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Ovine Observer

Infectious ovine keratoconjunctivitis (pink eye) in sheep feedlots

Michael Laurence (Murdoch University)
Author correspondence: m.laurence@murdoch.edu.au

This article contains excerpts of Dr Fraser Murdoch’s PhD thesis modified for publication here by Dr Laurence. This research was funded by the Live Export Programme (Meat and Livestock Australia).

Introduction

Infectious ovine keratoconjunctivitis (IOK), also known as pink eye, is an infectious disease that is not uncommon in a feedlot situation, including pre-embarkation feedlots where sheep are in close contact. Although sheep will typically recover from IOK with appropriate treatment, there is a cost associated with this treatment in medication, labour hours and additional feeding. IOK has also been recognised as having a detrimental effect on animal welfare and identified under the Australian Standards for the Export of Livestock legislation as one the reason animals would be unfit for travel. It is essential that animals suffering from IOK receive effective treatment to minimise any possible impact on their welfare and long-term health.

General aims

Some of the general aims of this extensive body of research were to describe the normal bacteria present in the eyes of sheep in a feedlot and assess the efficacy of several commonly used treatments for IOK, and the most appropriate treatment for different stages of disease.

Experiment 1 - Identification of causes and treatments of infectious ovine keratoconjunctivitis (pink eye) in a pre-embarkation quarantine feedlot. 

Several treatments are used throughout the world to try to alleviate the clinical signs of IOK. Although some work has been done to assess treatments in other countries information on the success of treatments used in Australia is sparse. 
 
IOK lesions were graded on scale of severity (Figure 5).
Figure 5 Grades of severity of Infectious Ovine Keratoconjunctivitis.
Figure 5 Grades of severity of Infectious Ovine Keratoconjunctivitis

Materials and methods

Fifty-one merino and merino cross sheep with no clinical signs of eye disease were selected from the pre-embarkation feedlot. Both eyes were swabbed to test for bacteria and Mycoplasma. New gloves were worn for each sheep to minimize the risk of contamination between animals. Eighty merino and merino cross sheep with naturally occurring IOK (pink eye) infections were also selected. Sheep were selected based on the clinical grade of infection. Only sheep with grade 2 (conjunctivitis) or grade 3 (corneal oedema) in both eyes were selected for this experiment.

Affected sheep showed no other signs of illness. All sheep were transported to the Murdoch University on-campus farm (Perth, Western Australia) where they were randomly assigned on passing through the race into 8 groups (n=10) and housed in group pens in an open sided shearing shed (animal house). This allowed for ease of monitoring and for treatments to be easily administered. Several common treatments were tested (Table 3).

Table 3 Infectious ovine keratoconjunctivitis (pink eye) treatments in animal house setting. The two most effective treatments are bolded.
Group
Treatment
Administration
Frequency
1
Oxytetracycline
Alamycin intramuscular injection
Single dose
2
Cloxacillin
Orbenin eye ointment
Single dose
3
No treatment
 
 
4
Oxytetracycline
Powder added to drinking water
Daily
5
Oxytetracycline
Terramycin pink eye powder
Twice daily for 5 days
6
Oxytetracycline
Oxytetracycline
Terramycin pink eye powder
Alamycin intramuscular injection
Twice daily for 5 days
Single dose
7
Oxytetracycline
Terramycin pink eye aerosol
Twice daily for 5 days
8
Oxytetracycline
Alamycin intramuscular injection
Two doses 4 days apart

Results

A total of 20 different organisms were cultured from the 51 sheep that had no clinical signs of eye disease (102 control eyes). Of these organisms, Bacillus species were the most commonly isolated with 87.3% positive. Moraxella ovis was the second most commonly isolated organism with 38.2% followed by Staphylococcus chromogens with 33.3%. Only 4.9% of control animals were positive for Mycoplasma species, only one of which was positively identified as Mycoplasma conjunctivae. Bacteriology was not conducted on sheep with clinical IOK, however, during the treatment experiment, it was found that two injections of long acting Oxytetracycline (OTC) treatment (Group 8) and the combination of OTC topical powder and OTC injection (Group 6) were the most effective treatments. These two treatments were equally effective.
 
Both topical OTC preparations, powder and aerosol were less effective than giving no treatment at all. Giving injectable OTC twice 4 days apart (Group 8), was found to be significantly better than most treatments, except a single dose of injectable OTC (Group 1) or the combination treatment (Group 6). The combination of OTC topical powder and OTC injection was found to be better than giving no treatment (Group 3), giving just the powder (Group 5) or applying the aerosol (Group 7). The study showed that Group 3, the control group, was significantly worse than Groups 6 and 8.

Discussion

Up to 20 different species of organisms were isolated from the eyes of healthy sheep including Moraxella ovis and Mycoplasma conjunctivae. This result highlights the potential carrier status of healthy sheep that can act as a source of infection to naïve animals. This experiment has also highlighted the limited value of using topical treatments for IOK in sheep. The use of topical OTC products has been found to worsen the clinical signs of IOK and giving no treatment is more effective that using either product. Injectable OTC was shown to be the most effective treatment, and this result is similar to studies carried out under more extensive situations in other countries. Although not the most effective treatment tested, in-water OTC (Group 4) was more effective than giving no treatment.
 
