The Vet Report: Routine Blood Testing in Reptiles is Encouraged
"Bad laboratory data are worse than no laboratory data at all." Considering the variety of species of herp patients that I see, I think this old saying is more significant than any other in all of veterinary medicine. This is because there are so many different types of herps, and so little is known about them.
There are lots of published papers about blood values in different herp species. However, they are not all easily accessible in the veterinary world, and more importantly, many research papers use different technologies in their data accumulation and analysis. So, where it may seem like a simple matter to look up the normal blood values for a western diamondback rattlesnake, for instance, the results are rarely a case of comparing apples to apples.
Collecting blood from the ventral tail vein of a green iguana. Most patients tolerate this procedure well, with only minimal restraint.
Especially with herps, there are many physiological influences that can affect an animal’s blood values, including age, sex, diet, the time of year and temperature of the animal at the time of the blood collection. On top of these are the physical factors, such as the type of collection method that was used (anticoagulant or none), type of collection tube (plastic vs. glass), type of analyzer used, temperature of the testing methods, etc.
This all said, even with all the variables, I still recommend that both routine and situation-specific blood testing be done with herps. The main key point here is how the blood results are interpreted. This is where experience and knowledge is critical.
Standard Sampling Techniques
Obtaining samples for hematology and clinical chemistry in reptiles is not difficult in most cases, but can be a clinical challenge in certain situations, especially in the case of miniscule patients. I have no problem getting blood from a 10-pound sulcata tortoise, but how does one safely take blood—not to mention how much can you take—from a 10-gram sulcata hatchling?
Standard sampling techniques such as cardiocentesis (heart puncture), jugular, axillary, femoral, buccal and tail vein venipuncture can all be used with reptiles. Each has both advantages and limitations, yet with practice and experience, any clinician can develop proficiency in the techniques that suit him or her best.
In snakes, tail vein venipuncture and cardiocentesis are the best techniques for collecting blood. I routinely use cardiocentesis, and after 30 years of collecting blood from literally thousands of snakes I have never had a problem. I have had owners express concern over using this technique, but when done properly it is very safe, and there are published studies showing that cardiocentesis is an acceptable technique.
With lizards, I prefer tail vein and jugular bleeding techniques. Either can be done, but in species that autotomize their tails, such as geckos, the tail technique should not be used.
When collecting blood from chelonians, the jugular vein or the axillary or femoral plexus can be used. The former is the preferred method in gentle animals, but in large or strong chelonians, it is still possible to collect blood from the latter two locations without sedation. There is a lot of talk about using the subcarapacial vein (located just over the top of the neck and under the shell), but I don’t recommend doing so because there have been several reports of tortoises suffering accidental spinal damage from aggressive sampling in that location.
With crocodilians, depending on their size, I use either the tail vein (for small animals) or the dorsal occipital sinus.
I do not encourage toenail bleeding in any species. With all the other convenient alternatives available, I see no reason to destroy perfectly normal tissue (the nail). Not only does this technique require more time, it yields a lymph-contaminated sample and undoubtedly causes considerable pain to the patient. If you don’t believe me, try clipping off the tip of your finger, just under the nail bed!
How Much Is Needed?
Blood volume in reptiles comprises 5 to 8 percent of their total body weight. Of this amount, up to 10 percent can be safely collected for analysis without harming the patient. As a rough approximation, the sample size should never be larger than 1 percent of an animal’s total body weight. For example, up to 4.5 cc’s of blood can be safely removed from an iguana that weighs 450 grams. This amount is way more than what would be required for even the most comprehensive blood testing.
A standard blood capillary tube holds 70 microliters of blood. This means that the volume of a single capillary tube is the maximum amount of blood that should be taken from any patient weighing a minimum of 7 grams. Even though this is a small sample, it is enough to yield some valuable diagnostic information.
The total (plasma) protein (when using a heparinized capillary tube), packed cell volume (which provides a good estimate of red cell amount, itself a good test for anemia and dehydration) and icterus index (plasma or serum color) are all easily measured from the amount of blood in a single capillary tube. Microscopic analysis of the blood-filled capillary tube may reveal microfilaria, and after centrifugation, will also provide a buffy coat percentage (estimate of white blood cells). If a thin blood smear is made, an estimated white blood cell count with a differential can be performed, and the red cells can be evaluated for any abnormalities or hemoparasites. After the cells have been spun down, the remaining plasma can be used for a limited number of specific chemistries.
So, potentially, even in an animal as small as 7 grams, you can still get a large-enough sample to get some valuable information. That is, if —and that’s a big if —you can safely collect the sample. That said, with very small patients, a single drop of blood may be all that is necessary to collect, and a thin, well-prepared blood smear will always be a valuable diagnostic aide.
Douglas R. MADER, MS, DVM, DABVP (C/F, R/A), DECZM (HERPETOLOGY), is a graduate of the University of California, Davis. He owns the Marathon Veterinary Hospital in the Conch Republic, and is a world-renowned lecturer, author and editor. He sits on the review boards of several scientific and veterinary journals.