RETROSPECTIVE ANALYSES OF THE NEUTROPHIL TO LYMPHOCYTE RATIO IN PRIMATES HOUSED AT A SINGLE ZOO

The neutrophil to lymphocyte ratio (NLR) is a hematological parameter that has been identified as a biomarker of inflammation and stress. High NLR has been associated with cardiovascular disease, rheumatoid arthritis, septic shock, and advanced tumors in humans, but research in animals is more limited. Studies utilizing NLR in nonhuman primates have mostly focused on the impacts of age and sex but have also analyzed the effects of rearing and acute stressors. This study included 377 individuals from 23 species of nonhuman primates housed at the Saint Louis Zoo and investigated 1) the effect of demographic differences, including age, sex, and rearing, on NLR; 2) NLR as a marker of stress by comparing NLRs from samples taken during routine examinations versus those taken during clinical events; 3) whether the method of immobilization was associated with differences in NLR; and 4) whether routine NLR was associated with mortality risk. Data were analyzed by species using generalized linear mixed-effect models or, for mortality risk, by taxonomic groups using Cox proportional hazards models. The effects of demographic variables differed by species; for example, the effect of sex on NLR was significant in only four species: mongoose lemurs (Eulemur mongoz), black-and-white ruffed lemurs (Varecia variegata), Goeldi's monkeys (Callimico goeldii), and cotton-top tamarins (Saginus oedipus). At the taxonomic group level, NLRs from clinical events were significantly higher than those from routine events in lemurs and African and Eurasian monkeys but not Central and South American monkeys or great apes; at the species level, significant differences were observed only in black lemurs (Eulemur macaco), ring-tailed lemurs (Lemur catta), lion-tailed macaques (Macaca silenus), and gorillas (Gorilla gorilla). When NLR varied significantly by the method of immobilization, inhalation and hand injection were associated with the lowest NLRs. Routine NLR did not predict mortality risk for any taxonomic group.