Infectious bovine keratoconjunctivitis: Pinkeye in cattle (Proceedings)

Etiology and pathogenesis

The number of gram negative bacteria proven or implicated to cause pinkeye (IBK) in cattle has expanded over the years. Additionally, many of the organisms have changed names over the years as bacteriologic nomenclature has changed. The nominal cause of IBK is Moraxella bovis; however, the mechanism of damage to the eye may be the most important consideration as will discuss later. In addition to M. bovis, other organisms that have been implicated as a cause of IBK include Neisseria (Brahmanella) catarrhalis, N. ovis, and M. ovis. Recently, hemolytic gram negative cocci isolated from IBK cases have been characterized and the name Moraxella bovoculi has been proposed.

Two microbial factors are important for the development of IBK. The first are the pilin proteins which allow the organisms to adhere to the corneal epithelium and to colonize the surface of the cornea. The second are the cytotoxins produced by the hemolytic strains of M. bovis and perhaps the other pathogens. The M. bovis cytotoxin (cytolysin/hemolysin) is a pore-forming protein that appears to "punch" holes in the surface of the corneal epithelium. This cytotoxin is a 110 Kd protein antigenically related to the RTX toxin of uropathogenic E. coli. Other cattle pathogens such as Mannheimia hemolytica, also contain RTX toxins.

Predisposing factors for IBK

There are a host of environmental and management factors that can exacerbate the occurrence of IBK cases or that help precipitate outbreaks. An important factor is UV radiation—sunlight—IBK occurs more often the summer and UV radiation damages the corneal epithelium. Another factor is mechanical irritation and damage of the eye by dust, plant pollen, or weed seeds. These materials can also irritate the eyes of "carrier cows' in the herd causing M. bovis or other potential pathogens to be shed in the lacrimal or nasal secretions of these animals. Plant awns or foxtails also irritate the eyes of cattle and can cause lesions similar to IBK. The handling of these cattle—if they are concurrently harboring pathogens—can aid in the spread of these pathogens iatrogenically to other cattle. Face flies not only irritate the eyes but they also can easily spread the bacteria to susceptible animals in the herd. Therefore, fly control is central to IBK prevention in most instances. Any factor that irritates the eyes, causing cattle to have excess lacrimal and/or nasal discharges, or transfers potentially infectious material from one animal to another will be an important factor in predisposition to IBK.

Diagnosis

Clinical diagnosis is straightforward and based on the appearance of a central corneal ulcer and any additional sequelae—corneal clouding, conjunctivitis, uveitis. The lesions caused by plant awns lodged in the eyes are typically eccentric and the plant awn can usually be visualized. The eye can be cultured for bacterial isolation and identification and antimicrobial sensitivity testing. In most cases this is not done, but may yield important information in severe or prolonged outbreaks.

Treatment

Antimicrobial therapy is indicated and will be the basis of outbreak management along with the elimination of any predisposing factors. The most productive treatment regimens based on research in beef cattle are outlined below.

1. Long-acting Oxytetracycline (200 mg/ml—Biomycin 200 or LA-200). Dosed at 9 mg/lb (20 mg/kg) SQ or IM at 48 to 72 hour intervals.

2. Florfenicol (Nuflor). Dosed at 9 mg/lb (20 mg/kg) IM repeated in 24 to 48 hours.

3. Florfenicol (Nuflor). Dosed at 18.2 mg/lb (40 mg/kg) SQ as a single treatment.

4. Ceftiofur (Excede). Dosed at 3 mg/lb (6.6 mg/kg) SQ as a single treatment.

5. Tulathromycin (Draxxin). Dosed at 1.1 mg/lb (2.5 mg/kg) SQ as a single treatment.

In addition to these treatment options, procaine penicillin G (300 IU) given subconjunctivally in the bulbar conjunctiva for 3 days has been shown to be efficacious. The use of dexamethasone (1 mg) also given subconjunctivally has been shown to cause no measurable advantages or disadvantages in IBK management. Neither florfenicol or ceftiofur are currently labeled for IBK therapy and a veterinarian's prescription and appropriate withdrawal times are necessary for use.

