Hantavirus Field Investigation:

In this article, we will look at is the outbreak of Hantavirus  in the southwestern United States. 

On May 14, 1993, a young man who was physically fit and seemingly healthy suddenly collapsed. He was rushed to a hospital in New Mexico, but died of acute respiratory failure within a couple of hours.  His fiancée had died just days earlier from similar symptoms.  This was just the beginning of an outbreak of a sudden, unexplained, and highly fatal respiratory illness of unknown etiology. May 17 saw three more deaths in the Four Corners area; the area in the southwest where Colorado, New Mexico, Arizona, and Utah all meet.  The unique detail is that all the victims who died were young and healthy prior to exhibiting their symptoms. This was a cluster of cases of severe respiratory illnesses affecting previously healthy young adults. Autopsy results show noncardiogenic (disease of the heart), pulmonary (lung) edema and hemodynamic compromise without an identifiable cause. 

The Causative Agent:

The New Mexico Department of Health contacted CDC for help on May 18, 1993, but initial laboratory tests failed to reveal a known disease common to all the victims. The CDC Special Pathogens Branch then initiated a joint investigation with the State Health Departments of the four involved states as well as with Indian Health Services, the Navajo Indian Nation, and UNMH (University of New Mexico Hospital) in Albuquerque, NM. Within three weeks of this collaborative response launched by state and local health agencies, laboratory studies from the federal CDC suggested that an acute infection with a previously unrecognized Hantavirus species was responsible for the unexplained deaths. Two major forms of hantavirus disease are known; hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS or HPS).

Hantavirus is considered to be a zoonosis, a disease that a human contracts from some sort of animal.  This strain of Hantavirus was different than other strains found elsewhere around the world. This was a very deadly version of the illness; Hantavirus pulmonary syndrome (HPS) that affects the heart and lungs.  The mortality rate of HPS is about 50 percent, which in the modern world is considered to be extremely deadly. Between 1993 and 1995 HPS killed more than 45 people in the Four Corners region of the United States.

The Vector:

The deer mouse transmits Hantavirus to humans through bites that break the skin, though this is a very rare occurrence.  Most of the infections occurred through aerosolization, where the victim breathes in saliva, droppings, or urine deposited by the animal when they inadvertently stir up areas where the mice live.  This was very common when individuals hunted pinion nuts, for they found great caches of them where the deer mice had collected the nuts. 

Additional testing of the animals revealed that the animals were not sick or dying from the virus themselves. This was considered to be a smart virus that coexists peacefully with the host, prolonging the live of the virus.  It was established that the virus has co-evolved with its host for hundreds of thousands of years or more. The question to be answered is why the human contact with the carrier rodents has increased. Answering such questions is at the very core of epidemiology; trying to understand the factors that change the potential risk people may have for contracting a particular disease.

Satellite data mapping of animal populations was used, starting from a period BEFORE people were contracting Hantavirus. One hypothesis was that the 1991 – 1992 El Nino year contributed to the outbreak because the extra rain dramatically increased the rodent population by increasing the vegetation the mice use for food. Very extensive investigation showed that there is a relationship between precipitation and Hantavirus.

Overreaction?

You may think that the Health departments and the CDC overreacted to the threat of Hantavirus in that fewer than 50 people actually died from the outbreak, however, it is important to note that there are really not that many infectious diseases in the U.S., so even if 25 to 30 people die of something, that is really quite frightening.  It is especially frightening for the people who live in the locality of the outbreak, especially before they know what the disease is and how it is contracted.  The Hantavirus outbreak in the Southwest has been compared to the anthrax outbreak because no one knew where it was coming from and how many people would ultimately be affected by it. In fact, by the end of July 1993, the mortality rate for Hantavirus was 78% for the Southwestern outbreak. For weeks, CDC did not know what the pathogen was or how it was spread.  This unknown made it impossible to put into place any sort of controls or preventive measures. The problem becomes one of knowing just how many people have been exposed and if they are at risk of coming down with the disease.

Control/Preventive Measures:

Once it has been established that the pathogen is spread through the aerosolization of the droppings and urine of deer mice, then the appropriate control and preventive measures were established.

Most of those affected were individuals who had been cleaning their homes or vacation properties.  As the weather in the region becomes colder, the mice look for warmer lodgings and enter people's homes.  Hikers who sleep on the ground or in old "ghost-town" buildings are at higher risk as are those who routinely harvest pinion nuts.  The use of rubber gloves, bleach, and respiratory masks when cleaning are methods that are used to prevent infection. Clothing can be washed in a regular washing machine with normal detergent as this process breaks down the outer lipid membrane of the virus. Keep rodents out of your home and your workplace. Plug holes or gaps, remove sources of food, keep firewood and other nesting sites as far away from the home as possible.

The symptoms of Hantavirus are very similar to the flu and can develop anywhere from one to five weeks after exposure to an infected area; fever over 101.0 F, myalgias, headache, nausea, vomiting, and sometimes diarrhea. Within a few days, the illness progresses rapidly to serious respiratory distress and difficulty breathing. There is no treatment except for supportive care. Evidence points to lower mortality rates with professional medical care. The virus does not spread human to human, so members of the same household are not necessarily at risk unless they have been directly exposed to the virus.