ADVERTISEMENT

Cat Disease Threatens Endangered Monk Seals

On the beaches of the Hawaiian islands, monk seals are dying from a pathogen in cat feces that is carried to the ocean in polluted runoff and sewage



Barbara Billand/NOAA

Hoku endured some rough days before he died last spring. Three dogs chased him off one of his resting beaches, and he battled a minor tsunami that left him wedged between a pair of boulders in a lava field far from shore.

Observers noticed him looking thin in the few months before fishermen found him dead on a beach near the east Kaua`i town of Kapa`a.

In the end, disease took him.

Nicknamed “Star” in Hawaiian for the small white spot on his forehead, Hoku was a large, 10-year-old Hawaiian monk seal, an endangered species.

Hoku may likely have been the second Hawaiian monk seal to die this year from Toxoplasmosa gondii, a parasite transmitted primarily through cat feces and carried to the ocean in polluted runoff and sewage.

The first suspected toxoplasmosis case of the year came in January. While conducting his weekly seal search along the coastline of Moloka`i’s Kalaupapa National Historical Park, marine ecologist Eric Brown discovered a stillborn pup in a tide pool. His mother, apparently in good health, lay nearby.

David Schofield, marine mammal response coordinator for the National Oceanic and Atmospheric Administration’s Pacific Islands Regional Office, believes the pup may have been the first Hawaiian monk seal to die from a toxoplasmosis infection transmitted in the womb.

With only about 1,100 monk seals left in the wild, the deaths are “very concerning and put toxo as one of our primary concerns” for the species, says Charles Littnan, lead scientist for NOAA’s monk seal research program.

The overall population of monk seals is declining at a rate of about 4.5 percent a year. The good news is that in recent years, their numbers have been growing in the main Hawaiian islands. Now resource managers worry that in the midst of so many humans, interactions will likely increase, as will the seals’ chances of encountering diseases and contaminants.

No studies have been done in Hawai`i on how and where toxoplasmosis reaches the ocean and there are few efforts to control it. In California, however, researchers have found that it infects sea otters mainly though runoff from urban areas.

Flushing cat litter down the toilet is one pathway, since sewage treatment does not always kill the parasite’s hardy eggs, called oocysts. Studies have found that oocysts can live for at least two years in sea water.

Over the past ten years, the cat parasite has killed at least four monk seals in the main Hawaiian islands -  two from Kaua`i, one from O`ahu, and one from Moloka`i - and perhaps six, experts estimate.

Those deaths “should be considered an absolute minimum since there are dead seals we never know about and ones we sample but are unable to determine a cause of death for,” Littnan said.

“We are only just beginning to understand the prevalence of the disease in the population and determine ways to mitigate the impact.”

Hunted to near extinction in the late 19th century, the Hawaiian monk seal was federally listed as endangered in 1976, after populations plummeted during the 1960s and 1970s, largely due to military disturbance.

Today, it is considered the most endangered pinniped in the United States. With a potential peak population of about 3,000 seals, NOAA predicts numbers will drop below 1,000 in the next few years.

The population’s core has long been in the remote, largely uninhabited Northwest Hawaiian Islands, but more and more, conditions there are killing them. Low juvenile survival due to starvation is by far the biggest problem facing monk seals. Some researchers speculate that overfishing may have caused a shift in predator dominance that is now making it nearly impossible for young seals to compete for food.

But diseased cats also are among the seals’ worst enemies, since their feces flow into the ocean via runoff and sewage.

The state Division of Forestry and Wildlife estimates that 300,000 to 400,000 free-ranging cats live on Maui alone. That’s roughly two cats per resident.

“Cats are all over the place in Hawai`i,” explains Thierry Work, a wildlife disease specialist with the Geological Survey’s Honolulu field station who studied toxoplasmosis in Hawaiian crows, known as `alala, about a decade ago. “Wherever there are cats, there’s the potential for toxoplasmosis.”

Although domestic cats are considered the main source, feral cats in remote areas also transmit the disease. Nearly 40 percent of 67 cats captured from the slopes of Mauna Kea, on the island of Hawai`i, tested positive for toxoplasmosis, according to a 2007 article in the Journal of Wildlife Diseases.

In the past decade or so, toxoplasmosis has been regularly found in a wide range of marine mammals, including whales, dolphins and sea lions. In Hawa`i, it also has killed seabirds and endangered Hawaiian crow and geese.

Perhaps most famously, southern sea otters in California began dying off in alarming numbers in the 1990s. Toxoplasmosis was found in 52 percent of fresh, beach-cast otter carcasses and 38 percent of live otters sampled along the California coast, according to a 2005 International Journal of Parasitology article.

In Hawa`i, it also has killed seabirds and endangered Hawaiian crow and geese.Whether the increased diagnoses indicate improved testing techniques or a rise in disease prevalence is difficult to determine.

“Probably both things are at play and it’s going to be awfully hard to tease those out,” says Scott Wright of the U.S. Geological Survey’s National Wildlife Health Center.

Sometimes a pathogen is discovered in a species and it’s unclear what it means in terms of the disease moving through a population, he says. “In some cases, circumstances change and the disease takes hold and causes a problem and other times it doesn’t,” he says.

For some seals, determining a cause of death is impossible since they are often highly decomposed when discovered.

“If we’re not there within 24 to 48 hours, the insides of the seal are soup,” Littnan says.
But even when his team confirms the presence of T. gondii, that doesn’t necessarily indicate an infection, let alone a fatal one. So, in addition to hunting for the pathogen, his team looks for signs, such as swelling of the brain, lymph nodes, or lungs, that are typical of toxoplasmosis.

