Venom

Simply put, snake venoms are slightly complicated cocktails that have been tailored by natural selection to maximize the capacity of a venomous snake to successfully immobilize and digest its prey. In other words, it is thanks to venom that venomous snakes can feed, hence, it is considered to be a very significant evolutionary innovation. Snake venom consists of "peptides" and "proteins", often called "toxins", referring to nothing more than smaller (peptides) and bigger molecules (proteins) that are made up of amino acids. Snake venom is not as abstract as people think and one can comprehend it fairly easily. The venom of a given individual is composed of a few tens to hundreds of different toxins and compounds. Each of these toxins make up a specific percentage of the venom. The most prevalent of them within the venom are the most important. For the Milos viper (Macrovipera schweizeri) these are the following: svMP (snake venom MetalloProteinase), PLA2 (PhosphoLipase A2), svSP (snake venom Serine Protease), CTL (C-Type Lectins), DI (DIsintegrins ) and LAAO (L-Amino Acid Oxidase). These names are of no significance to the average reader, what matters is that the biggest percentage is monopolized by only a few toxins. This allows us to develop effective antivenoms, targeting the most prevalent ones and thus neutralizing the venom efficiently. 

Some confusion also exists around the words used to describe the effects of different snake venoms; being "neurotoxic", "haemotoxic", "cytotoxic", "cardiotoxic" and so on. However, we try to keep things to a level that are still interesting and steer clear of being absolute. As such we like to say that a particular snake species has venom that is predominantly "haemotoxic" and/or "neurotoxic" thus not excluding other toxin families. As one can infer from the aforementioned, different categories mean that one can expect specific, and even combined, symptomatology depending on the toxin families a snake venom predominantly has. In reality it is tough to say that all individuals of a specific species have venom that falls within a single category. This is particularly true in the case of a species that has a very wide distribution range both horizontally and vertically as this means that its different populations inhabit different habitats, have a different diet and have faced different evolutionary pressures. This is probably not the case for the Milos viper as it inhabits only four and close to each other West Cyclades islands (Milos, Kimolos, Polyaigos and Sifnos) with fairly similar habitats and pressures. Additionally,  most of the venom analysis tends to be biased against venom collected from a specific population, usually the one where individuals are easier to find. To make matters worse, it is often the practice to get venom directly from snake farms that have kept individuals in captivity for generations. Captivity is also known to have an effect on the venom composition of an individual as its diet drastically changes from a variety of free ranging live prey items usually to a single frozen-thawed prey item. To tackle this issue we have collected and are currently analyzing venom from all four island populations of the Milos viper. Luckily the previous work from Schulte et al. 2023 has offered the first insights on the venom composition of the Milos viper venom from captive individuals kept at Latoxan laboratory in France (following figure). These first results highlight the presence of the previously mentioned toxin families (first paragraph) in the Milos viper venom, thus making it similar to other Macrovipera species as it is mainly cytotoxic and coagulotoxic/haemotoxic. Symptoms following a snakebite with a considerable amount injected would typically include pain, edema, hypotension, tissue necrosis, consumption coagulopathy and hemorrhage.

Snakebites from the Milos viper have been recorded in the past, though not as many as one would think considering that the species is infamous on both Milos and Kimolos islands and most often directly confronted when seen. There have been, however, no recorded deaths from a Milos viper bite in Greece. Only a few snakebites are reported annually from these islands. Considering the vast numbers of tourists that surpass the 100,000 mark each year, chances of a snakebite are near zero. This brings us to an another important point; not every bite is a wet bite (meaning that venom is injected) and most bites are evidently dry bites (with no venom is injected) or bites with a minimal amount injected. As previously mentioned snake venom has not evolved for defensive purposes (i.e. against humans) but for offensive ones (i.e. prey immobilization, digestion) which means that an individual would not waste the full amount available for a defensive attempt unless heavily persecuted. The vast majority of Milos vipers choose to retreat when confronted. The only case that we have found where it will stand its ground is when surrounded by people from all directions. Much like all venomous snakes, the Milos viper has got two venom glands (following image). These can be full with venom (after a complete meal digestion and rest), empty (right after capturing a big prey item) or anything in-between. Coupled with its unwillingness to spend any venom on a defensive bite, the unknown venom level within the glands lowers chances of severe symptoms even more. Luck of the draw.

