INTRODUCTION

Selenium (Se) is a trace element that plays a significant role in the health and performance of cattle (Sapkota, 2020). The incidence of selenium deficiency is distributed in various regions worldwide and has become more important in recent years because of the intensification of the systems (Mehdi and Dufrasne, 2016). In cattle, this deficiency affects both performance (Hall et al., 2013b) and immune functions (Hall et al., 2013a; Hugejiletu et al., 2013).

Young animals are more susceptible to clinical disorders associated with selenium deficiency because the metabolic pathways in developing organisms are more easily deregulated than those in adult cattle (Hall et al., 2014). In newborn calves, weakness at birth and high perinatal mortality rates associated with secondary infections can be observed (Bostedt and Schramel, 1990). In cases where the deficiency is severe, cases of nutritional muscular dystrophy may occur (Ammerman and Miller, 1975). The transfer of selenium from cows to newborn calves is done through the placenta and milk (Enjalbert et al., 1999). For this reason, it is essential to ensure the correct supplementation of pregnant cows during the last third of pregnancy (Bayril et al., 2015).

This paper describes an acute case of muscular dystrophy associated with high mortality rates in dairy calves in an intensive dairy farm in northwestern Argentina. Clinical, pathological, epidemiological, and biochemical aspects are highlighted.

CASE DESCRIPTION

Between March and May 2022, the Specialized Veterinary Diagnostic Service from INTA Salta (SDVE-INTA Salta) provided diagnostic assistance to a dairy farm in the town of “Campo Quijano” in Salta province due to a gradual increase in the perinatal and neonatal mortality rates of calves in an artificial rearing system. In the mentioned period, fifteen deaths out of forty-five births were registered. The newborn animals were immediately separated from their mothers and taken to a calf barn where they were offered three liters of colostrum each (by voluntary intake or by tube). Injectable iodine was applied to them too. On average, protein determination in the serum of colostrated animals using a refractometer ranged from 6.8 to 7.1 mg/mL. Before admission, the floor of the rearing area was incinerated, and each calf was housed in an individual pen with bedding and shelter. A total of seven complete necropsies were performed, including sampling for microbiological and histopathological studies. Additionally, blood samples were collected from prepartum and postpartum females and newborn calves.

RESULTS

Clinical findings and epidemiological findings

The affected calves were less than 10 days old. The animals died within one to two days after the onset of signs; a moderate degree of dehydration, extreme weakness, prostration, respiratory signs, and vocalizations were observed. In four animals, death occurred without previous signs. Only three of the affected calves showed diarrhea. The monthly mortality data recorded are shown in fig. 1.

Microbiological findings

Lung, spleen and liver samples were taken and cultured from all necropsied animals. All samples were negative for relevant pathogens.

Pathological findings

Pathological findings consistent with muscular dystrophy were present in three of the seven dead calves (42%). In the remaining four calves, no lesions were detected in the muscles evaluated. The findings are shown in fig. 2. The macroscopic lesions were characterized by a diffuse pallor in extensive muscle areas, especially in the muscles of the hind limbs. The most severely affected muscles were the semimembranosus, semitendinosus, and quadriceps. In two animals, whitish lesions were observed at the level of the myocardium. A histopathology study revealed severe polyphasic degeneration and necrosis of the muscle fibers, consistent with acute muscular dystrophy. The muscle fibers were hyper-eosinophilic, fragmented, and vacuolated and showed loss of cross-striation with the influx of macrophages and hypertrophy of satellite cells. A small number of muscle fibers retained some striation, showed increased basophilia and centrally localized nuclei, suggesting regeneration.

Biochemical findings

The concentrations of Se in the whole blood of prepartum and postpartum cows and newborn calves are shown in table 1. Briefly, low levels of selenium were observed in a high percentage of pre-partum (97,7±36,4 ppb) and post-partum (71,2±10,0 ppb) cows, and in newborn calves (68,3±10,5 ppb). In cattle, selenium values greater than 210 ppb are considered normal. In this case, the levels were much lower than those established in the bibliography (Kinkaid, 2000).

DISCUSSION AND CONCLUSIONS

Nutritional muscular dystrophy (NMD), or “white muscle disease”, is a pathological entity associated with severe Se and/or vitamin E deficiency (Andersonet al., 1977; Maas, 1984). In the present work, the clinical and pathological findings associated with the low levels of Se detected in calves and cows suggest that the deficiency of this mineral was the trigger for the high mortality rates observed. It should be noted that NMD is a late expression of selenium deficiency (Mehdi and Dufrasne, 2016). Other diseases can cause similar muscle injuries, including poisoning by ingesting Senna occidentalis (Carmo et al., 2011) or ionophore toxicity, such as monensin poisoning (Brito et al., 2016). Those causes were ruled out based on the anamnestic and epidemiological data. On the other hand, no studies could be conducted on blood tocopherol levels. Because of this, the possibility of a combined deficiency of Se and vitamin E could not be excluded. The response to the treatment with Se reaffirmed the origin of the disease.

The severity of the pathological changes produced by selenium deficiency depends on age, which also determines whether the consequences are fatal or not (Sapkota, 2020). Pathological conditions and deaths owing to selenium deficiency occur primarily in newborn animals and those in the early and last postnatal stages of development. The reasons why selenium deficiency results in such a diverse range of unusual pathological conditions are still unclear (Mehdi and Dufrasne, 2016). Because vitamin E and selenium have interdependent antioxidant functions, a deficiency of one of these compounds may increase the requirements of the other needed to prevent abnormalities (NRC 2000). It is likely that the cause of selenium-deficient animals manifesting one or any of the other symptoms depends on the concomitant vitamin E deficiency (Sapkota, 2020).

Kinkaid (2000) mentions that values of selenium in blood below 60 ppb indicate “severe deficiency”, values between 60 and 200 ppb are “marginally deficient”, values between 210 and 1200 ppb are “adequate” and greater than 1200 ppb are highly adequate. In this report, the results of selenium in the whole blood obtained from cows and calves showed low levels of this micro-mineral. About 66% of the calves and approximately 90% of the cows showed severe Se deficiency.

In Argentina, cases of nutritional muscular dystrophy have been registered in cattle (Rodriguez et al., 2018) and sheep (Micheloud et al., 2018), although information about that disease in dairy herds is scarce. However, selenium deficiency is widespread throughout the country (Lizárraga, 2021). In this regard, in a study conducted by Cseh et al. (2013) in which the level of glutathione peroxidase (GPx) was determined in 3621 blood samples of beef cattle from different farms in Argentina, it was found that 36% of the animals presented values below 30 IU/g of hemoglobin, which were compatible with Se deficiency.

Another author (Minatel et al., 2004) conducted a study in the northwestern region of Buenos Aires province, Argentina, and observed that 77% and 68.5% of the beef cattle blood samples presented compatible levels with deficiency of Se during winter and summer, respectively.

Several studies have suggested an immunosuppressive effect of selenium deficiency in ruminants (MacPherson, 1994). For example, this deficiency has been associated with sepsis in young cattle and mastitis in adult cows (Dalir-Naghadeh et al., 2006). It has been suggested that marginal selenium deficiency may be associated with general morbidity and/or poor performance in calves (Waltner-Toews et al., 1986). Consistent with this, Spears et al. (1986) show a lower incidence of neonatal diarrhea and a reduction in mortality in beef calves supplemented with this mineral, and how stillbirths and neonatal mortality were significantly reduced following the application of injectable selenium to prepartum cows.

Finally, due to the productive and reproductive importance of this micromineral, selenium deficiency must be considered when addressing morbidity or mortality issues in artificially fed calves.

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