How dangerous is it to swim in the Amazon river?

Question

I have a long term dream of visiting the Amazon rainforest. I have been trying to determine how dangerous it is to swim in the Amazon river. I realise it is a large river, with various side channels - but broadly speaking, how dangerous is it to swim in it? My preliminary research suggests that I do not need to be overly concerned by caimans (there are apparently almost(?) no confirmed human fatalities) or piranhas (despite the perception to the contrary created by various media), and that the possibility of some parasitic fish swimming up, ahem, an uncomfortable area is largely an urban legend. I have not however been able to find good statistics, and I cannot exclude other possible dangers. e.g. how about large pythons?

So, apart from the danger of drowning due to currents, how dangerous is swimming in the Amazon? I understand the greatest danger, aside from drowning, to be various infectious bacteria, protists, etc. Can anyone quantify this danger in a meaningful way?

Answer 1

There's no way to answer this question without asking another - where on the Amazon do you intend to swim?

This is the largest river system in the world, with the greatest outflow of any river - so big that it's bigger than the next 6 largest combined! It is generally accepted that it is the second longest. It starts somewhere deep in the Andes in the Mantaro River, which has a very steep gradient - 5 m/km, meaning that it will have rapids/white water, it then descends through a range of river systems into lowland forest and finally out into the Atlantic Ocean. It has a wide range of biomes from untouched forest, to alpine zones, to big cities to a delta.

This means there are a huge range of places that you could potentially swim - how safe each of those is, is very very dependent on where it is. Generally lowland rivers will be relatively safe to swim in (assuming no flooding and a competent swimmer), so long as the swimmer stays aware of hazards in the water - tangling vines/roots, floating or submerged branches/trees, rocks, eddies etc.

Swimming downstream of industrialized cities and high intensity agriculture is relatively risky in terms of infection from bacteria and other pathogens, as well as pollution, but the risks are fairly low still, unless you ingest the water or have open wounds that could get infected.

There are guided tours on the Amazon to see things like the Amazon River Dolphin, some of which apparently will let people swim with them. Based on this, it's probably safe to swim in those areas, but like any river with wild-life there are no guarantees.

Answer 2

If you are worried about wildlife, not very dangerous. Swim with other people and from a boat rather than from shore. Biggest threat is probably drowning. The major threats:

1. Sting Rays – Purported by Smithsonian Zoo to inflict most injuries to people in Amazonian rivers. Considered docile, but will sting if stepped on as they bury themselves in the sand to hunt for prey. Bacterial infection of wound may led to death. The number of injuries per year is unknown because often unreported or medical attention is sought out well after the fact. This study in the Brazilian Amazon used 84 injuries over a 3 year period. Can be avoided by swimming from a boat, rather than entering water from shore. Haddad Jr, V., Neto, D. G., de Paula Neto, J. B., de Luna Marques, F. P., & Barbaro, K. C. (2004). Freshwater stingrays: study of epidemiologic, clinic and therapeutic aspects based on 84 envenomings in humans and some enzymatic activities of the venom. Toxicon, 43(3), 287-294.

2. Electric Eels – These snake-like fish stun prey and scare off predators with about 600v shock, which is enough to knock a person unconscious. Again, very few reported interactions with people, as above, they tend to avoid them. They hunt in shallow water, so again, swimming off a boat will reduce your risk of interaction. https://nationalzoo.si.edu/animals/news/its-electric-fishes

3. Caiman – There are several species of this relative to the alligator in the Amazon. They can range in size from about 1-6m, making most too small to be a real threat to people. The largest species, the black caiman, has been known to (rarely) prey upon humans. People (and this is well-documented) hunt caiman for skins and meat, so they tend to avoid people. Sideleau, B., & Britton, A. R. C. (2012, May). A preliminary analysis of worldwide crocodilian attacks. In Crocodiles: Proceedings of the 21st Working Meeting of the IUCN–SSC Crocodile Specialist Group (pp. 22-25).

4. Piranha – Most reports of piranha attacks on people are single bites as part of defending young or territories. But they tend to hunt near shore, so again swim from a boat. Haddad Jr, V., & Sazima, I. (2003). Piranha attacks on humans in southeast Brazil: epidemiology, natural history, and clinical treatment, with description of a bite outbreak. Wilderness & Environmental Medicine, 14(4), 249-254.

5. Candiru- This is a parasitic catfish that attaches to the gills of larger fish with spines and feeds on their blood. Although there is no actual evidence of them ever bothering people, urban legends suggest that they are attracted to human urine and will swim up the urethra and use their spines to attach to the bladder. Bauer, I. L. (2013). Candiru—a little fish with bad habits: need travel health professionals worry? A review. Journal of travel medicine, 20(2), 119-124.

