Off the coast of Oʻahu, a Hawaiian monk seal rests in shallow water — one of around 1,600 remaining statewide. Nearby, a pod of spinner dolphins drifts through a bay. On land, kilometers away, a free-roaming cat moves through a watershed. What connects them is invisible — but it is real.
Simple, everyday choices can reduce a tiny parasite's journey from land to sea — and help protect Hawaiian monk seals, spinner dolphins, and the ocean they call home.
ʻĪlioholoikauaua · nai'a · endangered wildlife of HawaiʻiA common parasite with an uncommon pathway — and one that matters deeply for Hawaiʻi's ecosystems.
Toxoplasma gondii is a single-celled parasite that infects an estimated one-third of the global human population. For most healthy adults, it causes no symptoms at all — the immune system keeps it quietly in check.
However, it matters a great deal for two groups: people who are pregnant (where it can affect fetal development) and people with weakened immune systems (where a dormant infection can reactivate and become serious).
In humans, T. gondii is primarily transmitted through eating undercooked or raw meat containing tissue cysts (especially pork, lamb, and venison), and through consuming unwashed raw produce or water contaminated with oocysts. Direct contact with cats is far less common as a route of infection than most people assume — and an indoor cat that doesn't hunt is very unlikely to be a source at all.
In wildlife — especially marine mammals — T. gondii can be fatal. Hawaiian monk seals and spinner dolphins face elevated risk from this land-based parasite, and confirmed deaths from toxoplasmosis have been documented in both species in Hawaiʻi. Because only about 5% of marine mammal carcasses are ever recovered, the true scale of impact is likely higher than recorded deaths alone suggest.
Cats are the only animals in which T. gondii can sexually reproduce — making them the parasite's sole definitive host. This sexual reproduction happens in a cat's intestine and produces oocysts that are shed in feces. Every other warm-blooded animal, including humans, monk seals, and dolphins, can be infected but cannot complete this reproductive stage. These oocysts are extraordinarily tough — they survive in soil and water for months, even years.
A large share of the world's population has been infected. Most never know it — healthy immune systems contain it silently.
A first-time infection during pregnancy can affect the fetus. Simple precautions dramatically reduce risk.
Undercooked meat and unwashed produce are also major sources of human infection — often bigger than direct cat contact.
During a first infection, a cat sheds millions of oocysts daily for about 1–2 weeks. Most adult cats have already had their one shedding period and won't shed again.
Once in the environment, oocysts become infectious within a few days. Hawaiʻi's warm, wet climate is ideal for their survival — months to years in moist soil.
Rain washes oocysts from soil into streams and storm drains. Watersheds act as highways — delivering land-based contamination to the coast.
Oocysts concentrate in nearshore coastal waters. Filter feeders and marine mammals ingest them while feeding — with no natural immunity to protect them.
In Hawaiian monk seals and spinner dolphins, T. gondii can cause encephalitis and death. Multiple confirmed deaths have been documented in both species in Hawaiʻi. Because only ~5% of marine mammal carcasses are recovered, the true number of deaths is likely higher than confirmed counts alone (NOAA; Landrau-Giovannetti et al., 2022).
Hawaiʻi's islands are uniquely vulnerable — short watersheds, heavy rainfall, large free-roaming cat populations, and endemic wildlife that face elevated risk from this land-based parasite.
Every time it rains on Oʻahu's windward side, water runs downhill — through neighborhoods, across areas where free-roaming cats live, picking up whatever is in the soil. By the time that water reaches Kāneʻohe Bay or the coastline near Waimānalo, it carries with it whatever the watershed collected. Toxoplasma gondii is one of those passengers. Hawaiian monk seals, spinner dolphins, and native waterbirds like the ʻalae keokeo don't choose to swim near storm drain outlets. They simply live there.
Critically endangered, found only in Hawaiʻi, with around 1,600 individuals statewide (NOAA). Multiple deaths from T. gondii encephalitis have been documented since 2001. Because carcass recovery rates are low, confirmed deaths likely undercount the true toll.
Rests in shallow bays during the day after feeding at night — exactly the nearshore zones where watershed runoff concentrates. Confirmed deaths from toxoplasmosis have been documented in Hawaiian spinner dolphins (Landrau-Giovannetti et al., 2022), and NOAA notes total deaths are likely higher than recovered carcasses reflect.
The ʻalae keokeo (Hawaiian coot), nēnē, and other native waterbirds feed in wetlands and ponds connected to watershed systems — making them vulnerable to the same runoff pathways.
Unlike continental coasts, Hawaiʻi's watersheds are short and steep — rainfall reaches the ocean quickly, with little time or distance for filtration. What happens on land matters here more than almost anywhere else.
Transmission pathway · land to sea
Sheds oocysts in feces near watershed
Oocysts persist for months in warm, moist soil
Rain washes oocysts into streams & storm drains
Oocysts concentrate in nearshore water
Wildlife ingests oocysts while feeding
This isn't about blame — it's about small habits that make a real difference. Cats are wonderful companions. These simple changes protect wildlife without changing that.
Indoor cats don't hunt, so they're rarely exposed to T. gondii in the first place. No exposure = no oocyst shedding into the environment.
Highest impactFreshly deposited feces aren't immediately infectious — oocysts need 1–5 days to become infective. Daily cleaning removes them before that happens.
High impactFlushing litter puts oocysts directly into the water system. Sewage treatment doesn't reliably remove them. Bag and trash instead.
High impactCats become infected by eating infected prey or raw meat. Keeping them on commercial diets eliminates the main route of primary infection.
High impactSoil near areas where free-roaming cats live may contain oocysts. Gloves and handwashing after gardening protect you and keep oocysts from spreading.
Protective habitA catio (cat patio) is an enclosed outdoor space — attached to a door or window — that lets your cat enjoy fresh air and outdoor enrichment safely, without the risks of roaming. It's a win for your cat and for local wildlife.
Enrichment + safetyLocal and regional research — including this study — paints a clear picture of why Hawaiʻi requires its own response to this global issue.
Fecal, soil, and water samples collected along a Hawaiʻi watershed, tested by PCR and confirmed by DNA sequencing.
Water samples showed higher positivity than feces or soil — consistent with the hypothesis that watershed runoff concentrates and transports oocysts from land to coastal environments.
How oocysts move · land to sea · illustrated
T. gondii oocysts travel from free-roaming cat territories through watershed runoff into nearshore coastal waters where monk seals and spinner dolphins feed and rest
You love your cat. You love the ocean. These two things are not in conflict — a few simple habits connect both.
I'm a student researcher from Hawaiʻi with a deep interest in the connections between land and sea — and the invisible ways human choices ripple through ecosystems. This project grew from a simple question: can we trace Toxoplasma gondii from free-roaming cats through our watershed system into coastal waters?
The data says yes. And the encouraging part is that meaningful change doesn't require anything dramatic — just awareness, and a few small shifts in how we care for our cats and our land.