Subclinical infection

Typhoid Mary, pictured above in a 1909 tabloid, was a famous case of a subclinical infection of Salmonella enterica serovar.

A subclinical infection—sometimes called a preinfection or inapparent infection—is an infection by a pathogen that causes few or no signs or symptoms of infection in the host.[1] Subclinical infections can occur in both humans and animals.[2] Depending on the pathogen, which can be a virus or intestinal parasite, the host may be infectious and able to transmit the pathogen without ever developing symptoms;[3][4] such a host is called an asymptomatic carrier.[3] Many pathogens, including HIV, typhoid fever, and coronaviruses such as COVID-19 spread in their host populations through subclinical infection.[3][4][5]

Not all hosts of asymptomatic subclinical infections will become asymptomatic carriers. For example, hosts of Mycobacterium tuberculosis bacteria will only develop active tuberculosis in approximately one-tenth of cases;[6] the majority of those infected by Mtb bacteria have latent tuberculosis, a non-infectious type of tuberculosis that does not produce symptoms in individuals with sufficient immune responses.[7]

Because subclinical infections often occur without eventual overt sign, in some cases their presence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction (PCR) tests.[8][9]

Transmission

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In humans

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Many pathogens are transmitted through their host populations by hosts with few or no symptoms, including sexually transmitted infections such as syphilis and genital warts.[10] In other cases, a host may develop more symptoms as the infection progresses beyond its incubation period.[3][7] These hosts create a natural reservoir of individuals that can transmit a pathogen to other individuals. Because cases often do not come to clinical attention, health statistics frequently are unable to measure the true prevalence of an infection in a population. This prevents accurate modeling of its transmissibility.[11][12]

In animals

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Some animal pathogens are also transmitted through subclinical infections. The A(H5) and A(H7) strains of avian influenza are divided into two categories: low pathogenicity avian influenza (LPAI) viruses, and highly pathogenic avian influenza (HPAI) viruses.[13] While HPAI viruses have a very high mortality rate for chickens,[14] LPAI viruses are very mild and produce few, if any symptoms; outbreaks in a flock may go undetected without ongoing testing.[14]

Wild ducks and other waterfowl are asymptomatic carriers of avian influenza, notably HPAI, and can be infected without showing signs of illness.[13][15] The prevalence of subclinical HPAI infection in waterfowl has contributed to the international outbreak of highly lethal H5N8 virus that began in early 2020.[13][16]

Pathogens known to cause subclinical infection

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The following pathogens (together with their symptomatic illnesses) are known to be carried asymptomatically, often in a large percentage of the potential host population:

See also

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References

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Further reading

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