A disease's potential to cause an outbreak determines whether it should be under surveillance. An ‘alert threshold' (or ‘epidemic threshold') indicates the level of incidence above which a disease requires an urgent response. Each disease has a specific threshold that depends on its infectiousness, other determinants of transmission, and the degree to which it is locally endemic.
Disease control measures must be specifically developed to halt transmission of the disease agent that causes the outbreak. Often, knowledge of the agent is already available to guide the design of appropriate control measures. In general, response activities include: controlling the source or preventing exposure (for example, by improving water outlets to prevent cholera); interrupting transmission or preventing infection (by mass vaccination to prevent measles, or use of LLINs to prevent malaria); or modifying host defences (by prompt diagnosis and treatment, or chemoprophylaxis).
- Diseases for which a single case may indicate an outbreak: cholera, measles, acute flaccid paralysis/polio, yellow fever, viral haemorrhagic fevers.
- Malaria: 1.5 times the baseline over the previous 3 weeks.
- Watery diarrhoea: 1.5 times the baseline over the previous 3 weeks.
- Bloody diarrhoea: 5 cases.
- Meningitis: 5 cases or 1.5 times the baseline over the previous 3 weeks.
The above standards apply to emergencies and long-term situations.
UNHCR Public Health Section, Division of Programme Support and Management. At: [email protected]