Infection patterns can be assessed by two epidemiological parameters: the proportion of individuals infected (prevalence of infection) and the average number of worms which infected individuals harbour (intensity of infection).
Prevalence of infection
Prevalence of infection is the more easily assessed indicator for STH and schistosomiasis as it requires only identifying the proportion of individuals with one or more eggs detected in their stool or urine sample, and for LF, the proportion of individuals found with detectable parasites in their bloodstream. Because of this relative simplicity of measurement, WHO recommends use of prevalence to determine the need for control, with mass treatment of whole populations recommended where prevalence exceeds a pre-set threshold.
For LF, an antigen-detection test that is simple, sensitive and specific, called an immunochromatographic test (ICT), is now available and routinely used to estimate prevalence and identify areas requiring mass treatment.
Intensity and density of infection
Intensity of infection is a measure of the number of eggs per gram of faeces (for STH and Schistosoma mansoni) or eggs per 10 ml of urine (for S. haematobium). It is a key determinant of transmission within communities and of the risk of morbidity among individuals.
For lymphatic filariasis, density of infection is usually based on the recovery of microfilariae from a blood sample. In individuals, there is no clear relationship between density of infection and risk of clinical disease.
However, measurement of infection intensity (and density) requires time-consuming, quantitative laboratory methods. In the case of LF, blood samples need to be collected between 10pm and 2am to coincide with the presence of microfilariae in the blood. Intensity (and density) of worm infection is consequently not routinely assessed in field surveys.