Gross necropsy, histopathology, and ancillary test results from neonatal beef calves submitted to a veterinary diagnostic laboratory

Luis A. Rivero Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO

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Shuping Zhang Department of Veterinary Pathobiology, College of Veterinary Medicine, University of Missouri, Columbia, MO
Veterinary Medical Diagnostic Laboratory, University of Missouri, Columbia, MO

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Loren G. Schultz Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO

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Pamela R. F. Adkins Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, MO

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Abstract

Objective

The objective of this study was to evaluate the prevalence of abnormal findings in gross necropsy, histopathology, and ancillary test results from neonatal beef calves submitted to a veterinary diagnostic laboratory.

Samples

This retrospective clinical case study was conducted by reviewing necropsy reports submitted between 2015 to 2020. Case inclusion criteria were animals had to be a bovine, 2 to 21 days of age, and a nondairy breed.

Procedures

Gross necropsy, histopathology, and laboratory test results were recorded. Identified lesions and abnormal test results were categorized based on body systems and infectious agent type. Age and system affected were analyzed using a 1-way ANOVA and Bonferonni pairwise comparisons.

Results

Overall, 1,060 reports were reviewed and 95 met the inclusion criteria. Median age of enrolled calves was 9 days (range, 2 to 21). A total of 252 lesions were identified with a median of 3 lesions/calf (range, 0 to 7) and 2 different body systems involved/calf (range, 0 to 5). The most common disorders were classified as digestive (42.1% [106/252]), respiratory (12.7% [32/252]), and multisystemic (11.1% [28/252]). With respect to age and system affected, calves with neurologic lesions were significantly younger (mean age, 5.1 days) than calves with digestive lesions (mean age 9.6 days).

Clinical Relevance

These data suggest a high prevalence of infectious diseases, mainly digestive, respiratory and multisystemic in origin. These findings could help guide producers and veterinarians when assessing factors contributing to neonatal beef calf loss.

Abstract

Objective

The objective of this study was to evaluate the prevalence of abnormal findings in gross necropsy, histopathology, and ancillary test results from neonatal beef calves submitted to a veterinary diagnostic laboratory.

Samples

This retrospective clinical case study was conducted by reviewing necropsy reports submitted between 2015 to 2020. Case inclusion criteria were animals had to be a bovine, 2 to 21 days of age, and a nondairy breed.

Procedures

Gross necropsy, histopathology, and laboratory test results were recorded. Identified lesions and abnormal test results were categorized based on body systems and infectious agent type. Age and system affected were analyzed using a 1-way ANOVA and Bonferonni pairwise comparisons.

Results

Overall, 1,060 reports were reviewed and 95 met the inclusion criteria. Median age of enrolled calves was 9 days (range, 2 to 21). A total of 252 lesions were identified with a median of 3 lesions/calf (range, 0 to 7) and 2 different body systems involved/calf (range, 0 to 5). The most common disorders were classified as digestive (42.1% [106/252]), respiratory (12.7% [32/252]), and multisystemic (11.1% [28/252]). With respect to age and system affected, calves with neurologic lesions were significantly younger (mean age, 5.1 days) than calves with digestive lesions (mean age 9.6 days).

Clinical Relevance

These data suggest a high prevalence of infectious diseases, mainly digestive, respiratory and multisystemic in origin. These findings could help guide producers and veterinarians when assessing factors contributing to neonatal beef calf loss.

Introduction

Neonatal morbidity and mortality are major causes of economic loss for US beef cattle producers.1 In 2017, the majority of US beef operations (51.2%) had at least 1 calf that was born alive but died or was lost prior to weaning.2 Overall, 2.2% to 3.5% of beef calves born alive die before weaning.24 Based on producer surveys, the most common causes of death in calves < 3 weeks of age are calving-related problems (24.7%), unknown (24.0%), predators (13.8%), weather related (12.5%), digestive diseases (11.9%), and respiratory diseases (8.1%).2,3 Overall, a high proportion of calves die from unknown reasons, leaving an outcome related knowledge gap for producers. Additionally, while producer driven data are valuable, as many neonatal calf diseases are tentatively diagnosed and treated by producers, determining causes of death based on history and clinical signs alone can be misleading.5

