A new genetic disorder was recently identified within the Holstein breed.

This genetic condition has now been named Early Onset Muscle Weakness Syndrome (MW). It was previously known as calf recumbency. MW is a genetic disorder that affects Holstein calves’ ability to stand. Calves affected by MW are unable to stand at birth or lose the ability to stand shortly after birth. Some affected may recover, but most do not survive beyond 6 weeks of age.

The genetic disease has been traced back to a Roylane Socra Robust and his son Seagull-Bay Supersire. It is likely that more animals will be identified as further testing is carried out.

It is unclear what the population frequency is of MW, however it is thought likely to be present in all AI breeding programs to some degree. A gene test has been developed using research by scientists at Penn State University that can identify carrier animals, but a haplotype test is not yet available.

The likely causative mutation in the CACNA1S gene on chromosome 16. It appears to be a recent mutation within a very common Holstein haplotype. The CACNA1S gene provides instructions for making the main piece (subunit) of a structure called a calcium channel. Channels containing the CACNA1S protein are found in muscles used for movement (skeletal muscles). These skeletal muscle calcium channels play a key role in muscle contraction. The defect results in weakness and/or paralysis. The affected calves must be homozygous for the condition to be present.

From a clinical perspective, the condition would be difficult to distinguish from a weak calf after a protracted birth, a traumatic injury to the calf at or shortly after birth or other congenital condition.