The recent foot-and-mouth disease (FMD) outbreak in Great Britain has served as a rude reminder to all of us about the risks and threats that we are exposed to and often fail to recognize in animal agriculture. As of May 1, more than 1,500 farms have been infected with FMD in Great Britain, and more than two million animals have been slaughtered. And, the disease is not yet eradicated.

History is full of examples of epidemics of "new" diseases. Smallpox devastated the American Indian population when introduced by European settlers who were immune carriers. Influenza epidemics in people were triggered when a new strain of the virus jumped from a different species, such as ducks or swine, and then spread.

Veterinary medicine contains many examples, also. Twenty years ago, Johne's disease was described as rare by most of my veterinary professors, and hairy heel warts were not even mentioned. Now, about 25 percent of U.S. herds have some level of Johne's infection, while hairy heel warts can be found on 50 percent of dairies.

As a nation, we must be vigilant against emerging diseases, such as FMD. But, each dairy must also be concerned about its own re-emerging diseases, including Johne's disease, salmonella, bovine viral diarrhea, and others.

How it happens
Diseases emerge or re-emerge because susceptible populations of animals (or people) are exposed to infectious organisms in sufficient numbers to produce the disease. Populations may be susceptible because they have never been exposed to the disease or vaccinated. Even with prior vaccination, animals can be susceptible when infectious particles overwhelm them or environmental stress reduces their immunity.

To prevent or control a disease, producers and veterinarians must use one of the following three approaches or a combination:

1. Test and slaughter. This eliminates the reservoir of disease in a population of infected and carrier animals.

2. Environmental isolation and sanitation. This method eliminates the reservoir of disease in the environment, thus reducing the likelihood of infection.

3. Vaccination. This reduces the disease organism's ability to survive in the individual animal, which also reduces clinical effects.

Use of these methods will vary, depending on the characteristics of the disease and the circumstances. For example, if a herd has only one or two cows that test positive for Johne's disease, the test-and-slaughter approach makes sense. However, if 30 percent of the herd tests positive, removing all positive animals would not make sense economically.

Veterinarians, producers and public health officials teamed up early in the 20th century to control several diseases that threatened human health, as well as animal health. Brucellosis was controlled through a combination of test, slaughter and vaccination. Tuberculosis was controlled through a test-and-slaughter program. More recently, vaccination has been the main approach to controlling several diseases of economic significance on the dairy.

In many ways, disease prevention on a dairy reflects the same approach - a combination of culling infected animals, reducing the pathogen load in the environment, and vaccination.

It could be a mistake, however, to rely too heavily on any one of the three methods and overlook a more multi-faceted approach. For example, we may rely on vaccination to control IBR, yet fail to observe adequate biosecurity protocols when moving new cattle onto the dairy. Or, we attempt to control Johne's disease through a test-and-slaughter protocol, but don't address environmental factors that increase the exposure level.

Take a moment to review your herd health program. Ask your veterinarian if you have a tendency to rely on one method of disease control, or if you take the multi-faceted approach needed to keep disease from emerging and re-emerging on your dairy.

Brian Gerloff is a veterinarian and operates Seneca Bovine Service in Marengo, Ill.