Pinkeye is a highly contagious condition in cattle that is caused by multiple strains of Moraxella bovis bacteria. One report (NAHMS, 1997) indicated pinkeye infections were found in 1.1 percent of all unweaned beef cattle, and this was second only to scours (1.7 percent infection rate) for these cattle. Pinkeye is generally nonfatal, but there are significant economic losses incurred from pinkeye infections. Kirkpatrick and Lalman at Oklahoma State reported reduced weight gain and reduced milk production in infected cows, and treatment costs account for over $150 million annually to the industry. These losses include up to $100 in less value for cattle with pinkeye at markets, and a 20-year review study has shown 19 to 40 pounds less weight at weaning (Larson, 2012.)
The bacterial cause of pinkeye is usually M. bovis, but Barz (2012) has indicated the M. bovoculis strain has become more prevalent in winter infections, possibly because vaccine use has made the M. bovis strain less prevalent in the environment. Chlamydia and mycoplasma are other bacteria that have been implicated in pinkeye infections, and the infectious bovine rhinotracheitis (IBR) virus has also been associated with infections as a predisposing factor or to contribute to more severity of infection.
Early in my career I worked with a field veterinarian who thought all cattle had the M. bovis bacteria present in the eye, and infection was caused by an irritant to the eye. We routinely observed pinkeye infections to spike as soon as cool-season grasses headed out and the seeds provided the irritant. Other pathologists suggest the infection is caused by the combination of the irritant and a vector such as flies bringing the bug to the eye. The Oklahoma State report indicated flies ingest the mucous from the infected eye and transmit it to others as an irritant. The bacteria may be present in the fly for up to three days after ingestion. Other factors known to contribute to the infection include dust and ultraviolet (UV) light. The latter may contribute to higher infection rates for cattle with little or no color pigment around the eye that would intensify the effect of UV light.
Cattle that become infected first appear to have some watering around the eye, and this may progress to squinting eyes. The next stage is infection of the cornea of the eye that may first appear as a cloudy eye, but can quickly progress to yellowing and ulceration of the eye. Without treatment, the ulceration may progress and actually burst and empty the eye contents with resulting blindness. Cattle that recover from ulcerated corneal infections will have scar tissue that will make the eye appear cloudy for life with varying degrees of blindness in the eye.
Early diagnosis and treatment are keys to preventing expanding infections. The most useful drugs for treatment include oxytetracycline, penicillin and sulfonamides (Larson, 2012). Treatment with long-acting antimicrobials, such as LA-200, can be effective, particularly if they are repeated in three to four days. Injections of penicillin directly in the eye can also be effective, but this procedure should be done by a veterinarian to prevent eye damage. Powders and sprays are also used, but are generally not as effective as systemic treatments because they would have to be administered multiple times each day, and this irritation may actually compromise their effects. Eye patches are used to prevent further irritation and should be used with an antimicrobial. Vaccines are available for pinkeye, but should be considered as a partial preventative. As one farmer put it, "The vaccine does not keep my cattle from getting pinkeye, but they do prevent the wreck." Multiple strains of moraxella cause pinkeye, and not all strains are included in vaccines. Some larger herds are making their own vaccine so specific, local bacteria are included. A report from Whittier et al (2009) indicated to not vaccinate for IBR during pinkeye outbreaks, as the IBR vaccine can damage eye cells making it easier for M. bovis to invade the tissue.
Prevention of pinkeye includes four steps:
1. Vaccinate the herd so a general control is in place to help prevent a wide-spread, excessive infection rate.
2. Limit the irritation to the eye as much as possible. This is particularly true for seedheads appearing in pastures. Well-managed pastures seldom should have seedheads present.
3. Control flies as much as possible. Fly tags, rubs and feed-through control measures can all be effective. Spraying cattle has limited value because the life of the sprayed-on material in not very long.
4. Have a vaccination schedule in place for IBR, bovine virus diarrhea (BVD) and the respiratory complex to help prevent these organisms from increasing the onset or the severity of pinkeye infection.
Dr. John Comerford is associate professor of dairy and animal science at the Pennsylvania State University.