Vaccines

There are a number of commercial IBK vaccines on the market; however, there are a relatively small number of antigens in these vaccines. One company (Addison) utilizes 8 isolates for vaccine production which produces 9 vaccines marketed by 4 companies. Novartis makes a vaccine using 4 strains of M. bovis isolates for their vaccine. Schering-Plough uses 3 strains to make vaccines marketed by 3 different companies and another set of 3 strains for another vaccine they market. The bottom-line is while there are about 15 vaccines marketed there are 4 antigenic "types" in those vaccines. Additionally, some vaccines require two doses for immunization others require just one. The primary problem with vaccination is that producers do not vaccination soon enough in the year for their susceptible cattle to develop immunity prior to the onset of the IBK "season". This is an important part of prevention that is often neglected until an outbreak has occurred. Also, autogenous vaccines are not uncommon and likewise difficult to evaluate because often the vaccine arrives well after the outbreak has started.

Prevention and control

The use of vaccines well ahead of fly season and IBK season is indicated if a premise has had problems in the past. For two dose vaccines it may take 6-8 weeks for the protective immunity to develop in susceptible animals. Since IBK is frequently more severe in calves, it is important they are the focus of the management activities. Face fly control is a very important factor in preventing IBK, since they are a major vector in the spread of the bacteria. The use of insecticide ear tags can be an effective part of this preventive measure. Face flies develop resistance to insecticides over time. For maximum prevention, it is advisable to switch the class of drug used each year or two. If an organophosphate ear tag was used last year, recommend a pyrethroid ear tag this year. Additionally, if you plan to recommend a pyrethroid ear tag this year, use an organophosphate spray this year. Alternating the classes of drugs in this manner will increase the success of the program. It is also recommended that application of ear tags be delayed until the fly population is relatively high so that the possibility of the flies developing resistance this year is lowered. Sprays, back rubbers, face rubbers, and dust bags can be helpful in reducing the fly populations early in the season, before ear tag application. Then, as the fly populations increase, apply the fresh ear tags to achieve maximum benefit. Always follow the manufacturer's label directions for ear tag application. If they call for two ear tags—use two ear tags! If you need ear tags to prevent Pinkeye in the calves—use the tags in the calves. In the fall always remove the ear tags. If the ear tags are left in the cattle the flies that over winter—particularly the face flies—will develop resistance to the drug used and it will no longer be as effective. Face flies and horn flies lay their eggs in cow manure and the larvae can only develop in cow manure. Therefore, some of the compounds that are fed or given orally that kill the larvae in the manure pat can be very effective. One example of this is the insect growth regulator methoprene. This compound is an insect growth regulator (IGR), which is safe, and resistance does not develop to this product. It can be used in "feed through" products, where the drug passes through into the manure unchanged and kills the fly larvae in the manure. Other insecticide products are available that can kill the fly larvae when used as a "feed through", such as Rabon. Rabon is an organophosphate and resistance can develop to this compound.

In addition to face fly control a number of other management procedures can be helpful in terms of prevention and control of IBK at the herd level. Another aid in the prevention of IBK is to clip the pastures if grass is too long and headed out. This will decrease much of the irritation to the eyes of the cattle. When examining the eyes always recommend the use of disposable latex gloves. The IBK agents will bind to hands and this can effectively transmit the agent. Clothing can easily become contaminated. Therefore, it is best to treat any cases after all the routine animal handling procedures on healthy animals are done. Alternatively, recommend changing clothes after handling pinkeye cattle and before handling normal cattle.

Producers should routinely use a disinfectant for equipment used on animals with IBK. Nolvasan (chlorhexidine; Fort Dodge) is an excellent choice because it is not irritating to tissues and works well as a disinfectant. Common items to be disinfected include (1) forceps, hemostats, or tweezers used to remove foxtails, (2) nose tongs for restraint, or (3) rope or nylon halters. It may be a good idea to clean and disinfect the head catch or head restraint area of the chute as it may be an area of contamination and spread of the agents.