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Asymptomatic Carrier

An asymptomatic carrier (healthy carrier or just carrier) is a person or other organism that has become infected with a pathogen, but who display no signs nor symptoms. ^[1]

Although unaffected by the pathogen themselves, carriers can transmit it to others or develop symptoms in later stages of disease. Asymptomatic carriers play a critical role in the transmission of common infectious diseases such as typhoid, C. Difficile, influenzas, and HIV. While the mechanism of disease carrying is still unknown, researchers have made progress towards understanding how certain pathogens can remain dormant in a human for a period of time. ^[4] A better understanding of asymptomatic disease carriers is crucial to the fields of medicine and public health as they work towards mitigating the spread of common infectious diseases.

Types of Asymptomatic Carriers

Asymptomatic carriers can be further categorized by their current disease state.^[12]  When an individual transmits pathogens immediately following infection but prior to developing symptoms, they are known as incubatory carriers. Humans are also capable of spreading disease following a period of illness. Typically thinking themselves cured of the disease, these individuals are known as convalescent carriers. Viral diseases such as hepatitis and poliomyelitis are frequently transmitted in this manner. Considered to the be classic asymptomatic carriers, “healthy carriers,” exhibit no signs or symptoms of disease yet are capable of infecting others. ^[12]

Significance in Disease Transmission

The limited information on the prevalence of asymptomatic carriers creates a considerable difficulty when planning public health initiatives. Given that disease surveillance is dependent on estimates for both the asymptomatic rates and symptomatic rates of disease, the lack of information on the prevalence of carriers can lead to insufficient initiatives for the mitigation of common public health concerns such as C. Difficile or influenza. ^{[13] [14]}

Researchers have expressed desire to better predict transmission methods in order to determine the appropriate public health response. ^[15] For example, a disease with a known low asymptomatic rate may lead to increased surveillance of symptomatic cases, whereas a higher asymptomatic rate could lead to more aggressive methods such as travel-bans, compulsory quarantines since the number of infectious cases would be unknown.^[13]

Possible Explanations

While an exact explanation for aymsptomatic carriage is unknown, reasearchers have been dedicating their efforts towards understanding how specific bacterium thrive in human hosts in the hopes of determining a universal understanding of asymptomatic transmission.

A biological mechanism utilizing Salmonella

An August 2013 research publication from Stanford University Medical School demonstrated how salmonella is able to remain in immune cells and alter their metabolic systems in order to further transmit the disease. ^[4] Utilizing a closely related strand of bacterium (S. typhimurium), scientists created a mouse model that mimicked the persistent salmonella cases seen in carriers of typhoid. Knowing that the bacterium can reside in mice for their entire lives, researchers were able to determine that the bacterium tends to reside in macrophages. Further examination of the gut lymph nodes of the mice revealed that S. typhimurium changes the inflammatory response of the macrophages. Instead of eliciting an inflammatory response from the attack cells, the bacterium is able to convert them into an anti-inflammatory macrophage, allowing for optimal survival conditions. In the words of lead scientist Dr. Monack, “It wasn’t that inflammatory macrophages were invulnerable to infection, but rather that, having infected a macrophage, S. typhimurium was much more able to replicate in the anti-inflammatory type”^[4]

Investigators have also found that the presence of peroxide-proliferation-induced receptors, or PPARs, correlated to the presence of salmonella bacterium. PPARs, thought of as roaming genetic switches, are responsible for the fat metabolism needed to sustain anti-inflammatory macrophages in which S. typhimurium hides. ^[4]

Latest Research

Screening for Asymptomatic Bacteriuria in Adults

Medical researcers have spent a great deal of time exploring asymptomatic individuals who test positive for bacteria in their urine. Known as asymptomatc bacteriuria, this condition typically impacts 3-5% of women, with the most vulnerable populations being the elderly and those diagnosed with diabetes ^[17] . Within the female population, the risk of bacteriuria increases with age. Escherichia coli is the most common organism found during urine anylsis though the variety of potentially infectious organisms is diverse and can include Enterobacteriaceae, Pseudomonas aeruginosa, Enterococcus species, and group B streptococcus.^[18] With mounting costs and concerns about the lack of information regarding asymptomatic carriage, researchers from the Agency for Healthcare Research and Quality conducted a lengthy study in order to determine a set of screening recommendations as well as some inisght into the mechanism of bacteriuria^[17] Results of the meta-anylsis produced no clear explanation for asymptomatic carriage, but did yield new evidence that strengthened the support for screening for asymptomatic bacteriuria in pregnant women only.^[18]

Infectious Diseases Impacted by Asymptomatic Carriers

Asymptomatic carriers have furthered the spread of many infectious diseases. A common principle in epidemiology, the 80-20 rule speculates that 80% of the disease transmission is conducted by only 20% of people in a population. ^[16] Carriers are responsible for significant bacterium transmission in many common diseases such as the flu, but may also contribute to the spread of less common infections.

