European College of Equine Internal Medicine consensus statement on equine flaviviridae infections in Europe

Abstract

Horses and other equids can be infected with several viruses of the family Flaviviridae, belonging to the genus Flavivirus and Hepacivirus. This consensus statement focuses on viruses with known occurrence in Europe, with the objective to summarize the current literature and formulate clinically relevant evidence-based recommendations regarding clinical disease, diagnosis, treatment, and prevention. The viruses circulating in Europe include West Nile virus, tick-borne encephalitis virus, Usutu virus, Louping ill virus and the equine hepacivirus. West Nile virus and Usutu virus are mosquito-borne, while tick-borne encephalitis virus and Louping ill virus are tick-borne. The natural route of transmission for equine hepacivirus remains speculative. West Nile virus and tick-borne encephalitis virus can induce encephalitis in infected horses. In the British Isle, rare equine cases of encephalitis associated with Louping ill virus are reported. In contrast, equine hepacivirus infections are associated with mild acute hepatitis and possibly chronic hepatitis. Diagnosis of flavivirus infections is made primarily by serology, although cross-reactivity occurs. Virus neutralization testing is considered the gold standard to differentiate between flavivirus infections in horses. Hepacivirus infection is detected by serum or liver RT-PCR. No direct antiviral treatment against flavi- or hepacivirus infections in horses is currently available and thus, treatment is supportive. Three vaccines against West Nile virus are licensed in the European Union. Geographic expansion of flaviviruses pathogenic for equids should always be considered a realistic threat, and it would be beneficial if their detection was included in surveillance programs.

Keywords: equid; equine; gastroenterology; hepatitis; hepatology; microbiology; neurology; viral; zoonoses.

FIGURE 1

Equine pathogens in the Flaviviridae family: Japanese encephalitis virus (JEV), Murray Valley Encephalitis Virus (MVEV), West Nile virus (WNV) and Usutu virus (USUV) are classified in the Japanese encephalitis antigenic complex transmitted by ornithophilic mosquito species, mainly Culex spp. Japanese encephalitis virus and MVEV are not endemic in Europe. Tick‐borne encephalitis virus (TBEV) and louping‐ill virus (LIV) are closely related zoonotic flaviviruses belonging to the tick‐borne encephalitis (TBE) serocomplex transmitted by the same ixodid tick vector, Ixodes Ricinus. The manner of equine hepacivirus (EqHV) transmission is unknown

FIGURE 2

West Nile Virus (WNV) and Usutu virus (USUV) undergo sylvatic circulation between birds and mosquitoes. Birds serve as an amplifying reservoir host. Culex spp. are the major mosquito vectors of both viruses worldwide. Mosquitoes can transmit vertically to maintain the virus through the winter. Transmission occasionally occurs from mosquitoes to horses or humans, but mammals are dead‐end hosts

FIGURE 3

Tick‐borne encephalitis virus and LIV transmission involves a complex ecology of reservoir species, ticks and indicator species that support tick populations. Small vertebrates including rodents, insectivores, and wild carnivores serve as amplifying reservoirs, developing high titer viremia and transmitting to ticks. Ticks transmit transstadially through egg, larvae, nymph, and adults, and transmit to a wide variety of vertebrate species. Large mammals such as deer and livestock maintain tick populations and are susceptible to disease, but do not transmit back to ticks due to low virus load, short duration of viremia or a combination of the 2. They are considered indicator species because they are valuable sentinel species for disease and antibody prevalence. Humans and horses are considered accidental hosts because they are not involved in sustaining transmission or feeding tick populations

FIGURE 4

Global distribution of WNV infections reported to OIE 2005‐2021 and TBEV infections according to Im et al. WNV and TBEV cocirculate in Europe. Countries in gray: no data reported

FIGURE 5

Global distribution of EqHV., , , , , , , , , , , , Countries in gray: no data reported

FIGURE 6

Recommended diagnostic algorithm for WNND. Results of rapid IgG tests must be interpreted in the context of IgM results, or with virus neutralization to differentiate between WNV, USUV, TBEV, and LIV infection