by: Lee Jones, MS, DVM
University of Georgia

No, the germ theory is still alive and well and scientific fact. The theory states that many infectious diseases are caused by microorganisms such as bacteria and viruses. It was proposed as far back as the 16th century, but not until Louis Pasteur's research in the mid-1800s demonstrated a relationship between germs and disease did the theory actually gain acceptance. Now we hold it to be solid fact. By infectious, I mean diseases caused by organisms such as bacteria and viruses that cause an infection. There are other things that aren't infections that cause disease too like cancer, ingested or inhaled toxins or injury or physiologic diseases like diabetes when things in our body no longer work like they are supposed to. And infectious doesn't necessarily mean contagious or transmittable. Some diseases –such as those caused by Clostridia bacteria species (tetanus, enterotoxemia, botulism, blackleg, etc.) - are acquired from the environment and not spread from animal to animal like many of the viruses. Sometimes, maybe we forget that other things can contribute to infectious diseases in us and our livestock, and we put too much emphasis on the germ theory and not enough on other factors in our control.

Basically, infectious disease in humans and animals occurs when the exposure to disease-causing organisms is greater than our ability to resist or fight off the pathogen (organism that can cause disease). Our bodies and those of our livestock have an ingenious immune system to ward off disease by resisting infection from exposure. However, there are times when the pathogens win; they are able to overwhelm our defenses and cause disease.

First, there is exposure to a pathogen (disease-causing agent). Many times, exposure isn't obvious and nothing visible happens for most healthy people or animals. The organism enters our body but the immune system deals with it quickly. Sometimes the organism gets a foothold and begins to multiply, and we might feel mild symptoms such as a low-grade fever, soreness at the infection site if it's a bacterial infection, or generalized achiness if it's a systemic infection common with viruses. But the immune system is able to quickly eliminate the threat and the symptoms quickly subside. At other times, however, disease can actually make animals sick and symptoms are severe and medical treatment is required. So if every exposure doesn't cause disease, what is the difference between exposures that do and those that don't? The answer is resistance.

To explain resistance, we need to study the immune system. It begins at the surface. For humans, it's our skin; but for animals, it's skin and fur or hair coat. The skin is a barrier, like the walls of a fort or a fence keeping bad things out. Intact skin is a formidable barrier that most organisms can't penetrate. But there are openings in the skin. We have to eat, drink and breathe, so we have a nose and mouth as well as openings in the skin to remove waste like urogenital tract and anus for the GI tract. All these openings have specialized 'gate keepers' for protection, not unlike gate security at a military base. The mucosal linings of our mouth, throat and nasopharynx contain specialized immune cells and patches. Our tonsils are special organs made of lymphoid tissue that help capture and deal with bacteria and viruses that try to enter our bodies through our throat. In fact, the mucosa throughout our bodies, including the GI tract, has this tissue to constantly deal with harmful bacteria. If a pathogen does make it into the body, then other parts of the immune system, generalized and specialized cells and antibodies attack the invading organisms to protect the body.

The immune system can be trained or programmed. If it is exposed to a bacteria or virus, it learns to recognize that particular organism so that it can respond faster the next time the organism attacks. That is how vaccines work. A vaccine contains a weakened version of a disease-causing organism. It is administered to healthy animals to train the immune system - so if it is exposed to that organism, the immune system responds quickly so the infection doesn't cause disease. However, vaccines don't work the same in all animals and some vaccines don't work as well as others. For instance, the 7 -way clostridial vaccine, also known as 7 -way, is really good at stimulating immunity to the toxins produced by Clostridia spp. However, other vaccines may not stimulate the immune system as well, or an individual may not be able to respond to the vaccine. This can be a temporary condition or permanent if there is something wrong with the individual's immune system.

It helps to think about herd protection in the same way as our body's immune system. A biosecurity program is like the skin or outer barrier; a quarantine and test program is like the 'gate keepers' of our nose and mouth; and vaccines and good nutrition are like our systemic immunity.

The best protection is to not get an infection in the first place. If our animals aren't exposed to an infectious organism, then no disease should ever occur. Swine and poultry production facilities operate with this concept. The heightened biosecurity of these operations helps to keep disease very low and production high. However, this level of biosecurity isn't practical for cattle operations, but there are some things cattle operations could do to improve biosecurity: Manage human and other animal traffic. People can be fomites - spreaders of disease organisms. People who work cattle from one farm to another need to be careful that they don't accidentally bring something that can be harmful onto the farm. Disinfecting equipment between farms and wearing new clothes or protective clothing help to reduce the potential for transmission. It is essential to carefully evaluate new herd additions, such as replacement bulls and heifers, and make sure they come from clean sources.

Once the new additions are on premises, then it is best if they are kept separate from the herd for at least 5 weeks. This is the 'gate keeper' level of biosecurity or immunity. Also, at this time animals can be tested for exposure to diseases such as Neospora or Anaplasmosis, to name a few. During the quarantine period, animals should be watched closely for any signs of disease. Pregnant animals need to be watched for abortion or rechecked to make sure they are still safe in calf. Not only does this short-term separation help to protect the herd; it also helps the new additions to acclimate to their new surroundings. The stress of entering a new herd can also put new additions at risk of getting sick. Therefore, quarantine periods have a double benefit to the existing herd as well as the new additions.

We are seeing an increase in the number of Anaplasmosis cases in Georgia. In some cases, naive cows have been introduced into herds that have Anaplsmosis and have died shortly after arrival. While we typically think of new additions as potential carriers of disease, we also need to realize that they could be at risk from a disease that might be stable in our own herds. BVD, Neospora and Anaplasmosis are examples of diseases that might already be present in a herd and could cause problems for new additions.

A strategic vaccine program and good nutrition are like the immune system. The animals need to be healthy with a good nutrition program, so that their immune system functions the way it should. Animals that are stressed or have poor nutrition are at risk of being infected and may not be able to ward off disease effectively. Good stockmanship is essential for healthy cattle.

Transportation has been shown to dramatically increase risk of disease. Stressed cattle have compromised immune systems. Also, animals may become dehydrated or experience 'shrink' (loss of body weight due to dehydration) during transportation. This severely reduces the body's ability to prevent infection, and animals need access to clean water immediately after arriving at their destination. Rest is just as important. Anyone who has been sick knows that adequate rest is essential in trying to recover from disease; it is just as important for preventing disease. Sometimes, we can fail to remember that there are many similarities between us and animals. Our immune systems have many similarities, and keeping that in mind when we manage our cattle operations can help to keep our animals healthy, comfortable and productive.

The germ theory is still valid, but we also know that there's a lot more to protecting our herds than just avoiding germs or vaccinating our cow herd.

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