Newly discovered parasite linked to coastal seabird deaths

Along the Gulf Coast, black skimmers have long been a familiar sight — gliding just above the water with their bright orange beaks slicing through the surface to feed.

But when more than 160 chicks died in Texas nesting colonies during the summers of 2022 and 2023, researchers uncovered something no one had documented before — a previously unknown parasite that may be part of a much larger conservation challenge.

To investigate the die‑off, researchers from the Texas A&M College of Veterinary Medicine and Biomedical Sciences (VMBS) partnered with the Gulf Coast Bird Observatory and colleagues in the Texas A&M College of Agriculture and Life Sciences.

The cross‑disciplinary team — including Dr. Laura Bryan, Dr. Raquel Rech, Dr. Guilherme Verocai and Dr. Jacquelyn Grace — combined expertise in pathology, parasitology and wildlife biology to determine what was killing the chicks.

Through genetic testing, the team identified a parasite belonging to the Eimeria group — which typically infects the digestive tract of wild and domestic animals — and revealed two previously undocumented forms: one found in the kidneys and cloaca and another in the small intestine.

“To the best of our knowledge, this is the first report of Eimeria infection in black skimmers,” said Bryan, a clinical associate professor in the VMBS Department of Veterinary Pathobiology.

While the team is not yet able to attribute the birds’ death to the parasite, the finding adds a new piece to the puzzle behind the decades-long decline in black skimmer populations. It also highlights how little scientists still know about the disease affecting wild coastal birds.

“It’s really hard to tell what caused the death because these are wild birds with a lot happening at once,” said Grace, an assistant professor in the Department of Ecology and Conservation Biology. “When they start to get into poor condition, they get hit by more things and become immunocompromised.”

An unknown parasite detected

Early in 2018, the Gulf Coast Bird Observatory, which monitors chick colonies along the coast, started banding black skimmers for better tracking and found some well-nourished chicks that still failed to fledge and died.

In 2022, Grace’s Ph.D. student, Elena Duran, noticed more chicks dying, prompting the team to investigate further. Because colony access is limited, the team examined banded chicks that had passed the expected fledging age and discovered tiny white spots on the kidneys of six chicks; analyzing tissue samples from the kidneys, small intestine, and cloaca using genetic testing, they identified Eimeria as the parasite responsible for the damage.

“We knew they fell within the broader Eimeria group, but we couldn’t determine the exact species,” Bryan said. “They appear to be previously undescribed, and we don’t yet have molecular markers that would allow precise identification.”

Finding an undescribed parasite species may sound unusual, but Verocai says it isn’t surprising.

“Parasitism is a successful lifestyle in nature,” he said. “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.”

Although the parasite remains unknown to science, researchers believe it spreads much like other Eimeria parasites: birds likely become infected by ingesting parasite eggs, called oocysts, in contaminated food or water and possibly even while grooming their feathers.

“The parasite might be present in healthy adult birds without causing disease,” Verocai said. “Younger birds with not‑yet‑fully developed immune systems are likely more affected by the parasite.”

Researchers also still don’t know exactly how the parasite moves through the birds’ bodies.

“If other stages of the parasite have to develop in different cells throughout the body, there is typically a mid‑stage in the intestine for most renal Eimeria before they migrate,” Bryan said. “We do not yet know all of the transmission dynamics of this organism.”

A tangled mix of disease pressures

Researchers believe the parasite is only part of the story. Changes to the birds’ coastal habitat may also be increasing disease risk.

Along the Gulf Coast, shipping channels require constant maintenance dredging to stay navigable. The sediment brought up during this process is piled rather than being hauled away, which creates artificial islands that seabirds now use as nesting sites. While these islands offer some refuge, they may also have unintended effects on the birds.

“We think these populations are probably denser than they were naturally,” Grace said.

“This hasn’t been tested, but the birds that once nested more spread out along the coast are now concentrated mostly in artificial habitats, and that increased density likely contributes to disease spread.”

She added that unlike the short‑lived natural habitats the birds once used, these islands are more stable — conditions that may favor parasites that persist in the soil.

Shipping channels running through coastal industrial corridors or carrying vessels that transport oil and chemical products may also expose birds to contaminants. Toxins can accumulate in fish and other prey species that black skimmers eat, potentially placing additional stress on the birds.

“We’ve seen higher amounts of heavy metals in the skimmers,” Grace said. “We realized it was probably some combination of toxins and disease that was causing the mortality, and there was definitely disease involved.”

Rather than pointing to a single cause, the study suggests disease, habitat changes and environmental contaminants may all be contributing to chick mortality.

Now, the team’s biggest challenge is monitoring the colony long-term to better understand what is driving the die-off. Doing so will require additional samples, continued monitoring and more funding.

“If we can figure out whether something in the environment is contributing to the disease, that might be easier to modify than trying to treat a whole bunch of birds,” Grace said.