Urinary Calculi in Lambs and Calves
Phil Rogers MRCVS
Grange Research Centre, Dunsany, Co. Meath, Ireland
Urinary calculi in lambs
The national incidence of obstructive ovine urolithiasis is very low but winter urolithiasis can be a major problem for individual farmers. It is the second biggest cause of death behind respiratory disease, accounting for 18-38% of lamb deaths in winter (Causes of mortality in an intensive lamb fattening unit: Malone et al, 1985, Irish Vet. J., 39, 86-90).
Urinary sediment forms initially in the kidneys. It usually passes, via the ureter to the bladder. Fine sediment may pass harmlessly through the urethra, or may aggregate as larger particles (capable of obstructing the urethra), or as large stones (which remain in the bladder). Obstruction of the kidney or ureter is rare. The diameter and length of the urethra relative to the diameter of the calculus or sediment is critical to whether or not urethral blockage arises. The risk of obstruction is greatest in young male lambs, especially castrates. Obstruction is rare in females, as the urethra is short and its diameter wider than in males.
Clinical signs of obstruction (off feed, depression, abdominal colic, "water-belly") are rare unless the urethra becomes blocked (usually at the S-bends, or at the tip of the penis). Then, the bladder (or, sometimes, the pelvic urethra) may rupture, which may cause death within hours to days.
Causes of urinary calculi: Calculus formation has many causes, of which the mineral (P, Mg, Ca, Na) level in feed is only one factor. It involves: reduced water intake, reduced urine output, cold environment, alkaline urine pH and genetic susceptibility. Other possible factors are: urinary infection; mucilaginous material in urine; low roughage intake and saliva secretion; increased P retention; vitamin A deficiency; excess intake of fluoride, oxalate, sodium bicarbonate and silica.
However, precipitation of magnesium phosphate is the basic cause in intensively fed sheep. Urolithiasis is very rare if the total feed contains P < 0.46% and/or Mg < 0.23% DM respectively. The higher the P and Mg levels, the greater the risk. Low Ca/P ratios increase P absorption. Mg availability is high on cereal-based diets. Lambs on concentrates need less Mg, possibly 50% less, than lambs on grass diets.
Control and prevention of urolithiasis in lambs
Include 1-2% salt to enhance water intake and increase urine production (diluting the Mg and P level in urine), thus reducing the risk of formation of magnesium phosphate crystals.
Acidify the urine?: On concentrate- (but not forage- or grass-) based diets, inclusion of ammonium CHLORIDE (0.5-1.5% of feed DM) reduces crystal formation but increases the risk of demineralisation of the skeleton. This could be important in pedigree stock to be kept for breeding or showing, but may be unimportant in lambs fed for slaughter.
Use of ammonium SULPHATE as a urinary acidifier is NOT RECOMMENDED, as it may lead to a form of cerebro-cortical necrosis (CCN, polioencephalomalacia), which is poorly responsive to thiamin treatment.
The level of P should not exceed 0.57% or Mg should not exceed 0.28% DM in any complete concentrate diet for lambs:
if feed P is < or = 0.54%, feed Mg should be < 0.251% DM
if feed P is < or = 0.51%, feed Mg should be < 0.281% DM
Urinary calculi in calves
The same basic principles that apply to lambs also apply to intensively-fed calves (see lambs). Young males, especially castrates, are at greater risk of obstruction than females or older males. However, clinical signs (obstruction etc) are not as common in calves as in lambs.
It is inadvisable to feed cow-minerals or cow-concentrates to calves, as the Mg levels in cow-supplements can be high. Similarly, it is inadvisable to allow calves access to water-troughs medicated with soluble Mg-salts (chloride, sulphate, acetate) for tetany-control in cows. That said, outbreaks of obstructive urolithiasis in calves with access to Mg-medicated water, or to mineral-blocks or mixtures designed for cows, are rare.
[(1) All feed values are expressed on a DM basis. The usual DM of concentrate ration is about 88%. If the guide values are to be used for concentrate feeds on an as fed basis, they should be multiplied by 0.88].