Zoonosis

Photograph

By: Karen Kasmauski

Date: July 3, 2000

Source: Corbis Corporation.

About the Photographer: Karen Kasmauski has worked on twenty major stories for National Geographic since 1984, and is a photographer-in-residence. She is known for her photographs of complex social issues, which have been finalists for the National Magazine Awards. She also works on newspapers, and has published a book of her photographs, Hampton Roads, and contributed the images to Impact: Dispatches from the Front Lines of Global Health with text by Peter Jaret.

INTRODUCTION

A zoonosis is an infectious disease such as rabies or plague that can be transferred from wild or domestic animals to humans. Humans can contract zoonoses, the plural of zoonosis, from mammals, birds, fish, amphibians, reptiles, and even insects. Depending on the disease, humans contract zoonotic diseases in different ways, including direct contact or close proximity with an animal, or by sharing an animal's water source.

Epidemiologists and other scientists who study diseases describe the transmission of zoonotic diseases in terms of hosts and vectors. A host organism is an animal that is infected with or carries a bacterial, viral, or parasitic disease. The disease can be transferred directly from the host to a person via a mechanism such as the exchange of fluid, or through a vector organism that is an intermediary between a human and the host. An example is the deer tick vector for the well-known illness, Lyme disease. The tick picks up the disease while feeding on an infected mouse or other small mammal. The tick carries the bacterial disease in its gut, and can pass it to a new host during a later feeding. Normally, Lyme disease is transmitted between small mammals, but a human can contract Lyme disease if bitten by the infected tick vector. Although deer do not become infected with Lyme disease, they are responsible for carrying and transporting ticks. There are some diseases which humans contract from animal vectors that are not classified as zoonoses; such as the transmission of malaria by mosquitoes. To complete their life cycle these non-zoonotic diseases require a human host. A true zoonotic disease is capable of being transferred to humans from animals, but does not need a human body in order to survive.

The Hantavirus Pulmonary Syndrome (HPS) is an often fatal zoonotic disease contracted by humans who breathe in the aerosolized virus transmitted by the urine, droppings, or saliva of infected rodents. This deadly pulmonary illness was unknown when it appeared in May of 1993, in the so-called "Four Corners" region of the United States, where the states of New Mexico, Utah, Arizona, and Colorado meet. Initially, it was unclear what was able to quickly kill healthy adults in this region. Virologists from the Center of Disease Control (CDC) used techniques that allow an analysis of a virus at the molecular level, to link the pulmonary illness to a previously unknown type of hantavirus, which was later named Sin Nombre (Spanish for "Without Name").

PRIMARY SOURCE

ZOONOSIS

See primary source image.

SIGNIFICANCE

Scientists are learning about HPS through the study of rodent populations, which often requires the trapping and collection of various mice species. Research in the Southwestern United States has linked the years having higher levels of precipitation with a larger population of rodents, as the moisture leads to a greater supply of food for rodents, as well as higher vegetation growth, which provides ample habitat and protection for the rodents. Due to the weather phenomenon El Niño in 1991 and 1993, there was much more rain in the Southwest. The population density of deer mice in New Mexico grew from one deer mouse per hectare to twenty to thirty per hectare during that time period. It is thought that this large population of mice led to the first outbreak of HPS in May 1993. Although rare, HPS has since been found throughout the United States. Rodent control around the household is the primary defense against the hantavirus.

There are some rare zoonotic diseases for which the host is still unknown. Ebola, for example, has several very deadly forms that periodically infect communities in Central Africa. Once a human is infected with Ebola, the disease can spread rapidly from human to human through the direct contact of bodily fluids. Research has shown that perhaps primates, elephants, or bats, could be responsible for carrying Ebola and transmitting it to humans. One hypothesis is that the disease spreads to a human who handles or ingests meat from an infected primate. Bats are considered possible vectors, since they do not die from Ebola, and could therefore be a host that maintains the disease in tropical forests. A much less dangerous form of Ebola is found in the Asia Pacific region.

One zoonotic disease, tularemia, which is similar to plague and found in rodents, is highly infectious. In its aerosolized form, it has been considered a possible biological weapon as far back as World War II, since it takes a very small amount of the bacteria to infect an individual. It is naturally spread by a type of biting flies, contaminated water, and through handling infected animal carcasses. These bacteria can survive in low-temperature water and soil. The United States is said to have maintained stocks of tularemia bacteria until the 1970s.

FURTHER RESOURCES

Books

Preston, Richard. The Hot Zone: A Terrifying True Story. New York: Anchor, 1995.

Periodicals

Bunk, S. "Weathering Hantavirus: Ecological Monitoring Provides Predictive Model." The Scientist. (1999): 13(14)14.

"Tularemia." Journal of Environmental Health (April 2003): 65(8)31.

Web sites

Centers for Disease Control and Prevention. "Ebola hemorrhagic fever." ?http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola.htm? (accessed January 16, 2006).

Centers for Disease Control and Prevention. "Hantavirus Pulmonary Syndrome (HPS)" ?http://www.cdc.gov/ncidod/diseases/hanta/hps/? (accessed January 19, 2006).

Centers for Disease Control and Prevention. "Lyme disease." ?http://www.cdc.gov/ncidod/dvbid/lyme/index.htm? (accessed January 16, 2006).

Disease Control and Environmental Epidemiology Division, Colorado Department of Public Health and Environment. "Zoonotic Diseases." ?http://www.cdphe.state.co.us/dc/zoonosis/zoonosis.asp? (accessed January 16, 2006).

World Health Organization. "Ebola." ?http://www.who.int/mediacentre/factsheets/fs103/en/index.html? (accessed January 16, 2006).