Pinkeye, as it is usually named and defined in textbooks, is caused by a bacterial infection of the surface of the eyeball and the inner surfaces of the lids. Its technical name is infectious bovine keratoconjunctivitis (infectious=caused by an infecting organism; kerato=referring to the cornea of the eye; conjunctiv = referring to the pink tissues of the eyelids and the other soft red tissues of the eyeball; and "itis" = inflammation). The cause for this disease is usually given as the bacteria, Moraxella bovis. In recent years a couple of additional bacteria have been incriminated as causing pinkeye. Unfortunately, the bacteria that cause this disease are commonly carried by a few animals that show no signs of disease and serve to introduce it to a new herd when those animals are added to it or that keep the bacteria on the farm to expose a new group of susceptible animals to it. Most animals that recover from the disease clear the infection from their eye tissues; sometimes all of them do. The body's immune response is responsible for this and serves to help keep the animal from getting it again. This is why sometimes it appears in a cow herd with both cows and calves affected by it, but it eventually disappears by the end of the summer. If one or more carriers remain, it sometimes appears again the next year but usually only in the animals that weren't affected the previous year such as new calves or heifers housed at another location the previous year.
The presence of seed heads on tall stems does not, by itself, cause pinkeye. The presence of seed heads can be a factor causing some eye irritation as the cows and calves graze. Of course, so are dusts, pollen, strong sunlight, and face flies - all considered predisposing factors. But the causative bacteria have to be present, and at least a few non-immune animals have to be present, before the disease appears. And not all the predisposing factors have to be present to have the disease appear. Pinkeye in a non-grazing dairy herd can be a nightmare to deal with to which I can attest from personal experience; several times.
The disease often disappears from a herd after a couple of grazing seasons without any special preventive efforts like vaccination or pasture clipping. I suspect that this is caused by the eventual exposure and development of a good immune response by almost all the cows with the carrier cows eventually clearing the infection from their eyes. Unfortunately, there are several strains of Moraxella bovis out there, and it is a pretty safe bet that the available vaccines do not provide good protection for all of them. Therefore, it is not uncommon to see the disease reappear in a herd previously infected if new animals carrying a different strain are introduced or if the herd grazes close to another herd that has the disease. Face flies can carry the bacteria on their body for up to three days, and can transmit it between animals and between herds. The disease can also be spread cow-to-cow by close facial contact such as around feeders. The bacteria are in the tears. I have never seen any evidence to support that seed heads or other inanimate objects are involved in the spread of the disease from animal-to-animal, but the bacteria certainly are mechanically transmitted by flies, so I suppose it could occur in just the right circumstances.
Pinkeye demonstrates a well-known principle in infectious diseases. Disease usually occurs only when there is a susceptible host (in this case a non-immune cow), an infectious agent (Moraxella bovis for pinkeye), and environmental conditions that favor infection of the host (irritation of the eye to create tears that attract the flies and that favor the attachment of the bacteria to eye tissues). Infectious agents involved in many diseases are relatively common in most cattle herds, but disease isn't usually observed until the other two criteria are present. I have written about this concept in several past articles in the Ohio BEEF Cattle Letter especially in regard to calf scours. This is why management of animals and their environment to reduce the concentrations of infectious agents and the stresses on the animals is so important in the reduction of disease.
If you are currently experiencing a pinkeye outbreak or if you are purchasing stocker calves from multiple sources for summer grazing, clipping pastures to remove irritating seed heads and stems may be helpful. If it really costs $15 per acre to clip pastures, I suspect that an equal amount of money spent on other pinkeye prevention strategies, such as not mixing groups of animals just before or during face fly season if carrier animals might be present, judicious vaccine use, and effective face fly control, might be more cost effective than pasture clipping for routine prevention of pinkeye.
Regarding the use of vaccinations to prevent pinkeye, there are some considerations. There are multiple strains of Moraxella bovis in the US cattle population. This "strain" designation is based on the nature of the little hairs that project out of the bacteria called "pili". These structures allow the bacteria to attach to the eye so it can do its dirty work. Without them, it can't attach. The body's immune system responds to these pili as well as other components of the bacteria. If the immune response can correctly and efficiently target the pili, the bacteria can't attach and the disease is prevented. Vaccine manufacturers have improved vaccines significantly in the past 20 years by learning how to grow and harvest the bacteria so that the pili are expressed in the vaccine and stimulate a response. The "fly in the ointment", so to speak, is that there are more strains out there than we have vaccines for. If the body makes an immune response to pili in a vaccine that are different from those on the bacteria to which the animals are exposed, you likely won't get good protection. Since laboratory testing for the different strains isn't widely available, you don't know until you try a vaccine in the face of an exposure whether it is likely to help. To further complicate, the vaccines are given by injection and we are trying to stimulate a local response on the surface of the eye. This is called "mucosal immunity" and it is notoriously difficult to achieve with injectable vaccines. This may change in the future, but for all the diseases that occur on mucosal surfaces (like in the respiratory and GI tract), it is a problem to overcome. The currently available vaccines can be very helpful, but they are not always completely protective. There is a lot of research going on with this disease, especially in Australia and some here, because of the major economic losses it creates. Vaccines should always be administered according to manufacturer's directions and the timing should be such that the vaccine series is completed before typical pinkeye season by about 4-6 weeks to allow time for the maximal effect. This is especially important if you are using products that require, or even suggest, two doses in previously unvaccinated animals. Lastly, there is good evidence that modified live IBR vaccines, when administered during an outbreak of pinkeye, can make the problem worse so that should be avoided.
In the past decade or so diagnostic laboratories have found that at least two other bacteria may cause a disease that looks a lot like pinkeye. One is Branhamella ovis and the other is Mycoplasma bovoculi. Vaccine against Moraxella bovis doesn't do any good for these bacteria, and to my knowledge there are no vaccines for these bacteria. I received an email a couple of weeks ago from a veterinarian dealing with a stubborn herd problem where vaccine didn't seem to be helping. In those kinds of situations, it is always good to culture several typical eyes, before any treatment is given, and to perhaps take a small tissue biopsy to send to the diagnostic laboratory to see what kind of problem you are dealing with.
Source: Dr. William Shulaw, Ohio State University Extension Veterinarian