Coral fina
Micrurus filiformis

Only one single case of mild envenomation by a 50-cm Micrurus filiformis specimen from Pará, Brazil has been published to date [1]. Reported symptoms include mild pain immediately after the bite, mild swelling (edema) on the bitten finger one hour after the bite, epigastric pain, and vomiting [1]. Envenomation symptoms associated with more severe grades of envenomation for M. filiformis remain unknown given that no cases of snakebite accidents caused by this species have been recorded in Colombia’s National Public Health Surveillance System (SIVIGILA, Spanish acronym). In addition, there is no information on the composition of its venom. Nevertheless, the symptoms of envenomation by M. filiformis are likely similar to those that have been documented for envenomation by other coralsnake species. 

In addition to the symptoms mentioned above, mild envenomation by coralsnakes can result in other local symptoms such as redness of the skin (erythema), and potentially, muscle pain (myalgia) and muscle twitching (i.e., fasciculation) [2,3]. Moderate envenomation can lead to more severe redness of the skin, painful burning or aching (dysesthesia), burning or prickling sensations (paresthesia), drooping of the upper eyelid (palpebral ptosis), weakening of the muscles used to produce speech (dysarthria), and altered sense of taste (dysgeusia) [2,3]. Severe envenomation can lead to flaccid paralysis within the first 30 minutes of the bite, excessive salivation (sialorrhea), severe dysarthria, paralysis of the flexor muscles of the neck (“broken neck sign”), and shortness of breath (dypsnea) [2–4].   

Given the rapidity with which severe symptoms of envenomation by M. filiformis can appear, the most effective approach for treating envenomation by this species, as for bites by any coralsnake, is to rapidly seek medical treatment, especially antivenom therapy [5]. In the only reported case of envenomation by M. filiformis, administration of 100 mL of Micrurus antivenom was sufficient to promote the full recovery of the patient within 24 hours [1]. 

Immediately after a bite by M. filiformis and while en route to the hospital, the bitten limb should be immobilized [6]. Other treatments (e.g., the application of ice, various plants, or oils to the site of the bite) should be avoided, given that they have no proven clinical efficacy and would only delay the arrival of patients to medical facilities. 

Upon arrival at a medical facility, the antivenom treatment regimen should be administered once envenomation is confirmed (mild, moderate, or severe), and the geographical origin of the accident should be considered [2]. In cases occurring in the Andean, Caribbean, and Pacific regions, five vials of INS antivenom may be required, while for cases in the Amazon or Orinoquía regions, 10 vials are needed [2,3]. These antivenom dosages are the same for both children and adults [7], and the application of antivenom should only be performed by qualified medical personnel. 

A herpetologist (William W. Lamar) was bitten twice on the palm of the hand by a 20-cm specimen of Micrurus filiformis in Vaupés, Colombia that was swimming in a river, but no symptoms of envenomation were observed [8]. Otherwise, no clinically significant snakebite accidents by M. filiformis have been reported in Colombia. However, the occurrence of this species in diverse lowland habitats, including near human settlements, and its tendency to defend itself vigorously when threatened, predispose it to being a cause of clinically significant bites. Some indigenous groups have been reported to handle this species without taking any precautionary measures to obtain medicinal substances for the treatment of various ailments [9]. 

Given the small size and short fangs of this species, envenomation can occur even if fang marks are not visible at the site of a bite. Handling of this species should thus be regarded as dangerous, and all bites by this species should be treated as life-threatening whether or not fang marks on the skin are present. 

filiformis is a slender, small to medium-sized tricolored coralsnake with 12–23 body triads (repeating units consisting of three black rings separated by two white rings that together separate the red rings) and 1–2 triads on the tail. The primary black ring in each triad (the black ring in the middle of each triad) is typically longer than the two accessory black rings (the two outer black rings in each triad), and the red rings are longer than the white rings. The tip of the snout is black, which is followed by a white ring (that can either be complete or incomplete), a black ring, a red ring, and then the first complete body triad. 

