When worlds collide: human, critter and virus

Two of the more intriguing health stories this past summer didn’t involve rogue sharks, swine flu or any of the other usual suspects. The stars of this particular show were the hantavirus and the West Nile virus, with supporting roles played by rodents and mosquitoes.

It’s a reminder that we don’t just share the environment with other people; we also share it with a wide assortment of animals and germs, and the inevitable collisions among species can have consequences for human health.

News late this summer of a deadly outbreak of hantavirus in Yosemite National Park may have caught many people by surprise. Nine visitors to the park have gotten sick since July, and three have died.

The virus is carried by deer mice and transmitted to humans via inhalation of airborne dust particles contaminated with mouse droppings. Symptoms resemble influenza and can progress to hantavirus pulmonary syndrome, which affects the lungs and is sometimes fatal.

The Yosemite outbreak isn’t new. I first heard about hantavirus back in 1993, when a severe and puzzling disease broke out in the Four Corners area of New Mexico, Arizona, Colorado and Utah. A healthy young Navajo man suddenly developed a flu-like illness, was rushed to a hospital and died.

What happened next reads like a fascinating detective novel. I’ll let the U.S. Centers for Disease Control and Prevention tell the story:

While reviewing the results of the case, medical personnel discovered that the young man’s fiancee had died a few days before after showing similar symptoms, a piece of information that proved key to discovering the disease. As Dr. James Cheek of the Indian Health Service (IHS) noted, “I think if it hadn’t been for that initial pair of people that became sick within a week of each other, we never would have discovered the illness at all.”

An investigation combing the entire Four Corners region was launched by the New Mexico Office of Medical Investigations (OMI) to find any other people who had a similar case history. Within a few hours, Dr. Bruce Tempest of IHS, working with OMI, had located five young, healthy people who had all died after acute respiratory failure.

An intensive investigation revealed that the mystery disease was a previously unrecognized type of hantavirus. More investigation and research identified the main carrier as the deer mouse, and concluded that exposure to the virus was occurring in rural and semi-rural areas where mice and people lived in close proximity.

But why would the virus suddenly have emerged in the spring of 1993? In fact this form of the hantavirus wasn’t new; Navajo medical tradition had long recognized the disease and accurately linked it to deer mice. Experts think the cluster of cases in 1993 was the result of rapid growth among the Four Corners deer mouse population, brought on by heavy snow and rainfall early that year that increased the food supply and allowed deer mice to reproduce in greater numbers than usual. More mice created more opportunities for them to have contact with humans, hence greater opportunities to pass along the hantavirus.

We know now that there are several types of hantavirus that cause pulmonary syndrome and that the virus is carried in the U.S. by several different rodents, including the white-footed mouse, the cotton rat and the rice rat. Nearly 600 cases have been confirmed in the U.S. since 1993. Sporadic individual cases and small clusters break out from time to time throughout North and South America.

Nevertheless, it’s concerning to see a new group of cases, the largest in the U.S. since the Four Corners cluster almost 20 years ago. And in a reminder of how complex the causal factors can be, experts are still puzzling over the reasons for the outbreak. A booming deer mouse population in the Yosemite Valley might be part of the explanation, but it’s thought that the park’s tent cabins could play a role as well. The cabins, built in 2009, apparently are insulated well enough for deer mice to crawl indoors and nest, and the indoor environment could then allow the hantavirus to survive and flourish. All but one of the Yosemite Valley visitors who came down with the hantavirus had stayed in one of the cabins this summer.

The chances of being exposed to hantavirus can be significantly reduced by making the habitat less favorable for mice – sealing openings in homes and workplaces and keeping ┬ácampsites and outdoor areas clean.

The West Nile virus, which is carried by birds and transmitted by infected mosquitoes, offers a similar illustration of how closely entwined the animal and human worlds and surrounding ecosystem can be.

The story of how West Nile virus arrived in North America is another one worthy of a TV crime detection show. First identified in Africa in 1937, it was unknown in the western hemisphere until 1999, when a cluster of severe encephalitis cases popped up in New York City late that summer. Epidemiologists discovered the victims all had three things in common: All were previously healthy, lived within the same 16 square miles and had recently been active outdoors.

Testing and investigation led to the culprit: West Nile virus. Exactly how the pathogen arrived on American shores remains a mystery. What’s clear is that it has established a permanent presence. By the summer of 2002, the number of confirmed cases in North America had reached an unprecedented level and the disease was entrenched coast to coast.

Although we’ve learned much about West Nile virus over the past decade, it seems there’s still a lot that’s unknown about the complex interplay among bird, mosquito and human populations, immunity and weather patterns, and how these all can fluctuate from one year to the next.

Cases surged this year to the highest level since 2003. Some states, such as Texas, have been hit especially hard. A theory favored by many experts is that the weather may be playing a significant role. An unusually mild winter, followed by an early spring, might have allowed mosquitoes to repopulate more quickly. A hot summer may then have increased the virus’s ability to replicate, since it’s known to grow faster in hot conditions. Meanwhile, drought conditions may have prompted the bird population, which is the reservoir for the West Nile virus, to seek water in areas that are more urban, with mosquitoes following closely behind. The drought also has expanded the amount of stagnant water in sewers, catch basins, ponds and so on where Culex mosquitoes, the main transmitters of West Nile virus in North America, like to breed.

Health experts interviewed last month by CBS News offered yet another suggestion for why Texas has been hit so hard: Perhaps the recession resulted in many homes in urban neighborhoods being abandoned with half-empty swimming pools, decorative ponds or other sources of water, creating an environment for mosquitoes to multiply.

No one knows for sure, however, and the complete picture, as with the hantavirus, is likely complex and multi-faceted.

We don’t always notice the critters among us or pay attention to the social, environmental and climate conditions that favor or discourage their presence. All of these factors have a very real impact on human health, however, in ways that are often complicated, sometimes surprising and not to be underestimated.

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