A postmortem assessment of pathological findings can provide clarity regarding underlying disease processes. Necropsy data can be useful to confirm disease status of animals especially when morbidities and mortalities are higher than expected, help producers understand outcomes when there is a perceived treatment failure, provide confirmation of a presumptive diagnosis, and potentially identify outcomes that could be controlled through adjusted management strategies.6,7 A comparison of expected postmortem findings based on clinical signs and administered treatments and actual postmortem findings obtained at necropsy found limited agreement, indicating that necropsies and additional laboratory-based testing should be part of a systematic postmortem examination of causes of death.5 That said, in the US, only half of beef cow-calf operations consult with a veterinarian for any reason.8 In a Canadian study,9 only 9.8% of producers had a veterinarian perform a postmortem examination on ≥ 1 calf in 2010 and 5.2% had veterinarians submit tissues or biological samples to a veterinary diagnostic laboratory.

There are limited numbers of studies that have evaluated beef calf deaths unrelated to birthing or reproductive diseases by use of gross necropsy, histopathology, and laboratory tests. In 1 prospective study that evaluated 203 herds in western Canada, among 558 calves that were > 3 days of age, the most common final diagnoses based on history, field necropsy, and histologic examination were starvation (16%); abomasal ulcer, torsion, and perforation (12%); and enteritis (11%). Although starvation was the most common summary diagnosis, most of the cases had additional lesions, suggesting the calves were compromised by underlying disease.10

A summary of the common results from postmortem evaluations may prove beneficial to producers and show value in necropsy results. Therefore, the objective of this study was to describe lesions and abnormal test results among neonatal beef calves presented to a veterinary diagnostic laboratory.

Materials and Methods

Case selection

Electronic medical records from the University of Missouri’s Veterinary Medical Diagnostic Laboratory (VMDL) were retrospectively reviewed. Dates included were from May 15, 2015, to April 8, 2020. Animals were eligible for inclusion if they were a bovine, between 2 and 21 days of age, and of a nondairy breed.

Medical records review

The search terms used included the following: bovine, necropsy (gross, histopathology, and labs), necropsy (gross and histopathology), necropsy (gross), and necropsy (mail-in). Data collected from the medical record included the following: breed, age (days), sex, weight, type of necropsy performed, lesions identified, and number of lesions per animal. All gross examination and histopathologic based lesions were categorized based on system, and the associated pathogen type was recorded. All ancillary test results were recorded, including culture and susceptibility of bacterial agents when available. Bacteria identification was completed by the VMDL Bacteriology Laboratory. Bacteria genus and species were determined using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Undifferentiated coliforms were defined based on phenotypic appearance alone and were evaluated on Columbia blood and MacConkey agar.

Definitions

A lesion was defined as an abnormal change in a tissue. Both gross and histopathologic lesions were categorized. Terminology used to describe each lesion was based on the terminology stated by the board-certified pathologist that originally reviewed the case. System categories included the following: urogenital, CNS, digestive, hemolymphatic, cardiovascular, muscloskeletal, multisystemic, respiratory, and other known system. Urogenital system was utilized to categorize calves that had lesions associated with the urogenital tracts, including umbilical disease. The hemolymphatic system was used to categorized those cases with lesions in the spleen, thymus, or other lymphoid organs. Multisystemic disease was used to categorize cases with either confirmed or suspected sepsis on the basis of the pathologist’s report. Sepsis was determined by either isolating the same bacterial pathogen in 2 or more sites or by identifying multiple foci of inflammation. Calves were categorized only as septic if classified as such by the pathologist and the above conditions were met. Other known systems categorized calves with disorders in other body systems that were known but did not fall into one of the primary categories listed above, which included integument, endocrine, special senses, and peripheral nervous system. Gross necropsy was used to describe the evaluation that was based on visual inspection of the organ systems. Necropsy with histopathology included visual inspection as well as histopathologic examination of the tissues. Ancillary tests were requested based on the pathologist’s findings and could include the following: bacterial culture, bacterial antimicrobial susceptibility testing, fecal examination for parasites, PCR, or ELISA testing. Cryptosporidium spp were identified using a direct smear of fresh feces followed by a modified acid-fast strain and microscopic examination, while rotavirus and coronavirus were identified using PCR. Pathogens identified were classified by pathogen type (bacterial, viral, parasitic, fungal, unknown, or none identified).

