Hantavirus: the gamble on asymptomatics, still unclear if they are contagious

There is still uncertainty about the transmission of hantavirus in the asymptomatic phase. Should it be confirmed that it is almost exclusively people with symptoms who transmit the infection, given the severity of the clinical picture, their identification and subsequent isolation would be relatively easy. Tracing contacts and quarantining them should allow outbreaks to be contained, although, given the long incubation times, this would not be quick. But if asymptomatic people were also transmitting the virus, the picture could change. For the worse.

According to Campus Bio-Medico epidemiologist Massimo Ciccozzi 'there may be new contagions. Having lost contact tracing and not having quarantined all the passengers, the group that left the ship early may have seen other people, infecting them'. Given the lack of knowledge about the danger of asymptomatic people, experts said, more care was needed, especially after the first death, and to quarantine everyone who could have been infected. But the ship Hondius did not do this. And even in the isolation procedures, a precautionary criterion should and still should be observed: close contacts should be quarantined together and non-close contacts isolated separately. In short, procedures similar to those adopted during the Covid pandemic: strict today to reduce emergencies tomorrow.

For the diagnosis of hantaviruses there are kits certified for diagnostic use, but for the Andes virus the situation is more complex: the available tests are few, come mostly from China, and all have Ruo (Research Use Only) status, i.e. they are approved exclusively for research and not for clinical diagnostic use. A picture that makes it anything but easy for regional public laboratories, which are now grappling with how to organise their response on the basis of the Health Ministry's circular. "We are in the active screening phase, i.e. we are intercepting people who have been in contact with confirmed cases, and for this it is crucial to have a clinically sensitive test," says clinical microbiologist Francesco Broccolo, of the University of Salento, head of the UOSD Microbiology and Virology University of P.O V. Fazzi - it makes no sense to track only symptomatic people because even those who have no symptoms could be infectious'. This is demonstrated, for example, by the fact that in one of the 11 cases so far ascertained by the WHO, symptoms appeared after the test had been performed.

Asymptomatic transmission is also considered possible by the International Society for Hantavirus Research (Ish) and is documented in the article published in October 2023 by the Robert Koch Institute in Berlin in the journal Emerging Infectious Diseases. This research, in particular, provided the first animal model to study the interhuman transmission of the Andes virus, showing how viral elimination begins as early as the first day after infection through oral mucosa and urine. "Focusing only on manifest cases means ignoring the submerged part of transmission: asymptomatic patients, pre-symptomatic patients, and those who remain infectious even after clinical recovery,' Broccolo observes. 'In the prodromal phase, particularly in the period between 3 and 10 days after infection, the patient already has the virus, and the molecular test is ideal for active surveillance because it is the most sensitive,' he adds, 'and it is based on plasma because the virus is mainly found there. Now, the expert continues, 'it is important to intercept positive cases to avoid outbreaks, and this is precisely the task of active surveillance'.

Partially reassuring news, however, comes from the genetic sequence of the virus isolated from the patient hospitalised in Zurich, which is now freely accessible: it is 99% similar to the sequence detected in Argentina in 2018. At first glance, this indicates that the virus has not accumulated many mutations, essentially retaining its original physiognomy. "This is a relevant finding on two fronts: on the one hand," notes the expert, "it confirms that the virus is relatively stable; on the other, it suggests that the currently available Ruo kits, developed on Argentinian strains, have a very good chance of also working on the currently circulating virus - a non-negligible margin of diagnostic reliability pending ad hoc certified tools.