It is the time of year when cow-calf producers occasionally need to deal with a frustrating disease known as pinkeye.
The term pinkeye refers to any kind of inflammation of the conjunctiva of the eye. This is perhaps more of an “umbrella diagnosis” or clinical syndrome as there are numerous causes of these infections including foreign bodies, some viruses such as infectious bovine rhinotracheitis virus and a number of bacterial infections.
When most veterinarians and producers talk about pinkeye they are referring to a condition of the eye that typically occurs on pasture that is characterized by ocular discharge, squinting or sensitivity to light, excessive tearing, and inflammation (redness) of the cornea and the soft tissues around the eye. It is more common in younger animals and in 75 percent of cases only one eye is affected.
This is a very painful condition and as a result it can affect the animal’s appetite and cause some degree of weight loss.
In severe cases, the infection can lead to a corneal ulcer or an abscess within the cornea itself and occasionally even a corneal rupture and permanent blindness in the affected eye.
When I went to veterinary school, we were told that the cause of pinkeye on pasture was a single bacteria known as Moraxella bovis. This particular bacteria has pili or hair-like appendages that are found on their surface that allow the bacteria to attach to the cornea of the eye.
The bacteria then release a number of toxins that cause the inflammation resulting in the syndrome known as pinkeye.
However, now there are other bacterial pathogens that have been described in association with outbreaks including Mycoplasma bovis and Mycoplasma bovoculi. These bacterial organisms have been potentially implicated in outbreaks as well, but their role is still not fully understood and they can be found in many animals that are not suffering from pinkeye.
Carrier animals that harbour some of these bacteria probably play a role in outbreaks occurring, but more research is needed to identify how commonly this occurs and what factors affect shedding.
Cases of pinkeye tend to respond to antimicrobial therapy and we have a number of antibiotics approved in Canada that have label claims for pinkeye including long-acting oxytetracyclines, florfenicol and tulathromycin.
The major challenge to dealing with this disease is identifying cases on pasture early and then the logistical challenges of treating animals while on pasture.
The disease can spread rapidly throughout the herd and often occurs in an outbreak situation, which makes treatment even more challenging.
If you are dealing with a pinkeye outbreak, consult your veterinarian about the best treatment protocols and your veterinarian can also provide some additional potential options for dealing with more severe cases.
In most situations we would really prefer to prevent an infectious disease from occurring in the first place and this is probably one of the most frustrating diseases as we have very few tools that can effectively prevent pinkeye and we can’t predict where and when the disease may be more prevalent.
There are commercial vaccines available for pinkeye and some veterinarians have attempted to have autogenous vaccines made as well. However, the efficacy of these vaccines (both the autogenous and licensed) is somewhat questionable and there have been few clinical trials that show strong evidence of effectively preventing pinkeye infections with the use of vaccines.
There are a number of environmental factors that have been traditionally associated with pinkeye as well. Could we address these factors to prevent pinkeye?
Ultraviolet radiation has been shown in experimental infections to be a factor that makes Moraxella bovis more likely to cause pinkeye. In addition, pinkeye outbreaks tend to occur after lengthy exposure to bright sunlight. However, providing shade is not easily done on many of our prairie pastures and how big an effect it would have on reducing pinkeye infections is unknown.
Irritation of the eye from grass awns has been sometimes referred to as a potential risk factor. This could result in foreign bodies in the eye and result in a condition that would mimic pinkeye. However, there is no conclusive evidence that grazing tall grass is actually an initiating event for pinkeye and good forage management would actually make us want to have longer grasses in our pasture.
The final and most researched environmental risk factor that is often described is the presence of face flies. It is important to differentiate face flies from horn flies. Most of the flies we see around cattle on pasture are horn flies. Horn flies are small in size and are usually found on the backs, sides and poll area. Face flies are similar to a slightly larger, darker house fly and tend to cluster around an animal’s eye, mouth and muzzle area. Face flies can potentially cause corneal damage, which may allow infections to occur and there is some evidence that these flies can both harbour Moraxella bovis and transmit it.
However, face flies have only been in North America since the early 1940s when they were accidentally introduced and we know that pinkeye occurred before the introduction of the face fly.
A few studies have shown a weak correlation between the number of face flies and the prevalence of pinkeye but the results are far from conclusive.
The bottom line is that the only environmental factor for pinkeye that we can attempt to control at all is face flies. There is some scientific evidence that the use of insecticide ear tags, extended-release eprinomectin or other fly control methods will reduce the number of pinkeye cases to some degree.
Talk to your veterinarian about the best options for treating pinkeye cases. Unfortunately, there aren’t a lot of practical things we can do to prevent this disease from occurring other than face fly control and it remains one of the more frustrating summer pasture conditions that some producers have to deal with in their cattle.
John Campbell is a professor in the department of Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.