User:Dballouz/Super-spreader

Transmission
For an organism to be a superspreader, it should transmit the disease at a higher rate than the transmission rate for the majority of the population. For directly transmitted infectious diseases, R0 allows scientists to see the potential of an infection to spread throughout a susceptible population (transmission potential). R0 is “the average number of infections produced by an infected individual in a susceptible population”. When R0 is less than one, each infected individual infects on average, fewer than one additional person, making an epidemic impossible. However, if R0 is greater than one, an epidemic is possible. If R0 is large, that helps ensure that early random fluctuations do not eliminate infections and that an epidemic does occur.

The number of other individuals that the superspreader may infect is determined by a mixture of host, pathogen, and environmental factors. Host factors include activities that the hosts partake in that spread the pathogen—like bodily contact, food handling, and hygiene—and factors that determine if they will become (and remain) infected by the pathogen—like immunocompetence and tendency to seek treatment. Pathogen factors include how the pathogen affects the host: for example, pathogens that cause severe coughing are usually transmitted through the air. Lastly, environmental factors have a strong influence on pathogen transmission and infection. How crowded specific environments are and how medically knowledgeable the area is determines how efficiently the pathogen is able to move throughout the population. All of these factors combine to the transmission of the disease. When the factors all combine in a certain way so that only a few organisms infect much of the population, superspreading is in play.

20/80 rule
This is the general rule that refers to how a small portion of the population can cause a majority of infections. The 20/80 rule states that 20% of infected individuals cause 80% of transmission. This 20% is considered a high-risk core group that infects the majority of the population and ultimately causes an epidemic. This “core” group is the superspreaders. They are different for each infection, depending on the host, pathogen and environmental factors mentioned previously. For example, in the case of gonorrhea, the superspreaders were a fraction of the population who were very sexually active, allowing them to spread gonorrhea to many people very quickly.

SARS epidemic
By far the most publicized and popular case of superspreaders is in the case of the SARS outbreak. SARS, or severe acute respiratory syndrome, is characterized as a flu-like atypical pneumonia. The superspreading event in 2003 that started in Beijing was eventually contained, but not until after 8,273 cases, 775 deaths and SARS spreading to 37 countries within weeks.

The first cases of SARS occurred in mid-November 2002 in the Guangong Province of China. It quickly made its way to local hospitals where many health care workers, other patients, and guests were infected. Eventually SARS reached Hotel Metropole in Hong Kong, where the first superspreading event was recognized outside of mainland China (pictured above). The disease was transmitted to others in the hallway of the hotel, and the guests that were then infected traveled all over the world, creating a global epidemic. Guests transmitted SARS to Canada, Germany, England, and the United States. From a specific hospital in Beijing, a diabetes patient was infected. 33 of the 74 people she had close contact with had SARS develop, for a secondary infection rate of 45%. Second generation patients then transmitted SARS to 32% of their total close contacts. In this infection chain, four patients transmitted to at least eight other people, and were labeled as superspreaders. Many of the transmissions happened in hospitals between patients or workers.

Typhoid Mary
Mary Mallon, known as Typhoid Mary, was a superspreader of typhoid fever in the early 1900’s. She worked as a cook for many families and because of her poor hygiene and handling of raw foods she transmitted typhoid to 49 people, three of whom died. Her case was unusual because she was an asymptomatic carrier of the disease, as mentioned in Typhoid Mary. Mallon was the first documented case of an intermittent carrier of typhoid in the United States. She was healthy and displayed no symptoms, but she sometimes excreted the disease. Her urine and stool samples came back positive and negative for the disease, causing her not to believe that she had typhoid. When the New York City Health Inspector told her she was a carrier and to stop cooking, she refused and continued to transmit the disease, even after she was kept in isolation. It was her disbelief of her rare condition and poor hygiene habits that caused her to be a superspreader and infect the large number of people that she did.

Preventative Measures
As the core 20% of the population is responsible for most of the transmissions, stopping the disease must focus on finding and treating the 20% very quickly. Control programs that target the core 20% group are potentially very effective, especially compared to those that try to treat the whole population without focusing on this core. However, finding this group is difficult because of the different sources of heterogeneity including genetic, behavioral and spatial factors. If the cost to treat the 20% is less than treating the whole population, control interventions should be taken for this specific 20%. These include vaccinations, drug treatments, or exposure protection.

In South Africa, tuberculosis is also usually spread by superspreaders. It is increasing, and there are not enough beds in hospitals to keep all the patients, so they must send patients back to their homes where they still might spread the disease. However, they are now identifying superspreaders using a cough aerosol sampler to measure the number of TB bacteria in the cough droplets. If the patient has a high, live number of TB bacteria in their cough, they are considered a superspreader, and will be kept in hospitals or containment until they are no longer infectious.