Is the patient envenomed?
Is it likely that a clinically relevant injection of venom has taken place?
Inquire re:
- time of the bite,
- local pain,
- nausea, vomiting, abdominal pain.
Assess:
- state of consciousness.
Measure:
- blood pressure/pulse,
- respiratory rate.
Observe/investigate:
- bite marks,
- extent and intensity of local swelling,
- enlargement and painfulness of regional lymph nodes,
- swelling in the facial region, including the larynx/pharynx (angio-oedema),
- clinical signs of shock,
- bleeding from bite marks and other injuries,
- subcutaneous bleeding in the region of the swelling,
- gingival bleeding,
- blood-stained sputum, vomit ("coffee ground vomitus"), stools (melaena) or urine,
- acute abdomen (intra-abdominal bleeding!),
- focal neurological deficits, meningismus (intracranial bleeding!),
- cranial nerve deficits, such as ptosis, ophthalmoplegia, dysphagia, dysarthria,
- paralysis of the skeletal musculature including the respiratory musculature (→ respiratory insufficiency/respiratory failure),
- flank pain and renal bed sensitive to percussion.
Determine:
- cause of the accident by determination of specific venom antigen and venom concentration in the serum using the ELISA method (if clinically proven and standardised test kits are commercially available),
- clotting time (bedside test),
- urinary output.
The symptoms and degree of envenoming depend not only on the amount of venom injected and numerous other variables, but also on the time that has elapsed since the bite. This variable factor must be taken into account when making the following decisions:
- exclusion of envenoming (see below),
- the time interval between clinical examinations (see Therapy phase: Hospital),
- emergency care (see below).
The fact that a patient has been bitten by a known venomous snake and the presence of bite marks do not automatically allow the conclusion that a clinically relevant injection of venom has taken place.
On the other hand, local signs, e.g. swelling at the site of the bite, following crotalid bites are an indication that a significant injection of venom has occurred. There do, however, appear to be exceptions (see Diagnosis and treatment: Terrestrial snakes: North America: Hospital).
In contrast to the Crotalus durissus subspecies from the southern regions of the distribution area of this species, those in the north (now Crotalus simus) cause strong local effects. Following Micrurus sp. bites, however, local effects such as swelling are minimal or absent.
Noticeable bleeding may be absent during the initial phase or the entire course of envenoming following crotalid bites, even though defibrinogenation may be present to such a degree that the blood is completely incoagulable (clotting time test). This is true in particular for those species whose venom primarily contains components that have direct fibrinogen-coagulating activity ("thrombin-like" activity).
The simple clotting time test should always be performed if a crotalid bite is suspected.
Exclusion of clinically relevant envenoming
Monitoring for signs and symptoms (see above) that would indicate systemic envenoming for at least 24 h (recommended examinations, see Therapy phase: Hospital).
Preparalytic phase:
Elapids (Micrurus sp.): from 30–60 min to several hours, in some cases >12 h.
Preclinical phase of haemostatic defects:
Crotalids: systemic bleeding can occur within the first hour after the bite. Even severe haemostatic defects that can be detected on laboratory tests may not become clinically evident for a long period or even not at all. However, on laboratory investigations (clotting time), haemostatic defects can become apparent as early as <1 h after the bite (Rosenfeld 1971), although they may also appear after a delay of several hours.
The absence of signs of envenoming in the first hours after the bite does not exclude the possibility that a relevant injection of venom has taken place. There can be a long delay before systemic signs of envenoming develop.