Physiological alterations and predictors of death in neonatal calves with weak calf syndrome

Abstract

Background

Weak calf syndrome (WCS) is characterised by weakness, poor vitality and difficulty standing or suckling. Early identification of physiological alterations and prognostic indicators is critical for improving the management and survival of weak calves.

Methods

Twenty‐six neonatal calves, aged 1–5 days, that were unable to stand or suckle were analysed for electrolytes, blood gases, serum biochemistry and protein electrophoresis. The prognostic utility of these variables was then evaluated using receiver operating characteristic analysis.

Results

Twenty calves survived, while six died—four on the first day, one on the fourth day and one on the seventh day. Surviving calves lived for at least 90 days. Hypothermia occurred in 16 calves, elevated pCO₂ in 23 calves, acidaemia in 17 calves and hypoproteinaemia in all calves. Calves that did not survive had lower pH and alkaline phosphatase (ALP) than those that survived, while their pCO₂ and L‐lactate were higher. pCO₂ showed excellent performance as a prognostic indicator, while pH and ALP showed good prognostic performance.

Limitations

The small population size, absence of a control group and non‐standarsdised timing of diagnosis limit the generalisability of the findings. Furthermore, postmortem investigations were not conducted, so the causes of death could not be definitively identified.

Conclusion

Weak calves exhibit hypothermia, respiratory acidosis, hyperlactataemia and passive immunity failure. Parameters such as pH, pCO₂ and ALP are important prognostic indicators in these calves.

INTRODUCTION

Newborn calves experience drastic changes in their environment immediately after birth, which is a highly vulnerable stage due to the abrupt transition to extra‐uterine life, requiring independent regulation of respiration, metabolism, body temperature and immunity. Calf loss mainly occurs within the first 7 days after birth, ¹ , ² , ³ with death classified as perinatal (full‐term birth to 2 days of life) or neonatal (1 day of life to weaning). ⁴ Perinatal mortality remains high globally, ⁵ with reported rates ranging between 2% and 9.7%. ⁶

The term weak calf syndrome (WCS) has been broadly applied to calves that are stillborn or born small and weak and show difficulty or an inability to stand and nurse promptly. ⁷ , ⁸ , ⁹ Such calves often die within a few days of birth and are referred to as dummy or fading calves. ⁹ Previous studies have reported that environmental, genetic, infectious and nutritional causes can lead to calves being born weak or contribute to the risk of perinatal mortality. For example, weak calves can result from factors such as hypothermia caused by cold and wet climates, ⁹ , ¹⁰ , ¹¹ , ¹² dystocia, ² , ⁹ , ¹² , ¹³ the dam's age, parity and nutritional status at parturition, ¹⁴ , ¹⁵ breed differences, ¹⁵ , ¹⁶ infectious agents associated with abortion (bovine viral diarrhoea virus, Brucella abortus, Coxiella burnetii, Leptospira Hardjo, Neospora caninum, Pasteurella multocida and Salmonella Dublin), ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² trace mineral and vitamin deficiencies, ²⁰ , ²³ and mutations in the IARS gene (which encodes isoleucyl‐tRNA synthetase). ²⁴ Among these various causes, dystocia is one of the most important. Difficult or prolonged parturition often results in trauma, hypoxia and acid‒base imbalances, which not only directly cause death but also reduce the vitality of newborn calves. ³ , ¹³ , ¹⁶ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰

During birth, newborns must survive independently as they are separated from the physiological functions previously supported by the mother through the umbilical cord. ³¹ Newborn calves must maintain blood oxygen saturation, regulate their acid‒base balance, activate metabolic pathways for energy production and maintain body temperature. ³¹ , ³² , ³³ In addition, newborn calves need to absorb maternal antibodies from colostrum to be protected from infection during the early neonatal period. ³³ , ³⁴ , ³⁵ The vitality of newborn calves is crucial for survival, as those with reduced vitality often struggle to stand and fail to consume colostrum. ³⁶ , ³⁷ , ³⁸ Adequate colostrum intake is essential for acquiring energy, maintaining body temperature and ensuring the passive transfer of immunoglobulins. ³⁴ , ³⁸ , ³⁹ , ⁴⁰ Failure to achieve these functions increases the mortality of newborn calves both in the short and long term. ²⁷ , ⁴⁰ , ⁴¹ Although healthy calves successfully adapt to the external environment during these processes, weak calves fail to properly undergo these physiological transitions, resulting in persistent hypoxia, metabolic and respiratory acidosis, and/or hypothermia, which are the major factors that increase the risk of death. ² , ³¹ , ³⁴ , ³⁵ , ⁴¹ , ⁴² , ⁴³