Even though Greece is home to five viper species (Vipera ammodytes, Vipera berus bosniensis, Vipera graeca, Montivipera xanthina and Macrovipera schweizeri), it procures antivenom (AV) raised only against the horned viper (Vipera ammodytes) which happens to be the most common. On top of this, the single antivenom currently procured is the intramuscular Bul Bio product from Bulgaria. Unfortunately, intramuscular antivenom is considered "of the dark ages" and intravenous antivenoms should be procured again as done previously with VIPERFAV being the best candidate if a single antivenom against all five species approach is kept. Having said that, the BulBio product has been used in treating snakebites from the three most common snakes in Greece successfully (Vipera ammodytes, Montivipera xanthina and Macrovipera schweizeri), while it increased the hospitalization time of the patients. This makes us wonder if in these few reported cases the antivenom had any effect or if the patients simply had to ride it out on their own along with the added pressure of a low output intramuscular antivenom and antibiotics as per standard practice in the country. The most effective antivenom against the Macrovipera schweizeri bite seems to be the Razi Antivenom from Iran which is a hexavalent antivenom raised also against Macrovipera lebetina and Echis carinatus. Inoserp European Antivenom, which is a Pan-African antivenom has also been proposed to work against Macrovipera schweizeri venom by Schulte et al. 2023, however, Inosan seems to include a fraction of the active ingredient needed into a vial of AV. This means that in essence one would certainly need many tens of vials for a successful treatment. In the above cases, even though not directly raised against Macrovipera schweizeri venom, the AVs have cross-reactivity against venoms that are close in composition to those they have been originally raised against.

Clinical effects aside, the venom of the Milos viper also happens to hold at least some interest in terms of medicinal potential due to its ability to inhibit growth of bacterial strains. As proposed by Schulte et al. 2023, M. schweizeri venom could potentially be worth of consideration in future bioprospecting programs looking for novel anti-infectives. But even without a direct medicinal application, the Macrovipera schweizeri venom is of immense importance as part of our biodiversity with the wide variety of bio-active compounds that lie within. Demystifying the species and  raising awareness for its conservation status is crucial. There is no need for panic or even fear especially if one considers that during the hot summer months (June - September) the species is predominantly nocturnal and exceptionally hard to encounter. The comparison between the red and grey phenotypes also showed that there is virtually no variation between the venom composition. As such, the local myth that the red phenotype is more toxic than the grey, has been busted. 

Literature cited:

Calvete, Juan J., Libia Sanz, Yamileth Angulo, Bruno Lomonte, and José María Gutiérrez. "Venoms, venomics, antivenomics." FEBS letters 583, no. 11 (2009): 1736-1743.

Calvete, Juan J., Paula Juárez, and Libia Sanz. "Snake venomics. Strategy and applications." Journal of mass spectrometry 42, no. 11 (2007): 1405-1414.

Casewell, Nicholas R., Timothy NW Jackson, Andreas H. Laustsen, and Kartik Sunagar. "Causes and consequences of snake venom variation." Trends in pharmacological sciences 41, no. 8 (2020): 570-581.

Chippaux, J-P., Vaughn Williams, and Julian White. "Snake venom variability: methods of study, results and interpretation." Toxicon 29, no. 11 (1991): 1279-1303.

Daltry, Jennifer C., Wolfgang Wüster, and Roger S. Thorpe. "Diet and snake venom evolution." Nature 379, no. 6565 (1996): 537-540.

Mackessy, Stephen P. Handbook of venoms and toxins of reptiles. CRC press, 2016.

Schulte, Lennart, Maik Damm, Ignazio Avella, Lilien Uhrig, Pelin Erkoc, Susanne Schiffmann, Robert Fürst et al. "Venomics of the milos viper (Macrovipera schweizeri) unveils patterns of venom composition and exochemistry across blunt-nosed viper venoms." Frontiers in Molecular Biosciences 10 (2023): 1254058.

Oliveira, Ana L., Matilde F. Viegas, Saulo L. da Silva, Andreimar M. Soares, Maria J. Ramos, and Pedro A. Fernandes. "The chemistry of snake venom and its medicinal potential." Nature Reviews Chemistry 6, no. 7 (2022): 451-469.