Diagnosis and Investigation

Diagnosis
• Identification of the snake
• Grading severity of envenomation
Definitive diagnosis requires positive identification of the snake and clinical manifestations of envenomation. History should include the time of bite, description of the snake, type of field therapy, underlying medical conditions, allergy to horse or sheep products, and history of previous venomous snakebites and therapy. A complete physical examination, including baseline measurements of limb circumference proximal and distal to the bite site, should be done.
Snake bites should be assumed to be venomous until proved otherwise by clear identification of the species or by a period of observation.
Snake identification: Patients often cannot recall details of the snake's appearance; however, pit vipers differ from nonvenomous snakes (see Fig. 1: Bites and Stings: Identifying pit vipers ). Consultation with a zoo, an aquarium, or a poison control center can help in the identification of snake species.



Coral snakes in the US have round pupils and black snouts but lack facial pits. They have blunt- or cigar-shaped heads and alternating bands of red, yellow (cream), and black, often causing them to be mistaken for the common nonvenomous scarlet king snake, which has alternating bands of red, black, and yellow. The distinguishing feature in the coral snake is that the red bands are adjacent to only yellow bands, not black bands. (“red on yellow, kill a fellow; red on black, venom lack”). Coral snakes have short, fixed fangs and inject venom through successive chewing movements.
Fang marks are suggestive but not conclusive; rattlesnakes may leave single or double fang marks or other teeth marks, whereas bites by nonvenomous snakes usually leave multiple superficial teeth marks. However, the number of teeth marks and bite sites may vary because snakes may strike and bite multiple times.
A dry pit viper bite is diagnosed when no symptoms or signs of envenomation appear over 8 h.
Severity of envenomation: Severity of envenomation depends on the following:
• Size and species of the snake (rattlesnakes > cottonmouths > copperheads)
• Amount of venom injected per bite (cannot be determined by history)
• Number of bites
• Location and depth of the bite (eg, envenomation in bites to the head and trunk tends to be more severe than in bites to the extremities)
• Age, size, and health of the patient
• Time elapsed before treatment
• Patient's susceptibility (response) to the venom
Severity of envenomation can be graded as minimal, moderate, or severe, based on local findings, systemic symptoms and signs, coagulation parameters, and laboratory results (see Table 2: Bites and Stings: Severity of Pit Viper Envenomation ). Grading should be determined by the most severe symptom, sign, or laboratory finding.
Envenomation may progress rapidly from minimal to severe and must be continually reassessed.
If systemic symptoms begin immediately, anaphylaxis should be assumed.
Table 2

Severity of Pit Viper Envenomation
Grade Description
Minimal Changes at bite site only
No systemic symptoms or signs or abnormal laboratory findings
Moderate Changes extend beyond the bite site
Non–life-threatening systemic symptoms and signs (eg, nausea, vomiting, paresthesias)
Mildly abnormal coagulation or laboratory changes without clinically significant bleeding
Severe Changes involving the entire extremity
Severe systemic symptoms and signs (eg, hypotension, dyspnea, shock)
Markedly abnormal coagulation and laboratory changes with or without clinically significant bleeding


Investigations
Basic investigations'-' in snakebite include; hll blood
counts, blood grouping and cross-matching,
prothrombin time, ECG, serum electrolytes, urea,
creatinine and where available venom detection from
the snakebite wounds, using venom detection kits2.
Unknown Snake
In cases of snakebite, effective treatment often depends heavliy on identifying, to an adequate degree, what type of snake was responsible. While the exact species is not always important, some knowledge of the identity can be valuable in prognosis and in selection of the most appropriate antivenom.
Snakes are notoriously variable in their appearance, and visual identification can usually be considered unreliable. Consequently, the health professional must weigh the information from a number of sources judiciously.
1. If a swab can be made of the bite site, or there is a urine, blood or serum sample available, then identification of immunotype may be made through use of the Snake Venom Detection Kit.
2. In some cases, the field can be narrowed by reference to the clinical signs and symptoms.
3. Depending on the locality where the envenomation took place, species distribution may aid in confirming putative identification. This , of course, does not apply in cases where the snake is non-endemic, such as in private collections or zoos.
4. If identification of an endemic snake cannot be made, antivenom - when indicated - should be administered according to the table below. The table does not apply in Victoria or Tasmania in the case of non-endemic snakes, for which the Snake Venom Detection Kit should be used.
5. If no animal was seen, but snakebite suspected, the presence of two puncture wounds close together should suggest either spiderbite or snakebite (some small snakes' fangs are closer together than those of large spiders). In the absence of evident fang marks, differential diagnoses may be considered:
• Insect or other arthropod sting or bite (ant, bee, wasp, fly, beetle, bug, caterpillar, centipede)
• Puncture wound from an inanimate object (e.g., thorn, needle), with or without the introduction of antigens or toxins