Stone cold nightmares: Small ruminant urolithiasis

By Emily Nogay, VMD, MS, OSU Large Animal Services 

There isn’t a veterinarian out there who enjoys seeing a suspected blocked small ruminant on their schedule. Being “blocked” refers to when stones or crystals are created in the bladder and subsequently obstruct the outflow of urine out of the urethra. This is called urolithiasis, and it is similar to kidney stones in humans, except that sheep and goats tend to form stones in the bladder rather than the kidneys. This disease is difficult to treat and even more difficult to prevent, making it frustrating for both veterinarians and owners.

Small ruminant urolithiasis is a multifactorial disease, meaning it results from many factors (which can make prevention difficult). There are many different types of urinary stones; however, sheep and goats typically create two types: calcium carbonate and struvite stones. Struvite stones are part of a larger group called phosphatic stones; however, because phosphatic stones are difficult to distinguish from one another without laboratory testing, they are often lumped together and collectively referred to as struvite. Other types of stones are possible; however, calcium carbonate and struvite stones are most common in ruminants due to their higher urine pH.

Aside from the high urine pH of ruminants, why else do these stones occur? Well, the short answer is because of diet. Struvite stones occur when there is excess phosphorus and magnesium in the diet (usually from pelleted feeds). Calcium carbonate stones occur when there is too much calcium in the diet (usually from legumes such as alfalfa). When too much mineral is absorbed, the kidneys filter them out of the blood and into the urine. Over time, these minerals form into crystals, which can eventually turn into stones.

Hydration status and urine concentration are other key factors in stone formation. More concentrated urine contains more minerals that can come together to form a stone. Encouraging water intake can be one helpful way to prevent stone formation. This is especially true in winter, as animals dislike drinking cold water. Providing water at an appropriate temperature, depending on the environment, is necessary for proper hydration (especially with goats).  

Castration age has previously been described as one contributing factor to stone formation. It has been shown that if castrated young (<6months), the urethral diameter is smaller, thus less likely to pass stones if formed. In reality, this is controversial as the urethral diameter difference is just a few millimeters, which is often not enough to pass most stones, and unfortunately, not all studies agree across small ruminant species.

Other factors that contribute to stone formation include excess protein in the diet and vitamin A deficiency. These two essentially lead to organic material in the urine, which predisposes to stone formation. Lastly, although it hasn’t been well documented yet, it is strongly suggested that genetics play a major role in stone formation as well. This can be frustrating, as sometimes it can feel like you are doing everything correctly in the diet and still end up with urolithiasis.

Treatment of small ruminant urolithiasis is complicated and often unrewarding. Treatments can vary greatly depending on the patient’s stability, stone type, and owner goals. Stone type is important when considering treatment and prognosis, as struvite stones can be dissolved by increasing urine acidity, whereas calcium carbonate stones cannot. This means struvite stones offer more treatment options and usually have a better prognosis, depending on the situation. No matter the type of stone, these animals often require surgery or hospitalization, and even if they are able to pass urine normally again, they may form new stones and re-obstruct.

Since treatment isn’t always successful, it is even more important to focus on preventing urolithiasis. Unfortunately, because this is a multifactorial disease, prevention can also be frustrating and complicated. In short, don’t feed male sheep and goats anything besides grass hay (free choice mineral should still be available). For most pet animals, there is no need for any pelleted feeds or richer forages such as alfalfa. These animals should have free-choice grass hay, as forage will naturally increase water consumption, and chewing increases the amount of phosphorus excreted in saliva rather than urine. Free-choice hay also helps maintain urine output, preventing urine from becoming too concentrated. Increasing water intake can also be accomplished by adding salt blocks or feeding salty treats such as peanuts or chips (but no corn chips).

If you have animals that aren’t freeloaders (pets), then diet control becomes much more complicated. These diets need to be heavily managed to ensure appropriate levels of calcium, phosphorus, and magnesium. This might mean conducting regular feed testing or working closely with your veterinarian and the feed supplier to ensure appropriate levels of these minerals. For show animals that don’t have regular access to forage, water consumption becomes much more important. Offering these animals free-choice grass hay for 30 minutes at night can help decrease the incidence of stones.

One common practice to prevent urolithiasis in animals that cannot be on grass or grass hay alone is to add anionic salts, such as ammonium chloride, to the diet. This practice has some merit; however, it needs to be closely monitored to ensure it is done correctly, and even when done appropriately, it may make matters worse. Adding ammonium chloride to the diet will result in more acidic urine. This creates a poor environment for calcium carbonate and struvite stones to form, and if the urine is sufficiently acidified, it can dissolve any struvite stones that might already be present. This sounds like a great idea in theory; however, in reality, it is a bit more complicated. When this is done continuously, with the same amount of ammonium chloride in the feed, it stops working. Essentially, the kidneys are smarter than we will ever be and learn to overcome the anionic salts in the feed, and the urine does not become acidified. This means you are likely not preventing stones. Another complication with this practice is that when the urine is acidified, the kidneys are more likely to excrete calcium into the urine. The more calcium in the urine, the greater the likelihood of forming calcium carbonate stones (the bad kind!). After all that, if you still want to feed ammonium chloride, it must be closely monitored. It works best if the dose is pulsed (take it away for a few days, then add it back in for a few days, sometimes change the amount) and urine pH needs to be regularly tested to ensure that the urine is getting acidified enough to work, but also not causing too much acidification that results in more calcium being deposited in the urine.

With all of that being said, the best way you can avoid urolithiasis is to only own female small ruminants. Females still create the stones; however, their anatomy is different enough that they very rarely become blocked by them. Small ruminant urolithiasis has plagued veterinarians and owners for decades and, unfortunately, will continue to do so, as more research is required to understand the best treatment and prevention strategies. It is a very complex disease that can be difficult to manage, but working closely with your veterinarian and nutritionist can greatly reduce the incidence of urolithiasis in your herd.