– Dr. Michelle Arnold, DVM (Ruminant Extension Veterinarian, UKVDL)
Corneal ulceration in the early stages of pinkeye. Photo from Veterinary Clinics of North America, Food Animal Practice 26 (2010), page 489.
Infectious Bovine Keratoconjunctivitis (IBK) or “Pinkeye” is a costly and exasperating disease for the beef producer and industry. For the producer, the economic costs of pinkeye include lower average weaning weights, pinkeye treatment and labor costs, and discounts received for calves with corneal scars. Despite ongoing research to combat this disease, prevention has proven difficult because of the complicated interaction of pathogens (bacteria), host (cow/calf), and environmental factors that result in pinkeye’s development and its fast spread. Frequent observation of the herd allows early recognition and prompt treatment of affected eyes, resulting in better healing and less transmission to herd mates. However, preventing a pinkeye outbreak does not start with the first bad eye of the season. Once pinkeye cases begin, it is highly contagious and the bacterial pathogens spread rapidly by direct contact and by mechanical vectors, especially face flies. In an outbreak, on average 10% of calves and 3% of cows in a herd are affected in 30 days or less. Although knowledge gaps exist in our understanding of immunity in the bovine eye, prevention starts early by maximizing the herd’s ability to fight disease, and through reduction of sources of eye irritation, injury, and transmission. Pinkeye prevention for individual herds is best accomplished with the help of your local veterinarian because there is no “one size fits all” approach to control.
Recent research is changing much of what we thought we knew about the bacterial cause of pinkeye (the “pathogen”). Previously, the cause was thought to be invasion of the bacterium Moraxella bovis (M. bovis) because, in research trials, it was the only bacterium recovered from diseased eyes that resulted in pinkeye when placed in healthy calves’ eyes. However, we now know M. bovis colonizes the eyes at a young age and is part of the normal flora (the “microbiome”) of the eye. Cattle are the only known reservoir of M. bovis and adult cows harbor this organism year-round without problems. However, Moraxella bacteria have two known virulence factors, pili and cytotoxin, that change this organism from a harmless inhabitant to an aggressive pathogen. “Pili” are hairlike projections on the bacterial surface that enable attachment to a damaged or injured cornea. After attachment, “cytotoxin” is the poison released by the bacterium that kills corneal surface cells and the white blood cells recruited to fight infection in the eye, resulting in erosions that coalesce to form a corneal ulcer. After these virulence factors are triggered, these “hot” bugs spread within the herd and can cause a full-blown pinkeye case within 24 hours of infection. New molecular diagnostic techniques including PCR and next generation sequencing (NGS) are now being used to analyze the DNA of other organisms present in normal and diseased eyes to get a better understanding of how each contributes to disease and to aid in vaccine development. Studies reveal that two more organisms, Moraxella bovoculi and Mycoplasma bovoculi, are found in over 90% of pinkeye cases. Moraxella bovoculi is an extremely diverse bacterium that also has pili and cytotoxin, and it carries multiple genes that code for antibiotic resistance. To make the picture even more complicated, there are genetically distinct strains of Moraxella bovoculi that do not play any role in pinkeye. Mycoplasma bovoculi causes low level corneal irritation and damage but does not have pili or toxin. Because of this complex mixture of organisms, it is difficult to make an effective, protective vaccine against the pathogens associated with pinkeye.
“Host” factors that contribute to pinkeye development include immune deficiencies due to poor nutrition, lack of trace minerals, inadequate hydration, and sub-par vaccination status. Meeting nutritional requirements, providing vitamins and trace minerals, establishing a comprehensive vaccination program including the respiratory viral disease infectious bovine rhinotracheitis (IBR), and parasite control are all exceptionally important in improving the cow’s or calf’s innate ability to fight pinkeye. No scientific evidence supports feeding excessive levels of any vitamin or mineral, including Vitamin A, to prevent diseases of the eye. However, if trace mineral levels (especially selenium and copper) are very low in an animal, immune function is severely impaired. Cool, clean drinking water (instead of stagnant creek or pond water) improves intake and provides the necessary fluid for tear production to protect the corneal surface. This is exceptionally important in dry, dusty, and/or windy conditions. Tears are essential to wash away pathogens and tear proteins include antibodies to fight infection. Do not forget to regularly check and clean automatic waterers, especially in the summer. Other host factors unrelated to management can contribute to pinkeye. White-faced cattle, particularly Herefords, are more prone to the disease, likely due to enhanced reflection of UV radiation on the cornea. Reported heritability of pinkeye is low to moderate so any control through genetics is unlikely or will be slow, at best.
