A newly identified parasite in the kidneys of a coastal seabird may be one more pressure on the bird's declining population

Researchers investigating the deaths of roughly 160 black skimmer chicks in two Texas nesting colonies identified a previously uncharacterized lineage of the parasite Eimeria in the birds' kidneys, which the authors say is the first documented report of Eimeria infection in the genus Rynchops. A second, distinct lineage was found in the small intestine of one bird.¹

The findings, published in April in PLOS One, suggest a parasite may be one more thing driving down black skimmer populations along the Gulf Coast, already in decades-long decline from habitat loss, human disturbance, predation, and sea-level rise. Black skimmers are not federally listed as endangered, but their numbers have been falling for several decades, and relatively little is known about the diseases affecting them.¹

The mortality events occurred in the East Matagorda Bay colony during the 2022 and 2023 breeding seasons, and in the West Galveston population in 2023.¹ Chicks were found dead or dying at 2 to 4 weeks of age, emaciated and in poor body condition. Six chicks were examined through necropsy, histopathology, and genetic analysis at Texas A&M University's Department of Veterinary Pathobiology.

In 3 of the 6 birds, the kidneys were enlarged with pale tan foci. Microscopic examination revealed multiple developmental stages of coccidian parasites within the epithelial cells of the renal tubules, collecting ducts, and cloaca.¹ Genetic sequencing of the 18S rRNA gene showed the renal parasite was 96.25% identical to Eimeria reichenowi, a species found in cranes—a difference large enough to indicate a distinct, previously uncharacterized lineage. The intestinal parasite, detected in a single bird, most closely matched an uncharacterized Eimeria species reported in Northern gannets in Canada.¹

In the birds the researchers examined, the parasite "appeared to cause morbidity and likely contributed to the recent mortality outbreak," the authors wrote, though they cautioned that its broader significance for black skimmers "is unknown."¹

The research team, which combined pathologists, a parasitologist, and wildlife biologists across two Texas A&M colleges and the Gulf Coast Bird Observatory, does not attribute the deaths to the parasite alone. The chicks were in poor body condition, leaving them more susceptible to disease, and the birds carried elevated levels of heavy metals, pointing to disease, habitat change, and environmental contaminants as overlapping pressures.²

"It's really hard to tell what caused the death because these are wild birds with a lot happening at once," Jacquelyn Grace, PhD, an assistant professor in the Texas A&M Department of Ecology & Conservation Biology, said in a Texas A&M release.² "When they start to get into poor condition, they get hit by more things and become immunocompromised."

Laura Bryan, DVM, PhD, DACVP, a clinical associate professor in the Department of Veterinary Pathobiology, said the parasite has not been matched to a known species, falling within the broader Eimeria group but resisting more precise identification.²

"They appear to be previously undescribed, and we don't yet have molecular markers that would allow precise identification," Bryan said.²

An undescribed parasite is not as surprising as it may sound, according to Guilherme Verocai, DVM, PhD, MSc, an associate professor in the same department.

"Parasitism is a successful lifestyle in nature," Verocai said.² "Possibly half of the world's biodiversity is composed of parasitic species, though this may still be an underestimation because so many species have not yet been found or formally described."

Eimeria parasites are generally host- and tissue-specific and spread when birds ingest sporulated oocysts from contaminated food or water. Younger birds, whose immune systems are not fully developed, are likely more vulnerable, while adults may carry the parasite without showing signs of disease.

The authors noted that adult skimmers were not examined, and that it is possible they are subclinically infected.¹ How the organism moves through the body remains unclear, including whether it passes through an intestinal stage before migrating to the kidneys.²

Changes to the birds' coastal habitat may also be raising disease risk. Maintenance dredging of shipping channels creates artificial islands that skimmers now use for nesting, which Grace said concentrates populations more densely than would occur naturally.² Those islands are also more stable than the short-lived natural habitats the birds once used, conditions Grace said may favor parasites that persist in the soil.²

Grace said the team had also measured elevated levels of heavy metals in the skimmers, pointing to a combination of contaminants and disease rather than a single cause.

Formal description of the new parasite lineages will require, in the authors' words, "further studies based on the morphology of sporulated oocysts."¹ Beyond that, key questions remain about how widespread the infection is and how much it contributes to the decline: whether adult birds carry the parasite, whether it occurs in other colonies, and whether it is driving mortality at the population level.

References

1. Groch KR, Sanders TL, Duran EMI, Clontz D, Grace JK, Rech RR, et al. Renal coccidiosis in black skimmers in North America reveals an uncharacterized Eimeria lineage (Apicomplexa: Eimeriidae). PLoS One. 2026;21(4):e0345982. doi:10.1371/journal.pone.0345982
2. Chen Y. Newly discovered parasite may be contributing to black skimmer decline. Texas A&M College of Veterinary Medicine & Biomedical Sciences. July 28, 2026. Accessed August 26, 2026. https://vetmed.tamu.edu/news/press-releases/black-skimmer-parasite-discovery/