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Definition of terms

Term World Health Organisation (WHO) definition Our definition
Control Reduction of disease incidence, prevalence, morbidity and/or mortality to a locally acceptable level as a result of deliberate efforts; continued interventions are required to maintain the reduction. Control may or may not be related to global targets set by WHO. Reducing the number of new infections, the number of people currently infected, and the number of people who become sick or die from a disease in local settings. This is achieved through deliberate efforts.

E.g. schistosomiasis control focuses on reducing disease through periodic, large-scale population treatment with praziquantel.

Elimination as a public health problem (EPHP) A term related to both infection and disease, defined by achievement of measurable targets set by WHO in relation to a specific disease. When reached, continued action is required to maintain the targets and/or to advance interruption of transmission.

Documentation of elimination as a public health problem is called validation.

EPHP targets:

Schistosomiasis: <1% proportion of heavy intensity schistosomiasis infections
STH: <2% proportion of soil-transmitted helminth infections of moderate and heavy intensity due to Ascaris lumbricoides, Trichuris trichiura, Necator americanus and Ancylostoma duodenale.

Reducing the number of people with moderate and heavy infections (where an individual is infected with many worms) i.e., those most vulnerable to becoming very sick and dying from schistosomiasis. This is achieved through deliberate efforts (as with control).

E.g. soil-transmitted helminth infections are of lower public health concern when moderate and heavy infections are below <2% in a community. Interventions need to be continued to maintain this target or increased/modified to interrupt transmission (elimination).

Elimination (interruption of transmission) Reduction to zero of the incidence of infection caused by a specific pathogen in a defined geographical area, with minimal risk of reintroduction, as a result of deliberate efforts; continued action to prevent re-establishment of transmission may be required.

Documentation of elimination of transmission is called verification.

Interruption of transmission definition:
Absence of schistosomiasis infection in humans, snails and, in some context animals which act as reservoir hosts.

Stopping transmission of a disease (reduction to zero of new cases) in a specific geographic area, but not worldwide. When a disease stops circulating in a region, it’s considered eliminated in that region, but ongoing control efforts are needed to avoid re-introduction.

E.g. Polio was eliminated in the United States by 1979 after widespread vaccination efforts but vaccination is still required until the goal of global eradication is achieved.
Although interruption of transmission of schistosomiasis has been achieved in Japan, regions of China and in multiple countries in the Americas the process of verification is yet to be conducted. These achievements in elimination have been achieved through environmental and behavioural interventions to reduce the risk of transmission and exposure and critically through improved standards of living with safe water and sanitation infrastructure.

Endemic (disease) (Disease) in constant presence or usual presence found in a population within a geographic area. Whether a disease is always or usually found in a given population or country. In contrast to epidemics or outbreaks, diseases are considered endemic when they are constantly present in a geographic area or a specific population.

E.g. Schistosomiasis is endemic in Uganda.
A schistosomiasis outbreak occurred in Corsica however France is not endemic for schistosomiasis.

Endemicity The level at which a disease is found in a particular area or population (low, moderate, high). The frequency of an infection/infectious agent which causes disease varies across populations and geographic areas due to the conditions which increase or decrease transmission of the infectious agent.

E.g. there is high endemicity of schistosomiasis near to Lake Volta in Ghana as the parasite needs freshwater snails for its life cycle.

Eradication Permanent reduction to zero of the worldwide incidence of infection caused by a specific pathogen, as a result of deliberate efforts, with no risk of reintroduction. Documentation of eradication is termed certification. The complete and permanent worldwide reduction to zero new cases of a disease through deliberate efforts. This means no infections, with no possibility of further transmission, anywhere in the world.

E.g. to date, only one human disease has been eradicated: smallpox.

Health equity Equity is the absence of unfair or avoidable differences among groups of people, whether those groups are defined socially, economically, demographically, or geographically or by other dimensions of inequality (e.g., sex, gender, ethnicity, disability, or sexual orientation). Health is a fundamental human right. Health equity is achieved when everyone can attain their full potential for health and well-being. In line with WHO.
Incidence The number of new cases that develop in a given period of time. The number of people newly infected with schistosomiasis or soil-transmitted helminthiasis (STH) over a period of time.

E.g. the incidence of yellow fever in the Democratic Republic of the Congo was 4.4 per 1,000,000 total population in 2010 compared to 0.1 in 2021 (WHO).

Morbidity Detectable, measurable clinical consequences of infections and disease that adversely affect the health of individuals. Evidence of morbidity may be overt (such as the presence of blood in the urine, anaemia, chronic pain, or fatigue) or subtle (such as stunted growth, impeded school or work performance or increased susceptibility to other diseases). Severity of disease.

E.g. morbidity resulting from STH infection is directly related to the number of worms within an individual’s body.

Prevalence The number of cases of a disease that are present in a particular population at a given time. The percentage of the population in a certain area (country, district) infected with schistosomiasis or STH at a given time.

E.g. the prevalence of schistosomiasis among Ugandan school-aged children is x%.