Various clinical scoring systems have been developed to evaluate the vitality of newborn calves, with the most well known being an adaptation of the Apgar score originally designed for human infants. ³⁶ , ³⁸ , ⁴⁴ The Apgar score assesses five clinical criteria: heart rate, respiration, muscle tone, reflexes and mucous membrane colour. ⁴⁵ Based on this scoring system, various modified systems have been developed to incorporate specific calf behaviours or to consider their correlations with physiological indicators. ³⁶ , ³⁸ , ⁴⁴ The dairy calf vigor score evaluates calves' visual appearance, initiation of movement, general responsiveness, oxygenation, and heart and respiratory rates, serving as a comprehensive tool to assess calf vitality and objectively evaluate their health status and survival potential. ⁴⁶ However, the practical application of these scoring systems is often limited due to their complexity and lack of consistency. ³⁶ , ³⁸ , ⁴⁴ Developing simpler and more practical evaluations based on blood parameters, such as L‐lactate and pH, alongside existing scoring systems could provide a more effective approach. ³⁶ , ³⁸ , ⁴⁴ , ⁴⁷

Because successfully impregnating cows, birthing healthy calves and raising them well are crucial aspects of cattle farming, neonatal calf mortality is economically significant and remains a major concern from an animal welfare perspective. Considering concerns regarding animal welfare and resuscitation of weak calves, identifying clinically useful prognostic factors is desirable. In addition, it is essential to gain a clear understanding of the causes and clinical outcomes of WCS and, based on this knowledge, develop strategies to reduce calf mortality.

We hypothesised that the recovery of weak calves is associated with their initial physiological state and that identifying specific blood parameters could predict their survival outcomes. The objective of this study was to report a comprehensive biochemical profile in weak calves and to identify prognostic indicators for death.

MATERIALS AND METHODS

Animal population and management

Between October 2023 and March 2024, 26 indigenous Korean (Hanwoo) calves were recruited to this study. The calves originated from 23 farms, each raising fewer than 100 cattle. No clinical signs of infectious diseases were observed in other cattle on the farms where the neonatal calves were born during the 30 days preceding their birth. All cattle were housed indoors at their respective farms of origin. Pregnant dams were monitored by farmers and managed in individual pens within barns during the late gestation period. Bedding was provided in all farms, and heat lamps were used to maintain a warm environment.

After birth, the dams were tied within their pens. The farmers placed the calves in sternal recumbency, rubbing their bodies with a dry towel to stimulate respiration and remove any remaining moisture. Mucus from the nose and mouth was cleared manually. Within two hours of birth, calves were encouraged to suckle from the dam. If suckling was insufficient, they were additionally provided with 1–2 L of either frozen colostrum or a solution containing 300–500 g of colostrum replacer with at least 30% immunoglobulin G. The frozen colostrum had an immunoglobulin G concentration of more than 50 g/L. A bottle or an oesophageal tube feeder was used for feeding.

Inclusion criteria and intervention

When calves did not display normal vitality, farmers contacted a veterinarian from the College of Veterinary Medicine at Jeonbuk National University. Veterinarians arrived on‐site within an hour, and all evaluations and treatment procedures were performed by the same veterinarian (B. K.). Calves were included in the study regardless of the presence of dystocia or comorbidities if they could not maintain a standing position on all four legs or exhibited a weak or absent suckling reflex (Figure 1).

FIGURE 1.

Weak calf syndrome in Korean indigenous (Hanwoo) calves. (a) Calf unable to stand on its own and lacking a suckling response. (b) One of the calves showing weakness (left) following biparous birth

After measuring rectal temperature and collecting blood samples, the veterinarian administered treatments, including intravenous fluids, whole blood transfusion from the dam, glucose administration, antibiotic therapy and umbilical disinfection with povidone. All treatments and care were performed on‐site at each calf's respective farm. The veterinarian visited each farm daily for 7 days to check the calves’ conditions. Treatments were no longer administered when the calves either stood unaided with strong suckling reflex or died. Calves were fed according to standard feeding guidelines for the Hanwoo breed. ⁴⁸ To ensure adequate nutritional intake, farmers were instructed to provide an extra 1–2 L of milk replacer per day and to use oesophageal tube feeders when necessary. Among the weak calves, those that survived for more than 7 days were classified as surviving calves, while those that died within 7 days were classified as non‐surviving calves. The survival of the calves at 90 days after birth was also recorded.