Statistical analysis

Results were descriptively analyzed and reported as medians and range. Results were normally distributed with equal variances and therefore data was analyzed using 1-way ANOVA with statistical software (Stata IC version 13.1; StataCorp) to compare age, system affected, and pathological agent types. If an interaction was identified, then a Bonferroni pairwise comparison was used to identify differences. The χ2 or Fisher exact test was used to compare system categorizes. Analysis was completed using commercial software (SigmaPlot 14.0; Systat Software), and significance was determined at P < 0.05.

Results

A total of 1,060 bovine necropsies were performed during the study period. Of these, 95 (8.9%) animals met the inclusion criteria, with a median of 15 cases/y (range, 13 to 19 calves/y). Median calf age was 9 days (range, 2 to 21 days). Median weight was 33.5 kg (range, 11.5 to 58.3 kg). Overall, 51.6% (49/95) of included calves were male, 42.1% (40/95) were female, and 6.3% (6/95) had no sex reported. More calves were submitted for necropsy in the spring (34/95 [35.7%]), followed by the fall (30/95 [31.5%]), winter (21/95 [22.1%]), and summer (10/95 [10.5%]). Animals represented included mixed breed (42/95, 45.1%), Angus (36/95, 38.7%), Charolais (5/95 [5.3%]), Simmental (3/95 [3.2%]), Limousin (2/95 [2.1%]), South Poll (2/95 [2.1%]), Longhorn (1/95 [1.0%]), Salers (1/95 [1.0%]), and Dexter (1/95 [1.0%]). There was a total of 252 lesions identified, with a median of 3 lesions/calf (range, 0 to 7). When more than 1 lesion was identified within 1 system, duplicate representation of each system were removed to represent the number of system-based lesions. A total of 190 system-based lesions were identified, with a median of 2 systems affected/calf (range, 0 to 5). A total of 3 (3.2%) calves had no identified lesions.

Overall, gross necropsy only was performed on 6.3% (6/95) of cases. Gross necropsy and histopathology were performed on 10.5% (10/95) of cases. Gross necropsy, histopathology and laboratory testing were performed on 82% (78/95) of cases. Most cases (99%; 94/95) were from calves submitted to the VMDL while 1% (1/95) of cases were from field necropsies with samples submitted by the referring veterinarian. Among the calves with no identified lesions, 1 was examined by gross necropsy and histopathology only and 2 were examined by gross necropsy, histopathology, and ancillary laboratory tests.

Among the 252 lesions, digestive system (42.1% [106/252]) lesions were the most common (Table 1), followed by respiratory (12.7% [32/252]), multisystemic (11.1% [28/252]), and CNS (8.3% [21/252]) lesions. Among all systems, congenital abnormalities were noted among 8.4% (8/95) of calves. The most common congenital abnormalities identified included patent foramen ovale (3/8) and patent ductus arteriosus (3/8). When considering the system-based categorization (n = 190), most calves (57.9% [55/95]) had 2 or more system affected while only (38.9% [37/95]) had only 1 affected system. With regard to the calves that had 2 or more systems affected, 26.3% (25/95) had 2 affected systems, 20% (19/95) had 3 affected systems, 9.5% (9/95) had 4 affected systems, and 2.1% (2/95) of calves had 5 affected systems. The majority of calves with only 1 affected system were diagnosed with digestive lesions (73% [27/37]), followed by respiratory lesions (8.1% [3/37]). Among calves with 2 affected systems, the most common combination was digestive and respiratory system lesions (36% [9/25]), followed by a digestive system and other known system lesions (12% [3/25]) and digestive system and multisystemic lesions (12% [3/25]). Among calves with 3 systems involved, the most common combinations included the following: digestive, respiratory, and multisystemic (26.3% [5/19]); digestive, respiratory, and CNS (10.5% [2/19]); and digestive, multisystemic, and other known system (10.5% [2/19]). Among calves with 4 systems involved, the only combination represented more than once was digestive, CNS, musculoskeletal, and multisystemic (22.2% [2/9]). Among calves with 5 systemic involved, 1 of 2 had digestive, respiratory, musculoskeletal, hemolymphatic, and CNS lesions and 1 of 2 had urogenital, hemolymphatic, cardiovascular, CNS, and multisystemic lesions. Among calves with ≥ 3 affected systems diagnosed (n = 30), 43.3% (13/30) were not classified as multisystemic by the pathologist.