Environmental factors contributing to pinkeye are those that can irritate or injure the eye’s corneal surface and predispose the eye to infection, include ultraviolet rays from the sun, face flies, dust, seed heads and tall weeds. UV radiation promotes formation of “dark cells” in the cornea; these are damaged cells that are targets of M. bovis attachment. Dust particles, seed heads, tall weeds, and sharp stubble can scratch the corneal surface. Face flies irritate eyes when feeding, with abrasive blotting mouthparts that rasp, scrape, and penetrate the conjunctival tissues, triggering tear and mucus production that feed the insects. Bacteria in the secretions of pinkeye-infected cattle can survive on or in face flies for 2 to 3 days and infect other animals when the flies feed again. Cattle display “fly avoidance behaviors” including head throws, tail flicks, and bunching together with heads directed inward when face flies feed. Preventing eye damage with good face fly control, removing irritant seed heads and weeds, and by providing shade for UV protection reduces the opportunity for pinkeye to strike.
Face fly control is challenging and is not the same as horn fly control. Face flies are “blotters” that feed on tears instead of “piercers” that feed on blood as horn flies do. Fly control methods that depend on insecticides delivered in the bloodstream have no effect on face fly numbers. Face flies spend only a few minutes at a time on or around the head, which is a difficult area to protect. They are also strong fliers and may move two miles or more during their life so they can easily transfer pinkeye from herd-to-herd and farm-to-farm. Face flies can be partially controlled with feed-through insecticides such as IGRs (insect growth regulators), offered in feed or mineral. Both horn fly and face fly females lay their eggs in very fresh manure. IGRs can reduce the number of fly maggots developing to adults, provided the IGR is started early enough in the spring and cattle consume a sufficient daily dose. IGRs should be started in mid-spring, 30 days prior to fly season, and removed 30 days after fly season is over. Supplemental adult fly control is still needed to control adult flies moving in from nearby herds. Adult face flies can be somewhat controlled with repellents and insecticides applied directly to the face and eyes of cattle.
Figure 1: Screenshot of the Veterinary PestX Database dashboard.
Insecticide impregnated ear tags or forced-use dust bags provide the most consistent reductions in face fly numbers. Back rubbers with fly flyps or fly bullets tied at 4-6” intervals along their length and placed in forced-use areas like mineral feeders or entrances to water sites will consistently deliver insecticide to the face. The goal is to reduce face flies to less than 10 flies per head. The Veterinary PestX Database (Figure 1), available at https://www.veterinaryentomology.org/vetpestx is a tremendous resource to identify insecticide ear tags, dusts, feed-throughs, pour-ons, sprays, and premise treatments and their mode of action (MOA) group to control almost any type of cattle pest (see Figure 2). Fly control products must be rotated annually to a different MOA to preserve effectiveness. Repeatedly exposing face flies to insecticides in the same MOA group or to sublethal doses and killing most, but not all flies, allows survivors to develop resistance to all insecticides within the same MOA group. That genetic resistance can then be passed to their progeny.
Figure 2: Example of Mode of Action (MOA) groups. The group number represents the MOA followed by a letter that relates to chemical structure. In the example above, there are 2 different MOA groups: 1B (organophosphates), and 3A (synthetic or synergized pyrethroids).