Blood sample collection

Blood samples were taken from the jugular vein of the calves using a 21‐gauge needle and a 10‐mL syringe (BD Emerald Syringe; Becton Dickinson) and anaerobically collected into 5‐mL serum‐separating tubes (Vacuette serum tube; Greiner Bio‐One) before any therapeutic intervention. The tubes were transported to the laboratory in a refrigerated state within 1 hour of collection. The serum‐separating tubes were left to stand for at least 1 hour, and the serum was then separated by centrifugation at 3000 × g for 10 minutes. The separated serum was stored in a freezer at –24°C and analysed within 1 week. Frozen serum samples were thawed at room temperature for 30 minutes before analysis.

Measurement of parameters

Body temperature was measured before blood sampling using a portable electronic thermometer inserted into the rectum of the calves. The L‐lactate concentration was measured using a portable lactate meter (Nova Biomedical) immediately after the collection of whole blood. Electrolytes, blood gases, acid‒base parameters and haematological parameters were measured in whole blood immediately after blood collection using an i‐STAT device and an EC8+ cartridge (Abbott Point of Care). The measured parameters included sodium ions (Na⁺, mmol/L), potassium ions (K⁺, mmol/L), chloride ions (Cl⁻, mmol/L), total carbon dioxide (mmol/L), venous pH, partial pressure of carbon dioxide (pCO₂, mmHg), bicarbonate ions (HCO₃ ^–, mmol/L), base excess of the extracellular fluid (mmol/L), anion gap (mmol/L), haematocrit (%) and haemoglobin concentration (g/dL).

Biochemical parameters were measured in separated serum samples using an IDEXX Catalyst One chemistry analyser (IDEXX Laboratories). The measured parameters were creatinine (mg/dL), serum urea nitrogen (mg/dL), calcium (Ca, mg/dL), inorganic phosphate (P, mg/dL), total protein (g/dL), albumin (g/dL), globulin (g/dL), alkaline aminotransferase (U/L), aspartate aminotransferase (U/L), alkaline phosphatase (ALP, U/L), γ‐glutamyltransferase (GGT, U/L), total bilirubin (mg/dL), bile acid (µmol/L), cholesterol (mg/dL) and glucose (mg/dL) concentrations.

The identification of elevated and decreased levels of these parameters was conducted using reference ranges. Due to the lack of consistent and well‐documented reference ranges for neonatal calves, reference ranges for adult cattle were used as an alternative. ⁴⁹ , ⁵⁰ , ⁵¹

The separated serum samples were also used to perform agarose gel electrophoresis to analyse five protein fractions (albumin, α₁‐globulin, α₂‐globulin, β‐globulin and γ‐globulin). This process was performed using a semi‐automated agarose gel electrophoresis system (HYDRASYS 2; Sebia) according to the manufacturer's protocol. Briefly, 300 µL of serum was subjected to a microtechnique assay, electrophoresed for 35 minutes, stained for 5 minutes and de‐stained for 5 minutes. The samples were then cleared for 30 seconds, the excess solution was removed with a glass rod and the samples were dried for 10 minutes before being measured by optical density scanning.

Statistical analysis

The data were analysed using the SPSS statistical software package (SPSS 21.0; IBM) and GraphPad Prism 8 (GraphPad Software). Descriptive statistics were calculated for the dataset. The normality of the data was assessed using the Kolmogorov‒Smirnov and Shapiro‒Wilk tests. For comparisons between surviving and non‐surviving calves, the independent‐samples t‐test or Mann‒Whitney U‐test was used, depending on the results of the normality tests for each variable. Statistical significance was set at a p‐value of less than 0.05.

Receiver operating characteristic (ROC) curve analysis was performed to evaluate the ability of the parameters to predict death in weak calves. Predictive performance was confirmed by calculating the area under the ROC curve (AUC), and was classified as excellent (0.9–1.0), good (0.8–0.89), fair (0.7–0.79), poor (0.6–0.69) or fail (0.0–0.59). ⁵² , ⁵³ The optimal cut‐off value for each prognostic indicator was determined using Youden's index, ⁵⁴ and the positive and negative predictive values were calculated.

RESULTS

Population characteristics

Of the 26 calves included in the study, all but two were delivered naturally; two were assisted deliveries due to delayed parturition. All calves were full‐term birth, and one was the weaker calf from a biparous birth. The median age (range) was 1.5 (1–5) days, with 12 females and 14 males (Table 1).

TABLE 1.

Demographic information for 26 calves diagnosed with weak calf syndrome (WCS), grouped according to whether or not they died within 7 days

Housing type	All calves with WCS	Survivors	Non‐survivors
Number of cases	26	20	6
Sex
Female	12	10	2
Male	14	10	4
Age at diagnosis
1 day	13	8	5
2 days	9	9	0
3 days	2	2	0
4 days	1	1	0
5 days	1	0	1

Thirteen calves regained the ability to stand and showed improved suckling behaviour within the first day of treatment, while seven calves recovered by the third day. Six calves died: four within the first day, one on the fourth day and another on the seventh day. Weak calves that recovered within the first week survived for at least 90 days.