M. filiformis can be distinguished from all other co-occurring coralsnakes bearing triads (M. helleri, M. hemprichi, M. isozonus, M. nattereri, M. obscurus, and M. surinamensis) in Colombia by its slender body (diameter less than 0.5 cm in M. filiformis and greater than 0.5 cm in all other triad-bearing species) and higher number of body triads (10–24 in M. filiformis and 14 or fewer in all other triad-bearing species) [10].

In Colombia, M. filiformis occurs in lowlands in the Orinoquia, Amazon Basin, and eastern versant of the Andes up to 400 m in elevation (Figure 1) [10,11]. It has been documented in the following departments: Amazonas, Caquetá, Casanare, Cundinamarca, Guainía, Meta, Putumayo, Vaupés, and Vichada [12–16]. The potential area of the distribution of M. filiformis in Colombia is 531153,64 km2 (Figure 1).

Figure 1. Geographic distribution of Micrurus filiformis in Colombia and its habitat suitability model. Based on bioclimatic variables, the habitat suitability model predicts the species' potential distribution in Colombia, identifying zones with suitable or unsuitable environmental conditions for its occurrence across the country. Values close to 1 indicate optimal environmental conditions (high probability of presence), while values close to 0 indicate unsuitable conditions (absence likely).​

Uncommon. This fossorial snake occurs in humid primary and secondary forests, riparian areas, savannas, and various disturbed habitats (e.g., open fields) in lowland areas in Colombia [11,17]; it has often been sighted  near streams and rivers [8,9,18]. It is primarily active on the surface at night or near sunset [9]. Micrurus filiformis in Brazil and Peru has been observed to occur in similarly diverse habitats and display similar activity patterns [19–21]. It might be more common in disturbed areas than in natural habitats across its range [22–24], and a niche model of the potential distribution of M. filiformis in Brazil indicates that the probability of occurrence of this species is highest in areas closest to the Amazon watershed [25]. This species has been detected in several short-duration (weeks to months) herpetological surveys in Brazil and Peru, suggesting that it might be reliably detected in appropriate habitat with sufficient sampling effort [21,26–28].

No prey items for this species have been documented in Colombia. In Brazil and Peru, M. filiformis has been reported to prey on amphisbaenians, small snakes, and invertebrates [23,29]. Species that have been documented to prey on M. filiformis include the smooth-fronted caiman (Paleosuchus trigonatus) and the black spot piranha (Pygocentrus cariba) [22,29–31]. An attempted predation event on M. filiformis by the gray-winged trumpeter (Psophia crepitans) has also been reported [32]. 

This species typically flees when threatened, but it has also been observed to vigorously defend itself, move erratically, and engage in a tail display to thwart attack by predators [24,33]. In Brazil, M. filiformis is the host of the intestinal parasite Eimera micruri [34]. 

Micrurus filiformis is oviparous, but no information on the reproduction of this species has been collected to date. Much remains to be learned of the natural history and ecology of this widely distributed but poorly known snake in Colombia (Table 1). 

The scientific name of this species is derived from the Greek roots Mikros (small) and oura (tail) referring to the short tails, which is a general trait of species from the Micrurus genus. The specific epithet (filiformis) is derived from two Latin roots, filum (thread) and forma (shape or figure), which allude to the slender body of this species. [8,24]. In Colombia, this species is often referred to by locals as coral or coral fina or in English thread Coralsnake.

Table 1. Summary of important biological, venomic, epidemiological and medical traits.

★ Poorly Reliable: Confidence for this species is poor due to the lack of data and information on Colombian populations. Therefore, most of our knowledge comes from a few studies and populations outside the national territory. LD50: median lethal dose; MCD: minimum coagulant dose; MDD: minimum defibrinating dose; DEM: minimum edema-forming dose; DHM: minimum hemolytic dose; PLA2: phospholipases A2; SVSP: serine proteases, SVMP: metalloproteinases; NGF: nerve growth factor; CRISP: cysteine-rich secretory protein; CTL: C-type lectin/lectin-like, DIS: disintegrins, KUN: Kunitz peptides; BPPs: bradykinin-potentiating peptides; VEFG: vascular endothelial growth factor; 3FTx: three-finger toxins; LAAO: L-amino acid oxidases