By Russ Daly
Special to the Farm Forum
In recent columns, I’ve discussed diseases such as red nose, Bovine Viral Diarrhea (BVD), and anaplasmosis, that could be brought into a herd with new cattle. These are diseases that can cause trouble in an operation for years to come.
As bad as those can be, they’re probably a cake walk compared to the ones I’ll talk about this week.
We’ve already discussed how we can block transient (temporary) cattle diseases such as red nose and salmonella from a herd by isolating new arrivals for 30 days. We also discussed persistent infections, such as BVD and anaplasmosis, that we can test for, allowing us to treat or divert those animals before they make the rest of the herd sick.
This week, we’ll discuss a couple other diseases that aren’t as easily dealt with. They’re slow-burning infections that don’t show up for years after they infect cattle, making a 30-day isolation period of no use. Furthermore, while we can test for these diseases, the tests have major shortcomings.
One of these diseases is one I get more and more calls about every year – Johne’s Disease. Once thought to only affect dairy cattle, it’s found its way to many beef herds across the country. Johne’s shows up as a persistent diarrhea and weight loss; most affected animals still have a good appetite.
If these signs would occur soon after the animal caught the germ, we could quickly identify and remove them before the illness spread. Unfortunately, this is not how Johne’s works. Johne’s-affected cattle are first exposed to the bacteria as calves, but the slow-growing infection doesn’t create signs of illness for a long time – usually years later. Signs of Johne’s usually first appear in cows or bulls aged 3-4 years or more.
Current Johne’s tests do a good job of detecting the antibodies (in blood) or the germ (in manure), but neither of those show up in the infected animal until right before they start getting sick. In other words, even if that new yearling bull is harboring Johne’s, a blood test won’t detect it until years down the road.
Knowing this, testing younger animals for Johne’s before they enter a herd is pretty fruitless. The proper application of the test, however, is on a herd level. New bulls or heifers from a seedstock herd that has a long history of Johne’s testing (and negative results) are less likely to carry Johne’s than from an outfit of unknown (or positive) herd status. Therefore, knowing the testing history of the source herd is more valuable than an individual test on a new bull or heifer.
The other disease is more problematic yet – tuberculosis. Like Johne’s, tuberculosis (TB) is a persistent disease that won’t show up for years after the initial infection. More problematic, TB doesn’t cause sickness in infected animals. It’s almost always picked up at slaughter inspection. Producers can have TB in their herd and not know it’s there.
The caudal fold test is used for TB. A vet injects antigen into the tail fold and feels for a swelling there three days later. It’s not perfect, either, though. Early infections in younger animals can be missed.
As with Johne’s, source herd testing rather than individual animal testing could be a possible strategy for keeping TB out of a herd. Because – fortunately – TB is so rare (13 herds in the whole US in 2017), not many seedstock herds, understandably, are interested in testing. While I’m confident cattle TB is not going away anytime soon, I’m also pretty sure it’s not going to reach the level where TB testing becomes fashionable among seedstock suppliers.
So, unfortunately I don’t have much that’s practical to offer up about keeping TB out of a herd. Cattle producers should take comfort in the fact that TB is still an exquisitely rare occurrence in beef herds, and that there are a lot of people working to better understand, test for, and contain it once it’s found.
Despite these challenges, cattle producers should work to control what they know they can control. Isolating new arrivals for 30 days and using appropriate tests before animals enter the herd will help prevent the most common cattle diseases from walking into a herd off a cattle trailer this spring.
Russ Daly, DVM, is the Extension veterinarian at South Dakota State University. He can be reached via e-mail at [email protected] or at 605-688-5171.