Given the relative ease of administration of medication in water to large numbers of animals, this type of treatment warranted further investigation. A further study showed that while in-water OTC was clinically efficacious, the use of OTC-medicated pellets yielded better results. In addition, in-water OTC depressed appetite significantly and caused changes to the rumen microbiome that were detrimental to animal health, making it a less suitable option. Establishing injectable OTC as the most effective enables this treatment to be used as a useful standard with which to compare any treatments used in subsequent experiments. 

Experiment 2 - The treatment of infectious ovine keratoconjunctivitis with in-feed medication in a pre-embarkation feedlot. 

It was important to test whether the positive responses seen clinically in sheep treated with OTC-medicated feed in controlled animal house environments were replicated in an actual pre-embarkation feedlot. Up until this point, all experiments had focused on small numbers of animals with frequent and close individual monitoring. The experiment reported here mimicked the feedlot environment. This experiment was designed to validate the previous results in an industry context so that the efficacy of in-feed treatment of IOK could be demonstrated as a practical, affordable and realistic option for exporters. Both productivity and animal welfare were hypothesised to improve should the results concur with previous research.

A range of severity of infection was considered, based on selection of sheep with eye grades between 1 and 6. It was hypothesised that OTC medicated feed would be effective in treating mild, up to and including grade 3, IOK and that two injections of OTC would be effective against all grades of IOK.

Methods

Two hundred and seven Merino cross, mixed age sheep with naturally occurring clinical IOK were selected from those rejected at a pre-embarkation feedlot. All sheep had clinical IOK in both eyes, not necessarily of equal severity. Following grading, sheep were drafted into two groups, one group with clinical eye grades 2-4 and the other with clinical eye grades 5-6. From these groups, sheep were randomly drafted into 3 treatment groups (Table 4). 
Table 4 Infectious Ovine Keratoconjunctivitis treatments in feedlot setting
Group
Treatment
Administration
Frequency
1
No treatment
 
 
2
Oxytetracycline
Alamycin intramuscular injection
Two doses 4 days apart
3
Oxytetracycline
Terramycin added to feed
 
 
Group 1 had 69 animals, 40 with grade 2-3 and 29 with grade 4-5 lesions. Group 2 had 70 animals, 45 with grade 2-3 and 25 with grade 4-5 lesions. Group 3 had 68 animals, 36 with grade 2-3 and 32 with grade 4-5 lesions.
 
Sheep were housed in three separate raised pens in a standard feedlot. Free access to water was given to all sheep. Those in groups 1 and 2 had ad lib access to pelleted feed as would be typical in a feedlot.
Sheep in group 3 received medicated feed for 5 days.
 
Ad lib access to non-medicated pellets was given following cessation of medicated pellets. Eye grades were recorded every second day from day 0 up to day 10 using the same grading system as before.

Results

Eye score on Day 0 was significantly associated with eye scores post treatment (P <0.001). When treatment means were corrected for score on Day 0 there was a significant effect of Treatment (P<0.001) on daily eye score but this effect interacted with day-post-treatment such that the effect of treatment on eye score was different on different days’ post treatment (P<0.001) (Figure 6).
Figure 6 Adjusted pink eye mean scores for each treatment and day.
Figure 6 Adjusted pink eye mean scores for each treatment and day
At the end of the experiment, animals treated with injections had lower eye scores than those treated with in-feed medication. Both treatments had lower mean eye scores than the control animals. 
 
In-feed OTC for 5 days is an effective treatment for sheep with IOK up to and including grade 3, whereas injectable OTC is effective in treating IOK up to and and including grade 5 (Table 5).
Table 5 Treatment recommendations (Treat vs Not effective) of different routes of administration (In-feed vs Injection) at different degrees of severity of disease (Score) on day 8 of the experiment
Score
In-feed
Injection
1
Treat
Treat
2
Treat
Treat
3
Treat
Treat
4
Not effective
Treat
5
Not effective
Treat

Discussion

The results of the feedlot experiment are consistent with results seen in the previous experiments. Although, subjectively, feed intake in the medicated feed group did decrease, the amounts of residual feed remaining were small. In-feed OTC is considered effective in treating sheep with eye grades up to and including 3. The experiment demonstrated that those sheep with grades 4 and 5 need to be treated with intra-muscular OTC to have the best chance of recovery to the point where they could be exported. 
 
The use of in-feed medication is significantly less labour intensive than injecting individual animals, which greatly reduces the cost of the treatment. Within a cohort of sheep at a pre-embarkation feedlot there is likely to be variation in severity of clinical signs of IOK. Given the labour required to inject sheep with mild disease it is encouraging that in-feed medication is an effective treatment. As IOK is a painful condition having an effective treatment available that requires minimal labour could contribute to improved welfare of animals in pre-embarkation feedlots.