While fly tags can be an effective method to reduce face flies, it is important to use 2 tags (one in each ear) for optimal control of face flies. Some manufacturers offer “insecticide cattle strips” that can be slipped onto the shaft of an existing ID tag, alleviating the need for two tags in one ear. Read the label and look for tags that “control” face flies, instead of those that “aid in control” and observe the length of time control of face flies is expected. Additional insecticide products will be needed for late season fly control when the tags begin to lose effectiveness. All insecticide ear tags should be removed at the end of the season to decrease development of resistance and, most importantly, rotate fly tags to a different mode of action (MOA) each year (see Figure 3).
Figure 3: Example of fly tag rotation system (Y-tex Corporation). [UK does not specifically endorse any fly control product; example above is for illustration only]
Figure 4: Fly trap made with fly paper wrapped around a protein tub with chicken wire, placed near water and mineral sites. Photo courtesy of Gregg Brann, Grazing Specialist, TACD, and Synergistic Grazing Management Consultant [https://gregbrann.com]
Early detection of animals with the first clinical signs of pinkeye (excessive tearing, squinting, and blinking) and then prompt, effective treatment is essential to reducing spread to herd mates and limiting scar formation in the eye. Long-acting prescription antibiotics such as long-acting oxytetracycline (LA-300®) or tulathromycin (Draxxin®) are specifically labeled for pinkeye treatment. If those two antibiotics lose effectiveness, a veterinarian may prescribe florfenicol (Nuflor®), ceftiofur (Excede®), or other antibiotics to be used in an extra-label manner for treatment. Injectable antibiotics are considered the best option because of their long duration of activity and effectiveness in eliminating bacteria, enabling the cornea to heal. Other remedies may reduce pain and allow healing, but the bacteria can be shed for weeks if not eliminated. When severe ulceration exists, the cornea may need extra protection with either a patch, a third eyelid flap, or the eyelids may need to be sutured (stitched) together. Remember, preventing spread by recognizing and treating affected animals as soon as they show the first symptoms is crucial in controlling a disease outbreak. Active cases of pinkeye with excessive tearing attract flies that widely spread the aggressive bacteria. Additionally, topical application of a fly repellant to the face of an affected animal and quarantine away from the herd will also help reduce spread.
Pinkeye vaccines, whether commercial or autogenous (custom-made), will usually help reduce the number of affected animals or lessen the severity of clinical signs but cannot be completely relied upon to prevent pinkeye. Immune responses to pinkeye vaccines have been shown to be protective in some studies where animals are vaccinated with pili of a certain type and then challenged with a similar strain. A high degree of diversity among genes coding for pili is likely responsible for why some herds see a benefit from vaccination while other herds do not; if the vaccine strain stimulates immunity to a pilus type that is also present in the herd, there should be good protection. In clinical trials, approximately half reported significant protection from commercial pinkeye vaccines. When commercial vaccines are ineffective, an “autogenous” or custom-formulated vaccine can be manufactured from bacteria cultured from affected eyes within a certain area. To make a vaccine, all samples for bacterial culture must be taken early in the course of disease; preferably when the eye is just beginning to tear excessively and before any medications are used. These autogenous vaccine formulations, especially those that include M. bovoculi antigens, often show beneficial results in the field. Autogenous vaccines do lose effectiveness within one to two years as mutations and unpredictable recombinations create new bacterial strains and a new batch of vaccine must be made from new cultures. Timing is very important with pinkeye vaccine administration. Peak immunity occurs 1-2 weeks after the booster (2nd) dose and most vaccines require 2 doses, 1-2 weeks apart. Therefore, the optimal pinkeye vaccine program must begin 4-6 weeks before ‘pinkeye season’ starts.
In summary, pinkeye is one of the most common diseases of cattle and is of major economic importance to Kentucky cattle producers. Although research is ongoing to understand this complex disease, the keys to prevention and control of pinkeye still rely on the basics of maximizing the herd’s immune status, face fly control and maintaining as irritant-free environment as possible. Vaccines, either commercial or autogenous, will help but cannot be completely relied upon to prevent pinkeye. Once cases begin, antibiotic treatment decisions are best made with your veterinarian who will consider effectiveness and cost of the antibiotic, withholding times, and provide a prescription for the product. For more information on insect control, ask your local county extension agent for the UK Extension Publication “ENT-11: Insect Control for Beef Cattle”.