White muscle disease (WMD), or nutritional muscular dystrophy (NMD), is a myodegnerative disease of cardiac and skeletal muscle caused by a dietary deficiency in selenium (Smith, 2015; Valberg, 2014). In ruminants, WMD predominantly affects young, rapidly growing animals, particularly if their dams consumed a selenium-deficient diet during gestation (Smith, 2015). It most commonly presents in lambs 3-6 weeks old and calves 1-4 months of age (Abbott, 2018; Parkinson et al., 2019). Selenium intake in ruminants is highly correlated to the selenium content of the soil on which they graze (Witchel, 1998). Selenium deficient soils are widespread across the Central Tablelands of NSW (Robson et al., 2016; McCormick, 2014). Factors that predispose animals to selenium deficiency include clover-dominant pastures, high rainfall (>500mm), application of superphosphate and acidic volcanic soils (Robson et al., 2016; Dove et al., 2007). Thus, outbreaks of WMD in lambs are most likely to occur in this region during a lush spring on heavily fertilised, clover-dominant pastures (Robson et al., 2016).
On 26 November 2020, three composite lambs ranging from 3-4 weeks of age were examined on a property south of Black Springs in the Central Tablelands of NSW. The owner reported that 12 lambs were markedly lame in the hindlimbs and appeared ‘tucked up’. On mustering the mob of ewes and lambs to the yards, several lambs were reluctant to walk and a few collapsed. The owner noted that over the past three weeks, three lambs ranging from 2-3 weeks of age had died within 12 hours of becoming recumbent and weak.
The affected lambs were from two mobs of scanned, dry, maiden composite ewes (500 in total) that had been purchased in July 2020 from the Riverina region. However, 120 of these ewes lambed during October-November. The enterprise runs crossbred or composite ewes and beef cattle and normally lambs in winter. The soils are basalt derived and the owner heavily fertilises with superphosphate biannually, with the last application in August 2020. The mob had been in a paddock consisting of 80% clover and 20% phalaris, fescue, cocksfoot and ryegrass. Selenium supplementation was not a routine management practice on this property as there had been no clinical evidence of selenium deficiency previously.
All three lambs exhibited similar clinical signs. The lambs appeared ‘tucked up’ and lethargic. They were reluctant to move and when encouraged they walked with a stiff gait and an arched back (see Figure 1). No other abnormalities were detected and all ewes appeared clinically well.
Blood samples were obtained from the three affected lambs and five randomly selected, clinically normal, ewes from the same mob. Levels of the selenoenzyme glutathione peroxidase (GSH Px) were measured in all eight samples (see Table 1). Creatinine kinase (CK) levels were determined in the samples from the three diseased lambs (see Table 1). The CK levels in ewes were not tested as they were not exhibiting clinical signs.
|Test||Lamb 1||Lamb 2||Lamb 3||Ewe 1||Ewe 2||Ewe 3||Ewe 4||Ewe 5||Reference Range|
|GSH PX (U/gHb)||36||13||10||36||26||35||103||25||50 - 550|
Table 1: Summary of blood tests performed on three affected lambs and five randomly selected ewes.
The results (Table 1) indicate that only one ewe had adequate blood selenium levels. In the eight animals tested three had marginal selenium deficiency (30-50 U/gHb), and four had selenium deficiency (<30 U/gHb). Most of the affected lambs had low GSH Px levels. The CK levels in all lambs were elevated. WMD was diagnosed based on the low GSH Px levels in the ewes and lambs, the elevated CK levels and the clinical observations on the affected lambs.
On 28 November 2020, all lambs were treated with a subcutaneous injection of a 6-in-1 clostridial vaccine containing selenium (Glanvac 6S, Zoetis, containing 1 mg/ml sodium selenate, at a dose rate of 1 ml). The owner stated that approximately two weeks later most of the afflicted lambs had responded to treatment and appeared normal. However, a few of the severely affected animals continued to walk with a stiff gait. Following treatment, no additional cases were reported.
Differential diagnoses for stiffness of gait, with no alteration to mental state, that could be considered initially for this case include polyarthritis, myositis, myelitis, meningitis, tetanus, organophosphate poisoning, spinal cord compression, cerebellar disease and traumatic injuries (Valberg, 2014; Smith, 2015). However, WMD was the most likely diagnosis for this case as all affected lambs exhibited clinical signs consistent with a skeletal myopathy. Additionally, the environmental conditions were conducive to selenium deficiency in that the property is on basalt soils, there was a history of superphosphate application and a high rainfall year led to a productive spring with clover-dominant pasture (Robson et al., 2016; Dove et al., 2007). The low GSH Px levels confirm that both the ewes and lambs were deficient in selenium. The elevated CK levels are consistent with myopathy, supporting the diagnosis of WMD (Constable et al., 2017). The response to treatment further supports the diagnosis of WMD.
The previous sudden death of three lambs were presumably due to the acute cardiac form of WMD. It causes lesions in the heart, diaphragm and intercostal muscles and is characterised by signs of acute myocardial decompensation (Smith, 2015). It is typically sudden in onset and manifests as a profound weakness and recumbency, with death within 24 hours (Smith, 2015; Pugh & Baird, 2012). This presentation is consistent with the observations from the owner. Histopathology would have helped to confirm a diagnosis of WMD but as these were valuable lambs that were expected to respond to treatment, it was not undertaken as part of making a diagnosis.
This case report documents the first outbreak of WMD seen on this property. No cases were witnessed in the May-August drop lambs, or on neighbouring properties. Only the composite ewes that lambed in October-November had clinically affected lambs. This finding is presumably due to the clover dominance of the pastures grazed in spring, whereas May-August pastures comprised 20-30% clover. Typically, clovers are lower in selenium than grasses and are also high in polyunsaturated fatty acids (PUFA) (Abbott, 2018). High dietary PUFAs lead to free radical release and without enough antioxidants (such as selenoenzymes) muscle degeneration occurs (Smith, 2015; Abbott, 2018). Additionally, the rapid pasture growth rates in spring may have diluted the selenium in the pasture (Dove et al., 2007).
The owners will consider supplementing the whole flock with selenium in future. While the clostridial vaccine with selenium contains an appropriate dose (1mg) for short term supplementation in lambs, it is not enough to prevent selenium deficiency in ewes (requiring a dose of 5mg) (Robson et al., 2016). As most of the ewes (4/5) were deficient in selenium, supplementation should be considered. There are numerous methods of selenium administration such as licks, drenches, intraruminal boluses, depot injections, feed additives and fertilisers (Witchel, 1998). For this property an annual injection of 1 ml of barium selenate (such as Selovin LA, Elanco, containing 50 mg/ml of barium selenate) to ewes at crutching (1 month prior to lambing) and then selenium combined clostridial vaccine to lambs at marking (such as Glanvac 6S, Zoetis, containing 1 mg/ml sodium selenate, at a dose rate of 1 ml) seems most practical. Supplementation of ewes during pregnancy will boost the selenium status of lambs at birth and increase transfer of selenium in colostrum/milk, minimising the risk of WMD (Suttle, 2010). In addition to preventing WMD, selenium has also been demonstrated to improve live weight gain, wool production, fertility, and immune function in deficient flocks (Wilkins et al., 1982; Witchel, 1982; Whelan et al., 1994). Monitoring of the flock selenium status should be considered as selenium supplementation may not be required in less favourable seasons.