Hoku, an up-and-coming dominant male, died of “severe meningoencephalitis caused by a protozoan,” Littnan says. Although toxoplasmosis is suspected, it has not yet been confirmed as the culprit. He notes that only Hoku’s brain was inflamed but not other organs that toxoplasmosis has a predilection for. This suggests that the parasite was “inactive for a long period,” he says, “but some event, such as immune suppression, may have led to activation. At this point, the findings are speculative until all the results come back.”

Hoku visited an area known as Salt Ponds four times in the six months before he died. There are well-known, and well-fed, feral cat populations there and at Kalaupapa National Historic Park. But Littnan says seals “move across a pretty large range. It’s hard to trace it back to where they were likely exposed.”

Throughout most of Hawai`I, water quality in surface waters ranges from “slightly impaired to severely impaired” by pathogens and pollutants, according to a state assessment. In particular, runoff from densely populated watersheds on Maui and O`ahu likely contains pathogens, according to the state’s polluted runoff control implementation plan.

Sewage is also a significant source of pathogens as tropical storms overwhelm aging transmission pipes and inadequate treatment systems. What’s more, injection wells on Maui have created giant wastewater plumes at some popular beaches.

Cat feces also contaminate livestock. A study of pig farms on O`ahu found that nearly half of more than 500 pigs tested positive for toxoplasmosis. Most rivers that flow to the ocean traverse through an agricultural site, according to a 2006 EcoHealth article by Littnan and researchers from NOAA and the Hubbs-SeaWorld Research Institute.

Although the state doesn’t test for toxoplasmosis, the Natural Resources Defense Council’s Testing the Waters 2010 report provides some insight into the role runoff plays in pathogen transmission in Hawai`i. Storm water was responsible for 99 percent of beach closures/advisories in 2009. Sewage spills accounted for the remaining one percent. Kaua`i beaches exceeded daily maximum bacterial standards most often.

According to the EcoHealth article, nearly 30 million gallons of sewage were spilled between 2000 and 2004. That number was exceeded in a matter of days in 2005, when a broken main caused the city of Honolulu to divert 48 million gallons of raw sewage into Waikiki’s Ala Wai canal.

In addition to toxoplasmosis, other pathogens have infected monk seals.

A few years ago, Littnan, NOAA contract veterinarian Robert Braun, and Brent Stewart and Pamela Yochem of the Hubbs-SeaWorld Research Institute tested seals for pathogens while trying to assess the disease threat.Their results, published in EcoHealth, suggest that seals encounter a variety of pathogens, including Sarcocystis neurona and Neospora caninum, as well as T. gondii. S. neurona causes equine protozoal myeloencephalitis, which creates lesions on the spinal cord and brain stem. Neosporosis can cause abortions in cattle and neuromuscular degeneration in dogs.

Leptospirosis is among the biggest concerns. Suspected in the death of two pups born near a stream mouth on the island of Hawai`I, it is mainly transmitted through contact with surface water contaminated with infected rodent or mongoose urine.

Although there is limited data on leptospirosis prevalence in Hawai`i, it’s considered ubiquitous. The reported incidence rate among people in Hawai`i (1.29 cases per 100,000 persons) is about 30 times the national rate, with the highest rates on Kaua`i and Hawai`i islands.

In another effort to gauge threats, Hawai`i Pacific University graduate student Jessica Lopez is evaluating 77 industrial chemicals and pesticides found in about 60 monk seals over the past decade.

“We don't know anything about what levels can cause an effect in monk seals, whether lethal or sub-lethal, so it will be difficult to speak to whether the levels measured are ‘safe’ or not,” she says.

Lopez also plans to evaluate seal movements and locations of sewage outflows and agricultural and industrial complexes to determine high risk areas that may factor into decisions on managing the seals.

For Littnan, filling data gaps is his top priority, such as tracking the mother of January’s stillborn pup to see if she has signs of infection.

“It would be very interesting to learn more about seals that [test positive for toxoplasmosis] but are not showing any clinical signs,” he said.

If the mother tests positive, drugs might be available for treatment. NOAA is partnering with California’s Marine Mammal Center to build a monk seal hospital in Kona.

To raise public awareness about toxoplasmosis and cat feces, NOAA has begun talking with the Hawai`i Humane Society, various interest groups and the health department.

But given his experience with Hawaiian birds, Work of the U.S. Geological Survey says generating the political will to control cats is “very difficult.”

“The `alala is a classic case in point,” Work says. Ten years ago, toxoplasmosis was identified as a threat at a national wildlife refuge in Kona that was created for the birds. Yet today, the cats remain at the refuge, while the `alala have been extirpated from the area.

Scientists are concerned about the role the disease may play in the seals’ recovery.

Under NOAA’s recovery plan, the population must grow from the current 1,100 to a minimum of 2,900 before it can be downlisted to threatened. In a recent article, NOAA scientists stated that “fishery interactions, direct killing, and disease could rapidly undo the current fragile positive trend” on the main islands.

“The greatest need for seals right now is fostering the co-existence of seals and people. That’s the most immediate threat to population growth in the main Hawaiian islands,” Littnan says. Compounding the threat, “toxo and lepto will be there and will probably continue to operate in a background level.”

Share this Article:

Comments

You must sign in or register as a ScientificAmerican.com member to submit a comment.
Scientific American Dinosaurs

Get Total Access to our Digital Anthology

1,200 Articles

Order Now - Just $39! >

X

Email this Article

X