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.