Table 1

Number and percentage of lesions (n = 252) identified that were categorized into each system category and number of beef calves (95) with at least 1 lesion within each category. Most calves had multiple lesions impacting multiple body systems.

Body system category Total No. of lesions identified (%) No. of calves with lesion(s) identified within each system
Digestive 106 (42.1%) 67 (70.5%)
Respiratory 32 (12.7%) 31 (32.6%)
Multisystemic 28 (11.1%) 23 (24.2%)
CNS 21 (8.3%) 17 (17.9%)
Urogenital 19 (7.5%) 16 (16.8%)
Cardiovascular 14 (5.6%) 6 (6.3%)
Musculoskeletal 13 (5.2%) 12 (12.6%)
Other known systems 11 (4.4%) 11 (11.6%)
Hemolymphatic 8 (3.2%) 7 (7.4%)

Among calves with a digestive lesion (70.5% [67/95]), 43.2% (29/67) had multiple lesions within the digestive tract (Table 2). A total of 106 digestive lesions were identified, with enteritis being the most common (45.3% [48/106]) followed by rumenitis (13.2% [14/106]), peritonitis (5.7% [6/106]), and hepatitis (5.7% [6/106]). Overall, 50.5% (48/95) of calves that were included in this study had enteritis. Among the 48 cases of enteritis, the majority of cases (60.4% [29/48]) had 1 infectious agent type identified, followed by identification of 2 different infectious agent types (35% [17/48]) and identification of 3 different infectious agent types (4.2% [2/48]). The most common infectious agents associated with enteritis were bacteria (54% [26/48]), viruses (44% [21/48]), parasites (27% [13/48]), and unknown agents (15% [7/48]). A total of 77 infectious agents, including bacteria, parasites, and viruses, were detected in the small intestines of calves with enteritis. The most common bacteria isolated from the small intestines of calves with enteritis were E coli (79.2% [38/48]) and Clostridium perfringens (43.8% [21/48]). While these bacterial agents were commonly identified, they were only rarely associated with histologic lesion (Table 3). Cryptosporidium spp was the only parasitic agent and was identified in 22.9% of calves with enteritis (11/48). Salmonella spp were identified in the small intestines of 4 calves with enteritis. Among the Salmonella isolates identified, S enterica serotype Newport was detected in 2 out of 4 calves, while the remaining samples were identified as S enterica serotype Agona (1/4), and S enterica serotype Give (1/4). Viral PCR testing was not performed in all calves with enteritis, including no rotavirus or bovine viral diarrhea virus (BVDV) testing for 8 of 48 and no coronavirus testing for 6 of 48 cases. Among those that were tested for viral enteric pathogens, 26% (11/43) were positive for coronavirus, 34% (14/41) were positive for rotavirus and 0% (0/41) were positive for BVDV.

Table 2

Total number and percentage of each digestive lesion identified among 95 necropsied beef calves. The percentage represents the number of each specific lesion per total number of digestive lesions (n = 106) and the number of each specific lesion per total number of calves (95).

