A mathematical model for the control of swimmer's itch

Abstract

Swimmer's itch is an emerging disease caused by flatworm parasites that often use water birds as definitive hosts. When parasite larvae penetrate human skin they initiate localized inflammation that leads to intense itching. Concerns about this issue have been growing recently due to an apparent increase in the global occurrence of swimmer's itch and its subsequent impacts on recreational activities and associated revenues. Past study has identified the common merganser as a key definitive host for these worms in the United States; a number of snail species serve as intermediate hosts. Although previous attempts at controlling swimmer's itch have targeted snails, a handful of efforts have concentrated on treating water birds with the anthelmintic drug, praziquantel. We construct a mathematical model of swimmer's itch and its treatment within the infected merganser population. Our goal is to identify merganser treatment regimes that minimize the number of infected snails thereby reducing the risk of human infections. Optimal control of bird hosts is defined analytically and we include numerical simulations assuming different resource‐allocation strategies. Results from the study may help identify treatment protocols that lower merganser infection rates and ultimately reduce the occurrence of swimmer's itch in freshwater systems throughout the Midwest. Recommendations for Resource Managers Regardless of the time and monetary resources available, praziquantel treatment frequency should increase as mergansers arrive on the lake with continued treatments (albeit at reduced levels) until the end of the residency period. Allocating plenty of resources towards the treatment of mergansers predicted a sharp drop in infected birds, which then remained close to zero throughout the remainder of the residency period. This approach reduced schistosome infection in mergansers and kept snail infections within the idealized range during times of peak recreational activity. Consequently, human cases of swimmer's itch would be expected to be low to nonexistent. Furthermore, our treatment‐longevity computation suggested that subsequent praziquantel dosing would not be required for a number of years. Under more limited resources, the number of birds treated per day was much smaller throughout the residency period; however, even under these circumstances (which equated to treating approximately one bird every 5 days), simulated infected merganser densities were reduced to the point where snail infections remained below epidemic levels through to the end of the recreational period. Treatment longevity was shorter compared with the high‐resource option, but still extended 122 days into Season 2 (posttreatment). We also used our model to investigate situations where lake managers and/or federal agencies might be taxed in terms of the time available to continuously treat mergansers on a given lake. An individual scientist may only have a single day (or two) to dose birds, rather than continuously administering praziquantel throug...

Swimmer’s itch control: Timely waterfowl brood relocation significantly reduces an avian schistosome population and human cases on recreational lakes

“…Yet whether praziquantel alone can be as effective in whole-lake reduction of snail infections and SI cases needs to be studied. A disadvantage is that praziquantel impacts only the adult worms already present and does not prevent new infections, so young-of-the-year would likely need to be treated at least twice to prevent them from passing parasite eggs during the summer months [ 26 , 68 , 69 ]. Fortunately, praziquantel has very low toxicity for vertebrates, is metabolized quickly, and is tolerated well, hence its widespread and heavy use in the treatment of parasitic helminths in pets, fish farming, captive and free wildlife, livestock, and humans [ 70 , 71 ].…”

A novel schistosome species hosted by Planorbella (Helisoma) trivolvis is the most widespread swimmer's itch-causing parasite in Michigan inland lakes

“…Management options that target a single parasite species, such as by removing or treating the primary definitive host (e.g. Blankespoor and Reimink, 1991 ; Peirce et al ., 2020 ), may be ineffective in lakes that harbour other parasite species instead of – or in addition to – the target species (Rudko et al ., 2022 ). Management options that focus on preventing contact between swimmers and cercariae, such as floating booms or anti-schistosome skin creams, may be more effective in locations with multiple schistosome species (Salafsky et al ., 1999 ).…”

Scratching the Itch: Updated Perspectives on the Schistosomes Responsible for Swimmer’s Itch around the World

“…Global warming is likely to lower thermal barriers to colonization in some lakes, and thus may favor changes in the composition of species involved in dermatitis transmission or introduce species such as P. antipodarum that might serve as decoys and lower transmission [ 173 ]. Warming may also increase eutrophication rates, which are generally considered to be a factor favoring dermatitis outbreaks [ 16 , 174 ]. Increasing pollution by heavy metals such as lead may have negative effects on bird populations and the populations of parasites they harbor [ 74 ].…”