted that awareness campaigns in recent years had created public recognition of the potential for WNV to cause severe neurological disease and death in horses and humans in South Africa, and of the need for vaccination among horses.

“However, due to economic constraints, or perhaps ignorance regarding immunology, one may expect many South African horse owners to neglect regular vaccination during years when few cases occur, as is the case with other non‐government regulated vaccines.

“This may contribute to a decline in immunity, especially in younger, immunologically naïve horses, leading to increased WNV case numbers during outbreaks.”

There is also uncertainty amongst horse owners regarding the duration of natural immunity post‐infection, creating differing opinions regarding whether horses should be vaccinated.

The study team concluded that it was advisable for owners of competitive horses, or those younger than 5, especially purebreds such as Thoroughbreds, Warmbloods and Arabians, living in the eastern temperate to warm parts of the country with high summer rainfall, or travelling between provinces, to undertake routine, complete vaccination against WNV.

“These vaccines should be given annually during spring, in order to decrease morbidity and mortality by timeously increasing immunological resistance against WNV.”

Bertram, F.-M.; Thompson, P.N.; Venter, M. Epidemiology and Clinical Presentation of West Nile Virus Infection in Horses in South Africa, 2016–2017. Pathogens 2021, 10, 20.

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



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