Diagnosis No. of lesions Percentage of lesions (n = 106) Percentage of calves with lesion (n = 95)
Enteritis 48 45.3% 50.5%
Rumenitis 14 13.2% 14.7%
Peritonitis 6 5.7% 6.3%
Hepatitis 6 5.7% 6.3%
Abomasitis 5 4.7% 5.3%
Diarrhea 5 4.7% 5.3%
Colitis 4 3.8% 4.2%
Omasitis 4 3.8% 4.2%
Rumen acidosis 2 1.9% 2.1%
Hepatic lipidosis 2 1.9% 2.1%
Abomasal ulcer 1 0.9% 1.1%
Atresia ani 1 0.9% 1.1%
Duodenal ulcers 1 0.9% 1.1%
Enterocolitis 1 0.9% 1.1%
Esophagitis 1 0.9% 1.1%
Peritoneal effusion 1 0.9% 1.1%
Reticulitis 1 0.9% 1.1%
Ruminal drinking 1 0.9% 1.1%
Typhlocolitis 1 0.9% 1.1%
Salmonellosis 1 0.9% 1.1%
Table 3

Number of each microorganism identified at various body sites among the 95 beef calves that were presented to a veterinary diagnostic laboratory. Within each body site, the number of each organism associated with a lesion at the specific organ site is listed over the total number of each microorganism identified at each organ site location. Body site locations were included if bacteria were identified from a specific location > 10 times. Bacterial genus was included if identified ≥ 3 times. Bacteria were listed at the species level if identified > 10 times. If multiple species were identified but each species was identified < 10 times, they were grouped at the genus level. Isolates identified as coliforms were not identified to the genus or species level by the laboratory.

Bacteria identified No. of isolates* Small Intestine (%) Large Intestine (%) Lung (%) Liver (%) Umbilicus (%) Brain/CSF (%) Joint (%)
Escherichia coli 106 15/50 (30) 10/17 (59) 5/7 (71) 8/11 (73) 3/5 (60) 4/5 (80) 2/2 (100)
Clostridium perfringens 46 1/24 (4.2) 1/14 (7.1) 0/0 (0) 1/3 (33) 0/0 (0) 0/0 (0) 0/0 (0)
Coliforms 27 1/2 (50) 0/5 (0) 2/7 (29) 0/5 (0) 0/1 (0) 1/2 (50) 1/1 (100)
Streptococcus spp 17 0/3 (0) 0/1 (0) 0/5 (0) 0/1 (0) 1/4 (25) 0/2 (0) 0/0 (0)
Pseudomonas spp 12 1/4 (25) 0/0 (0) 1/3 (33) 0/0 (0) 0/1 (0) 0/4 (0) 0/0 (0)
Salmonella spp 12 4/4 (100) 4/4 (100) 0/0 (0) 2/2 (100) 0/0 (0) 0/0 (0) 0/0 (0)
Proteus spp 8 0/0 (0) 0/1 (0) 0/1 (0) 0/2 (0) 0/2 (0) 0/2 (0) 0/0 (0)
Staphylococcus spp 8 0/0 (0) 0/0 (0) 1/2 (50) 0/0 (0) 0/1 (0) 1/2 (50) 2/3 (67)
Campylobacter spp 7 2/4 (50) 0/1 (0) 0/0 (0) 0/0 (0) 0/1 (0) 0/0 (0) 0/0 (0)
Klebsiella spp 5 0/1 (0) 0/0 (0) 0/3 (0) 0/1 (0) 0/0 (0) 0/0 (0) 0/0 (0)
Mannheimia spp 4 0/0 (0) 0/0 (0) 1/2 (50) 0/0 (0) 0/0 (0) 0/1 (0) 0/0 (0)
Acinetobacter spp 3 0/1 (0) 0/0 (0) 0/0 (0) 0/1 (0) 0/0 (0) 0/0 (0) 0/0 (0)
Bacillus spp 3 0/0 (0) 0/0 (0) 0/1 (0) 0/0 (0) 0/0 (0) 0/0 (0) 1/2 (50)
Enterococcus spp 3 0/1 (0) 0/0 (0) 0/0 (0) 0/0 (0) 0/2 (0) 0/0 (0) 0/0 (0)
Moraxella spp 3 0/0 (0) 0/0 (0) 1/1 (100) 0/0 (0) 0/0 (0) 0/1 (0) 0/1 (0)
Truperella pyogenes 3 0/0 (0) 0/0 (0) 0/0 (0) 0/0 (0) 1/1 (100) 0/0 (0) 1/1 (100)
Total* 287 24/96 (25) 15/43 (35) 11/39 (28) 11/28 (39) 5/20 (25) 6/22 (27) 7/11 (64)

