Blood Collection in Snakes

Video

This video was produced by Katie Lennox-Phillibeck and narrated by Dr. Angela Lennox.

The video script was written by Dr. Christal Pollock and reviewed by Drs. Byron de la Navarre, Sean Perry, and Anneliese Strunk. More detailed information is available in the text below.

 

Introduction

Proper patient handling, blood collection technique and sample handling are all critical for accurate interpretation of snake hematology and biochemistry.

 

Equipment needed

To collect the sample, you will need:^5,11,13

• Needles, usually between 25 to 22-gauge and 25-38 mm (1 to 1.5-in) in length
• A 1 or 3-ml syringe or insulin syringe
• Antiseptic (Fig 1)
• Glass slides and hematocrit tubes
• Blood collection tubes
• Disposable gloves to minimize the risk of zoonotic disease

Visit “General Principles of Reptile Venipuncture” for advice on sample handling, use of anticoagulants, recommended sample volume, as well as lymph contamination or hemodilution.

 

 

Handling and restraint

You will also need an assistant. Snake venipuncture requires a minimum of two people:  one individual to restrain the snake and a second to collect the sample (Fig 2).¹¹ A good rule of thumb in manual restraint is to have one person per every 0.9 to 1.2 meters (3 to 4 feet) of snake.

 

 

Venipuncture sites

Blood samples are routinely collected from the heart and ventral coccygeal vein of snakes.¹⁰ The jugular vein, palatine-pterygoid vein, and dorsal vertebral vessels are also potential sites, but can be more difficult to access (Fig 3, Fig 4). ^2,4,5,11,12 Each venipuncture site has its advantages and limitations and the site selected can depend on the preferences and experience of the phlebotomist, the volume of blood needed, as well as patient species, body condition, and size (Table 1).^9,13,14

 

 

 

Table 1. Common venipuncture sites in the snake 2,5,10,11
Site	Advantages	Limitations
Ventral coccygeal vein	A better choice if the snake is more difficult to restraint.	More difficult to use in smaller snakes Risk of puncturing the musk glands or hemipenes in males Lymph contamination can occur
Cardiocentesis	A large volume of blood can be collected from this site Lymph contamination is rare	Rare complications can include pericardial effusion leading to cardiac tamponade and/or laceration of the great vessels*
*Comolli 2022 suggests that some degree of pericardial and cardiac pathology may be more common3

Ventral coccygeal vein

The ventral coccygeal vein or caudal “tail vein” sits on ventral midline, beneath the caudal vertebral bodies. It lies caudal to the vent between 25% and 50% down the tail (Fig 5).^5,10,11

Potential complications or disadvantages of the caudal tail vein

Collection of blood from the ventral coccygeal vein is usually only possible in large snakes, measuring more than 1.2-1.8 meters (4-6 feet) in length.^2,4,5,10 A few drops of blood adequate for making blood smears and filling hematocrit tubes can be collected from this site in smaller snakes.

If venipuncture is attempted too close to the vent there is a risk of puncturing either the paired musk glands or paired hemipenes in male snakes (Fig 6, Fig 7).^4,5,11,13 Contamination with lymphatic fluid can also occur with tail venipuncture (Fig 8) ⁵, however, the ventral coccygeal vein can be attempted first in any size snake.¹⁴

 

 

Technique

As a blind technique, venipuncture of the ventral coccygeal vein requires “perfect”, or at least optimal, patient positioning.¹³  Allowing the tail to hang towards the ground before and during blood collection can promote better blood flow.⁵

To perform ventral coccygeal venipuncture:^2,5,10,11

• Securely grasp the caudal tail in the palm of your hand (Fig 9).
• Aseptically prepare the venipuncture site.
• Insert the needle at a 45-60 degree angle between two ventral scales directly on midline (Fig 10, Fig 11).
• Maintain slight negative pressure as the needle is advanced craniodorsally.
• If the needle touches a vertebral body, slowly reverse direction while maintaining slight negative pressure.
• Once blood appears in the needle hub, hold the needle steady while continuing to apply gentle negative pressure (Fig 12).
• If the needle reaches bones without getting a flash of blood, then very slowly withdraw the needle a few millimeters and redirect more craniad or caudad while maintaining negative pressure.
• If a flash is noted but stops, gently rotate the needle and syringe while applying very gentle negative pressure and moving the needle and syringe in very small increments towards and away from the vertebrae until a better flow of blood is observed.

 

 

 

A lateral approach to the coccygeal vein has also been described.² The needle is inserted along the lateral aspect of the tail within the natural groove created by tail musculature.² Although this approach can be used in some snake species, it is more commonly applied in lizards and crocodilians (Fig 13).²

Cardiocentesis

Cardiocentesis is a relatively simple technique that can be used in snakes of all sizes.^1,4,12,14 The patient should ideally weigh more than 200 grams ^10,11, however use of this technique in snakes as small as 10 grams has been described.⁵ Cardiocentesis is the technique of choice when large volumes of blood are needed and it is also considered the best site to minimize the risk of lymph contamination.^1,5,7,13 Nevertheless, excellent restraint is necessary to avoid cardiac trauma and this technique should be reserved for snakes that are moribund, anesthetized, or when tail venipuncture has failed.^2,5,14

Complications with cardiocentesis are extremely rare, but can include pericardial effusion leading to cardiac tamponade, inadvertent venipuncture of the great vessels and/or laceration of the great vessels.¹⁵ Based on a study in ball pythons, even serial cardiocentesis (39 samples in six snakes over 120 days) was well tolerated and showed no significant histologic changes to the heart.⁸ However, in another study evaluating heart and lung tissue in ten healthy ball pythons undergoing cardiocentesis, all snakes showed some degree of pericardial and cardiac pathology.³

To perform cardiocentesis:^{1,2,4,5,10,11,13}

• Position the snake in dorsal recumbency (Fig 14, Fig 15)
• Identify the heart. Often the apex beat can be visualized, but the heart is located approximately one-fourth to one-third of the distance from the head to the tail (Fig 16). The heart can also be found at the cranial extent of the lungs (Fig 17).
• Clean the ventral scutes in this area.
• Since the snake heart is mobile within the rib cage, it will be necessary to stabilize the heart between the index finger and thumb (Fig 18).
• Slowly insert the needle at a 45-degree angle between the two ventral scales over the distal point of the beating heart (Fig 19). This will direct the needle into the thick-walled ventricle as opposed to the thin-walled atria.
• Once the needle is inserted, begin to apply gentle negative pressure. Blood will slowly fill the syringe with each heartbeat (Fig 20).
• Maintain digital pressure afterwards for 30 to 60 seconds (Fig 21).

 

 

 

 

 

 

 

If blood does not enter the syringe, withdraw and replace the needle to make a new attempt.⁵ Do not attempt to move the needle as one would do with the tail vein as this risks lacerating the ventricle or pericardium.⁵

 

Summary

Blood samples are routinely collected from the ventral coccygeal vein or heart in snakes. If a large volume of blood is needed, cardiocentesis is the technique of choice. If the patient is a relatively large snake or more difficult to restrain, the ventral coccygeal vein is the better choice, however, the ventral coccygeal vein can be attempted first in any size snake.

 

Acknowledgements:  Thank you to Drs. Charles Innis and Stacey Leonatti Wilkinson for helpful feedback.

 

References and further reading