Pinkeye: prevention and treatment.
Dr. Richard Randle, College of Veterinary Medicine, University of Missouri, says dealing with pinkeye is complicated by the fact there are several causes of eye lesions in cattle, all lumped together and called pinkeye; a number of things can create signs similar to those of infectious pinkeye. Thus the success of treatment or vaccination can be varied.
"In most cases eye lesions result from spread of contaminated eye material from an infected eye to a non-infected animal, and this can happen in several ways. Obviously, fly control is a big part of pinkeye control," says Randle.
Solid colored cattle with dark skin around the eyes have traditionally been thought to have less pinkeye, but this isn't always the case. "The color of the skin around the eye, and the related irritation from ultraviolet light is a factor, but because of the variety of agents that cause these eye lesions, there are a lot of things that make it to where color is not a big factor in whether or not an animal will get pinkeye," he says.
Dr. Annette O'Connor, Iowa State University, says Moraxella bovis is the bacteria most commonly associated with pinkeye, but other organisms can be involved. And having M. bovis in the eye isn't enough to cause pinkeye in cattle. "There must be trauma to the eye to enable the bacteria to establish an infection. M. bovis tends to stay in the conjunctiva (delicate membrane that lines the eyelids and the exposed surface of the white of the eye), in the lubricating fluid. Unless there is trauma to the eye, the bacteria can't bind to the cornea and cause the disease," she says. Things that cause trauma include high levels of ultraviolet light, infections by viruses and mycoplasma bacteria, plant pollens, tall grasses that scrape or cut the eye while the animal grazes, dusty feed, dusty or windy conditions, and face flies.
"Other flies like horn flies and stable flies may be involved also, but we're not sure. Pinkeye incidence usually hits a peak in summer- probably because there are a lot of flies, the grass is long, it's usually dry and dusty, and there's a lot of ultraviolet light. All of these things occur in summer and the conditions are right for trauma to occur to the eye. There is also evidence that infection with other agents such as mycoplasmas and infectious bovine rhinotracheitis (IBR) can affect the eye and then cattle are more likely to get pinkeye," says O'Connor. The adenoviruses (a group of viruses that cause a variety of respiratory diseases in humans) are also associated with severe outbreaks of pinkeye, and there is no vaccine available for these viruses.
There are several ways to prevent pinkeye, by preventing situations where trauma occurs. Reduce the dust, clip the grass, provide shade, control the flies, etc. But these practices are not always effective. It's difficult to control dust, almost impossible to control UV light, and on some properties, it's impossible to keep the pastures clipped. There's no one thing alone that's totally effective in prevention or treatment. It's a multi-faceted approach--doing a number of things together to try to minimize the risk.
O'Connor says often we don't adequately control face flies. "Unlike horn flies that spend almost all their time on the animal, face flies don't spend much time on cattle. At any one time there may be only five to 10 percent of the face fly population on the cows; the rest of the flies are out in the pasture. And when they're on cattle they don't stay there long-just long enough to feed on eye secretions and go back to where they were. So they're really not exposed to a pesticide (on the cattle) that much," she explains.
"Some prevention methods may work on some farms, while others Won't. On one farm the problem might be dust and the rancher institutes a fly control program that's effective, but it's not the main cause of the eye trauma. On another farm it might be dust and on another it might be long grass. So that's why you see variable responses to management practices," she says.
The other most frequently used preventative practice is vaccination. The goal of vaccinating is to boost the animals' immune response to the causative organism, M. bovis. "The results with vaccination are variable, however," says O'Connor. "It's not entirely clear what antigens we need to put in the vaccine to make it effective. M. bovis binds to the eye by means of things called pili." These are the filament-like appendages of certain bacteria, and they contain some of the antigenic properties of the bacteria.
"If a vaccine contains pilus antigens, it's thought to be more effective. But pilus antigens change more rapidly than vaccines can be changed. Most vaccines contain several pilus antigens, but they have to go through tests to show that they work, before being accepted for use. Then when they finally get on the market, if they are used on a farm where M. bovis has that particular pilus antigen, it will probably work," she says. However, if the pilus antigens change, the vaccine may not work.
"We don't know how this happens. Perhaps the antigens change, or another pilus antigen is introduced to the farm from new cattle or flies. There is some cross protection, but not much," she says.
Randle says there's been varied success with vaccination. "Trying to build a good immunity at the eye level is difficult. The thing to understand about vaccine is that in a lot of instances it won't prevent an outbreak. But in the face of an outbreak, vaccination can help by reducing the severity of the cases and maybe shortening the time period the eye is affected," he says.
"In herds with a severe eye problem that doesn't respond to normal therapy, an option is to have your vet take swabs from eyes and try to culture the organism and determine what is actually going on. This can help us redirect the therapy. In herds with severe problems it may warrant trying to pinpoint the cause and see what other agents may be responsible," explains Randle.