The number of isolates and the total number represents all isolates identified, including bacterial genus and species not shown in this table.

The most common respiratory lesion identified was pneumonia (72% [23/32]) followed by pleuritis (6% [2/32]). Other respiratory abnormalities identified included bronchiolitis (1/32), laryngitis (1/32), neutrophil sequestration in the lungs (1/32), pulmonary congestion (1/32), pulmonary edema (1/32), pulmonary hemorrhage (1/32), and tracheitis (1/32). Pneumonia cases were further classified as bronchopneumonia (26% [6/23]), multifocal (26% [6/23]), interstitial (18% [4/23]), embolic (4% [1/23]), and other (26% [6/23]). Of the 23 animals with pneumonia 13 had a bacterial culture performed on the lungs and common isolated organisms included Escherichia coli (31% [4/13]), undifferentiated coliforms (31% [4/13]), and Mannheimia haemolytica (15% [2/13]). The presence of these bacterial species was not frequently associated with histopathologic lesions (Table 3).

The most common reason for the system-based categorization of multisystemic was because of signs of sepsis (79% [22/28]). Other lesions that were characterized as multisystemic included bacteremia (3/28 10.7%), neutrophil sequestration in multiple organs (3.6% [1/28]), petechia (3.6% [1/28]), and subcutaneous hemorrhages (3.6% [1/28]). Among calves with a multisystemic diagnosis (n = 23), bacteria were cultured from several organ systems outside the gastrointestinal tract, including the lungs, liver, umbilicus, joints, brain, spleen, and kidneys. The most common bacterial species identified was E coli (78% [18/23]), followed by undifferentiated coliforms (22% [5/23]), and C perfringens (13% [3/23]). S enterica was identified among 17% (4/23) of calves with multisystemic categorization; however, among these cases, Salmonella spp was isolated only from the gastrointestinal tract. Among calves with a system categorization of multisystemic disease, common concurrent lesions included enteritis (61% [14/23]), pneumonia (39% [9/23]), meningitis (17% [4/23]), and septic arthritis (17% [4/23]).

Bacterial cultures were performed on samples from 81% (77/95) of the necropsied calves (Table 3). Among the 287 bacterial isolates recovered, the most common body sites of origin included the small intestines (33% [96/287]), large intestines (15% [43/287]), lung (14% [39/287]), liver (10% [28/287]), brain/CSF (8% [22/287]), and umbilicus (7% [20/287]). The most common bacteria species identified were E coli (37% [106/287]) and C perfringens (16% [46/287]). A total of 12 Salmonella isolates were recovered from 7 (7% [7/95]) calves, with most of the isolates originating from the intestines or feces (75% [9/12]). Salmonella was also isolated from the liver (17% [2/12]) and abomasum (8% [1/12]). Many calves that were culture positive for Salmonella spp also had enteritis (71% [5/7]). Antimicrobial susceptibility testing was performed on 7 Salmonella isolates, but not all isolates were tested for the same antimicrobials. The number of resistant isolates for the following antimicrobials are as follows: penicillin (7/7), sulfadimethoxine (5/5), oxytetracycline (4/7), ampicillin (3/5), ceftiofur (0/5). Antimicrobial susceptibility testing was performed on 9 E coli isolates from septic calves but not all isolates were tested for the same antimicrobials. The number of resistant isolates for the following antimicrobials is as follows: penicillin (9/9), sulfadimethoxine (4/8), oxytetracycline (8/9), ampicillin (6/8), and ceftiofur (0/9). While bacteria was often isolated, depending on organ site, only 25% to 64% of isolates were determined to be associated with a histologic lesion (Table 3).

