Bungarus

Distribution

Kraits are found in tropical Asia, from near Iran, through the Indian subcontinent (including Sri Lanka and Bangladesh) and on to Southeast Asia (including Indonesia and Borneo).

Description

Kraits usually range between 1.0 and 1.5 m (3 ft 3 in and 4 ft 11 in) in total length (including tail), although specimens as large as 2.0 m (6 ft 7 in) have been observed. The banded krait (B. fasciatus) may grow as large as 2.125 m (6 ft 11.7 in). Most species of kraits are covered in smooth, glossy scales arranged in bold, striped patterns of alternating black and light-colored areas. This may serve as aposematic colouration in its habitat of grassland and scrub jungle. The scales along the dorsal ridge of the back are hexagonal. The head is slender, and the eyes have round pupils. Kraits have pronounced dorsolateral flattening, which causes them to be triangular in cross section.

Diet and behavior

Kraits are ophiophagous, preying primarily upon other snakes (including venomous varieties) and are cannibalistic, feeding on other kraits. They also eat mice and small lizards.

All kraits are nocturnal. They are more docile during the daylight; at night, they become very active, but are not very aggressive even when provoked. They are actually rather timid, and often hide their heads within their coiled bodies for protection. When in this posture, they sometimes whip their tails around as a type of distraction.

Reproduction

Kraits are oviparous. The female lays a clutch of 12 to 14 eggs in piles of leaf litter, and stays with them until they hatch.

Venom

Bungarus contains some species which are among the most venomous land snakes in the world to mice based on their LD50. They have highly potent neurotoxic venom which can induce muscle paralysis. Clinically, their venom contains mostly presynaptic neurotoxins. These affect the ability of neuron endings to properly release the chemical that sends the message to the next neuron. Following envenomation with bungarotoxins, transmitter release is initially blocked (leading to a brief paralysis), followed by a period of massive overexcitation (cramps, tremors, spasms), which finally tails off to paralysis. These phases may not be seen in all parts of the body at the same time. Since kraits are nocturnal, they seldom encounter humans during daylight, so bites are rare, but a bite from a krait is potentially life-threatening, and should be regarded as a medical emergency.

Typically, victims start to complain of severe abdominal cramps accompanied by progressive muscular paralysis, frequently starting with ptosis. As no local symptoms are seen, a patient should be carefully observed for tell-tale signs of paralysis (e.g. the onset of bilateral ptosis, diplopia, and dysphagia), and treated urgently with antivenom. Frequently, little or no pain occurs at the site of a krait bite, which can provide false reassurance to the victim. The major medical difficulty of envenomated patients is the lack of medical resources (especially intubation supplies and mechanical ventilators in rural hospitals) and the ineffectiveness of the antivenom.

Once at a healthcare facility, support must be provided until the venom is metabolised and the victim can breathe unaided, especially if no species-specific antivenom is available. Given that the toxins alter acetylcholine transmission which causes the paralysis, some patients have been successfully treated with cholinesterase inhibitors, such as physostigmine or neostigmine, but success is variable and may be species-dependent, as well. If death occurs, it typically takes place about 6-12 hours after the krait bite, but can be significantly delayed. Cause of death is usually respiratory failure—suffocation by complete paralysis of the diaphragm. Even if patients make it to a hospital, subsequent permanent coma and even brain death from hypoxia may occur, given the potential for long transport times to get medical care.

Mortality rates caused by bites from the members of this genus vary by species; according to University of Adelaide Department of Toxicology, bites from the banded krait have an untreated mortality rate of 1–10%, while those of the common krait are 70–80%. Several websites state the mortality rate is 50% even with treatment, but no specific species is mentioned and no original source in the medical literature for this statement is given. In common with those of all other venomous snakes, the death time and fatality rate resulting from bites of kraits depend on numerous factors, such as the venom yield and the health status of the victim.

Polyvalent elapid antivenom is effective in neutralizing of the venoms of B. candidus and B. flaviceps and rather effective for B. fasciatus. In this last case, the monovalent B. fasciatus antivenom is also moderately effective.