Source: SDSU Extension | May 20, 2019
By Russ Daly, professor, SDSU Extension veterinarian, state public health veterinarian
Have you ever thought about how some years get labeled “bad years” for a certain animal disease? We remember that “bad year” for calf scours. Likewise, there are “bad years” for calf pneumonia and even “bad years” for breeding on pasture.
What is it about those “bad years”? Are there years when there are just a lot more germs around for some random reason?
The reasons behind cattle diseases exploding in certain years and not others have to do with the fact that our animals are beholden to their environment. Climatic conditions such as snow, rain, heat, or humidity can affect the ability of the animal to resist a certain disease or succumb to it. Additionally, weather conditions can be beneficial to the growth and survival of certain illness-causing germs or they can be detrimental. This is illustrated by the concept of the “epidemiologic triad” – the concept that the interaction among the animal, the germs, and the environment affects whether an animal gets sick or stays healthy. Cattle are among the domestic animals most affected by fickle weather conditions.
Plentiful moisture during the grazing season might contribute to what could be called a “bad year” for a certain cattle disease: pinkeye.
Not unfamiliar to cattle producers, pinkeye is a bacterial infection of the surface of the eye. What starts out as red, weepy eyes quickly progresses to severe inflammation and an eating-away of the clear portion of the eyeball. It’s a painful problem for affected cattle; left untreated, it can lead to blindness. Different types of germs have been identified in affected eyes, with Moraxella bovis being the one most commonly found.
So what creates a potentially “bad year” for pinkeye? Again, it’s not simply because there are more germs around. In fact, cattle pinkeye is quite different from some diseases where just being exposed to the germ equates to an illness (influenza, for example). Researchers tell us that pinkeye germs can be found in the eyes of normal cattle. What’s more, when they try to artificially create the disease experimentally, they can’t do it by just squirting the bacteria in the calf’s eye. It’s not just the germ; there has to be something more.
That something more is irritation. The normal eye is very good at resisting infection. In order to cause infection and inflammation, pathogenic bacteria have to stick to the eye surface, multiply, and release enzymes that damage the cornea. But this surface is constantly washed by tears, which also contain antibacterial substances. Simple blinking also brushes bacteria off the cornea. Even if some of the bacteria happen to stick, the cells of the cornea (the clear portion of the eye) turn over every 5-7 days.
But irritation to the eye surface is just the chink in all this armor that Moraxella is waiting for – and the opportunities are plentiful during moist summers. Tall grass can scratch the eye surface, and pollen is an additional irritant. Flies and other insects become more plentiful, and they are particularly drawn to tears and gunk that forms in the corner of the eye, causing more irritation. Even strong sunlight provides an irritation that the bacteria can take advantage of.
Because bovine pinkeye can’t easily be replicated by simple exposure to the bacteria, it could very well be considered an environmental disease. Reducing the pinkeye risk for pasture cattle means addressing their environment.
The role of the bacteria should not be entirely discounted, however. Pinkeye tends to affect multiple cattle in a herd. When the bacteria is successful at infecting an animal, there’s more of it around to spread to another by flies or direct contact. Exposure to higher bacterial numbers means less of an irritation is needed for an infection to start.
It’s easier said than done but controlling the environmental aspects of pinkeye through fly control shade and clipping tall grass should be considered. While the success of pinkeye vaccines is erratic, they should also be discussed with your veterinarian. Just because the environmental aspects of pinkeye may be hard to control, producers should still do what they can to protect cattle from this painful, production-robbing disease.