Measured parameters and prevalence

Hypothermia was observed in 16 (61.5%) calves, and anaemia was detected in five (19.2%) calves. Electrolyte imbalances were found in two calves, with one showing hyponatraemia and hypochloraemia and the other showing hyperkalaemia. Acidaemia was present in 17 (65.4%) calves, four (15.4%) of which had severe acidaemia (pH < 7.2). Elevated pCO₂ levels were observed in 23 (88.5%) calves, increased L‐lactate concentrations in 21 (80.8%) calves and elevated P concentrations in six (23.1%) calves. All weak calves had hypoproteinaemia, and 22 (84.6%) calves had hypoglobulinaemia. Creatinine concentrations were elevated in 14 (53.8%) calves, ALP and GGT activities were elevated in 16 (61.5%) calves each and three (11.5%) calves showed low GGT activities. Total bilirubin concentrations were elevated in 24 (92.3%) calves, and all calves had low cholesterol concentrations. Haematocrit and haemoglobin levels were within their reference ranges for all calves. Hyperglycaemia was observed in 15 (57.7%) calves, hypoglycaemia in five (19.2%) calves and normoglycaemia in six (23.1%) calves (Figure 2).

FIGURE 2.

Overview of parameter alterations in weak calves. The six calves highlighted in grey (nos. 1, 16, 19, 20, 22 and 24) represent those that did not survive. Pink ‘H’ boxes represent values that are above the normal reference range and light blue ‘L’ boxes represent values that are below the normal reference range Dark blue ‘S’ boxes represent cases of severe acidaemia (pH < 7.2), and light green ‘M’ boxes represent cases of mild to moderate acidaemia (7.2 ≤ pH < 7.35). Blank boxes represent values within the reference range. Reference ranges are detailed in Table 2. ALB, albumin; ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; BE, base excess; BT, body temperature; CHOL, cholesterol; CRE, creatinine; GGT, γ‐glutamyl transferase; GLOB, globulin; GLU, glucose; Hb, haemoglobin; HCT, haematocrit; L‐Lac, L‐lactate; pCO₂, partial pressure of carbon dioxide; SUN, serum urea nitrogen; TBil, total bilirubin; TP, total protein

Results in surviving and non‐surviving calves

Significant differences in pH, pCO₂, base excess, L‐lactate ALP and P levels were observed between surviving and non‐surviving calves. Although not statistically significant (p = 0.052), the median rectal temperature was lower in non‐surviving calves (37.6°C) than in surviving calves (38.0°C). Non‐surviving calves exhibited a lower pH (7.21) than surviving calves (7.33) (p < 0.001), while pCO₂ was higher in non‐surviving calves (69.6 mmHg) than in surviving calves (52.1 mmHg) (p < 0.001). L‐lactate concentrations were elevated in non‐surviving calves (6.7 mmol/L) when compared with surviving calves (4.0 mmol/L) (p = 0.011). P concentrations were higher in non‐surviving calves (7.4 mg/dL) than in surviving calves (5.7 mg/dL) (p = 0.047). In contrast, base excess and ALP activity were lower in non‐surviving calves than in surviving calves (0.0 mmol/L vs 1.1 mmol/L, p = 0.015, and 137.5 U/L vs 316.0 U/L, p = 0.011, respectively) (Table 2 and Figure 3).

TABLE 2.

Physiological and biochemical parameters in calves with weak calf syndrome (WCS)