Keeping good immunity to other diseases (such as IBR and BVD) can help prevent pinkeye. Some veterinarians feel viral diseases can hinder the immune system and lead to higher incidence of other problems. When ranchers vaccinate calves with modified live IBR-BVD vaccine, this dramatically reduces the incidence of pinkeye, foot rot, diphtheria and summer pneumonia in the calves.
IBR (infectious bovine rhinotracheitis, or "red nose") can also be mistaken for pinkeye. Randle says, "IBR has an eye form and can cause runny eyes and eye lesions that look very similar to infectious pinkeye. Being a virus, IBR does not respond well to antibiotic therapy. So some eye lesions may actually be an eye form of IBR. This is a herpes virus, and herpes can cause ulcers. Once the body is infected with herpes, it's always infected-like a person with recurring cold sores or shingles. Stress can bring it on again. IBR vaccines are very effective at preventing some of the respiratory or reproductive problems associated with IBR, but not always effective at preventing the eye lesions."
Total herd health is very important, and this includes cattle management and nutrition. There are many things involved in the pinkeye issue, such as trace minerals and basic deficits in mineral content of diet that can make a difference, according to Dr. Larry Letner, at Harris, Missouri. In order to reduce pinkeye, he feels a stockman needs to understand soil content, grass types, growing seasons, protein levels in forages, etc. In his practice, Letner focuses on total herd health and principles of management and nutrition before he starts with drugs and vaccines. A lot of people start at the top with drugs, and then finally get back to what some of the underlying causes might be, he says.
O'Connor says one type of prevention is treatment. "We don't usually think of treatment as a preventative practice, but if we can't keep the disease from occurring, if we treat early, before it becomes severe, we can limit the impact of pinkeye on things like weaning weight and milk production," she says. This may also save an eye that would otherwise be scarred or permanently damaged.
"People often think they have failed because they had to treat an animal, but I think that if they understand the disease they will realize why they sometimes can't prevent it. If they view aggressive treatment as a way to limit the impact of the disease, that's also successful," she says. By treating early, you minimize spread of pinkeye (via face flies), from animal to animal.
Pinkeye is easily spread. "If you get cattle into the corral to treat some, and run them all through the chute, you create an environment in which all the calves will be more susceptible. You may increase transmission of the disease through the herd, unless you can bring the affected one in by itself to treat. If you treat an animal with pinkeye, make sure your hands are clean before you touch the eyes of the next animal, or you'll defeat the purpose-you will be the fly, the spreader! It's very important when treating pinkeye, to try to limit that impact," she explains.
O'Connor has heard producers say, "I treated the herd and the pinkeye outbreak got worse!" It could certainly work that way. If you round up the herd, put them in a dusty corral and run all the calves through the chute, you might be making it more possible for them all to get pinkeye.
Pinkeye caught in early stages responds well to treatment, but even the eyes that are badly ulcerated and blind will recover. Two things are important in treating pinkeye- using an antibiotic to combat the infection, and protecting the eye from dust, sunlight, flies and other irritants while it heals. M. bovis is susceptible to many of the commonly used antibiotics, and some of them marketed as topical treatments for pinkeye. It's difficult to maintain adequate levels of drugs that are applied topically, however. They don't stay in the eye long enough. Tears wash the medication out of the eye within a few hours. Ointments, powders, sprays or squirts must be repeated at least twice daily to be effective.
Normal cattle produce slightly less than an ounce of tears daily. Cattle affected by pinkeye produce many times more tears than normal, which rapidly wash away any antibiotics applied to the surface of the eye. Certain antibiotics may be injected under the conjunctiva lining the inside of the eyelid, however. It lasts longer at the site than topical medication. Antibiotics given systemically are also helpful. LA-200 has been shown to be beneficial, injected intramuscularly or subcutaneously at a rate of 4.5 cc per 100 pounds of animal.
Eyes generally heal faster (and with less complications) if protected with eye patches or sewing the eyelids shut. The latter has the advantage of being more dependable for protecting the eye from bright light, dust and flies (eye patches sometimes come loose or rub off), and also keeps the eyelids immobile. There's no blinking to rub the protruding ulcer and keep irritating the inflamed eye. The eye is also constantly bathed in its own tears, which seems to have a healing effect. Keeping the eye stitched shut for a while keeps it from drying out; many seriously affected eyes have a tendency to bulge and prolapse--and become dried out, which is a detriment to healing, says Randle.
A twice-daily application of a topical pinkeye antibiotic may work if infection is caught early, but if pinkeye has several days' start and the eye is already turning blue or ulcerating, injecting the eyelid and protecting the eye for a couple of weeks gives much better results. Cases detected and treated early are less likely to develop complications (such as deep ulceration and scarring, "bubble eye" and permanent damage), but it is still important to treat eyes in late stages of pinkeye. Not only will treatment help an eye heal faster and with less damage than if you let the disease run its course, but will also eliminate the bacteria causing the infection- to ensure that the animal does not continue to serve as a source of pinkeye for the rest of the herd.
