In the present study, WBC, RBC, Hb and Ht were significantly lower in Fukushima monkeys than in Shimokita monkeys. Although direct comparison of this study with previous studies may be problematic, since methods and sites for blood sampling reportedly affect blood properties12, the blood cell counts of the present Shimokita monkeys are similar to those reported previously13,14. Nigi et al.14 reported that Japanese monkey populations, except for the Yakushima macaque (Macaca fuscata yakui) subspecies, show large individual variability in blood cell counts, with no difference between the local populations. Although the number is small, previous studies that investigated blood cell counts in wild Japanese monkey populations showed no evidence for different hematological values among the populations. It is possible that the low blood cell counts in Fukushima monkeys were caused by infectious diseases or malnutrition. However, our group has been investigating and has autopsied more than 1000 monkeys captured in Fukushima City since 2008, with no findings of infectious disease specific to the area that may have reduced blood cell counts. Moreover, fat indices did not vary significantly between Fukushima and Shimokita monkeys (Tables 1, 2), indicating that the low blood cell counts was not caused by malnutrition.

Because no cesium was detected in the muscle of Shimokita monkeys, the low hematological values in Fukushima monkeys could have therefore been due to the effect of other radioactive materials. Stepanova et al. conducted hematological studies of Ukrainian children between 1993 and 1998 after the Chernobyl disaster of 198615. They observed reduced blood cell counts, Hb and platelet counts in these children and found that the extent of the reduction in each child correlated with the level of radiocesium in the soil of the area of residence. This is similar to what we observed in the present study. Although blood cell counts varied significantly between Fukushima and Shimokita populations, no significant difference was observed between the 2 groups of Fukushima monkeys captured in areas with different levels of soil contamination. The study conducted in Ukraine that is described above also showed that WBC did not differ significantly near the border of 2 areas with different levels of soil contamination15. Further studies are needed to investigate monkeys inhabiting an area with a high soil contamination level. In addition, the muscle radiocesium concentrations in Fukushima monkeys are known to show seasonal variation, increasing 2–3-fold in winter5. This suggests that muscle cesium concentrations would vary greatly among monkeys captured in the same area, as in this study (Table 1). The biological half-life of cesium in monkeys is approximately 21 days5. Even if radiation damage is the cause of the low blood cell counts seen here, it is difficult to prove a causal relationship because of the time lag between uptake of the radioactive material and the appearance of radiation damage. The difficulty multiplies when comparing areas with relatively similar radiation exposure.

Despite these complex factors, a significant negative correlation was observed between WBC and muscle radiocesium concentrations in immature Fukushima monkeys (Table 3). In addition, WBC, RBC, Hb and Ht values–which were lower Fukushima monkeys compared with Shimokita monkeys–were significantly correlated with each other, suggesting that with more samples it will be possible to verify the correlation between the 4 hematological values and the muscle radiocesium concentrations. In immature Fukushima monkeys, WBC was significantly negatively correlated with cesium concentration in the muscle, but in mature Fukushima monkeys, no correlation between hematological values and muscle cesium concentration was observed. It is possible that WBC declined because immature monkeys were more vulnerable to radioactive materials. Moysich et al.16 conducted an epidemiological study to investigate the risk of leukemia among Europeans affected by the Chernobyl disaster and found that the risk was clearly higher among small children than among adults, suggesting that the hematological consequences of radiation exposure vary by age.

The hematological changes in the Fukushima monkeys might likely be the result of exposure to some form of radioactive material, but only radiocesium concentration was measured in this study. These hematological changes might have been caused by a decline in hematopoietic function in the bone marrow because the WBC differential did not differ between the Fukushima and Shimokita monkeys. We therefore plan to investigate in a future study the underlying mechanism in detail with the aim of detecting other radioactive materials, such as 90Sr. Presently, it is difficult to investigate Japanese monkeys inhabiting highly contaminated areas where entry is restricted. However, we intend to perform hematological tests and other measurements as soon as the relevant permissions can be obtained.

Low blood cell count does not necessarily mean that the health of individual monkeys is at risk. However, it may suggest that the immune system has been compromised to some extent, potentially making individual animals and the entire troop susceptible to, for example, epidemic infectious disease. It is therefore necessary to perform long-term immunological and other health-related studies of Japanese monkey populations in Fukushima.