	Total WCS (n = 26)	Survivors (n = 20)	Non‐survivors (n = 6)	Reference range 49 , 50 , 51	p‐value
Body temperature (°C) a	38.0 (37.5–38.8)	38.1 (37.7–38.8)	37.6 (33.0–38.2)	38.5–39.5	0.052
Na+ (mmol/L)	139.0 (136.8–141.3)	139.0 (136.3–141.8)	139.0 (136.3–141.3)	132–152	0.943
K+ (mmol/L)	4.7 (4.4–5.1)	4.7 (4.4–5.1)	4.6 (4.4–5.4)	3.9–5.8	0.864
Cl− (mmol/L) a	98.0 (96.0–101.0)	98.0 (96.0–101.0)	100.5 (95.5–104.3)	95–110	0.358
pH	7.31 (7.23–7.37)	7.33 (7.28–7.38)	7.21 (6.89–7.25)	7.35–7.5	<0.001
pCO2 (mmHg)	54.1 (50.4–64.9)	52.1 (49.6–54.7)	69.6 (68.0–78.6)	34–45	<0.001
tCO2 (mmol/L) a	28.0 (26.8–31.0)	28.0 (27.0–31.0)	30.5 (16.8–32.3)	20–30	0.284
HCO3 − (mmol/L)	26.6 (25.0–29.2)	26.2 (25.0–29.2)	28.0 (14.7–30.0)	20–30	0.755
BEecf (mmol/L)	0.0 (−2.0–2.0)	0.5 (−1.8–3.5)	0.0 (−18.3–0.3)	−3.5–3.5	0.015
Anion gap (mmol/L)	19.5 (17.0–21.3)	20.0 (18.0–21.0)	17.5 (15.8–24.3)	14–26	0.895
L‐lactate (mmol/L)	4.3 (2.6–6.2)	4.0 (2.5–4.9)	6.7 (3.6–14.5)	0.6–2.2	0.011
HCT (%)	30.5 (25.5–36.0)	30.0 (24.5–35.5)	32.0 (24.3–38.0)	24–46	0.755
Haemoglobin (g/dL)	10.4 (8.7–12.2)	10.2 (8.4–12.1)	10.9 (8.2–12.9)	8–15	0.749
Creatinine (mg/dL)	2.1 (1.6–2.5)	2.1 (1.5–2.5)	1.9 (1.4–2.8)	1–2	0.842
SUN (mg/dL) a	10.0 (7.8–11.8)	9.0 (7.3–13.3)	10.5 (9.3–13.3)	6–27	0.409
Ca (mg/dL)	10.5 (9.8–11.3)	10.5 (9.8–11.2)	10.6 (9.4–11.6)	9.7–12.4	0.820
P (mg/dL) a	5.8 (5.0–6.5)	5.7 (4.7–6.1)	7.4 (5.6–8.3)	5.6–6.5	0.047
Total protein (g/dL)	4.1 (3.8–4.4)	4.1 (3.7–4.3)	4.0 (3.7–4.7)	5.7–8.1	0.939
Albumin (g/dL)	1.9 (1.7–2.0)	1.9 (1.7–2.0)	2.1 (1.8–2.2)	2.1–3.6	0.130
Globulin (g/dL) a	2.0 (1.9–2.5)	2.1 (1.8–2.4)	2.0 (1.9–2.6)	3–3.5	0.667
ALT (U/L) a	44.5 (35.8–57.8)	45.0 (40.0–54.8)	38.0 (33.5–136.5)	11–40	0.604
AST (U/L) a	99.5 (79.0–140.3)	99.5 (83.3–141.0)	95.0 (49.8–538.5)	78–132	0.605
ALP (U/L) a	267.0 (157.0–513.3)	316.0 (196.3–535.8)	137.5 (114.5–263.5)	0–200	0.011
GGT (U/L) a	19.5 (8.0–97.3)	30.0 (10.5–119.8)	8.5 (6.0–22.8)	6.1–17.4	0.055
Total bilirubin (mg/dL) a	1.2 (0.7–1.7)	1.3 (0.8–1.7)	0.9 (0.6–1.5)	0.01–0.5	0.272
Bile acid (µmol/L) a	17.5 (12.6–28.9)	18.4 (11.8–29.0)	14.5 (12.5–34.8)	<50	0.738
Cholesterol (mg/dL) a	13.5 (9.0–26.3)	13.5 (9.0–24.0)	22.0 (9.0–42.0)	65–220	0.321
Glucose (mg/dL)	85.5 (51.5–111.0)	85.5 (59.3–108.3)	60.5 (25.0–119.8)	45–75	0.429

Note: Data are presented as median (interquartile range).

Abbreviations: ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; BEecf, base excess of the extracellular fluid; GGT, γ‐glutamyltransferase; HCT, haematocrit; pCO₂, partial pressure of carbon dioxide; SUN, serum urea nitrogen; tCO₂, total carbon dioxide.

^a Non‐parametric tests were used for comparisons between survivor and non‐survivor groups.

FIGURE 3.

Physiological parameters associated with survival in weak calves. The parameters shown are those with statistically significant (p < 0.05) differences between surviving and non‐surviving calves. These include (a) pH, (b) partial pressure of carbon dioxide (pCO₂), (c) base excess, (d) L‐lactate, (e) alkaline phosphatase (ALP) and (f) inorganic phosphate (P). The boxes represent the interquartile range (IQR) and the line inside each box indicates the median. Whiskers extend to the most extreme values within 1.5 times the IQR. Values beyond this range are shown as individual points. The shaded areas within the dashed lines represent reference intervals for healthy adult cattle. ⁴⁹ , ⁵⁰

Protein electrophoresis results

Protein fraction electrophoresis was performed on all 26 weak calves. All values are expressed as median (interquartile range). Total protein was 4.01 (3.78–4.44) g/dL, albumin 1.90 (1.80–2.00) g/dL, α₁‐globulin 0.15 (0.10–0.70) g/dL, α₂‐globulin 1.20 (0.95–1.30) g/dL, β‐globulin 0.40 (0.40–0.53) g/dL and γ‐globulin 0.10 (0.10–0.30) g/dL. No significant differences in any protein fraction were observed between survivors and non‐survivors.

