The three-legged stool of disease prevention

By Shaun Wellert, DVM, assistant professor Ohio State ATI

When I teach students about infectious diseases, I often use a model known as the infectious disease triad. This framework illustrates that disease occurs only when three essential factors are present simultaneously: a susceptible host, a pathogen, and a conducive environment. The host is the animal that becomes ill; the pathogen is the disease-causing organism; and the environment is the surrounding conditions that allow the pathogen to infect and multiply.

For example, if one of my calves develops respiratory disease, the host is the calf itself. The pathogen may be a virus such as bovine respiratory syncytial virus (BRSV). The environment includes pen or housing conditions, such as ventilation, stocking density, moisture, and temperature, that influence the development of disease. Illness occurs only when all three components intersect.

When we shift from understanding disease to preventing it, the same framework applies. The infectious disease triad can be thought of as a three-legged stool. Remove any one leg, whether pathogen, susceptible host, or conducive environment, and the stool collapses. By disrupting even one of these components, we can greatly reduce the likelihood that an animal will become sick.

Pathogen

Disease cannot occur without a disease-causing organism being present. Keeping infectious pathogens off your premises is a foundational principle of any sound biosecurity plan. Many new pathogens are introduced through the purchase or movement of infected animals. Testing incoming animals for common diseases prior to introduction is essential. Just as important is quarantining new arrivals. Isolation allows time to detect illness that may have been incubating during transport or shortly before departure, before animals commingle with the resident herd.

Animal sourcing also matters. Purchasing from reputable single-source farms with documented vaccination and health records carries far less risk than buying from high-traffic environments such as sale barns, where animals have unknown histories and have been stressed and intermingled with others.

It is also important to remember that not all pathogens originate on the farm. Many are already present in the environment. Thorough cleaning and disinfection of housing, feeding equipment, waterers, and handling facilities reduce pathogen loads and decrease the likelihood of those organisms causing disease.

Susceptible host

Exposure of an animal to a pathogen alone is not enough for disease to occur. The animal must also be vulnerable to the pathogen. Reducing host susceptibility is one of the most practical and effective disease prevention strategies available.

Proper nutrition is foundational. Animals fed balanced rations with appropriate vitamin and mineral supplementation are better equipped to mount an effective immune response. Minimizing stress, whether from overcrowding, transport, weather extremes, or social disruption, is equally important. Stress hormones such as cortisol can suppress immune function, increasing the risk of infection.

Vaccination is another critical tool. By exposing the immune system to specific pathogens in a controlled manner, we can stimulate protective immunity before natural exposure. When animals later encounter the pathogen, their immune systems are prepared to respond efficiently, preventing or significantly reducing disease. However, vaccines must be selected thoughtfully. Targeting the diseases most likely to be encountered in your region and management system is essential. Working closely with your veterinarian to design an appropriate vaccination program is therefore very important.

Environment

Management of the animal’s surroundings plays a significant role in disease risk. Proper ventilation, clean and dry bedding, and appropriate temperature control help maintain comfort and minimize stress. A clean environment limits the survival and multiplication of infectious pathogens, particularly bacteria. While we cannot control the weather, we can manage its effects. Providing adequate airflow and cooling during heat stress, as well as shelter and supplemental warmth during cold stress, supports immune function and lowers disease risk.

In conclusion, successful disease prevention requires deliberate attention to all three components of the infectious disease triad. What am I doing to reduce host susceptibility? How am I limiting environmental risk factors? What steps am I taking to prevent infectious pathogens from entering or persisting on my farm? When we address each of these consistently and intentionally, we greatly increase the likelihood of maintaining healthy, productive animals.