There was a significant association between median age and system affected (P = 0.012; Figure 1). Pairwise comparisons indicated that calves with CNS lesions were younger than calves with digestive lesions (P = 0.029). The χ2 or Fisher exact test (when needed) also identified that calves with digestive lesions were less likely to have urogenital lesions (P = 0.001) and cardiovascular lesions (P = 0.003). Calves with CNS lesions were less likely to have musculoskeletal lesions (P = 0.002), while calves with urogenital lesions were more likely to have musculoskeletal lesions (P = 0.014). No seasonal effects were identified.

Figure 1
Figure 1

Mean age (± 1 SD) of calves (n = 95) included in the study by system-based lesion categorization. *Significant (P < 0.05) differences among mean ages.

Citation: Journal of the American Veterinary Medical Association 260, 13; 10.2460/javma.22.03.0139

Discussion

Overall, the majority of lesions in this retrospective study were digestive, respiratory, and multisystemic in origin. This is in contrast to producer-driven data that identified weather-related causes, unknown causes, and digestive problems as the most common causes of calf loss in the first 3 weeks of life unrelated to birthing problems.8 This discrepancy could be based on case selection, as calves that have died during a difficult weather event are unlikely to be presented for necropsy. However, the addition of gross and histopathologic findings were able to help classify the death of necropsied calves that would otherwise likely be reported as unknown by producers. It should be noted that most calves had at least 1 lesion identified, while only 3 (3.2%) calves had no lesions identified. This finding supports the utility of necropsy data to help classify likely causes of death in neonatal calves.

Enteritis was the most common reported digestive disorder in this study. This is similar to a Canadian study10 that reported enteritis as the most common digestive disorder; however, that study had a much lower prevalence than our study. Overall, 45.3% of cases with digestive lesions included in this retrospective evaluation had enteritis,9 while only 16% of older calves (those that lived > 3 days) had enteritis in the Canadian study.10 A high proportion of enteritis cases in this study were associated with E coli, C perfringens, and Cryptosporidium sp. The significance of the identification of these bacterial organisms on culture of the intestines is questionable since E coli and C perfringens are normal inhabitants of the gastrointestinal tract and the latter can also rapidly overgrow shortly after death. Additional characterization of these bacterial species would be necessary to determine their virulence potential. Additional characterization of C perfringens was pursued in only 2 cases of suspected clostridial enteritis by enterotoxin ELISA, both of which were negative. Viral pathogens, coronavirus and rotavirus, were commonly identified among enteritis cases, 26% and 34%, respectively. Reports of prevalence of both coronavirus and rotavirus in beef calves are sparse, but a previous study10 found a higher prevalence of coronavirus (62%) in Canadian beef calves than what is reported in this study.

Respiratory disease is one of the most common causes of disease in preweaned calves and is one of the major reasons for antimicrobial usage in calves prior to weaning.2,11 Most previous studies2,11 base their classification of respiratory disease on producer records. Few studies have characterized the lesions within the lungs, and in the 1 study10 that did, cases were classified broadly as pneumonia. While bronchopneumonia is thought to be a common cause of disease in preweaned beef calves and is often associated with summer heat, little data can be found to determine the prevalence of bronchopneumonia among young calves. In this study, the majority of lesions classified as pneumonia were not characterized as bronchopneumonia, with 74% of cases being classified as either interstitial, multifocal, other, or embolic pneumonias. Multifocal and embolic lesions are likely related to a septic spread of disease rather than originating from the respiratory tract. Factors that have been proposed to have an impact on respiratory disease in preweaned calves include maternal transfer of immunity, dystocia, environmental stressors, and maternal nutrient restriction.12