Prognostic indicators for death in weak calves

ROC analysis was performed on the parameters that differed significantly between surviving and non‐surviving calves to determine their ability to predict death. ALP, P, pH and pCO₂ levels showed significant predictive performance (Figure 4). The AUC for pH was 0.89 (p = 0.004), indicating good predictive performance. At a pH cut‐off of less than 7.25, the sensitivity and specificity were 83.33% and 90.00%, respectively. The AUC for pCO₂ was 1.00 (p < 0.001), showing excellent predictive performance, with both sensitivity and specificity reaching 100.00% when the cut‐off exceeded 65.9 mmHg. The AUC for ALP was 0.85 (p = 0.011), indicating good predictive performance, with a sensitivity of 83.33% and a specificity of 95.00% when the cut‐off was below 156 U/L. For P, the AUC was 0.77 (p = 0.048), indicating fair predictive performance. When the cut‐off exceeded 6.65 mg/dL, the sensitivity and specificity were 66.67% and 90.00%, respectively (Table 3).

FIGURE 4.

Receiver operating characteristic (ROC) curves for predicting death in weak calves. The areas under the ROC curves are as follows: (a) pH, 0.89 (p = 0.004); (b) pCO₂, 1.00 (p < 0.001); (c) L‐lactate, 0.73 (p = 0.088); (d) base excess, 0.69 (p = 0.162); (e) alkaline phosphatase (ALP), 0.85 (p = 0.011); (f) P, 0.77 (p = 0.048). The red lines represent the ROC curves for each parameter

TABLE 3.

Receiver operating characteristic curve analysis for predicting death in calves with weak calf syndrome

	AUC	95% CI	p‐Value	Cut‐off	Sensitivity (%)	Specificity (%)	PPV (%)	NPV (%)
pH	0.89	0.76–1.00	0.004	<7.25	83.33	90.00	71.43	94.74
pCO2 (mmHg)	1.00	1.00–1.00	<0.001	>65.9	100.00	100.00	100.00	100.00
L‐lactate (mmol/L)	0.73	0.44–1.00	0.088	>5.9	66.67	85.00	57.14	89.47
BEecf (mmol/L)	0.69	0.46–0.92	0.162	<1.5	100.00	35.00	31.58	100.00
ALP (U/L)	0.85	0.61–1.00	0.011	<156	83.33	95.00	83.33	95.00
P (mg/dL)	0.77	0.54–1.00	0.048	>6.65	66.67	90.00	66.67	90.00

Abbreviations: ALP, alkaline phosphatase; AUC, area under the receiver operating characteristic curve; BEecf, base excess of the extracellular fluid; CI, confidence interval; NPV, negative predictive value; pCO₂, partial pressure of carbon dioxide; PPV, positive predictive value.

DISCUSSION

The first week of life has been identified as a critical period for calf survival. Among the six calves in this study that did not survive, four died on the first day, one on the fourth day and the other on the seventh day. In contrast, weak calves that recovered within the first week survived up to 90 days, underscoring the importance of timely interventions during this decisive period.

Hypothermia is a major physiological stress factor in newborn calves, especially those born in cold weather. Although no significant difference in body temperature was found between surviving and non‐surviving calves in this study, the median rectal temperature of weak calves was 38.0°C, with hypothermia observed in 16 of 26 (61.5%) weak calves. When a newborn calf is exposed to cold, glucocorticoid and catecholamine secretion increases, leading to activation of the sympathetic nervous system. This results in increased utilisation of substrates, such as fat, glycogen and protein, thereby enhancing heat production through catabolic processes. In weak calves, insufficient intake of colostrum as their primary energy source increases the risk of hypothermia. A 40 kg calf needs approximately 2.4 L of colostrum to maintain thermoneutrality, and for every 1°C decrease in environmental temperature below the lower critical temperature, an additional 125 mL of colostrum is required to meet energy demands. In addition, weak calves commonly suffer from hypoxia, which may worsen hypothermia. ¹² , ³⁵ , ⁵⁵ , ⁵⁶