6. Anaconda- There are 4 species of these tropical swimming snakes in the Boa genus. They are mostly active in the evenings and can move quickly in the water, but are slow on land. They also tend to hang out in shallow water to hunt, so again, swimming off a boat is good practice. They are non-venomous and kill their prey by wrapping around them and squeezing, cutting off circulation to the brain, then swallowing prey whole. They are probably physically able to kill humans, but attacks on humans are rare, and there are no substantiated reports of an anaconda killing a person. Rivas, J. A. (1998). Predatory attacks of green anacondas. Eunectes murinus, 157-159.

Answer 3

The danger specifically of piranhas depends on the time of year. Around Manaus the water is high during the rainy season (October to March) and it may be ok to swim (bearing in mind bob1's answer). But during the dry season piranha's, especially in left over pools, may be hungry and dangerous. https://piranhaguide.com/a-documented-list-of-all-known-piranha-attacks-piranha-victims/

Caiman will mostly try to avoid encounters with humans.

I swam in the Amazon river once, if i ever get another opportunity i will bring a mask.

Answer 4

I would just like to point out the one modern report of an attack on a human male by the candiru fish is highly likely to be spurious for a multitude of reasons, not least of which is the impossibility of such an attack from the perspective of simple fluid physics. From the Wikipedia article on the candiru fish:

Modern cases

To date, there is only one documented case of a candiru entering a human urethra, which took place in Itacoatiara, Brazil, in 1997. In this incident, the victim (a 23-year-old man known only as "F.B.C.") claimed a candiru "jumped" from the water into his urethra as he urinated while thigh-deep in a river. After traveling to Manaus on October 28, 1997, the victim underwent a two-hour urological surgery by Dr. Anoar Samad to remove the fish from his body.

In 1999, American marine biologist Stephen Spotte traveled to Brazil to investigate this particular incident in detail. He recounts the events of his investigation in his book Candiru: Life and Legend of the Bloodsucking Catfishes. Spotte met Dr. Samad in person and interviewed him at his practice and home. Samad gave him photos, the original VHS tape of the cystoscopy procedure, and the actual fish's body preserved in formalin as his donation to the INPA. Spotte and his colleague Paulo Petry took these materials and examined them at the INPA, comparing them with Samad's formal paper. While Spotte did not overtly express any conclusions as to the veracity of the incident, he did remark on several observations that were suspicious about the claims of the patient and/or Samad himself.

According to Samad, the patient claimed "the fish had darted out of the water, up the urine stream, and into his urethra." While this is the most popularly known legendary trait of the candiru, according to Spotte it has been known conclusively to be a myth for more than a century, as it is impossible because of simple fluid physics.

The documentation and specimen provided indicate a fish that was 133.5 mm in length and had a head with a diameter of 11.5 mm. This would have required significant force to pry the urethra open to this extent. The candiru has no appendages or other apparatus that would have been necessary to accomplish this, and if it were leaping out of the water as the patient claimed, it would not have had sufficient leverage to force its way inside.

Samad's paper claims the fish must have been attracted by the urine. This belief about the fish has been held for centuries, but was discredited in 2001. While this was merely speculation on Samad's part based on the prevailing scientific knowledge at the time, it somewhat erodes the patient's story by eliminating the motivation for the fish to have attacked him in the first place.

Samad claimed the fish had "chewed" its way through the ventral wall of the urethra into the patient's scrotum. Spotte notes that the candiru does not possess the right teeth or strong enough dentition to have been capable of this.

Samad claimed he had to snip the candiru's grasping spikes off in order to extract it, yet the specimen provided had all its spikes intact.

The cystoscopy video depicts traveling into a tubular space (presumed to be the patient's urethra) containing the fish's carcass and then pulling it out backwards through the urethral opening, something that would have been almost impossible with the fish's spikes intact.

When subsequently interviewed, Spotte stated that even if a person were to urinate while "submerged in a stream where candiru live", the odds of that person being attacked by candiru are "(a)bout the same as being struck by lightning while simultaneously being eaten by a shark."

Answer 5

Wear an armor suit. Look up Sobral Santos, 1981. It capsized while docking at Obidos, 2000 miles up river. No good records ; about 500 passengers , about 300 died ( the boat was docking - does not sound like a long swim). Almost no bodies were recovered ,Those that were had been partly eaten so no definitive information regarding how many drowned and how many were killed by water creatures. A small fish ,candiru . about 1" diameter and 5" is suspected of doing the most damage. BUT, on the other hand author Heiko Bleher ( the man that wrote the book on Discus fish) has searched the Amazon basin for aquarium fishes since he was 7 years old ( now an old guy). So if you know what you are doing and what fish may be present, it is possible to go into the water - generally he was snorkeling.