No magic bullet
Randle says, "In herds with outbreaks that appear to be pinkeye, there are variations in effectiveness of treatment. We don't know for sure which agent is causing the problem in every case." Some veterinarians take swabs from affected eyes to culture the causative organism, and make a specific vaccine for that herd, since there can be other organisms involved besides M. bovis.
O'Connor says she has never seen any scientific literature that says this works, or doesn't work. "The individual farm would have to attest to whether it worked, and that's hard to prove. For instance, one year you may have a really bad pinkeye problem, and the next year it might be better, or it might be caused by something else. You may have an incredibly dusty year, and the next year it rains," she points out.
"If the changed conditions-happen to coincide with the year your vet made the vaccine, you may think the vaccine worked, but in reality it might be you had less pinkeye due to less dust, fewer flies, or some other condition that changed so the eyes could stay healthy. The vaccine might only be effective one particular year, and the next year new antigens are involved. Or, in the year you vaccinated, maybe there was not going to be any pinkeye anyway. Then maybe three years later you get an outbreak and wonder why the vaccine doesn't work. People don't understand why it seemed to work for the last 3 years and then suddenly didn't work," she says.
"There are so many potential ways to get the disease, and also the pilus antigens are changing. The vaccine may work, and you see a decrease in pinkeye, but the next year perhaps a face fly or new cattle on the ranch bring in another strain of M. bovis with a different pilus antigen, and now your vaccine doesn't work."
Many people are sure that face flies are the cause of pinkeye, but as O'Connor points out, pinkeye existed in the U.S. before we had face flies. "Pinkeye was first identified in this country in the 1890's, and face flies didn't enter the U.S. until 1952 from Nova Scotia, gradually moving south across the country. Incidence of pinkeye has increased with the spread of the face fly population. But at the same time, agriculture has intensified. There are many changes in how people handle cattle now. Face flies contribute to the problem, but they are not the only factors." Veterinarians and cattlemen need to understand why preventative practices sometimes fail. Then they might be a little less frustrated.
Randle says, "There's also the problem that many of the things that might be potentially effective are costly. So this creates a challenge in determining the most appropriate thing to do, and it must be evaluated for each herd. If an outbreak occurs however, aggressiveness is always better than playing it along and waiting to see if it gets better. Having a plan of action and moving to it early is what we recommend, and it should be a multi-faceted approach, taking into account all the things that play a role."
If you are seeing more than sporadic cases of pinkeye in your herd, ask your veterinarian to help determine the cause, so proper measures can be taken to correct the problem.
In earlier years It was thought that face flies had sponging mouth parts similar to those of the common house fly, but in 1993 Kansas State University researchers discovered (with use of an electron microscope) that the face fly has very sharp microscopic teeth. Located on the end of the fly's tongue, these teeth are used to irritate the eye, making it water. This gives the fly access to the mainstay of its diet--the protein-rich eye secretions. Then pinkeye bacteria can readily gain access to the eye, via the tiny wounds In the cornea caused by the face fly. The scrapes reduce some of the eye's protective defenses. Also, if flies scrape an infected eye and then move to another animal, they transmit the bacteria to the next animal.
Ropert Cope, DVM, Salmon, Idaho says pinkeye is highly contagious. "In some herds 45 percent of calves have been affected over the course of a summer. Signs In calves are readily apparent: runny eyes, squinting, and avoidance of bright light. These signs are usually seen three to five days after Infection. At that point, small ulcers may be developing on the surface of the cornea. These ulcers often enlarge to as much as one-half inch in diameter by five to seven days after infection," he says.
"If the ulcer is due to trauma, such as a wound to the eye caused by a stick, the ulcer will generally be off to one side of the cornea. An ulcer caused by pinkeye will almost always be In the center. Blood vessels grow toward the ulcer, finally reaching the center of it by 14 to 16 days. Rupture of the eyeball is rare, but can occur with severe infection or following trauma when a blind animal runs into objects. Otherwise, the eyes of most infected cattle heal within 60 days, leaving scar formations on the cornea in the form of a white spot," says Cope.
"The normal cornea can be damaged by light, viruses and/or M. bovis, resulting in neutrophils gathering to combat the bacteria." Neutrophils are white blood cells that ingest and destroy foreign cells. "The neutorphils release enzymes that increase the size and depth of the ulcer, allowing perforation of the cornea and prolapse of the iris through the defect," he explains.
BY HEATHER SMITH THOMAS
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|Title Annotation:||Livestock health|
|Author:||Thomas, Heather Smith|
|Publication:||Countryside & Small Stock Journal|
|Date:||Sep 1, 2007|
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