The Costs and Cost-Effectiveness of Mass Treatment for Intestinal Nematode Worm Infections Using Different Treatment Thresholds

Abstract

BackgroundIt is estimated that almost a half of all of people living in developing countries today are infected with roundworms, hookworms, or whipworms or combinations of these types of intestinal nematode worms. They can all be treated using safe, effective, and inexpensive single-dose generic drugs costing as little as USD 0.03 per person treated when bought in bulk. The disease caused by intestinal nematodes is strongly related to the number of worms in the gut, and it is typical to find that worms tend to be aggregated or clumped in their distribution so that <20% of people may harbour >80% of all worms. This clumping of worms is greatest when the prevalence is low. When the prevalence rises above 50%, the mean worm burden increases exponentially, worms are less clumped, and more people are likely to have moderate to heavy infections and may be diseased. Children are most at risk. For these reasons, the World Health Organization (WHO) currently recommends mass treatment of children ≥1 year old without prior diagnosis when the prevalence is ≥20% and treatment twice a year when the prevalence is ≥50%.Methods and FindingsThe risk of moderate to heavy infections with intestinal nematodes was estimated by applying the negative binomial probability distribution, then the drug cost of treating diseased individuals was calculated based on different threshold numbers of worms. Based on this cost analysis, a new three-tier treatment regime is proposed: if the combined prevalence is >40%, treat all children once a year; >60% treat twice a year; and >80% treat three times a year. Using average data on drug and delivery costs of USD 0.15 to treat a school-age child and USD 0.25 to treat a pre-school child (with provisos) the cost of treating children aged 2–14 years was calculated for 105 low- and low-middle-income countries and for constituent regions of India and China based on estimates of the combined prevalence of intestinal nematode worms therein. The annual cost of the three-tier threshold was estimated to be USD 224 million compared with USD 276 million when the current WHO recommendations for mass treatment were applied.ConclusionThe three-tier treatment thresholds were less expensive and more effective as they allocated a greater proportion of expenditures to treating infected individuals when compared with the WHO thresholds (73% compared with 61%) and treated a larger proportion of individuals with moderate to heavy worm burdens, arbitrarily defined as more than 10 worms per person (31% compared with 21%).

Effects of Annual Mass Treatment with Ivermectin for Onchocerciasis on the Prevalence of Intestinal Helminths

“…Single-dose therapy with albendazole is more effective (72%) than mebendazole (15%) against hookworm. 28 In areas where annual distribution of ivermectin is ongoing, "piggybacking" an additional 27,29,30 and costs related to importation and transports to intervention areas. However, expanding coverage to new areas where piggybacking with other MDA programs could not occur would have a much higher incremental cost as the cost of drug delivery would need to be considered.…”

Soil-transmitted helminthiasis in Latin America and the Caribbean: modelling the determinants, prevalence, population at risk and costs of control at sub-national level

“…Both place Guatemala as at the top of the ranking and Trinidad and Tobago at the bottom and, while there are some significant differences in where in the ranking the methodologies place the countries in between (Nicaragua, Dominican Republic and Chile for example), there seems to be broad similarities in the trend. However, the range of prevalence values in the Hall et al (2009) is much wider -ranging from (a perhaps implausibly high) 92.6% in Guatemala to 4.5% in Trinidad and Tobago. Our model predicts a more homogenous situation across the countries.…”

“…Fig. 11a compares the national level STH prevalence predicted by this model to those presented by Hall et al (2009) (which are those originally arrived at by de Silva et al (2003) after applying to updated data the methodology originally employed by Chan et al (1994)). The figure reveals that the two methodologies result in a similar ranking by prevalence of the countries.…”

Parasitic nematodes in humans : exploring the host-parasite dynamic through historical, biological, and public health evaluations of infection.

“…Further research is needed to determine if the observed differences in mean egg count per gram of feces actually point toward physiological differences in fecundity across countries or whether the finding was just an artifact of diagnostic technique. It should also be noted that worm fecundity is dependent upon the density of wonns in an infection, with higher numbers of worms leading to lower individual rates of egg production (168,169).…”

“…Intraspecific interactions within a host can have an impact on the virulence, fecundity, and population structure of a parasitic infection (168,169,233). Studies on intestinal nematodes have shown that an increase in parasite density leads to decreased individual fecundity, likely a result of intraspecific competition for resources (168,169).…”