The number of hantavirus illnesses aboard cruise ships is still rising. According to the World Health Organization, there were eight cases as of May 6, three of which have been identified as hantavirus by laboratory testing. We learned that three travelers had passed away in recent days.
The cruise liner MV Hondius is currently undergoing a medical evacuation of some of her guests. Other travelers have gotten off and are heading back to their homes. A passenger on the ship is now a confirmed case, according to Swiss authorities, and is being treated in a hospital in Zurich.
As a public health physician, I have a particular focus on respiratory conditions. Additionally I looked into a hantavirus outbreak.
Investigators are interested in the following details on the current cluster of instances. To determine whether the infection is spreading from person to person, evidence must be gathered.
There was an unidentified pathogen back in 1993.
I was a young epidemic intelligence service officer at the US Centers for Disease Control in 1993. I was sent to the southwestern US deserts to assist in the investigation of a terrifying outbreak that primarily affected Navajo people.
Suddenly, adults in their twenties and thirties began falling ill. As fluid seeped into their lungs, they would first have a fever and cough before quickly developing severe respiratory failure. Some looked healthy enough to dance in the evening, but they passed away in a matter of hours.
The team conducting the investigation was anxious. As of yet, we had no idea what the infection was, how it was spreading, or if we were in danger.
Since a well-known runner was among the first cases to be identified, we originally questioned if infection could be connected to breathing in dust from the desert. The prevalent plague and a leak from a distant military biowarfare lab were also taken into consideration.
Following laboratory testing, a novel hantavirus—later dubbed the Sin Nombre virus—was found to be the culprit. Exposure to the urine, feces, and saliva of infected deer mice was associated with the virus's attack on the lungs' tiny blood arteries. In the impacted communities, the number of mice had sharply grown and were infiltrating homes and offices.
One important discovery was that the Sin Nombre virus does not seem to pass from person to person similar the majority of hantaviruses. Shared exposure to mice or rodent-infested settings, particularly during cleaning or other close contact with contaminated objects or dust, provided an explanation for family clusters.
Because of this, many of us were taken aback years later when it was discovered that the Andes virus, a South American hantavirus, could occasionally pass from person to person.
Although this is still rare, it has been reported, notably in outbreaks in Argentina, the nation from which the MV Hondius sailed prior to the present probable outbreak.
Now, what would a disease investigator do?
Verifying the diagnosis is to first stage in any investigation into an outbreak. The distinction between a "suspected" and "confirmed" instance is still important at this point.
Researchers must determine whether the cluster's severe respiratory infections are entirely caused by the hantavirus or if confirmed instances are happening in the context of another infection, such influenza or Covid.
Creating a timeline is the next step. The first indication of where and how they were exposed is frequently the timing of the onset of symptoms.
The ship left Ushuaia, Argentina, on April 1, according to WHO. On April 6, the first recorded patient began to exhibit symptoms. Later in April, symptoms appeared in other individuals.
Let us concentrate on the first three examples.
The respiratory symptoms that follow a hantavirus infection that mostly affects the lungs are referred to as hantavirus pulmonary syndrome. Usually, these appear two to four weeks following exposure. On the other hand, sickness can manifest up to eight weeks following infection or as early as one week.
Because of this, it is challenging to explain the first case as exposure obtained on the ship after departure. Five days after departing Argentina, on April 6, symptoms first appeared. That is even shorter than the lowest end frequently mentioned and less than the typical incubation period (the time between infection and the onset of symptoms).
In that scenario, it is more likely that the individual was infected in Argentina before to boarding. A bird-watching pastime that may have exposed people to rodents is being reported.
The later instances are less clear. They might have been discovered prior to departure, while engaging in shore activities in Argentina, or at another location. However, another option is also raised by their timing: transmission from the initial instance to close contacts on board.
The epidemiology becomes intriguing at this point.
Did individuals contract the virus from one another?
The first case was closely related to the second. This gives rise to two reasonable explanations. The same sick rodent (or its urine or droppings, for example) may have come into contact with them both. On the other hand, the second case most certainly got the infection from the first case.
The third example did not belong to the same close-knit family. Investigators may still be able to identify a common source for the outbreak if they discover that this individual went on the same trips in Argentina as the first two. However, suspicion of person-to-person transmission rises if there was no shared rodent exposure.
This does not imply that transmission from person to person is established. This makes it one of the most important theories to investigate.
Investigators would have to take into account a less neat sequence of events if human-to-human transmission is not the explanation.
Pre-boarding exposure with a brief incubation time would have occurred in the first example. Either the infection from the first instance or the same exposure with a longer incubation period would be required for the second case.
Either a separate exposure to infected rats prior to boarding or another exposure during the journey would be required for the third instance. None of these are unachievable. However, it becomes increasingly difficult to rule out the human-to-human theory when more cases emerge and if they coincide with contact with previous cases.
It is also crucial to consider the approximate difference in disease between the initial case and subsequent cases. Severe hantavirus disease is likely to be accompanied with an increased risk of being contagious and infecting others if person-to-person transmission is taking place. Therefore, we would anticipate that symptoms would appear two to three weeks following close contact with a previous severe case, and this is what the cruise ship is exhibiting.
What are the consequences for public health?
Therefore, both options must be covered by the practical public health response: a restricted spread from person to person and a common environmental source.
This entails conducting in-depth interviews including pre-boarding travel, shore excursions, rodent sightings, wildlife exposure, cabin locations, housekeeping tasks, shared meals, shared transportation, and intimate contact with sick people.
It also entails careful reconstruction of who had contact with whom and when sequencing of viral samples when feasible and laboratory confirmation in numerous cases.
Genetic fingerprinting can determine whether the virus has the same historical mutation that made it possible for human-to-human transmission to occur during earlier epidemics (which were readily contained with simple isolation and infection control). There would be worries about increased transmission hazards if a novel mutation was discovered.
The most important advice for the general public and health officials thinking about accepting the passengers from the ship under quarantine is to remain calm.
The majority of hantaviruses do not pass from person to person. Even with the Andes virus, person-to-person transmission is rare and typically necessitates close or extended contact. The risk to the world's population is now deemed low by WHO. Unlike Covid or influenza, this virus does not propagate.
However, this is precisely the kind of cluster that necessitates rigorous shoe-leather epidemiology for epidemic investigators: Verify the diagnosis, create the chronology, test the conflicting theories, and allow the sickness, exposure pattern, and lab results to tell the tale.
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