New tick-borne virus identified in China after study of unexplained SFTS-like illness

Researchers in China have identified a previously unknown tick-borne orthonairovirus associated with fever, fatigue, gastrointestinal symptoms and abnormal laboratory findings. The virus, named Asian longhorned tick nairovirus (ALTNV), was found in more than 10% of 3,163 patients investigated in areas where severe fever with thrombocytopenia syndrome is endemic.

The discovery, reported in the New England Journal of Medicine, began with a diagnostic problem: around 30% of patients presenting with signs compatible with Dabie bandavirus infection, the cause of severe fever with thrombocytopenia syndrome (SFTS), tested negative for that virus. Researchers therefore screened patients with recent tick exposure using RNA sequencing and virus isolation.

ALTNV was classified as an orthonairovirus in the family Nairoviridae, genetically distinct from known species in the genus. The virus could be isolated in cell culture, and electron microscopy showed morphological features consistent with orthonairoviruses.

More than 10% of patients tested positive

Among 3,163 patients, 10.4% were positive for ALTNV on the basis of viral RNA or IgM antibodies. Among patients who tested negative for Dabie bandavirus, 15.1% were ALTNV-positive. In patients infected with ALTNV alone, the most frequent signs were fatigue in 84%, gastrointestinal symptoms in 80%, thrombocytopenia in 38% and elevated aminotransferase concentrations in 36%. All patients with ALTNV infection alone recovered completely.

The clinical picture therefore overlaps partly with SFTS but appears, from these data, to be generally milder when ALTNV occurs alone.

Coinfection with Dabie bandavirus linked to more severe disease

The study also identified 38 patients coinfected with ALTNV and Dabie bandavirus. Compared with patients infected only with Dabie bandavirus, coinfected patients more frequently developed respiratory symptoms, 61% versus 38%, and kidney impairment, 84% versus 66%. Seven of the 38 coinfected patients died, corresponding to 18.4%. These findings do not yet establish whether ALTNV directly increases the severity of SFTS, but they raise an important diagnostic issue in areas where both viruses circulate.

Haemaphysalis longicornis appears to be the main vector

Researchers also examined 47,428 ticks collected from 15 Chinese provinces. ALTNV RNA was detected in 1.3% overall, with the highest prevalence, 1.8%, in Haemaphysalis longicornis, the Asian longhorned tick. Experimental work showed that H. longicornis could transmit ALTNV to mice, supporting its role as a vector. This tick species is already widely distributed in East Asia and is also established in several other parts of the world.

The study therefore identifies ALTNV as another pathogen circulating within a tick ecosystem already responsible for several important zoonoses.

A new differential diagnosis for SFTS-like disease

The epidemiological importance of ALTNV lies less in the discovery of another virus than in the number of infections identified. A substantial proportion of patients with an SFTS-like illness but negative Dabie bandavirus tests were infected with ALTNV. In endemic areas, relying on a single diagnostic target could therefore leave a significant fraction of tick-borne illness unexplained.

The authors conclude that the cocirculation of ALTNV and Dabie bandavirus, combined with their shared tick vector, makes improved differential diagnosis necessary in regions where SFTS is endemic.

For animal and One Health surveillance, the next questions concern the virus’s reservoir hosts, its geographical distribution beyond the Chinese provinces studied, and whether domestic or wild animals contribute to maintaining its transmission cycle.