The hematologic RIs obtained in this study are different from those previously reported in the literature, probably because those studies were developed in different experimental models of pigs and did not differentiate results according to the productive stage, BC or age [1,4,6,7,8,18]. Due to the evidence existence of factors like physiological state, weight can vary hematological values, we now propose new hematological RIs for sows, based on productive stage. These factors should be considered for porcine hematology data interpretation and could prevent incorrect clinical diagnoses for blood count test.
The changes in hematological parameters from day 35 of pregnancy showed a considerable decrease of blood cells and an increase in plasma proteins throughout sow gestation which were probably due to physiological pregnancy hemodilution occurrences, which has been previously reported in other species [1,15,16,17,18]. This hemodilution can produce a plasma volume increase of 30–50% during pregnancy, generating a constant decline of Hct and cell count, as pregnancy progresses [15]. The expansion in plasma volume depends on a physiological condition of pregnancy corresponding to increased activity of the renin-angiotensin-aldosterone system and increased water retention due to sodium (Na+) reabsorption from the kidneys [15]. The hemodilution in late pregnancy observed in domestic animals may have a physiological importance as it reduces blood viscosity and thereby, greatly increases blood flow in the small blood vessels, and facilitates the increase in blood flow to vital organs, such as the kidneys and uterus [15,19].
The effect of parity on blood cells might be due to the direct relationship with the amount hematopoietic cells. Several reports have indicated a physiological reduction in the production of erythrocytes and white blood cell count by lesser quantities of bone marrow cells, according to age of pigs [20]. Young sows, during the first gestations, are still in the growth and development processes and therefore, consumption of proteins is destined both to the fetuses and sow growth. Of note, the plasma protein concentration could be more sensitive to the amount of ingested protein [21]. The increase in plasma proteins is in agreement with other studies that associate the increase of age during the adult stage to a halt of the development process and growth [4]. In this study, it was essential to us to inform the RIs for each of the parities (1, 2, 3) and body condition (thin, optimal) at the different times evaluated, because interactions indicate the need to analyze hematological values together for each level, and it was also the main goal of these research.
The body condition generated an evident effect on first-parity sows. This could be due to the increase of protein demand during the sow’s growth stage, which has been informed to be notorious during gestation [21].
Among the hematological changes during the sow productive cycle, hypervolemia during pregnancy is also crucial to protect the mother and the fetus from the adverse effects of blood loss during delivery [15,17]. A reduction of the hematocrit value will result in false anemia during pregnancy [18]. Moreover, this hemodilution could explain the constant decrease of platelets throughout the gestation period and the lowest concentration of blood cells during the 90 days of gestation [22]. Besides, total white blood cell count decreases during gestation, and anemia during pregnancy has been documented in sows [4,18]. This could be explained due to the effect of hemodilution during pregnancy because there is a documented reduction of all the figurative elements [19]. Therefore, this physiological anemia could explain the gradual increase of plasma proteins during pregnancy mainly because secretion of albumin is stimulated by a decrease of osmotic pressure in plasma [23]. Moreover, some hormones, such as testosterone, estrogens, and growth hormone, promote an increase in plasma proteins because of their anabolic effects, to avoid edema, ascites and hydrothorax as a physiological response20. In our study, monocytes showed similar behavior to total white blood cell count and hematocrit, which could coincide with the dilution effect due to plasma increase.
Interestingly, we detected an inverse relationship between hematocrit and parity. The youngest sows (parities one and two) presented higher hematocrit percentages than sows of higher parities. This effect could be due to two factors. Firstly, hematocrit values tend to be higher in younger animals because the bone marrow is expanding, which contributes to hematopoiesis [18]. However, the demand for erythrocytes at maturity limits this function to long bones; thus, hematopoiesis decreases and red marrow is replaced by fatty tissue [18]. Secondly, young animals tend to be more susceptible to stress, which generates splenic contraction, producing a relative polycythemia by stimulation of the sampling process [22]. This hypothesis is reinforced by analyzing the white blood cell count, which is higher in younger animals and could be indicative of leukogram due to stress [4,18].
The interactions of hematocrit, plasma proteins, platelets and parity of sows during the time of the study are important and therefore, we believe it becomes necessary to inform the IR for each of the parities.
In this study, segmented neutrophils were highest at weaning and subsequent weaning and increased white blood cell count during these periods but decreased the Lymph count. In this respect, neutrophilia and lymphopenia frequently occur at parturition in sows, an effect that could be generated by the rise of cortisol associated to the accouchement [4]. Cortisol has a considerable impact on blood cells, including a reduction of lymph, a rise in the number of segmented neutrophils and total white blood cell count [23]. In this sense, it is well-known that leukogram stress is more neutrophilic than lymphophilic and is associated with a transient increase of hematocrit [4]. However, we must consider that sampling stress is the largest source of hematologic variation in swine and develops within two minutes, rapidly affecting the leukogram [4,24]. Another possible cause of increase is that segmented neutrophils play an important role in eliminating udder pathogens by phagocytosis and subsequently, killing bacteria, an important function during lactation [16].
Lymphocytes and segmented neutrophil counts increased more during early pregnancy (T3) than in the non-pregnant stages, as has been reported in mares, monkeys and women [15,17,25]. This is due to an increased inflammatory response during normal pregnancy, which can be a consequence of selective immune tolerance, immunosuppression and immunomodulation of the fetus [26]. All these factors may be related to the decreased immune function of maternal cells, the suppression of specific immune functions and a compensatory increase in non-specific immunity [19]. The different interaction level of eosinophils with farm origin could be due to the ventilation conditions of the farm, since the amount of ammonia perceptible at the time of sampling causes irritation of the respiratory system [22].
Regarding reference intervals, we can highlight that this study complies with all the recommendations of the ASVCP, in addition to the individual monitoring of sows during their reproductive cycle. Establishing reference intervals is essential to determine normality or alterations of some of the hemogram parameters, which provide fundamental information for the interpretation of exams [2]. Other reference intervals reported in the literature are broad and probably not representative because they did not consider the different stages of sow physiological state. Furthermore, these previous studies were established from smaller size samples or specific moments of the reproductive cycle [4,6,20,22]. Therefore, we strongly believe that the proposed RIs in our study are more precise, because we have included main of the factors that generated effects and interactions for variation of the reference intervals.
Despite our analysis, our study has some shortcomings, a) although we had a relatively appropriate sample size for analyses, fewer number of sows in each examined group could affect the statistical power, b) even though the two farms used to collect sows were similar in production and management terms, some differences in, for example, vaccination plans or microclimate, could affect homogeneity of the sample, c) some other potentially candidate factors were not evaluated in this study and other biochemical parameters (e.g. creatine kinase, aminotransferase activities, albumin, bilirubin, urea concentrations, among others) might be also important.
In conclusion, the present proposed reference intervals could constitute a sensitive tool to determine some hematological alterations in industrial production sows, particularly in highly industrialized swine production in Chile and other countries with similar productive management.