Index case

The index case is the first documented patient in the onset of an epidemiological investigation,or more generally, the first case of a condition or syndrome (not necessarily contagious) to be described in the medical literature, whether or not the patient is thought to be the first person affected. An index case will sometimes achieve the status of a 'classic' case study in the literature, as did Phineas Gage, the first known person to exhibit a definitive personality change as a result of a brain injury. The index case is the first documented patient in the onset of an epidemiological investigation,or more generally, the first case of a condition or syndrome (not necessarily contagious) to be described in the medical literature, whether or not the patient is thought to be the first person affected. An index case will sometimes achieve the status of a 'classic' case study in the literature, as did Phineas Gage, the first known person to exhibit a definitive personality change as a result of a brain injury. The index case may or may not indicate the source of the disease, the possible spread, or which reservoir holds the disease in between outbreaks, but may bring awareness of an emerging outbreak. Earlier cases may or may not be found and are labeled primary or coprimary, secondary, tertiary, etc. The term primary case can only apply to infectious diseases that spread from human to human, and refers to the person who first brings a disease into a group of people. In epidemiology, the term is often used by both scientists and journalists alike to refer to the individual known or believed to have been the first infected or source of the resulting outbreak in a population as the index case, but such would technically refer to the primary case. 'Patient Zero' was used to refer to the supposed source of HIV outbreak in the United States, but the term has been expanded into general usage to refer to an individual identified as the first carrier of a communicable disease in a population (the primary case), or the first incident in the onset of a catastrophic trend. In some cases, a known or suspected patient zero may be informally referred to as an index case for the purpose of a scientific study, such as the two-year-old boy in a remote village in Guinea who was thought to be the source of the largest Ebola virus outbreak in history. In genetics, the index case is the case of the original patient (ie. propositus or proband) that stimulates investigation of other members of the family to discover a possible genetic factor. The term can also be used in non-medical fields to describe the first individual affected by something negative that since propagated to others, such as the first user on a network infected by malware. In the early years of the AIDS epidemic, a 'patient zero' transmission scenario was compiled by Dr. William Darrow and colleagues at the U.S. Centers for Disease Control and Prevention (CDC). This epidemiological study showed how 'patient zero' had infected multiple partners with HIV, and they, in turn, transmitted it to others and rapidly spread the virus to locations all over the world (Auerbach et al., 1984). The CDC identified Gaëtan Dugas as a carrier of the virus from Europe to the United States and spreading it to other men he encountered at gay bathhouses. Journalist Randy Shilts subsequently wrote about Patient Zero, based on Darrow's findings, in his 1987 book And the Band Played On, which identified Patient Zero as Gaëtan Dugas. Dugas was a flight attendant who was sexually promiscuous in several North American cities, according to Shilts' book. He was vilified for several years as a 'mass spreader' of HIV, and seen as the original source of the HIV epidemic among homosexual men. Four years later, Darrow repudiated the study's methodology and how Shilts had represented its conclusions. A 2007 study published in the Proceedings of the National Academy of Sciences of the United States of America by Michael Worobey and Dr. Arthur Pitchenik claimed that, based on the results of genetic analysis, current North American strains of HIV probably moved from Africa to Haiti and then entered the United States around 1969, probably through a single immigrant. However, Robert Rayford died in St. Louis, Missouri, of complications from AIDS in 1969, and most likely became infected before 1966, so there were prior carriers of HIV strains in North America.