III.

Sin Nombre Virus

In 1993 a new hantavirus disease was recognized in the southwestern United States. The illness was at first referred to as Four Corners Disease, named for the area where the disease was first observed, where Arizona, New Mexico, Colorado, and Utah meet. The agent responsible was called the Sin Nombre (an area in New Mexico which in Spanish means “no name”) virus. The victims of the virus developed influenza-like symptoms—including fever, muscle aches, cough, and headache—which rapidly worsened. Fluid and white blood cells accumulated in the lungs, causing hypoxia (low blood-oxygen levels), shock, and, in many cases, death from a type of lung failure called acute respiratory distress syndrome, also known as hantavirus pulmonary syndrome. Within a short time, cases were found in other states. By the end of 1995, 123 cases, with a fatality rate of 51 percent, had been confirmed from 23 states. The disease was also identified in Canada, Brazil, Venezuela, and Argentina.

The search for the cause of this mysterious disease began with typical epidemiological studies that involved interviewing survivors and people who came in contact with the victims. Blood samples from victims shared evidence of antibodies against hantaviruses, an indication that they had been exposed to hantavirus in the past, and that this earlier exposure had initiated an immune response. To prove which hantavirus was indeed the cause of the victims' death, scientists used the polymerase chain reaction (PCR). This technique is used to rapidly amplify DNA strands. Once amplified, specific methods are used to identify the specific virus. Scientists determined with certainty that hantavirus was present in the tissues of the victims and determined that the disease was caused by a previously unknown hantavirus. Hantavirus specimens from different areas were compared, revealing that several previously unknown viruses were active in the United States. The entire genetic structure of the Sin Nombre virus was determined, and diagnostic tests were created. The same methods are being used in attempts to develop a vaccine. (For a description of vaccines, See Immunization.)

The primary host of Sin Nombre virus in the southwestern United States is the deer mouse, Peromyscus maniculatus. In large sections of this part of the country, 10 to 35 percent of deer mice are infected, and in certain areas about 80 percent of deer mice carry the virus.

Sin Nombre virus, like other hantaviruses, does not cause disease in its rodent hosts. The virus is shed in the saliva, urine, and feces of these animals for many weeks and perhaps for the lifetime of the animal. Human infection occurs when dust containing infected dried rodent excretions is inhaled. Sin Nombre and the other newly discovered hantaviruses probably have long been present in the region of the western United States inhabited by deer mice. The virus was recognized in 1993 only because of the number and clustering of human cases, after two particularly wet winters and an abundant supply of rodent food caused an increase in rodent populations, which then led to a rise in the incidence of the disease.