By Mike Corcoran, DVM, DABVP (REP/AMPH), CERT AQV
You recently saw a 2-year-old male bearded dragon in your clinic for lethargy, weight loss, and decreased appetite. You noted some husbandry changes that need to be addressed, most notably temperatures that are too low in the enclosure. On physical examination you observed numerous swollen joints. Radiographs showed lytic changes to the endplates of bones in the swollen joints. As part of the complete workup, you wisely submitted a serum biochemistry panel and a complete blood count to your veterinary diagnostic laboratory. You sent home supplemental feeding for the client and had them get an additional heat source for supportive care while waiting a day for the results. The results are returned the next day, but now you need to make sense of it. This is one of the first reptile CBCs that you’ve ever checked, and you look at it with anxiety. “What the flip is an azurophil?” you ask yourself. No worries, help is here to make the most sense out of your next reptile cbc.
There are a lot of similarities you’ll notice when looking at your first reptile CBC. Like dogs, cats, horses, and other mammals, the reptile CBC breaks down parameters of red blood cells and white blood cells. You’ll also see a packed cell volume(PCV) if you supplied hematocrit tubes in addition to a slide. Beyond absolute numbers, you’ll see relative percentages for white blood cells in the report from the veterinary pathology laboratory plus morphology for both cell lines. Together, these culminate in a snapshot of the clinical condition at the time the blood was obtained. Like any other clinical test with other species, results always need to be interpreted in light of the history, physical examination, and clinical signs of the patient. So, in this regard, reptile CBCs can be read similarly to other species’ CBCs.
We’ll get into details later, but there are some notable differences in the reptile CBC that need to be considered if you’re used to interpreting samples for other species. One of the most important things to remember is that when we say reptiles, we’re talking about more than 10,000 species in numerous orders within the class Reptilia. As such, it’s harder to generalize “reptile CBC” to the degree that we can “canine CBC.” You’ll also notice some unique cell lines; azurophils are a type of leukocyte that’s unique to reptiles and amphibians. Some of the familiar cell lines are present but may function differently in some species.
When you look at the reptile CBC, lymphocytes will be the most common white blood cells (WBC) in most species (Heatley, 2019). Exceptions to this rule in some common species are listed below:
The numbers in the individual cell lines do change in response to pathology. That may be best demonstrated in the literature by a published study of cold-stunned sea turtles in New England. Turtles with severe cold stunning did show significant overall increase in WBC count, with a decrease in the lymphocyte count and significantly increased heterophils. The condition reversed in convalescent turtles (McNally 2020). However, this is an extreme circumstance and there are a number of things that can cause significant differences in cell count. Marked differences in cell counts have been documented due to infection, inflammation, sex, age, nesting behavior, ovulation, and seasonality in various species (Sykes 2015, Howard 2021, Yang 2014). Neoplasia may also be a rare cause of alteration in the numbers (Hepp-Keeney 2021). Keeping in mind these species exceptions while generally operating under several generalities will make reading reptile CBCs easier going forward.
One notable difference with reptiles is the presence of nucleated red blood cells (RBCs). This difference is the reason that most automated cell counters are ineffective for reptile CBCs, and this was one of the key motivators for developing the AI diagnostics now used by Moichor. Prior to this system, accurate counts could only be done through a manual count, requiring a great deal of dedicated staff time and resulted in a large variation in accuracy.
Morphology changes may be triggered by similar conditions to what’s seen with mammals. Precursors of RBCs have larger, rounder nuclei and darker cytoplasm. With regeneration, mitotic figures may be seen in circulation (Nardini 2013). A left shift may therefore result in changes to the mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC).
Hemoparasites may be observed in reptiles, though most are considered to have little to no clinical significance (Nardini 2013). Most require an intermediate host and will be quickly eliminated in captive reptiles.
Anemia in reptiles should be approached in a similar manner to other species. Evaluate the RBC parameters and morphology to first decide if the anemia appears to be regenerative or non-regenerative. Polychromasia is a reliable indicator of regeneration for reptiles. However, with RBC lifespans up to 800 days in some reptiles, this can be a little delayed compared to other species (Nardini 2013). Regenerative anemia can be caused by blood loss from hemorrhage or destruction of RBCs. Immune-mediated anemia has not been documented in reptiles, but some toxic agents have been linked to destruction of cells (Heatley 2019). Non-regenerative anemia may be the result of infectious disease, chronic inflammation, poor husbandry, or neoplasia (Heatley 2019, Nardini 2013, Sykes 2015).
Most of the reptile CBC reports from veterinary clinical laboratories will appear like other species’ with some notable exceptions. Eosinophils may not be counted in reports for snakes unless you’re using Moichor where we do certainly count them when observed. In general, eosinophils appear to be absent in most snake species with the exception of some cobras. When present in snakes, they may be a variant of the heterophil (Heatley 2019, Sykes 2015). Iguanas have green granules in the eosinophils, and the function of these cells isn’t fully understood (Nardini 2013). Snakes and lizards also have a unique type of WBC: azurophils. The general cell functions are listed in the chart below.
Toxic changes will be documented if they’re observed on the blood smear. Toxic changes observed in reptile WBCs include degranulation, vacuolation of the cytoplasm,, and in later stages, abnormal cytoplasmic granules (Nardini 2013).
Because the numbers in each cell line may be affected by so many extrinsic and intrinsic factors, it’s better for you to pay attention to trends in the counts in response to treatment and any morphologic changes in the cells. Again, consider these changes in light of the complete clinical picture. Changes in the CBC may be observed in 2 weeks or less when instituting a treatment plan, depending on the individual and the severity of the condition. Repeated testing is extremely valuable in guiding your therapy.
* See the article text for differences in iguanas, tegus, and snakes.
The CBC of your bearded dragon was consistent with severe systemic infection or inflammation. You have the client return, and you get samples of the swollen joints for reptile cytology and culture. You also submit a blood culture and start systemic, broad spectrum antimicrobial treatment. The cytology finds inflammatory cells and a population of gram-negative bacteria. The cultures show good sensitivity to the medication you started. Through 6 weeks of treatment, recheck CBCs show improvement in the cell numbers and morphology of the WBCs. Your patient becomes more active, and his appetite returns. Another success to add to your experience in reptile care!