Effect of estuarine wetland degradation on transport of Toxoplasma gondii surrogates from land to sea

Abstract

The flux of terrestrially derived pathogens to coastal waters presents a significant health risk to marine wildlife, as well as to humans who utilize the nearshore for recreation and seafood harvest. Anthropogenic changes in natural habitats may result in increased transmission of zoonotic pathogens to coastal waters. The objective of our work was to evaluate how human-caused alterations of coastal landscapes in California affect the transport of Toxoplasma gondii to estuarine waters. Toxoplasma gondii is a protozoan parasite that is excreted in the feces of infected felids and is thought to reach coastal waters in contaminated runoff. This zoonotic pathogen causes waterborne toxoplasmosis in humans and is a significant cause of death in threatened California sea otters. Surrogate particles that mimic the behavior of T. gondii oocysts in water were released in transport studies to evaluate if the loss of estuarine wetlands is contributing to an increased flux of oocysts into coastal waters. Compared to vegetated sites, more surrogates were recovered from unvegetated mudflat habitats, which represent degraded wetlands. Specifically, in Elkhorn Slough, where a large proportion of otters are infected with T. gondii, erosion of 36% of vegetated wetlands to mudflats may increase the flux of oocysts by more than 2 orders of magnitude. Total degradation of wetlands may result in increased Toxoplasma transport of 6 orders of magnitude or more. Destruction of wetland habitats along central coastal California may thus facilitate pathogen pollution in coastal waters with detrimental health impacts to wildlife and humans.

FIG. 1.

Schematic diagram of the bottomless flume used in Toxoplasma gondii surrogate transport studies. The flume was placed in either mudflat or vegetated wetland habitats, and water was pumped from the nearby tidal creek into the inflow end at a rate that achieved desired current velocities, while the weir at the outflow end was used to set the water depth to 10 cm for all experiments. Two flow straighteners were used at either end of the flume to reduce turbulence associated with inflow and outflow features. Surrogates were released downstream of the inflow-end flow straightener, and water samples were collected 160 cm downstream from the release point by using two peristaltic pumps that suctioned water separately from the top and bottom depths. The figure is not drawn to scale. (Courtesy of Alison Kent.)

FIG. 2.

Cumulative proportion of Toxoplasma gondii surrogate microspheres (Dragon Green) recovered in transport studies within vegetated and mudflat wetland habitats. Vegetated wetland experiments were performed in the Fall (dense vegetation) and in the Spring (sparse vegetation). The curve for the mudflat habitat represents the average of duplicate experiments (see Table 2).

FIG. 3.

Flux of Toxoplasma gondii surrogate microspheres recovered in a transport experiment representative of a vegetated estuarine wetland habitat in Fall. Microspheres were sampled separately in the upper and lower water columns (top and bottom sampling devices [Fig. 1]), and fluxes were calculated for the “top” and “bottom” halves of the water column by using measured current velocities as described in Materials and Methods.