In our previous article on snake envenomation, we discussed the important mechanisms behind envenomation. This is an important starting point in being able to correctly diagnose and begin to treat suspected snake bite cases. This next article will hone in on diagnosis of snake envenomation in dogs and cats, which can at times be considerably challenging. With such a wide variety of clinical signs, and snake bites often quite difficult to locate, veterinary professionals need to employ multiple diagnositic tests and techniques in order to confidently diagnose a presenting patient.
Now that we understand the mechanisms behind snake envenomation, this will help to guide our diagnostics, as haemolysis, rhabdomyolysis, and coagulopaties can be readily detected in many diagnostic tests. First, we’ll first discuss the tests you can use to assist your diagnosis, and then go into further detail about the all-important Snake Venom Detection Kit (SVDK).
Snake Envenomation – What Diagnostics Can I Use?
This simple, quick, and low cost test that should be performed on all patients suspected of snake envenomation. Once a PCV is performed, evidence of anaemia maybe present due to the presence of haemolysis. In many cases of envenomation, the serum will likely be icteric as well and should be noted. Occasionally a blood transfusion is required if global hypoxia is present from rapid cell destruction (you can read our Transfusion Medicine Guide for more on this).
Running full bloods, biochemistry, and electrolytes can be an incredibly valuable diagnostic test in snake envenomation cases, particularly with regards to electrolytes. This is due to the fact that metabolic acidosis can occur due to release of intracellular contents resulting from rhabomyolysis and haemolysis. Signs to look out for include elevated potassium, phosphate and magnesium.
Additionally, it’s important to regularly monitor PvCO2 and PvO2 to determine effects of pro and anti-thrombotic effects on the pulmonary vessels and
parenchyma. Although if the patient is coagulopathic, do not obtain an arterial or jugular sample.
Ideally, a partial thromboplastin time (PT) or activated partial thromboplastin time (APTT) should be performed in all suspected patients, but an activated clotting time (ACT) can be performed if other tests are unavailable or cost prohibitive. Due to the high fibrinogen consumption from pro-coagulant envenomation, clotting tests may not return to normal until 18-24hrs post venom neutralisation, when fibrinogen is re-synthesised. PT/APTT or ACT should be measured every 6hrs until results have normalised. It’s also important to note that many cats affected by snake envenomation don’t show signs of a coagulopathy.
When performing biochemistry on a suspected snake envenomation patient, creatine kinase (CK) and aspartate aminotransferase (AST) can be two of the hallmark indicators. These two parameters are are measured to determine the significance of rhabomyolysis, and if ongoing muscle damage is occurring. CK can take 2hrs to rise after envenomation, and half-life of CK is approx 3-6hrs and AST 12hrs, so any reduction in AST is significant.
Urea and creatinine are used to determine if there are any delayed effects from envenomation, and if there is impaired renal function. This can be due to effects from pigmenturia, hypovolaemia or direct nephrotoxins, though the latter are not described as yet. Total bilirubin (TBIL) can be also be measured to monitor effects of haemolysis and possible renal tubular damage.
Although it might not be your first thought to obtain a urine sample for a suspected snake envenomation patient, this can be quite helpful beyond its use in a snake venom detection kit. Obtaining a sample allows you to monitor for the presence of pigmenturia (myoglobin) +/- haematuria (red cells) as indications for possible
renal tubular damage.
Additionally, you’ll be able to measure the pH of the urine, as alkaline pH has been shown to solubilise myoglobin, leading to reduced incidence of renal tubular necrosis and improved prognosis. Although as with jugular blood collection, do not perform a cystocentesis to obtain urine from the patient if they are coagulopathic.
Indirect Identification – Snake Venom Detection Kit (SVDK)
In addition to a wide range of useful in-house diagnostics, use of a snake venom identification kit not only allows for detection of snake venom, but will be able to identify the venom immunotype and most appropriate antivenom for treatment.
A snake venom detection kit can be performed in serum/plasma, or urine in the dog and cat due to ease. When using blood as the chosen sample, spin the sample down and use the serum or plasma content. Collection by peripheral venipuncture is preferred due to a likely coagulopathy. It should also be noted that using a serum or plasma sample can produce false positives, so urine is the preferred sample method.
Urine is the preferred sample, as venom in urine is up to 4x more concentrated than in blood. If choosing urine as the sample for your snake venom detection kid, try gentle palpation to gain an adequate sample, or place a temporary urinary catheter to obtain a sample (as previously mentioned, cystocentesis is contraindicated in coagulopathic patients). If the patient is obtunded-stuporous and is to be admitted, a permanent urinary catheter may be placed instead and allows continuous monitoring.
For dogs, venom may be detected in the urine between 1 – 24 hours post envenomation, but anectodally may be sooner. And for cats, venom may be detected in the urine after 8hrs, but anectodally may be sooner. If the apparent period from time of envenomation and presentation is under 1 hour, this should not prevent the test from being performed (as the owner’s historical account may be inaccurate).
More information on snake venom detection kits, their use in clinic, and the detailed method for the procedure can be found via the official CSL website.
As we briefly touched on in our previous article on snake envenomation, identification of the snake that has caused possible envenomation is ideal, but not always possible. If the snake is obviously deceased, then identifcation can be made in-house referring to official government websites for guidance or by using the “scale count” method as detailed on the Australian Reptile Online Database.
Common Venomous Snakes:
Some common elapids (venomous snakes) found within QLD include:
- Brown Snake (Eastern, Western, Common, Speckled or Ringed) – this species is responsible for approximately 85% of bites treated at AES
- Tiger Snake (Easter or Western) – this species is responsible for approximately 5% of bites treated at AES
- Black Snake (King Brown/Mulga, Red-Bellied, Blue-bellied/Spotted) – this species is responsible for approximately 10% of bites treated at AES
- Taipan (Coastal or Inland)
- Death Adder (Common or Northern)
- Stephen’s Banded Snake
- Coiled Copperhead Snake
- Whip Snake (yellow-faced, black or collared) – can occasionally be the presumed cause of weakness in cats
- Crowned Snake (Dwarf, Northern or White)
- Rough-Scaled Snake
- Naped Snake (Red, Yellow or Orange)
- Black-Bellied Swamp/Marsh Snake
- Eastern Small-Eyed Snake – occasional bites from this species are treated at AES
- Coral Snake
- Myall Snake
- Pale-Headed Snake
- Colletts Snake
Although not always possible, being able to correctly identify the snake in question that has caused the envenomation can be highly helpful in guiding your treatment plan and prognosis of your patient. Though safety is paramount in any situation where a venomous snake is involved, with extreme caution to be taken and live venomous snakes to only ever be handled by qualified professionals.
Now that we’ve covered the pathophysiology behind snake envenomation, and the various diagnostic tools that can be used to help confirm snake envenomation in a patient, our next article discusses life-saving treatment and supportive care of snake envenomation that your patient needs. For more detail and a downloadable handout for your clinic, be sure to access the completely free Snake Envenomation Clinical Protocol at VetAPedia.