By Vidya Rajan, Columnist, The Times
I am in India. It’s a vibrant and bustling place. This month, a couple of friends and I are planning a trip to a wildlife sanctuary in Southern India*. We hope to see tigers and a plethora of other animals and plants.
What I am hoping we do not encounter closely are mosquitoes and snakes. I do not like mosquitoes, but I do like snakes, in principle. But many snakes here are venomous and getting bitten is not desirable. Luckily snakes are shy and do not seek out human company, which is just fine with me.
I was pondering the terms venomous and poisonous, and snakes seemed like a good place to start with differentiating between the two words. Snakes are venomous, not poisonous. Venom is injected into the body to cause distress, whereas poisons are ingested. That is straightforward and easy to remember. Being stung by bees is a good reminder that venom can be injected by teeth, claws, and stingers, all from different pointy ends of the animal. I saw a nifty description of the difference between poisons and venoms: “If you bite it and you die, it’s poisonous. If it bites you and you die, it’s venomous” [1]. But it leaves out toxins which are a big player in the world of nasty ways to die.
Let’s start with toxins. Toxins are biological secretions that cause an intoxication. Poisons and venoms are both toxins because they are made by living organisms, but toxins are also virulence factors made by microorganisms. Some infamous toxins are cholera toxin, tetanus toxin, and staphylococcus toxins which range from those that cause scarlet fever to toxic shock syndrome to causing “flesh-eating” symptoms.
Virulence is the factor that causes some pathogens to be more invasive and efficient at causing disease than others. Virulence factors compromise the host, making it more susceptible to the pathogen. Toxins compromise cell or tissue integrity and cause death indirectly. For example, cholera toxin causes the intestinal lumen to secrete cAMP and causes an efflux of chloride ions into the intestinal lumen. The chloride ions draw out water from the cells, causing a copious and watery diarrhea, which has the additional effect of spreading the bacterium to new hosts through fecal contamination of water sources. John Snow first identified the Broad Street sump pump as the source of a massive cholera outbreak in London due to the contamination of a water well by a sewage pipe [2]. Gram-positive bacteria such as diphtheria, tetanus, staphylococcus, and botulinum toxins are made by sporulating bacteria which kill the host and then grow in their bodies, which thus become foci of dissemination of the pathogen into the environment such as in the air or soil. Enteropathogens are usually gram-negative bacteria which generally produce toxins that cause diarrhea that cause the pathogen to wash into food and water sources which can be ingested by a new host. Another subtle way of distinguishing toxins is whether they cause an infection (by facilitating growth and dissemination of infectious bacteria within or outside the original host) or intoxication (by actual toxin production which causes death)[3]. Curiously, many “bacterial” toxins, like diphtheria, scarlet fever, and cholera toxin are actually due to the presence of a toxin-carrying virus that integrates into the bacterial chromosome. Bacteria cured of the virus are innocuous. Toxins make terrific immunogenic molecules for producing vaccines, and vaccines against cholera, botulinum, tetanus, and diphtheria are directed against their toxins.
Venoms are injected or otherwise inserted actively into the host. These are generally by biting, clawing, and stinging. The common feature of venoms is the ability to compromise tissue and cellular integrity due to the presence of one or more or four classes of compounds: neurotoxins, necrotoxins/cytotoxins, myotoxins, and hemotoxins. As can be inferred from their names, neurotoxins damage nerves, necrotoxins or cytotoxins cause cell death, myotoxins affect muscles, and hemotoxins lyse blood cells. Some snakes, Gila lizards, salamanders, stonefish, stinging insects, 1000 species of cone snails [4], and biting spiders are venomous. Surprisingly, some mammals are also venomous. Moles, shrews and solenodons have venomous saliva which they use to capture prey. The male platypus develops a crural spur on the hind legs with which to battle other males for mates. Some venomous animals transfer venom from a different part of their body to their teeth or spines or fur. For example, slow lorises are tiny primates whose venom gland is located in their elbow. When threatened, they cutely raise their arms and lick their elbows, but they are actually transferring venom to their mouth to bite their tormentor. Poisons can also be co-opted as venom for protection. European hedgehogs lick the poisons produced by toads and spread it on their spines, which can be deployed to transfer the toxins in to an attacker’s body, which is technically the definition of a venom.
Poisons lurk everywhere in quantities great and small. I have written about plant and animal poisons before (see newsletters from September 2020 and January 2021). Given that I mentioned I am in India, specifically in the city of Chennai, I will say here that Chennai is home to the first dedicated reptile park in India. It was established by the American-born naturalized Indian, Romulus Whitaker, and houses venomous and non-venomous Indian snakes as well as all three types of Indian crocodiles (marsh crocodile, saltwater crocodile, and gharials), lizards, turtles, chameleons, and geckos. There are over 2000 snake species in the world; 287 live in India – 87 of them classified as venomous. Probably that is too many to describe in this article. But the big four are: the common krait, the Russel’s viper, the saw-scaled viper, and the common cobra. Snake venoms are made of three major components: neurotoxins (kraits, cobras), hemotoxins (vipers), and cytotoxins (cobras).
But India has a complex culture with animistic underpinnings, where animals are worshipped at part of nature and as manifestations of gods. I will leave you with this extraordinary story of a village – Shetpal village in Maharashtra state –
where snakes are welcomed into the family – not just any old snake, but venomous cobras. Curiously and amazingly, there is no record of people being bitten by these normally trigger-happy venomous snakes. Watch the videos below and wonder. I did.
* Due to covid, our trip to the wildlife sanctuaries was cancelled.
References:
- @TheFactSite. What’s The Difference Between Venom, Poison & Toxins? – The Fact Site. 2020 2020-03-27; Available from: https://www.thefactsite.com/differences-between-venom-poison-and-toxins/.
- Outbreak, W.B.S.C. 1854 Broad Street cholera outbreak – Wikipedia. 2022; Available from: https://en.wikipedia.org/wiki/1854_Broad_Street_cholera_outbreak.
- Schmitt CK, M.K., O’Brien AD, Bacterial Toxins: Friends or Foes? – Volume 5, Number 2—April 1999 – Emerging Infectious Diseases journal – CDC. 1999, Centers for Disease Control and Prevention: Emerging Infectious Diseases.
- Wu, K.J., Cone Snails Are Liars and Murderers. 2021. https://www.theatlantic.com/science/archive/2021/03/cone-snail-venom/618270/
