The disease triad

Kelly Lechtenberg, DVM, PhD, Midwest Veterinary Services, Oakland, Neb., says there are three elements that make up the "disease triad" in feedlot cattle. Speaking at the 2011 Annual Western Veterinary Conference, Lechtenberg says those three elements are pathogen factors, host factors and environmental factors.

Pathogen factors include dose, reservoir, environmental hardiness, contagiousness, infectivity and virulence. Questions to consider are how contagious are the pathogens? What kind of contact is necessary for transmission (direct aerosol or fecal-oral such as IBR, BVD, Salmonella vs. intermediate host such as anaplasmosis, liver flukes, etc.)? Much of this, Lechtenberg says, is largely out of our control.

Environmental factors include sanitation (mud, dust), contact pattern (continuous vs. all-in/all-out), animal density, animal movement between groups, group size and social stress. "The likelihood of effective contact is the percentage of the group that will have contact with a pathogen-shedding individual that results in infection/disease," Lechtenberg explains.

What influences that likelihood is immune system competence/immunosuppression that are influenced by factors such as internal parasite burden, stress (environmental, social, disease), nutritional deficiency and level of heterosis/genetics. Also involved are animal density, group size and pathogen factors such as contagiousness/ virulence.

Host/animal factors in the disease triad are influenced by age, innate resistance, immune status, nutritional status and genetics /heterosis.

Lechtenberg says there some areas where we can intervene to have an effect on the disease triad as there are several environmental and animal contributors to transmission or disease. Questions that need to be asked include: Can housing (heat, cold, dust, moisture, etc.) impact likelihood and extent of transmission? Can animal source, age, or other factors affect disease risk? Can the way you are administering health, nutrition and animal care affect disease risk?

Lechtenberg notes that there are areas of intervention that include the reservoir, environmental and animal contributors to transmission, environmental and animal contributors to disease and the timing of transmission.