Beta-mannosidosis is a genetic defect of Nubians which is similar to G-6-S in its mode of transmission and in the method recommended for its management. Like G-6-S, it is confined to Nubian goats, it is caused by a simple recessive gene, and a DNA test is available to distinguish normal, carrier, and affected animals. As for G-6-S, the optimal management strategy, the one which offers the best balance between getting rid of the bad gene and saving the good genes, is to use only normal bucks.
Unlike G-6-S, beta-Mannosidosis is rapidly fatal and no affected goats grow up to breed. For that reason the incidence of this defect is lower than the incidence of G-6-S, with only about 13% of the population being carriers.
An affected kid is born with what looks like cerebral palsy. The kid is unable to stand or to hold up its head, and it shakes if it tries to do anything (intentional tremor). If it tries to reach the nipple it shakes violently and fails, but if the nipple is put in its mouth it calms down and is able to suck. It may also have skeletal deformities, and twitching eye movement and be deaf, but these symptoms are more obscure and variable.
The cause of these difficulties is the lack of an enzyme which normally removes certain sugars from the cells. These sugars accumulate in the cells, and the cells of the nervous system show the effects first. Even with the best care the kid will die within a few weeks at the most.
Unlike with G-6-S, it is immediately obvious that the affected kid is defective, although its difficulties might be mistaken for some sort of birth trauma or oxygen deprivation. At least there is no possibility of bad feelings resulting from the inadvertent sale of a defective kid.
The defect is described in Smith and Sherman, Goat Medicine, page 167. At the time the book was written the only test for beta-Mannosidosis measured the levels of the pertinent enzyme in the blood; this was quite unsatisfactory because there is a considerable overlap between the levels in normal goats and in carriers. Since then a foolproof DNA test has been developed at Michigan State University, and is now available in Texas (TVMDL).