In this study, most calves with WCS exhibited respiratory acidosis (a decrease in pH with an increase in pCO₂), with non‐surviving calves showing more severe acidaemia and a greater increase in pCO₂ than surviving calves. In utero, the pulmonary vessels are constricted and most of the blood bypasses the lungs and receives oxygen via the placenta. After birth, uterine contractions help remove lung fluid, and the first breath initiates a reduction in pulmonary vascular resistance, activating the pulmonary circulation. During this process, calves experience hypoxia and respiratory and metabolic acidosis. The severity of respiratory acidosis depends on the time between the cessation of blood supply from the mother and the onset of successful respiration, with healthy calves correcting hypercapnia and hypoxia within a few hours after birth. ³¹ , ³² , ⁵⁷

L‐lactate is the end product of the anaerobic glycolysis of pyruvate. When the fetal membranes rupture and the umbilical vessels separate, the maternal blood supply ceases. This promotes anaerobic glycolysis in tissues to maintain energy metabolism. ³¹ , ³² Therefore, L‐lactate is an indicator of hypoxia and tissue hypoperfusion. It has also been reported to be a prognostic marker in cattle with acute respiratory diseases and in calves undergoing surgery for acute abdominal emergencies. ⁵⁸ , ⁵⁹ , ⁶⁰ In the present study, L‐lactate concentrations were elevated in weak calves, with even higher concentrations observed in the non‐surviving group, likely because of anaerobic metabolism due to hypoxia.

Serum or blood urea nitrogen and serum creatinine levels were used to estimate the glomerular filtration rate. Although additional assessments, such as urinalysis, were not conducted, the elevated creatinine concentrations in the weak calves in this study may be due to acute kidney dysfunction caused by hypoxia. Urea is produced when ammonia, a byproduct of protein catabolism, is metabolised in the liver through the urea cycle and can be affected by liver function and diet. ⁶¹ Because weak calves have hypoproteinaemia due to poor colostrum intake, creatinine concentrations may be elevated, whereas serum urea nitrogen concentrations may not be. A decrease in the glomerular filtration rate can lead to hyperphosphataemia, ⁶¹ , ⁶² and an increase in P concentrations was observed in non‐surviving calves, with P showing a fair ability to predict death in weak calves.

This study's weak calves generally exhibited hypoproteinaemia and low γ‐globulin concentrations, although there were no significant differences between survivors and non‐survivors. This suggests that passive immunity transfer through colostrum may have been insufficient, possibly due to the reduced ability of the calves to suckle. Moreover, even considering the relatively low birthweight of Hanwoo calves (approximately 25–27 kg ⁴⁸ , ⁶³ , ⁶⁴ ) and fact that the mean concentration of immunoglobulin G in Hanwoo colostrum has been reported as 101.51 g/L, ⁶⁵ the insufficient quantity and quality of colostrum provided in this study could have led to these findings. According to previous studies, the criteria for classifying calves as having failure of passive immunity transfer are serum immunoglobulin G concentrations of less than 1 g/dL (10 mg/mL) 24–48 hours after birth. ⁶⁶ , ⁶⁷

Newborn calves non‐selectively absorb various macromolecules, including immunoglobulins, via pinocytosis from enterocytes of the small intestine during the first 24‒36 hours after birth. The absorbed immunoglobulins are transported across cells into the lymphatic system via exocytosis. Immunoglobulins then enter the circulatory system through the thoracic ducts. The non‐selectivity of this process leads to an increase in the concentration of other protein macromolecules, such as GGT, in newborn calves. ³⁴ , ³⁹ , ⁶⁸ Therefore, calves that successfully absorb colostrum may have significantly higher serum GGT concentrations than adult cattle. In 1‐day‐old calves, the serum GGT concentration should be approximately 200 IU/L. Calves with a serum GGT concentration of less than 50 IU/L within the first 2 weeks of life should be considered to have failed passive transfer. ⁶⁸ , ⁶⁹ Although endogenous sources are not excluded, ALP activity is also significantly elevated in newborn calves. ⁶⁹ , ⁷⁰ In this study, the serum ALP and GGT concentrations of weak calves were normal or slightly elevated compared with the reference values in adult cattle. Serum ALP concentrations were significantly lower in non‐surviving calves than in surviving calves and were found to be a good predictor of death in weak calves. Such failure in passive immunity can compromise defence against infections, potentially leading to an increase in neonatal calf mortality.

However, this study has certain limitations that must be acknowledged. The small sample size may limit the generalisability of the results. In particular, the inclusion of only six non‐surviving calves necessitates cautious interpretation of the ROC curve analysis and the associated cutoff values. Differences in the farmers’ experience could have influenced the selection process and the physiological outcomes of the calves included in the study. The inconsistent time taken to request assistance from the veterinarian was another limitation, as this variability, along with differences in the calves' ages, may have influenced their physiological status and parameters, potentially affecting survival outcomes. The absence of a control group comprising healthy and vigorous calves also limited the ability to distinguish whether the findings in the surviving group were specific to WCS or reflected normal neonatal physiology.

