Correct dietary formulation is vital for young stock to ensure adequate growth rates and development. Protein is particularly important for growth and immune function. Normally these needs are met by their dams and the pasture. During the drought of 2018 - 2020 the option to early wean has been taken by many producers to reduce nutritional stress on stock. Cows can be maintained on less with lower metabolisable energy and protein requirements. This reduced pressure on cows allows for improvements in body weight, leading to better joining percentages that help to maintain herd fertility. Early weaning calves allows for a better allocation of limited feed resources as feeding a calf through a cow is very inefficient. If the replacement diet fails to meet the calves’ minimum requirements, then reduced production and losses may occur.
A private vet in the Southern Highlands called in mid-February 2020 asking for assistance with a local producer who had been losing calves intermittently over a two-month period totalling 55 out of 250 calves. In the preceding two weeks the losses had dramatically increased (Figure 1). Affected stock initially showed symptoms of epiphora and light sensitivity, which progressed to corneal ulcers. The calves became moribund and recumbent prior to death. Circling and drooping of one ear had been noted in the occasional animal, although this clinical sign was not observed during any farm visits. The farmer also noted a "doughy" look to the face.
The calves were born in early November. They had been weaned from their mothers in late November onto milk replacer. At this time, they were given doses of Multimin, 7-in-1 vaccine, and AD&E as per label instructions. The calves were drenched with Cydectin in early December and given a booster 7-in-1 vaccination 6 weeks after initial weaning. The calves were given access to a good quality calf pellet with 18.7% crude protein. The available quantity was increased with growth from 1.1kg/hd/day to 1.96kg/hd for the mob of 120 head. This diet was providing 14.08-25.09 MJ ME/head/day and 18.7% CP per head per day. The calves also had access to 1/3 of a bale of barley straw per day (7.8 MJ ME/kg; 7.2% CP). An interruption in the supply of calf pellets during the first week of January 2020 meant the calves were transitioned on to a high fibre pellet shortly after drenching. Feed analysis showed that this pellet had a crude protein of 13.3%.
During the investigation post-mortem samples were collected and submitted to EMAI at Menangle. Analysis of the blood shown in Table 1 revealed biochemistry results consistent with recumbency prior to death and blood cell values consistent with dehydration and depletion of circulating white blood cells. The samples were tested for lead, Salmonella (individual and pooled cultures) and pestivirus antibody (AGID). Tests for both lead and Salmonella returned negative results whilst pestivirus antibody AGID returned a positive result. Histopathology showed pathology in the lungs, liver, small intestine and spleen suggestive of septicaemia and/or toxaemia. The origin of this infection could not be determined. The enteritis was quite severe and was most likely related to a viral cause but was not thought to be the main cause of death. The interstitial suppurative pneumonia was thought most likely to be hematogenous in origin given the nature and distribution of the histological changes, reinforcing the likely cause of death being septicaemia.
|AST||0-120||U/L||508 H||190 H|
|CK||0-300||U/L||61462 H||4241 H|
|URE/CREA||0.00-0.07||0.12 H||0.16 H|
|PROTEIN||60.0-85.0||g/L||58.4 L||58.0 L|
|SERUM HB||0.00-0.20||g/dL||0.62 H||0.29 H|
|PR/FI||15-100||8 L||11 L|
|RBC||5.00-8.00||1012/L||13.98 H||10.69 H|
|MCV||44-62||fL||39 L||39 L|
|MCH||14-20||pg||13 L||13 L|
|WBC||4.0-12.0||109/L||2.2 L||2.0 L|
|LYMPHO.||2.50-7.50||109/L||1.21 L||0.96 L|
It is not possible to determine whether the WBC depletion was caused by the pestivirus infection or the low protein diet. It is possible that both played a role in producing a lowered immune status as pestivirus is known to replicate in WBCs. Regardless of the cause of the immunosuppression, septicaemia secondary to pestivirus infection and/or a low protein diet as the cause of death is most likely. Appropriate dietary protein for young growing stock is vital. It is recommended that a diet with a minimum 12-13MJ ME/kg and 18-20% true protein is provided. It is important to consider the inclusion of roughage has on the overall dietary crude protein. In this case inclusion of straw (CP 7.1%) as 10% of the diet meant that the overall dietary crude protein fell to 12.74%, well below the required 18-20%.
A treatment regime of oxytetracycline (Alamycin LA300, 1ml/10kg, IM) and NSAIDs (Metacam 20mg/ml, 1ml/40kg, SQ) was instituted. A new calf pellet was sourced with a label claim of 18% crude protein. Deaths fell to nil with these two actions. Testing of all stock for pestivirus antigen was recommended but results were not available at the time of writing.