Do you like eating fish? Well so do some leeches. Most people only know about the leeches that feed on humans, but there are also numerous other species of leeches out there, and many of them are found in the sea. As the saying goes, "there are plenty of fish in the sea", so they are often the target of these leeches' appetites. And if you're going to be farming fish in the sea, there's a chance that you have inadvertently opened up an all-you-can-eat buffet for some hungry leeches.
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| Top left: Limnotrachelobdella okae attached to the base of a fish's dorsal fin, Top centre: a leech attached to a fish's tail fin, Top right: a lesion caused by the leech's feeding. Bottom: A mature Limnotrachelobdella okae leech Photos from Fig. 2 and Fig. 3 of the paper. |
The Mi-iuy Croaker (Miichthys miiuy) or 鮸鱼 is a popular food fish in China, where it is considered to be not just tasty and highly nutritious, but may also have medicinal properties. Because of the high demand for this fish, it is farmed in sea cages to ensure continuous supply without threatening wild stocks. But as with farming on land, having a lot of animals in a single place makes them extremely vulnerable to parasites.
In January 2025, there was an outbreak of marine leeches at a croaker farm off the coast of Raoping County in the Guangdong Province. Fish began dying off at the start of January, with fish mortality peaking towards the end of month, before tapering off over the course of February and March. At its peak, the sea cages that housed the croakers were swarming with leeches, and over a hundred thousand kilograms of fish were killed by their feasting.
So what exactly were the slithery killers that dined and dashed at that fish farm? Researchers were able to identify those thirsty blood-suckers as a species of leech called Limnotrachelobdella okae. It belongs to the Piscicolidae family - a group of leeches that mostly specialise in parasitising fish. When the leeches entered the sea cage, they just grab onto whichever part of a fish and start sucking, but they mostly focused on the part of the body which had thin skin and plenty of blood vessels. This included the fins, mouth, the edge of the gill covers, the tail, and the belly of the fish. Limnotrachelobdella okae are big leeches, reaching 12-15 cm in length, and having just a few of those big suckers on a fish would render it anaemic in no time.
When the researchers examined the parasitised fish, they found that not only did these fish have severely depleted blood cells as result of the leech's ravenous appetite, they also suffered internal injuries to their internal organs including their liver, spleen, and kidneys. This is due to depleted blood flow to those organs, because under anaemic conditions, blood flow is preferentially directed to organs with high oxygen demands such as the heart and the brain, and this comes at the expense of the other organs. And unlike our kidneys, the "head kidney" in fish also produces new blood cells, rather like our bone marrow. So the reduction of blood flow into the fish's head kidney also reduces its capacity to replenish that lost blood.
So why did the outbreak happen when it did? Well, it might have something to do with leech's temperature preference. Limnotrachelobdella okae seems to prefer water at a chilly 5-10°C. When the researchers kept some of the leeches in aquariums set at different levels of salinity and temperature, they found that the leeches thrived in cold, salty water, but if it gets above 20°C they start dying in droves. This might explain why the swarm of leeches suddenly appeared in January, since that is the coldest month in China. As the year progressed and the weather got progressively warmer, the number of leeches declined and their threat ended once the water temperature reached 20.5°C in April.
Parasites are just like any other organisms in the environment, their activities are deeply tied with climatic conditions. So understanding parasite ecology can help us better prepare for their emergence. Because sometimes winter doesn't just bring cold wind and snow, it could also be bringing leeches in tow.
Reference:
Che, S., Mo, Z., Zhang, H., Tang, H., Dan, X., & Li, Y. (2026). First report of marine leech Limnotrachelobdella okae infestation in miiuy croaker (Miichthys miiuy): an emerging threat to Chinese mariculture. Veterinary Parasitology, 341:110637.