Even healthy calves typically exhibit blood gas disturbances, acid‒base imbalances, high L‐lactate concentrations and low protein concentrations immediately after birth, which are compensated and restored to normal levels within 12‒24 hours. ⁴⁷ , ⁷¹ , ⁷² , ⁷³ , ⁷⁴ , ⁷⁵ For example, the pH is approximately 7.2 immediately after birth and gradually recovers within 12‒24 hours, with pCO₂ normalising in a similar timeframe. ⁴⁷ , ⁷¹ , ⁷² , ⁷³ L‐lactate concentrations are initially high but gradually decrease over several hours or days. ⁴⁷ , ⁷¹ , ⁷² These changes are considered the result of respiratory stabilisation and compensation. Protein concentrations are also typically low immediately after birth, with hypoproteinaemia being common, but they gradually increase as colostrum absorption occurs. ⁷⁴ , ⁷⁵

This study reflects the practical context of evaluating and treating WCS in clinical situations. To enhance the reliability of the study, farmers were guided to apply consistent management practices as much as possible, and all final evaluations and treatments of the calves were performed by a single veterinarian to minimise potential bias in the selection process and findings. Weak calves in this study exhibited variations in acid‒base parameters depending on their age at the time of diagnosis. Calves diagnosed later tended to show a normalisation of acid‒base imbalances (Tables S1 and S2). This suggests that time‐related changes in acid‒base balance may confound the prognostic value of these parameters, emphasising the need for caution when using them as standalone indicators (Table S3). Five out of the six non‐surviving calves were evaluated on the first day of life, which may have resulted in their acid‒base parameters being disproportionately poor at that time (Table S4). Abnormalities in biochemical parameters alone should not be used to predict death or to decide whether to discontinue treatment. Measuring indicators such as acid‒base imbalances or lactate concentrations continuously rather than at a single time point could be useful for assessing the extent and progression of improvements. In particular, indicators such as lactate clearance may play a critical role in evaluating prognosis, ⁵⁹ , ⁷⁶ , ⁷⁷ and this represents a valuable topic to be addressed in future studies.

This study primarily focused on the physiological and biochemical changes in calves with WCS and did not include a comprehensive investigation of its causes or postmortem examinations to determine the causes of death in the non‐surviving calves. These aspects are essential for achieving a more complete understanding of WCS.

Despite these limitations, this study evaluated the physiological and biochemical changes in calves with WCS within the therapeutic context of practical conditions, providing valuable evidence to improve the survival rates of weak calves. Management of hypothermia, correction of hypoxia and respiratory acidosis, normalisation of tissue perfusion and promotion of colostrum intake were identified as essential interventions for improving the survival rates of weak calves. These findings offer practical insights for developing effective treatment strategies and early intervention protocols, enabling veterinarians and farmers to manage WCS more effectively. This is expected to ultimately enhance animal welfare and positively impact farm productivity. Furthermore, it establishes a meaningful foundation for future studies on WCS.

AUTHOR CONTRIBUTIONS

Youngwoo Jung and Byoungsoo Kim designed the study. Mi‐Jin Lee provided methodological support. Byoungsoo Kim, Ji‐Yeong Ku, Youngjun Kim, Kwang‐Man Park and Jonghun Baek collected the data. Youngwoo Jung, Ji‐Yeong Ku, Youngjun Kim, Kwang‐Man Park and Jonghun Baek analysed the data. Youngwoo Jung, Ji‐Yeong Ku and Mi‐Jin Lee contributed to data visualisation. Youngwoo Jung drafted the manuscript. Youngwoo Jung, Byoungsoo Kim, Mi‐Jin Lee and Jinho Park reviewed and edited the manuscript. Jinho Park supervised the study.

CONFLICT OF INTEREST STATEMENT

The authors declare they have no conflicts of interest.

ETHICS STATEMENT

This study was approved by the Institutional Animal Care and Use Committee of the National Institute of Animal Science, Republic of Korea (JBNU IACUC no. NON2023‐123). Informed consent for blood sample collection was obtained from all the owners.

Supporting information

Supporting Information

Jung Y, Kim B, Ku J‐Y, Kim Y, Park K‐M, Baek J, et al. Physiological alterations and predictors of death in neonatal calves with weak calf syndrome. Vet Rec. 2025;e5327. 10.1002/vetr.5327

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.