Kevin D. Lafferty

Food web topology and parasites in the pelagic zone of a subarctic lake.

1. Parasites permeate trophic webs with their often complex life cycles, but few studies have included parasitism in food web analyses. Here we provide a highly resolved food web from the pelagic zone of a subarctic lake and explore how the incorporation of parasites alters the topology of the web. 2. Parasites used hosts at all trophic levels and increased both food-chain lengths and the total number of trophic levels. Their inclusion in the network analyses more than doubled the number of links and resulted in an increase in important food-web characteristics such as linkage density and connectance. 3. More than half of the parasite taxa were trophically transmitted, exploiting hosts at multiple trophic levels and thus increasing the degree of omnivory in the trophic web. 4. For trophically transmitted parasites, the number of parasite-host links exhibited a positive correlation with the linkage density of the host species, whereas no such relationship was seen for nontrophically transmitted parasites. Our findings suggest that the linkage density of free-living species affects their exposure to trophically transmitted parasites, which may be more likely to adopt highly connected species as hosts during the evolution of complex life cycles. 5. The study supports a prominent role for parasites in ecological networks and demonstrates that their incorporation may substantially alter considerations of food-web structure and functioning.

Brain cancer mortality rates increase with Toxoplasma gondii seroprevalence in France

The incidence of adult brain cancer was previously shown to be higher in countries where the parasite Toxoplasma gondii is common, suggesting that this brain protozoan could potentially increase the risk of tumor formation. Using countries as replicates has, however, several potential confounding factors, particularly because detection rates vary with country wealth. Using an independent dataset entirely within France, we further establish the significance of the association between T. gondii and brain cancer and find additional demographic resolution. In adult age classes 55 years and older, regional mortality rates due to brain cancer correlated positively with the local seroprevalence of T. gondii. This effect was particularly strong for men. While this novel evidence of a significant statistical association between T. gondii infection and brain cancer does not demonstrate causation, these results suggest that investigations at the scale of the individual are merited.

Cat ownership is neither a strong predictor of Toxoplasma gondii infection nor a risk factor for brain cancer

Using a dataset including 37 countries, we recently reported a positive correlation between the national seroprevalence of the protozoan parasite Toxoplasma gondii and the national incidence of brain cancer [[1][1]]. We further established the significance of this association in a second paper

Water, dams, and prawns: novel ecological solutions for the control and elimination of schistosomiasis

Abstract Background Dams have long been associated with increased burdens of human schistosomiasis, but how dams increase disease is not always clear, in part because dams have many ecological and socioeconomic effects. A recent hypothesis argues that dams block the reproduction of the migratory river prawns that eat the snail hosts of schistosomiasis. In the lower basin of the Senegal River, there is evidence that prawn populations decreased and schistosomiasis increased after completion of the Diama Dam in Senegal. Restoring prawns to a water-access site upstream of the dam has been shown to reduce snail density and reinfection rates in people. However, whether a similar cascade of effects (from dams to prawns to snails to human schistosomiasis) occurs elsewhere is unknown. The aim of this work was to assess whether the dam-associated prawn declines coincident with schistosomiasis increases observed in the Diama Dam watershed in Senegal might be generalisable across a broad geographic area. Methods We delineated prawn habitat boundaries for 24 large, marketable, migratory Macrobrachium spp prawns, based on the United Nations Food and Agriculture Organization species catalogue of shrimps and prawns. Using a published repository of schistosomiasis studies in sub-Saharan Africa, we compared infection before and after the construction of 14 large dams for people living in: (1) upstream catchments within historical habitats of native prawns, (2) comparable undammed watersheds, and (3) dammed catchments beyond the historical reach of migratory prawns. For each of these three scenarios, we calculated odds ratios (ORs) of schistosomiasis disease before:after dams using the number of people infected out of the total number examined, reported in the freely available GNTD database by using a generalised linear mixed model with binomial errors and a logit link function. Finally, by using published Gridded Population of the World estimates for 2015, version 3 (GPWv3) available from NASAs SEDAC data portal, we calculated the current human population living within areas around the world that are both endemic for schistosomiasis and occur within native prawn ranges. Findings Catchments located within prawn habitat had overall ORs after:before of 2·8 (95% CI 2·7–2·9) for Schistosoma haematobium and 4·4 (3·6–5·3) for Schistosoma mansoni , showing a strong overall increase in schistosomiasis. By contrast, undammed watersheds in nearby prawn habitat experienced overall schistosomiasis declines across the same time periods; ORs after:before 0·90 (95% CI 0·88–0·91) for S haematobium and 0·75 (0·73–0·78) for S mansoni . The dammed catchments outside of prawn habitats also experienced schistosomiasis increases, but to a lesser degree than did dammed catchments within prawn ranges; ORs after:before 1·15 (95% CI 1·1–1·2) for S haematobium and 1·5 (1·3–1·8) for S mansoni . Finally, we estimated that a third to a half of the global population-at-risk of schistosomiasis could benefit from restoring native prawns. Interpretation Because dams block prawn migrations, our results suggest that prawn extirpation contributes to the sharp increase of schistosomiasis after damming, and points to prawn restoration as an ecological solution to reduce human disease. Funding Stanford Woods Institutes Environmental Ventures Projects (SHS, IJJ, DL, and GADL), Stanford Institute for Innovation in Developing Economies Global Development and Poverty Initiative (SHS and GADL), Bill & Melinda Gates Foundation (SHS, MJ, and GADL), National Science Foundation Coupled Natural and Human Systems (grant number 1414102; SHS, KDL, AMK, GADL, and DL-C), National Institutes of Health (grant #1R01TW010286; SHS, CMH, PAC, and JR), Stanford Mentoring Undergraduates in Interdisciplinary Research programme (OC), Stanford Vice Provost for Undergraduate Education programme (JF), California State University Monterey Bay Research Experiences for Undergraduates programme (AK), and University of Michigans Society of Fellows (CLW).

Distribution and mapping of the snail Cerithideopsis (Cerithidea) californica at Carpinteria Salt Marsh, California USA, June to August 2012

We collected detailed spatial data on the density and size distribution of intertidal snails as part of a broader effort to understand food webs in California estuaries. The survey area was Carpinteria Salt Marsh, California USA, which comprises 9 Ha tidal channels, 2 Ha salt flats, 17 Ha upland habitat, 6 Ha tidal pans, 52 Ha vegetated marsh, 2 Ha tidal flats. Using nearly 4,000 transects in potential snail habitat, we mapped snails throughout the estuary. Specifically, we systematically placed transects at intervals stratified within targeted habitat types: channels, pans, or marsh (or planar habitat that was mixed marsh and pan). At a quarter of the quadrats, we also estimated snail size-frequency distributions. Because we recorded the location of each quadrat, these data can be used to map the distribution of snails throughout this estuary.