Carriers and subclinical transmission

Diseases in which there is an animal reservoir, intermediate host or vector are complex and difficult to control, but even in the simplified transmission cycle of direct spread from human-to-human, complications occur with the carrier state. A carrier is a person who can transmit the infective agent, but is not manifesting the disease. There are several types of carriers:

• asymptomatic carriers who remain well throughout the infection;

• incubating or prodromal carriers who are infectious, but unaware that they are in the early stages of the disease;

• convalescent carriers who continue to be infectious after the clinical disease has passed.

The carrier state can either be transient or chronic.

The important features of carriers are as follows:

1. The number of carriers may be far greater than the number of those who are sick.

2. Carriers are not manifest so they and others are unaware that they can transmit the disease.

3. As carriers are not sick, they are not restricted and, therefore, disseminate the disease widely.

4. Chronic carriers may produce repeated outbreaks over a considerable period of time.

Identification of carriers is a singularly difficult and generally unsuccessful exercise. If the carrier is asymptomatic, the organism is often in such reduced numbers or excreted at such infrequent intervals that routine culture techniques will not detect them. The investigation has to be repeated many times and is probably only successful at specific instances, for example, during a minor diarrhoeal episode in a suspected typhoid carrier. A further difficulty is that clinically unaffected people object to having investigations performed on them, making the coverage incomplete. Examples of diseases in which the carrier state is important are typhoid, amoebiasis, poliomyelitis, menin-gococcal meningitis, diphtheria and hepatitis B. Cholera can produce more carriers than those that are sick. More on carriers will be found in the sections dealing with each of these diseases.

In some diseases, the carrier state appears to be prolonged or is perpetuated when there are, in fact, no carriers. This may be due to cyclical sub-clinical transmission when infection is transmitted within a family or throughout a community, without the subjects being aware of any particular symptoms. One member of a family passes on the disease to another and becomes free of it him/herself. It is then passed on to other family members and eventually back again so that it is maintained in a sub-clinical cycle. When someone who is susceptible to the disease accidentally enters this cycle or the organism is more widely disseminated, then a clinical outbreak occurs. This is a mechanism by which poliomyelitis is maintained in the community.

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