The TIF direct smear as an epidemiological tool; with special reference to counting helminth eggs.

Thiomersal-iodine-formalin (TIF) has been used as a faecal preservative in many prevalence surveys of intestinal helminths and protozoa. In helminth surveys, however, estimates of worm burden are no less essential than those of prevalence. Direct-smear and dilution egg-counting techniques, using fresh faeces, have been developed to provide such estimates. This study originated in a field survey that required the use of a preservative. Attempts to estimate worm burden with TIF-preserved faeces led to the assessment of TIF direct-smear (TIF-DS) methods reported here. TIF-DS egg-counting provides reliable statistical estimates of hookworm, Ascaris and Trichuris burden; this is a satisfactory method for estimating worm burden when faeces must be preserved and transported from the field. TIF direct-smears also determine parasite prevalence "efficiently". Some previous studies have cast doubt on the value of TIF concentration (TIFC) and in this study also TIFC proved to be erratic or ineffective in concentrating eggs.