Often in pasture cattle, more treatment is done for pinkeye than any other diagnosis. Pinkeye rarely kills cattle, but it causes them discomfort and has economic consequences.
In addition to the expense of treatment, pinkeye is associated with weaning weight reductions of as much as 35 lbs. It can lead to disfigurement and blindness (which can result in market discounts), and it's often inconvenient to treat individual cattle in pastures.
Pinkeye is a term used for a set of symptoms. These include tearing and discharge from the eyes; inflammation of the tissues around the eyeball (conjunctivitis), squinting and aversion to bright light; and ulcerations or cloudiness in the center of the front part of the eye (cornea). The disease can take three weeks to a month to heal and often leaves a scar on the eye.
The most common "cause" of pinkeye symptoms is the bacteria Moraxella bovis. Strains of M. bovis that cause pinkeye have surface structures that allow them to bind to receptors on the eye. They then produce enzymes that damage the eye.
Having M. bovis in an eye is not enough for disease to occur. M. bovis can be cultured from 5% of normal eyes. The missing ingredient is eye irritation and damage.
This irritation can come from a variety of sources including face flies, sunlight (especially the ultraviolet component), dust, and tall grass. Pinkeye can occur any time during the year, but mostly peaks in summer when many of these irritators are at their peak. Infections with viruses, such as Infectious Bovine Rhinotracheitis (IBR) and other bacteria, can increase the severity of pinkeye due to M. bovis.
Other eye problems that exhibit pinkeye symptoms include infections with other organisms, such as IBR, Mycoplasma or Neisseria, and foreign objects in the eye, such as hay. With IBR, there may be accompanying signs of upper respiratory disease. With foreign bodies, the lesion on the eye will often be off-center instead of near the center of the eye.
What's Effective? Perhaps because pinkeye is so common, there are many strategies to manage the condition. Few, however, have ever been carefully evaluated for effectiveness. In recent years, we've started to see some well-done, controlled studies of pinkeye treatment, helping us understand what works and what does not.
Cattle eyes have good healing capacity and M. bovis is susceptible to a wide range of antibiotics. Prompt treatment, therefore, has a good chance at success. Early treatment also reduces permanent eye scarring.
The treatment of choice is long-acting oxytetracycline given according to label directions. Oxytetracycline injection - with or without oxytetracycline in the feed - has been shown to be as effective as subconjunctival injection of penicillin.
Corticosteroids will provide relief from symptoms, so the calf will look better in the short run, but in the long run they may inhibit healing of the eye. Topical applications of antibiotics should be done three times a day to be effective, which often isn't cost-effective or practical.
Young cattle are most affected by pinkeye. It's thought that older cattle develop resistance to M. bovis from previous exposure. Cattle that have recovered from M. bovis pinkeye will have moderately protective immunity against another occurrence caused by the same strain.
This suggests that artificially providing exposure through vaccination should reduce pinkeye rates. There are, however, many strains of M. bovis. These strains are different enough that exposure to one strain may provide only partial or no protection from other strains.
Prevention involves a multi-pronged attack. Like other difficult-to-manage diseases - such as scours and pneumonia - pinkeye is a multi-factoral disease.
Start With Face Flies Face flies aren't only a source of irritation, they're a means of M.bovis transmission. Prevention, therefore, centers on controlling these pests in fly season.
Proper use of dusters, back oilers, insecticide sprays, pour-ons, insecticide-impregnated eartags and feed-additive insecticides can help. Other strategies that may be practical include reducing exposure to sunlight by providing shade and reducing irritation by clipping pastures.
Pinkeye vaccine effectiveness is controversial, due theoretically in part to differences between M. bovis strains. Infection studies suggest that, while vaccines are unlikely to prevent infection, they may reduce the severity and duration of infection, especially if the infecting strain is similar to the vaccine strain.
IBR infection may predispose cattle to infection with M. bovis. Thus, vaccination of herds against IBR may reduce outbreaks of M. bovis.
Cattle should not be vaccinated against IBR during an outbreak with M. bovis, however, since there is one report that exposure to modified-live IBR vaccine at the same time that cattle are artificially infected with M. bovis results in more severe eye lesions. There is no evidence that exposure to the attenuated virus in vaccines will induce pinkeye or compromise the effectiveness of pinkeye vaccines.
Prevention strategies are difficult to evaluate in a production setting. Often, pinkeye rates vary greatly from year to year.
If infection with a certain strain one year induces protective immunity, we might expect to see reduced levels of pinkeye the next year. If so, any measure taken the second season might look effective.