Typhoid fever[edit]

Typhoid fever is an ailment caused by bacterium Salmonella enterica ser. Typhi. An individual can acquire this infection from consuming risky foods or drinks, or by consuming foods or drinks prepared by an infected individual. (Hence, Typhoid Mary.) Those who recover from this infection can still carry the bacteria in their cells, and therefore be asymptomatic.^[5]

Typhoid Mary in a New York Hospital

The Classic Carrier: Typhoid Mary[edit]

Mary Mallon, known as "Typhoid Mary", was an asymptomatic carrier of Salmonella enterica serovar typhi.^[2] A  cook for several families and soldiers in New York City during the late 1800’s, several cases of typhoid fever were traced to her by the Health Department. At the time, there was no way of eradicating the disease, and it was spread primarily through fecal-oral transmission. Most of Mary Mallon's transmission risk was thought to arise from her continued involvement in occupations involving food preparation and handling. New York City's public health officials initially sought to merely restrict her from such employment rather than permanently quarantining her. When she continued to be non-compliant, the Health Commission ordered that she be quarantined on one of the islands surrounding Manhattan. She remained there until her death.

Despite appearing perfectly healthy, it is estimated that Mary infected about 50 people before she was quarantined. Scientists calculate that between 1% and 6% of individuals infected with Salmonella typhi become chronic, asymptomatic carriers like Mary.^[3]

HIV[edit]

HIV infection has a long period during which the person is asymptomatic.^[6] Although the host may not be experiencing symptoms, the virus can still be passed on to others. It is also possible for the infection to become symptomatic after this latent period. Whether the host is showing symptoms or not, opportunistic infections can take advantage of the weakened immune system and cause further complications.^[7]

Epstein-Barr Virus (EBV)[edit]

Many carriers are infected with persistent viruses such as Epstein-Barr Virus (EBV), which is a member of the herpes virus family. Studies show that about 95% of adults have antibodies against EBV, which means they were infected with the virus at some point in their life.^[8]

Clostridium difficile[edit]

Clostridium difficile has also been shown to be spread by asymptomatic carriers, and poses significant problems in care home settings.^[9] Reports indicating that over 50% of long-term patients presented with fecal contamination despite a lack of symptoms have led many hospitals to extend the contact precautions period until discharge. ^[14]

Chlamydia[edit]

Chlamydia, an STI that affects both men and women, can also be asymptomatic in most individuals. Although the infection is not yielding any symptoms, it can still damage the reproductive system. If the infection goes unnoticed for a long time, the infected individual(s) are at risk of developing pelvic inflammatory disease (PID). Like Chlamydia, PID can also be asymptomatic. ^[10]

Poliomyelitis[edit]

A small number of asymptomatic carriers (referred to as chronic excretors) continue to produce active virus for years (or even decades) after their initial exposure to the oral Sabin vaccine.^[11] Carriers of the attenuated virus unintentionally spread the attenuated virus, inoculating others, giving them contact immunity; however some adults with weak immune systems have contracted paralytic polio from contact with recently immunized children. Carriers of virulent strains spread polio, increasing the difficulty of poliomyelitis eradication.

Tuberculosis

Tuberculosis (TB) is an infectious disease usually caused by the bacterium Mycobacterium tuberculosis (MTB). Tuberculosis generally affects the lungs, but can also affect other parts of the body^[19] . Active or symptoamtic tuberculoisis is spread from person to person through the air through baterium spores that are released into the air following a cough or sneeze. Some individuals may be infected with the mycobacterium tuberculosis but never display symptoms. Called latent turberculosis, these cases while uncontagious^[20] are particularly problemtatic from a public health perspective since approximately 10% of those diagnosed with latent TB will go on to develop an active (and contagious) case.^[20]

References[edit]

1.    Jump up^ "Dictionary Definition". Medical-dictionary.thefreedictionary.com. Retrieved 20 August2013.

2.    Jump up^ "Scientists get a handle on what made Typhoid Mary's infectious microbes tick". Med.stanford.edu. Archived from the original on 18 August 2013. Retrieved 20 August2013.

3.    ^ Jump up to:^{a b} "Scientists get a handle on what made Typhoid Mary's infectious microbes tick | News Center | Stanford Medicine". med.stanford.edu. 2013-08-14. Retrieved 2016-02-14.

4.    Jump up^ "Denise M. Monack". WikiGenes. Retrieved 2016-02-14.

5.    Jump up^ "CDC - Typhoid Fever: General Information - NCZVED". Cdc.gov. Retrieved 2016-02-14.

6.    Jump up^ Siliciano, Robert F. "HIV Latency". Cold Spring Harbor Laboratory Press. Retrieved 20 August 2013.

7.    Jump up^ "Asymptomatic HIV infection: MedlinePlus Medical Encyclopedia". Nlm.nih.gov. 2016-02-02. Retrieved 2016-02-14.