Multisystemic disease was the third most common system categorization in the population of necropsied calves in this study. This retrospective study10 has a higher prevalence of septic calves than what has been previously reported for neonatal beef calves (7.4%). Enteritis, pneumonia, meningitis, and septic arthritis were often identified among cases with septic lesions in this study, which is similar to previous data.13 This data support the notion that a focal site of infection is an important risk factor for sepsis in neonatal calves.14,15

Major risk factors for illness in neonatal calves include dystocia, calving in confinement, failure of transfer of passive immunity, poor maternal milk production, or weather-related conditions that make it challenging for the calf to nurse.1618 While no studies in the US have investigated the prevalence of failure of transfer of passive immunity in beef calves, studies from Ireland1921 have reported a failure of passive transfer prevalence of 22% to 29% which is higher than the 19.2% reported for US dairy herds. Identified factors that contribute to failure of transfer of passive immunity in beef calves include dystocia, poor mothering ability, parity of the dam, and nutrition of the dam.4,18,19 One study found that even mild complications in the birthing process that do not require assistance increase the risk of failure of transfer of passive immunity, highlighting how small difficulties in calving can impact the life and productivity of a beef calf.19 It is estimated that up to 20% of 2-year-old heifers may abandon their calf or not allow them to nurse and 12% of neonatal beef calf losses could be attributed to mothering problems.4,22 Furthermore, primiparous dams and those dams that have been nutrient restricted in the prepartum period have been shown to have decreased colostrum quality, which increases the risk of failure of transfer of passive immunity.18 Unfortunately, in this study, transfer of passive immunity status was not available for review.

Overall, lesions that likely contributed to illness were identified in a high proportion of cases. Previous research has identified noninfectious disease processes to be more common than infectious among necropsies done across all age groups of calves (abortions, still births, perinatal calves and neonatal calves).10 The results of this study differ, in that infectious diseases were common. This is not surprising because the previously mentioned study found that infectious causes increased as calves aged. Among perinatal calf deaths this previous report found lesions in the thyroid, cardiac muscle, and skeletal muscle, and therefore the role of trace minerals and vitamins in overall disease was speculated.10 Lesions in these organ systems were not common among the sample population in our study, but underlying trace mineral imbalances may be a factor to consider. No trace mineral profiles were performed among cases included in this study.

Limitations of this study include its retrospective nature and lack of uniformity of necropsy data. Standardizing nomenclature based on postmortem findings in simple and consistent terms can provide more useful information about significant lesions that may be associated with death.23 Additionally, the results of the gross, histopathologic, and ancillary tests are all listed in this study, however these results do not always agree, making interpretation challenging. For example, bacterial species are frequently isolated from body sites; however, they are less frequently associated with a lesion at that specific location. While, having access to common organisms identified from different organ systems may be useful to veterinarians when addressing sick calves, it is important for veterinarians to evaluate whether the bacterial species is associated with a lesion and therefore significant. Furthermore, there is an inherit bias of calves who presented for necropsy evaluation, as these may have been calves coming from outbreak situations or situations in which a presumptive diagnosis of the cause of death was made on farm and may not accurately represent the overall calf population. Finally, the inclusion criteria for this study required an age to have been recorded for each calf. This criterion may have led to a bias, as this selects for producers that maintain accurate records and delivered that information to the laboratory.

In conclusion, this study reports gross necropsy, histopathologic, and ancillary test results from beef calves submitted to a veterinary diagnostic laboratory. Common systems affected are in line with previous reports of digestive and respiratory diseases being common systems affected. Multisystemic diseases (ie sepsis) may be more common in calf populations than previously thought and could account for the high proportion of calf losses when producers cannot identify a cause of death. Future studies should focus on clinical prospective investigation to identify risk factors for these disease processes in neonatal beef calves.

Acknowledgments

No third-party funding or support was received in connection with this study or the writing or publication of the manuscript. The authors declare that there are no conflicts of interest.

We acknowledge the work of all faculty and staff from the University of Missouri Veterinary Medical Diagnostic Laboratory that contributed to the workup of these cases.

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