The leukocytes (white blood cells) in the bloodstream of all animals are one of the first lines of defense in the animal's innate (non-specific) immune system. They seek and destroy foreign particles and pathogens, and clear the body of debris from waste from normal metabolism and wound-healing. In all animals there are 5 different types of leukocytes, which each perform different duties in the immune system, but all working in concert. These are the lymphocytes, neutrophils, eosinophils, basophils and monocytes. In reptiles and birds, the neutrophil is replaced by heterophils, which perform the same function. Lymphocytes can be broken down into the T cells, B cells and natural killer (NK) cells, although these are not distinguishable on blood smears. Neutrophils (and heterophils) are short-lived, phagocytic cells that protect against bacterial and other pathogen infections. Eosinophils are involved in regulating the immune reaction or inflammation response and protect the body against metazoan (multicellular) parasites. The function of basophils is not clear, but they appear to be involved in the inflammation response. Monocytes are long-lived phagocytic cells that engulf bacteria and cellular debris.
The number of each cell type varies among species, and can also vary among individuals of a species, as can be seen from the reference tables in this website. The relative number of each cell is usually termed the 'leukocyte profile' or 'leukogram' for the animal in question. This profile is an important indicator of the well-being of the animal, because deviations from normal cell numbers is usually a sign of illness, infection or stress (see below). For this reason, researchers have been increasingly adopting leukocyte profiles into their investigations of wild animals. For example, an investigator may wish to know what effect air pollution has on the health of nestling bluebirds. He or she may therefore chose to examine white blood cell numbers of nestlings from sites with different levels of air pollution. This would involve collecting blood samples from each nestling and making standard 'blood smears' or 'films' (see fig), which allow the investigator to view and count blood cells through a microscope. The relative number of white blood cells counted on the smear indicates how many were in circulation at the time of the sampling.
One of the biggest problems researchers have when working with leukocytes of non-mammalian wildlife is that the 'normal' number of each cell type is rarely known for the species in question. When veterinarians examine leukograms from dogs and cats, they can easily consult their tables of reference ranges for these species, which have been established after examination of thousands of animals. Moreover, since mammal erythrocytes are non-nucleated, there are machines that can perform automated white cell counts! But, for a researcher studying the less well-studied tiger salamander, or indigo bunting, for example, they usually have to struggle to find relevant information on the white blood cells of these species, often relying on published information from the nearest related species. In addition, the nucleated red cells of amphibians, reptiles and birds also means that the white cells must be manually counted through a microscope.
The purpose of this website is to help wildlife researchers with leukocyte counts. From long hours spent at many university libraries, the author has painstakingly searched for published records of leukocyte profiles of amphibians and birds (reptiles will come soon), and these data have been compiled in the pages herein. These records come from veterinary, ecological and anatomical journals, and some even date back to the early 1900s. Some of these are records where the blood of an animal was examined specifically to determine the white cell numbers for the species in question (often along with blood chemistry data), while others were experiments involving treatments to captive or wild animals and blood data was obtained. In these cases, only the control values are presented in the table. Intermixed with these records are several profiles from the author's own data, where blood smears from a variety of amphibian and bird species were examined, but the data were not otherwise published. The collective data contained in these tables can give investigators a sense for the normal number of each of the five leukocyte types in most amphibians and birds, and provide a basis for comparing the numbers from their study species. In addition, the list of references presented with each table will no doubt save the next person a lot of time in literature-searching!
When the published data are compiled in the tables, one of the first things that becomes clear is that while there can be differences among species in the relative numbers of each cell type, closely-related species tend to have similar numbers, such as the anuran amphibians, which generally have 50-60% lymphocytes, 20-30% neutrophils, 1-10% eosinophils, 1-10% basophils and 1-5% monocytes. Regarding amphibians, another trend that becomes clear from the table is that all of the Ambystomatid salamanders examined thus far appear to have unusually high numbers of eosinophils, the cell that defends against metazoan (multicellular) parasites. In fact, there may not be any other animal species on the planet that has as many eosinophils as these salamanders! Another example is the heterophil numbers seen in most raptors, which tend to be high (above 60%), compared to those of passerines, which tend to be between 5-30%.
Finally, there is one other aspect the tables show. When there are multiple reports of cell numbers from the same species, there are often discrepencies among investigators. In most cases, these differences are in the relative number of neutrophils and lymphocytes reported. This is likely due to differences in stress levels of the animals examined in the different studies. Stress hormones are known to cause alterations in the relative numbers of these two cells in circulation (see page on stress and leukocytes). Specifically, stress causes increases in neutrophil numbers and decreases in numbers of lymphocytes in circulation. Thus, if one investigator captured and examined blood from wild individuals, and another examined blood from captive individuals of the same species, the two profiles may differ in the relative number of these cells because of the difference in stress levels of the animals. With this effect of stress in mind, one can also more easily interpret differences between studies that examined animals in breeding and non-breeding condition, since breeding animals often have higher stress levels.
The tables and this website represent a work in progress, and the tables will grow as new data becomes available. Please contact the author (akdavis[at]uga.edu) to contribute data or for questions.