8.    Jump up^ "The Broad Spectrum of Epstein-Barr Virus (EBV) Disease on". Medicinenet.com. Retrieved 2016-02-14.

9.    Jump up^ Riggs, M. M.; Sethi, A. K.; Zabarsky, T. F.; Eckstein, E. C.; Jump, R. L. P.; Donskey, C. J. (2007). "Asymptomatic Carriers Are a Potential Source for Transmission of Epidemic and Nonepidemic Clostridium difficile Strains among Long-Term Care Facility Residents". Clinical Infectious Diseases. 45 (8): 992–998. doi:10.1086/521854. PMID 17879913.

10. Jump up^ "STD Facts - Chlamydia". cdc.gov. Retrieved 2016-02-14.

11. Jump up^ Ousmane M. Diop; Cara C. Burns; Roland W. Sutter; Steven G. Wassilak; Olen M. Kew. "Update on Vaccine-Derived Polioviruses — Worldwide, January 2014–March 2015". Morbidity and Mortality Weekly Report. 64: 640–646.

12. "Carrier." Encyclopedia of Public Health. . Encyclopedia.com. 20 Oct. 2018 <http://www.encyclopedia.com>

13.  Furuya-Kanamori, Luis, et al. “Heterogeneous and Dynamic Prevalence of Asymptomatic Influenza Virus Infections.” Emerging Infectious Disease, vol. 22, no. 6, June 2016, pp. 1052–1056., doi:https://dx.doi.org/10.3201/eid2206.151080

14. Michelle M. Riggs, Ajay K. Sethi, Trina F. Zabarsky, Elizabeth C. Eckstein, Robin L. P. Jump, Curtis J. Donskey; Asymptomatic Carriers Are a Potential Source for Transmission of Epidemic and Nonepidemic Clostridium difficile Strains among Long-Term Care Facility Residents, Clinical Infectious Diseases, Volume 45, Issue 8, 15 October 2007, Pages 992–998, https://doi.org/10.1086/521854

15. Perlman, William. “Asymptomatic Influenza Infection Rates Deserve More Attention.” ContagionLive, Intellisphere, 20 May 2016, www.contagionlive.com/news/asymptomatic-influenza-infection-rates-deserve-more-attention

16. Emoryhealthsci. “Zeroing in on 'Super Spreaders' and Other Hidden Patterns of Epidemics.” EurekAlert!, Advancement of Science (AAAS, 25 Feb. 2016, www.eurekalert.org/pub_releases/2016-02/ehs-zio022516.php.

17. Evidence Summary: Asymptomatic Bacteriuria in Adults: Screening. U.S. Preventive Services Task Force. August 2016.

https://www.uspreventiveservicestaskforce.org/Page/SupportingDoc/asymptomatic-bacteriuria-in-adults-screening/evidence-summary11

18. Colgan, Richard, et al. “Asymptomatic Bacteriuria in Adults.” American Family Physician, American Academy of Family Physicians, 15 Sept. 2006, www.aafp.org/afp/2006/0915/p985.html.

19. “Tuberculosis (TB).” World Health Organization, World Health Organization, 2018, www.who.int/en/news-room/fact-sheets/detail/tuberculosis

20. “Latent Tuberculosis Infection (LTBI).” World Health Organization, World Health Organization, 8 June 2018, www.who.int/tb/areas-of-work/preventive-care/ltbi_faqs/en/.

Article Title: Hyperbaric Medicine

Content

-The majority of the article was on topic and very relevant. I thought the authors did a pretty decent job of including a large amount of fairly easy to understand information. My one critique would be that the Contraindications section occurred at a random point.

-While the article was thorough in including the scope and treatment methods, it did not seem to a thorough explanation for the sections, Possible Complications and Effects of Pressure.The article could also be improved with the addition of several infographics and images.

Evaluating Tone

-The article took a clinical and neutral perspective. It did not appear to have a strong opinon on the topic, but rather stated the medical information and research findings

Evaluating Sources

-There were numerous sources (Over 106!) and I found most to be up to date and accessible. There were a few sources that required an account for scientific journals, but I still found it possible to verify the information, and utilize the sources to add to my own paper.

Checking the Talk Page

-There is little to no activity on the Talk page. The most recent post was from November 2010. The conversatons centered on controversial treatments, proper definition of the term hyperbaric, and the varyng costs.

Possible Articles for Editing

1- Asymptomatic Carrier- This article needs a lot of work! It does not have a thorough overview of the possible causes, it does not list any research being conducted, and is written as a personal essay. It only focuses on the historical instances but even lacks enough information on that. It has only ten sources which I think can be greatly improved upon

2- Hyperbaric Medicine: While the topic has some substantial information, it did not offer enough insight into possible complications, the effects of pressure, and the growing trend of using hyperbaric medicine for alternative uses