Ataxic and weak lambs – is it swayback, white muscle disease or something else?
Differential diagnoses for white muscle disease include polyarthritis, respiratory disease, tick-borne fever, delayed swayback and rickets.
Given the time of year this has been a common question posed in recent weeks with some diagnostic options outlined below.
Swayback, both the congenital and delayed from, results from vacuolation and demyelination of the white matter in the central nervous system, beginning during mid-late pregnancy. Delayed swayback is most common in lambs aged four to twelve weeks old. This usually presents with an ascending paresis/paralysis and ataxia, most obviously when stock are moved. The cause is low copper levels during pregnancy, which can be secondary to high molybdenum and sulphur levels.
Blood and liver copper levels can be difficult to interpret in affected animals because lesions due to deficiency occur before the onset of clinical signs and copper status may have subsequently improved. Histopathological examination of the brain and spinal cord is the most useful test. Delayed swayback cases may respond to supplementation with concentrate feed, but more often they are permanently affected.
Differential diagnoses for delayed swayback include vertebral empyema, compressive cervical myeloencephalopathy, traumatic injury, vertebral body osteomyelitis, tick-borne fever, louping ill, white muscle disease (WMD) and sarcocystosis (usually in older lambs/hoggs).
Selenium and/or vitamin E deficiency can have a variable presentation. However, white muscle disease is one of the most identifiable syndromes. Pathology involves hyaline degeneration followed by coagulative necrosis.
In acquired WMD, signs develop three to 16 weeks after birth, with weakness, stiffness and tachypnoea being the most common presentations, often exacerbated by environmental stressors.
Blood GSH-PX reflects historic selenium intake and can be used to diagnose WMD in the live animal. Elevated CK and AST levels are also seen and there may be concurrent vitamin E deficiency.
At post mortem, areas of muscle and cardiac tissue are sometimes pale and gritty but no changes can be seen in many cases. Histopathology of skeletal muscle (+/- diaphragm/intercostal muscle) and myocardium is diagnostic. Low liver selenium/vitamin E levels can help to confirm the diagnosis. Lambs with acquired WMD may respond to treatment with selenium/vitamin E supplements.
Differential diagnoses for WMD include polyarthritis, respiratory disease, tick-borne fever, delayed swayback and rickets.
If you have suspect cases or farms with potential issues, please do not hesitate to get in touch and we can work out the best way to investigate further.
Posted by SRUC Veterinary Services on 19/05/2022