Emerging diseases

Introduction

Museums hold specimens and other types of samples that are useful for understanding not only the biodiversity, evolution, and ecology of organisms, but also for understanding problems that confront humans in our ever-changing world. One of these is helping to identify previously unknown diseases, also known as emerging diseases.

In 1993, a mysterious respiratory disease began to affect a small number of people in the Four Corners region of the southwestern USA. This disease came on suddenly and had a very high mortality rate; 70% of people who came down with the symptoms rapidly died. Researchers raced to determine the cause of the disease and figure out whether it was new to the United States. Collections held in the Museum of Southwestern Biology in Albuquerque, New Mexico, played a critical role in understanding the origin and spread of the mystery disease, now called hantavirus pulmonary syndrome (HPS).

1993 outbreak

The first cases of a mysterious new respiratory illness in the Four Corners region of the Southwest were identified by doctors and scientists in May 1993. The victims of the illness were a young, healthy New Mexico couple who died within several days of one another. More people began to come down with the illness and more cases that were previously overlooked were identified. Scientists and doctors gathered data from the patients and eliminated some possibly causes of the illness, like a chemical or the pneumonic plague. (The pneumonic plague is a respiratory form of the plague, a disease caused by a bacterium that is carried by rodents in the western US.)

One theory that researchers considered was that the illness was caused by a type of virus called a hantavirus. Hantaviruses are found in rodents in every part of the world. Only some hantaviruses can infect people, who usually contract the viruses from rodent urine, droppings, or saliva. In Europe and Asia, hantaviruses cause a group of human illnesses collectively known as hemorrhagic fever with renal syndrome (HFRS). HFRS affects the kidneys and other organs and causes internal bleeding, not the respiratory symptoms seen in people who had the mystery illness in the US. 

Scientists at the US Centers for Disease Control and Prevention (CDC) tested tissue samples from people who had contracted the mystery illness. Based on the results, they concluded that a previously unknown hantavirus was the cause of the disease. They eventually called the virus the "Sin Nombre virus" (nameless virus), or SNV. They also trapped and tested many rodents in the Southwest and identified the main rodent host of the virus as the deer mouse (Peromyscus maniculatus). Since 1993, more hantaviruses that can cause illness in humans have been found in North and South America.

Origin of the virus and the role of museum collections

Researchers wanted to know whether the Sin Nombre hantavirus that caused the 1993 illnesses and deaths was new to the Southwest or whether it had been present there for a long time. One way to figure out whether it was new or not was to look for evidence of the virus in deer mice that lived and died in the Southwest before the 1993 outbreak. But where could investigators find mice that had lived and died in the past?

The answer was to look in museum collections. Frozen deer mice held in the Museum of Southwestern Biology in Albuquerque, New Mexico, and at Texas Tech in Lubbock, Texas, were tested for antibodies to the virus. Scientists found antibodies in more than 10% of the deer mice that they tested and discovered that the virus was present in deer mice in the Southwest at least as early as 1979.

Other evidence backed up the finding that the virus was present in the Southwest before 1993. Scientists identified some people who died of hantavirus before the 1993 outbreak. They determined that these people had had hantavirus by reviewing their medical records and by testing tissue samples from the victims that had been preserved. One man even survived a case of hantavirus in 1959! The man was hospitalized with symptoms of the virus after exposure to rodents in Colorado in the 1950s. His blood was tested for antibodies to the Sin Nombre virus in the 1990s, confirming that he had been infected.

The role of climate

After the identity of the virus causing the mystery illness was sorted out and deer mice had been identified as the carriers, scientists began to examine the role of climate to determine whether Sin Nombre virus outbreaks are linked to fluctuating weather patterns. They found that wet years stimulated more plant growth which, after a short period of time, caused an increase in the density of the deer mouse population. For example, the year 1992 was very wet, which caused more plant growth in 1992, and a very high deer mouse population in 1993, the year of the outbreak of the Sin Nombre virus.

Scientists analyzed data from multiple years and concluded that rainy fall to spring seasons led to increases in rodent densities and higher numbers of hantavirus cases in the Southwest. Wet fall to spring seasons often occur in the Southwest during years when El Niño is active. El Niño is a climate pattern that brings increased precipitation to the Southwest. The delay between an El Niño year and a rise in hantavirus cases may be one to several years. Other factors, such as how many rodents are infected with hantavirus and whether people take steps to reduce the risk of catching hantavirus also play a role in Sin Nombre virus outbreaks.

The role of indigenous knowledge

The first victims of hantavirus pulmonary syndrome identified during the 1993 outbreak were members of the Navajo Nation. Navajo oral history recorded an illness associated with wet periods when many pinyon nuts (the edible seeds of the pinyon pine tree, Pinus edulis) were produced and mouse populations were high. Outbreaks of the illness had occurred in 1918 and 1933. Beyond this, Navajo traditional knowledge associates mice with disease and advises people to take practical steps to keep mice out of homes and food supplies. These details suggest that there may have been outbreaks of the Sin Nombre virus before 1993. These years were also associated with wet weather and abundant food for rodents.

Unfortunately, early media coverage of the hantavirus outbreak of 1993 was sensational and linked the virus to the Navajo people in ways that promoted discrimination. Native American people in the Four Corners region of the Southwest have been disproportionately affected by outbreaks of the Sin Nombre virus because their communities are in rural areas where infected deer mice are sometimes plentiful. Deer mice are also found in suburbs, although they are not as common in urban areas because they are displaced by other rodents.