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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

In humans

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

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

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

References

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Further reading

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