Alimentary Infections by Tick-Borne Encephalitis Virus

Abstract

Tick-borne encephalitis virus (TBEV) causes serious the neurological disease, tick-borne encephalitis (TBE). TBEV can be transmitted to humans by ticks as well as by the alimentary route, which is mediated through the consumption of raw milk products from infected ruminants such as sheep, goats, and cows. The alimentary route of TBEV was recognized in the early 1950s and many important experimental studies were performed shortly thereafter. Nowadays, alimentary TBEV infections are recognized as a relevant factor contributing to the overall increase in TBE incidences in Europe. This review aims to summarize the history and current extent of alimentary TBEV infections across Europe, to analyze experimental data on virus secretion in milk, and to review possible alimentary infection preventive measures.

Keywords: TBEV; alimentary infection; cows; goats; milk; sheep; tick-borne encephalitis virus.

Figure 1

Routes of TBEV transmission within the life cycle of ixodid ticks. The Ixodes ricinus, I. scapularis and Dermacentor reticulatus ticks are able to maintain TBEV in nature. The life cycle of ticks consists of four developmental stages (larva, nymph, adult, and egg). Each parasitic stage (except egg) needs to take a blood meal on a suitable host to develop into the next stage. The main animal reservoirs for TBEV are rodents. Larger mammals and birds may act as hosts for adult ticks. TBEV infects ticks while they are feeding on an infected reservoir. The main route of TBEV maintenance in nature is non-viraemic transmission from nymphs to naive larvae while co-feeding on the same rodent host. TBEV can be transmitted to humans via a tick bite (mostly nymphs) and via alimentary infection through the consumption of raw milk products from TBEV-infected ruminants (goats, sheep, and cattle) (Figure was created by BioRender).

Figure 2

Epidemiology anamnesis of human cases of TBEV in Slovakia (source: www.uvzsr.sk (accessed on 23 December 2021)). The dashed line represents a linear trendline for the alimentary infections.

Figure 3

Comparison of TBEV viral titers in milk (full line) and blood (dashed line) of experimentally infected sheep, cows, and goats (adapted from Ref. [84]).