Study: Slime-producing molecules help spread disease from cats to sea otters

sea otter

The spread of diseases from land animals to sea otters and other marine mammals is aided and abetted by gelatinous, sticky polymers produced by seaweed, reports a research team headed by a UC Davis veterinary infectious-disease expert.

These large, complex molecules form slimy biofilms and bind water-borne organic matter into larger particles, in which disease-causing microorganisms can become embedded and introduced to the marine food chain, the researchers discovered.

Using the parasite Toxoplasma gondii as a model, they showed how these sticky polymers increase the chance that disease-causing organisms would be picked up by marine snails, which graze on kelp and are among the common foods of some endangered sea otters.

Findings from the new study were published Oct. 8 in the journal Proceedings of the Royal Society B.

“Discovering the role that these invisible polymers play in disease transmission in the ocean is a tremendous step forward in helping us better understand and mitigate the impacts of coastal water pollution on the health of wildlife and humans,” said lead author Karen Shapiro, a research scientist in the School of Veterinary Medicine.

Toxoplasma gondii oocyst (arrow) associated with a a marine aggregate (i.e. ‘snow’). Sticky molecules (stained here in blue) were shown to enhance the association of the parasite with aggregates, which could lead to accumulation of the pathogen in ‘hot spots’ for infection in susceptible animals like sea otters. (Photo credit: Karen Shapiro)


Sea otters and humans at risk

Contamination of coastal waters with disease-causing microorganisms is known to pose a threat to the health of both humans and animals, but the mechanisms by which diseases are transmitted in marine ecosystems has until now remained a mystery.

The researchers focused on Toxoplasma gondii, a protozoan parasite that infects animals and humans worldwide.

The parasite actively reproduces in various cat species including domestic cats. Its egg cells pass on in cat feces and can persist in the environment for months to years, infecting marine mammals including the endangered southern sea otter in California.

Humans also can be infected by T. gondii when they consume contaminated water or undercooked shellfish.


Snails to sea otter transmission

Puzzled by the high rate of T. gondii infection in sea otters and other marine mammals, the researchers set out to track the route of transmission. Noting that T. gondii infections were 10 times more common among sea otters that fed heavily on kelp-grazing marine snails than among otters that fed on abalone and other ocean food sources, they investigated why the sea snails might be particularly effective carriers of the parasite.

Marine snail grazing on a blade of kelp in a ‘snail condo’ unit under laboratory conditions. Snails that were exposed to Toxoplasma concentrated the parasite in their feces by more than 150 fold and continued to excrete oocysts (eggs) for 10 days. (Photo credit: Karen Shapiro)

In laboratory tests, the researchers discovered that the gelatinous polymers, excreted by seaweed, act in two ways to provide an environment conducive to transmission of infectious diseases.  First, the polymers act like glue, binding together water-borne organic material into larger particles, in which infectious agents like the T. gondii egg cells can embed and more quickly settle to the ocean floor.

Secondly, the polymers help to form sticky biofilms, which can trap the T. gondii egg cells and coat kelp on which marine snails graze. The parasite then can be easily passed on when the snails are eaten by otters, completing the intricate chain of disease transmission from land-based cats to the endangered coastal sea otters.

Other researchers on the study were Colin Krusor, Patricia A. Conrad, John L. Largier and Jonna A.K. Mazet, all of UC Davis, and Fernanda F.M. Mazzillo and Mary W. Silver, both of UC Santa Cruz.

The National Science Foundation’s Evolution and Ecology of Infectious Disease program provided funding for the study.


About UC Davis

UC Davis is a global community of individuals united to better humanity and our natural world while seeking solutions to some of our most pressing challenges. Located near the California state capital, UC Davis has more than 34,000 students, and the full-time equivalent of 4,100 faculty and other academics and 17,400 staff. The campus has an annual research budget of over $750 million, a comprehensive health system and about two dozen specialized research centers. The university offers interdisciplinary graduate study and 99 undergraduate majors in four colleges and six professional schools.


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Aquatic polymers can drive pathogen transmission in coastal ecosystems
Karen Shapiro, Colin Krusor, Fernanda F. M. Mazzillo, Patricia A. Conrad, John L. Largier, Jonna A. K. Mazet and Mary W. Silver
Proc. R. Soc. B. 2014 281 20141287; doi:10.1098/rspb.2014.1287 (published 8 October 2014)


Gelatinous polymers including extracellular polymeric substances (EPSs) are fundamental to biophysical processes in aquatic habitats, including mediating aggregation processes and functioning as the matrix of biofilms. Yet insight into the impact of these sticky molecules on the environmental transmission of pathogens in the ocean is limited. We used the zoonotic parasite Toxoplasma gondii as a model to evaluate polymer-mediated mechanisms that promote transmission of terrestrially derived pathogens to marine fauna and humans. We show that transparent exopolymer particles, a particulate form of EPS, enhance T. gondii association with marine aggregates, material consumed by organisms otherwise unable to access micrometre-sized particles. Adhesion to EPS biofilms on macroalgae also captures T. gondii from the water, enabling uptake of pathogens by invertebrates that feed on kelp surfaces. We demonstrate the acquisition, concentration and retention of T. gondii by kelp-grazing snails, which can transmit T. gondii to threatened California sea otters. Results highlight novel mechanisms whereby aquatic polymers facilitate incorporation of pathogens into food webs via association with particle aggregates and biofilms. Identifying the critical role of invisible polymers in transmission of pathogens in the ocean represents a fundamental advance in understanding and mitigating the health impacts of coastal habitat pollution with contaminated runoff.

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About is dedicated to raising awareness about California's threatened sea otters and the role research plays in the species recovery and conservation. It's a collaboration of the Monterey Bay Aquarium, U.C. Santa Cruz, U.C. Davis, U.S. Geological Survey, U.S. Fish & Wildlife Service, California Department of Fish and Wildlife, among others. is also home of the world's first HD live stream of southern sea otters in the wild.