By: Russ Daly, extension veterinarian at SDSU Extension
I consider myself fortunate.
I’ve never seen brucellosis, nor have I been sick from it myself. The state I live and work in has been free of the disease in cattle for over a decade, longer for pigs. For my whole 23-year veterinary career, my direct experience with brucellosis can be illustrated by two objects – a bottle of brucellosis vaccine and an orange metal ear tag – objects used to vaccinate heifers against the disease.
Had I practiced veterinary medicine in the 1930’s or 40’s, my experiences with brucellosis would have been vastly different. I would have encountered infected cows on a weekly basis – cows needing treatment after aborting their calves, or infected cows that needed calving assistance. I would have likely contracted a case of it myself. In 1942, the USDA declared brucellosis to be the most important bacterial zoonotic disease (disease transferred from animals to man) present at the time. I would have been knocked out of work, home with the rising and falling "undulant fever" that goes along with human brucellosis. In 1934, the first year of the eradication program, about 15% of the cattle were infected with this disease – it was real and it was not uncommon. What it did to cattle was bad enough. What it did to farmers and their families was pretty bad too.
But the hard work done since then has paid off greatly for animals and people. Brucellosis is no longer on the list of causes of abortion for cattle and is not the public health scourge that it once was. The attack on cattle brucellosis was made possible by the development of blood tests (detecting antibodies in infected cows) and vaccine (preventing clinical signs and abortions in vaccinated cattle). It was not easy and it was not without contention.
However, brucellosis is not dead and gone from the United States. While the species that affects cattle, Brucella abortus, has essentially been wiped free from domestic cattle herds, it has a wildlife reservoir in elk and bison in the Yellowstone Park area. With diseases like this in farm animals, we have a chance to test and vaccinate. With wildlife, of course it’s a different story. States that border the park regularly deal with infections in cattle that have come from wildlife. Montana recently reported the second brucellosis-positive cattle herd within a short time period, which may affect their state’s status as "free." The Brucella bacteria is shed around the time of calving, so if domestic cattle have contact with infected bison or elk when they calve, transmission may occur.
Pigs have their own form of brucellosis that causes early farrowing or abortions in infected pregnant sows. Swine brucellosis is caused by a bacteria different from the one that affects cattle, Brucella suis. It’s important to note that there are several different species of Brucella bacteria, and they don’t tend to cross species all that much. For example, cattle brucellosis is not transmitted to pigs. Another difference is that there is no vaccination available for swine brucellosis; we tackle that disease mostly through blood testing. Other species of Brucella can affect sheep (Brucella ovis), goats (Brucella melitensus), and dogs (Brucella canis), and generally this species specificity holds true.
Swine brucellosis appears to have a wildlife reservoir as well. In this case the culprit is feral swine ("wild hogs"). Where the wildlife reservoir for cattle brucellosis is well-defined geographically, Brucella suis-infected wild pigs are widely scattered. In South Dakota, we have not identified established feral swine populations. But they are present in some of our neighboring states.
Brucellosis brought in by feral swine means potential headaches for pork producers. But it also creates headaches for public health people too. Over the years, a fair number of reports of human brucellosis have been attributed to people having contact with wild pigs – mostly through hunting them and the subsequent butchering process. While treatable with antibiotics, these infections can debilitate people for very prolonged periods of time.
While we’ve made great strides in tackling brucellosis on the farm animal front, wildlife populations will continue to be a potential source for these infections – for animals and people alike – for some time to come. These are diseases that we can’t let our guard down on just yet, meaning that the vaccine and orange Bangs tags will still be important tools for a while.