Mechanism of Infection and the Cruise Ship
I had been thinking about hantavirus recently just before the recent reports of three deaths from hantavirus aboard a cruise ship are extremely unusual. 149 people remain on board and have not been allowed ashore, and the boat hasn’t been allowed to dock.
As is illustrated above, the normal route of transmission of the virus to people is through the feces and body fluids of infected rodents. In the US, most of the activity has been centered in the Four Corners region (the area where Arizona, Colorado, New Mexico, and Utah meet).
In a worst-case scenario, infection can lead to hantavirus pulmonary syndrome (HPS), which can have a 35% mortality rate. In the US, the median age of patients with HPS (n=890 since 1993) is 38 and about 2/3 of them are male.
There is some behavioral explanation for this. It’s thought that many exposures have to do with going to cabins, barns, sheds, and outbuildings in the spring to clean them for summer use. This can involve sweeping up rodent droppings and dust contaminated with dried urine and saliva from rodents who used the structure for shelter during the winter. In addition, this also may be due to handling firewood or stored materials where deer mice have nested.
That’s what makes this cruise ship outbreak so unusual. The three who died are passengers, so their exposure to rodent infested areas on a cruise ship seems far less likely than that of the crew. However, it’s also worth noting that the Andes hantavirus (the most likely South American one – ANDV) does have a documented human-to-human transmission chain, although that doesn’t happen efficiently. This also could mean that the exposures happened before embarkation in Argentina.
It’s most likely that the rodents or contaminated materials came on board in Argentina. However, it will be interesting to see if there are hantavirus problems on St. Helena in the near future, given there has never been reported cases there. If that happens, one could conclude that an infected rodent left the ship at that port, which would mean that it was a rodent infestation problem on the ship before it left Argentina as opposed to contaminated materials being brought onboard. This is all conjecture though as a thought experiment.
El Niño and Unexpected Ecological Context for Rodent-Borne Viruses
The cruise ship wasn’t what originally put hantavirus back on my radar — climate models did. The trigger for my thinking is both European and US models for El Niño. About 15 years ago I wrote something on ProMED about the interaction between El Niño, West Nile Virus, and hantavirus and how that could lead to an outbreak of HPS in the Four Corners region, as occurred in 1993. My conclusions were what got me interested in the health impacts of climate change.
During El Niño, some parts of the US have more rain as seen in this map from NOAA, including areas near the Four Corners Region. The simplest description is the southwestern and southern US.
Hantavirus
Extra moisture in the southwest would cause far more production of grasses, which would have the seeds to help support a larger deer mouse population. Deer mice are prolific breeders. They breed year around, but peak during fall and spring (ie, when there is more grain and other food sources). Females produce 2-4+ litters each year and can become pregnant about a month after birth. This means that given enough food, exponential growth of rodent populations can happen quickly (ie, 10-20 fold in a single season), increasing the chances of spreading hantavirus into human structures.
West Nile Virus (WNV)
WNV is primarily spread by the Culex mosquitos, which have a range across most of the US. They simply need a stagnant source of water (used tires, clogged gutters, buckets, bird baths, flower pots, etc.) to breed, even better if there is some organic material in it. Think about that in context of more moisture in the southern US, particularly in late winter/early spring, when birds would be migrating north.
Corvids (crows, jays, and magpies) are the birds that often die quickly from WNV due to high viral loads. That’s not very important in relation to deer mice though. Owls and hawks are often infected through predation, either on infected rodents or consuming the carcasses of other infected birds.
The Perfect Climate/Biological Storm
That creates a perfect storm of high deer mice populations due to increased grain production from El Niño moisture and increased Culex mosquito (and WNV prevalence) due to the increased moisture, depleting raptor populations that would otherwise help control deer mice populations.
Cruise Ship Parting Thoughts
Cruise ships puzzle me. I’ve described them as prisons with better tasting food that has a chance of making you really sick or killing you. They simply are not for me. When I was searching for information about the cruise ship incident, I came across a much better quote about them. “I do not understand why people want to visit a floating petri dish disguised as a 1990s mall built inside a terrible hotel. Is it the buffet?“
This is even more puzzling to me during an ongoing pandemic, which leads me to my other thought about this event and a possible contributor. Even though COVID vaccines are no longer mandated by the cruise industry for passengers or crew, I wonder if the crew are more likely to keep up on vaccines and take precautions on their own due to what happened in the cruise industry earlier in the pandemic. Passengers who would choose to go on a cruise right now certainly throw caution to the wind and I suspect would have been repeatedly infected. The crew are aboard for their livelihood.
That might account for why it’s been passengers that have died even though I can’t figure out how they would have more likely been exposed than crew. It might have much more to do with less cautious people with repeated COVID infections having damaged immune systems, thereby being more likely to become diseased with a lower viral load than among a similar immunocompetent cohort.
A Bigger Perfect Storm
Hopefully this provides some context as to why the interaction of climate and zoonotic disease is so important. Toss in climate refugees and crowding and the problems become even larger than this simple discussion around two viruses and climate. The path we are on is pretty bleak.
