Horseshoe crab eggs in Thailand: tetrodotoxin, no antidote, and why to skip them

Two kinds of horseshoe crab live in the Gulf of Thailand. One, the mangrove horseshoe crab, called maeng da thuai (แมงดาถ้วย) in Thai, carries tetrodotoxin (TTX) in its eggs and meat, the same toxin found in pufferfish. Boiling, grilling or tossing it in a salad does not remove it, and there is no antidote. The other, maeng da chan (แมงดาจาน), is the one people mean to eat, but the two are very hard to tell apart. So this article takes the cautious line: the safest choice is not to eat horseshoe crab eggs at all.

Horseshoe crab eggs are a known dish in several coastal provinces, as a spicy salad (yam khai maeng da) and in an old-style sticky rice sweet. The point here is not to erase those stories, but to show what the risk is, what the real numbers look like, and what to do if someone has already eaten them.

Two species that look different, but are hard to separate

Thailand’s Department of Disease Control (DDC) describes the two species like this:

	Maeng da thuai	Maeng da chan
Scientific name	Carcinoscorpius rotundicauda	Tachypleus gigas
Size	Smaller	Bigger
Tail	Round, smooth	Triangular, serrated
Toxin	TTX in eggs and meat	Described by the DDC as edible

It sounds as if checking the tail settles it. But the DDC adds plainly that the two are very difficult to differentiate, and once the eggs are scooped out or made into a salad, a diner never sees a tail at all. A physician’s study from Chon Buri Hospital lists the belief that people can tell poisonous from edible crabs among the pieces of folklore that make prevention hard.

A crab that was safe to eat last time is not proof the next one will be. The Chon Buri study calls that belief inaccurate.

What TTX does in the body

TTX blocks the sodium channels in nerves, so nerve signals stop getting through. The result is numbness, then weakness, and with enough toxin the muscles used for breathing stop working.

The DDC says symptoms can start anywhere from 10-45 minutes to 3 hours after eating, depending on the animal, the season and the dose. The eggs cause more severe illness than the meat. Symptoms include:

• numb lips and tongue, spreading to the hands and feet
• dizziness, nausea, vomiting
• heavy sweating, drooling, slurred speech, blurred vision, a floating feeling
• in severe cases, paralysis of the breathing muscles, respiratory arrest, and death within 6-24 hours

TTX is heat stable, so cooking does not help, and the StatPearls clinical review states there is no known antidote. Hospitals give supportive care, keeping breathing and blood pressure going until the body clears the toxin.

Real numbers from Chon Buri Hospital

The largest case series on this comes from Chon Buri Hospital, covering 1994 to 2006. In that time 280 people were admitted after eating the eggs of C. rotundicauda; 245 medical records were available. Walk through them step by step:

1. Of 245 patients, 240 had numb lips and tongue: 240 ÷ 245 = 98%. It was the first sign in almost every case.
2. Stage 4, the most severe stage with respiratory failure: 68 patients, 68 ÷ 245 ≈ 28%, nearly one in three.
3. 61 patients had total paralysis and needed a ventilator. Most came off it in 12-24 hours; severe cases needed 48-72 hours.
4. 239 recovered fully (97.5%), 5 died (2%), and 1 had brain damage from lack of oxygen.

A 97.5% recovery rate sounds reassuring, but it describes people who reached hospital in time and got a ventilator. Far from a hospital, or after waiting too long at home, the outcome could be very different.

The DDC’s own surveillance for 2016-2020 shows how serious it can be: 13 TTX poisoning incidents from marine animals over five years, at least one every year, 6 from horseshoe crab and 7 from pufferfish. Of 37 patients, 11 died, or 29.7%, far higher than in the Chon Buri series.

When it happens most

The Chon Buri cases peaked from December through March. The DDC warns that the cool season is when people in coastal villages go out gathering seafood, so sporadic cases are expected then. If you are at the coast late in the year and see horseshoe crab eggs on a menu, that is all the more reason to pass.

If someone has already eaten them

• Any numbness, even slight, in the lips, tongue, hands or feet: call 1669 (Thailand’s emergency medical number) or go to hospital now. Do not drive yourself.
• Tell the doctor exactly what was eaten: horseshoe crab eggs or other seafood, when, and how much. TTX poisoning is diagnosed mainly from symptoms, so the history matters a great deal.
• Others who shared the dish should watch for symptoms and go to hospital if numbness starts.
• More advice: DDC hotline 1422 and the Ramathibodi Poison Center, 1367.

The Chon Buri physician recommends admitting all but the mildest cases for observation, and notes that after 24 hours without severe effects, life-threatening problems become unlikely.

Numb lips after eating seafood means go to hospital now, not wait and see.

Common mistakes

The Chon Buri study names four beliefs that keep people getting sick every year. All four are wrong.

• “You can tell the toxic crab from the edible one.” Not once it is just eggs or a cooked dish, and even whole crabs get mixed up.
• “A crab that was fine before is always safe.” Not true.
• “Cooking destroys the toxin.” TTX survives heat.
• “Non-toxic eggs burst when heated.” This does not work as a test.
• Waiting it out at home. Numbness can turn into paralysis within hours. Waiting risks respiratory arrest.

Summary

The mangrove horseshoe crab carries TTX in its eggs and meat. Heat does not destroy it, there is no antidote, and it is hard to tell from the edible species, especially once it is on a plate. The most cautious advice is not to eat horseshoe crab eggs. If someone has eaten them and their lips or tongue go numb, call 1669 straight away. For general street food safety, see Thai street food safety: what to look at before you order.

Sources

• Weekly Disease Forecast No.282: Tetrodotoxin Poisoning after Consumption of Poisonous Marine Animals — Department of Disease Control, Thailand
• Tetrodotoxin poisoning following ingestion of the toxic eggs of the horseshoe crab Carcinoscorpius rotundicauda, 1994-2006 — Southeast Asian J Trop Med Public Health
• Tetrodotoxin Toxicity (StatPearls) — Europe PMC