Contagious ophthalmia (infectious keratoconjunctivitis or &squo;pink eye&squo;) is a contagious bacterial infection of sheep, goats and cattle. Clinical signs include inflamed conjunctivae, ocular discharge, corneal opacity and impaired vision, with corneal ulceration and perforation occurring in severe cases (Motha, et al.., 2003). Bacteria commonly associated with this condition include Mycoplasma conjunctivae, Chlamydia pecorum and Moraxella (Branhamella) ovis. However, Colesiota (Rickettsia) conjunctiva and Listeria monocytogenes can also be involved (Abbott, 2018). M. conjunctivae has been identified as the major cause of contagious ophthalmia in sheep (Naglić, et al.., 2000); however, other mycoplasma spp. can be involved, for example Mycoplasma arginini has been isolated (Surman, 1973). Moraxella ovis may be present in infections with contagious ophthalmia, but can also be found as part of the normal conjunctival flora of healthy sheep (Bankemper, et al.., 1990 and Dagnall, 1994) and, therefore, may be the primary pathogen or an opportunist infection.
This report details four cases of contagious ophthalmia in lambs and ewes where M. conjunctivae was detected via PCR, with and without concurrent isolation of Moraxella ovis. This finding supports the view that an antibiotic that is effective against Mycoplasma spp. (such as oxytetracycline) is the preferred option in severe cases of contagious ophthalmia.
Four cases of contagious ophthalmia were investigated in the Central Tablelands, NSW, from March to June 2020.
Case 1. In March 2020, ocular disease was noted amongst a total of 2450 Merino, Merino x West Suffolk and Merino Dorset lambs running in several mobs. Approximately 75% of the lambs were affected; however, few had notable visual deficits. The eyes of three lambs were examined, all presenting bilateral or unilateral serous ocular discharge, cloudy oedematous corneas (Figure 1) and peripheral neovascularisation. One had a shallow corneal ulcer (Figure 2).
Case 2. In March 2020, 30 of 300 Merino lambs were noticed to be affected with ophthalmic disease. Some lambs had been treated with topical cloxacillin as benzathine salt (278mg OU; Opticlox, Norbrook) with no noticeable improvement. Six were completely blind. Three lambs and one ewe were examined. The lambs had cloudy, oedematous corneas, red conjunctivae and serous ocular discharge (Figure 3). The ewe had bilateral stromal abscesses, extensive neovascularisation and corneal opacity (Figure 4).
Case 3. In March 2020, a mob of 515 composite ewe lambs, mustered for investigation of ewe infertility, were noted to be affected with mild contagious ophthalmia. From a selection of ten ewes examined closely, four were affected. The affected lambs had serous ocular discharge and some also presented with red conjunctiva and a cloudy cornea.
Case 4. In June 2020, 20 Merino ewe hoggets from a mob of 200 presented with blindness and a high stepping gait. Four were examined, all with bilateral cloudy corneas, superficial corneal erosions, marked neovascularisation and serous ocular discharge.
Dry eye swabs were collected for all cases and submitted to the EMAI for bacteriology diagnostics. Mycoplasma spp. and Chlamydia spp. PCRs were performed for all cases. Three cases had aerobic culture (sheep blood agar plate incubated at 37°C with 5% CO2, and MacConkey agar plate incubated at 37°C) performed on all swabs, set up the day after the swabs were collected. One case (Case 3) did not have any culture performed.
Table 1 displays all laboratory results for Cases 1 to 4. Moraxella ovis was cultured in Cases 1 and 4. Mycoplasma spp. were detected via PCR from all cases. Employment of a real-time Mycoplasma spp. PCR for Cases 2-4 revealed that the relative amount of the target sequence was high (cycle threshold [Ct] values between 21-29).
The Mycoplasma sp. detected in all cases was determined to be M. conjunctivae by amplification and sequencing part of the 16S rRNA gene using in-house Mycoplasma-specific primers. Chlamydia spp. were not detected via PCR (individual or pooled) from any of the cases.
|Case||Lamb or Ewe||Aerobic culture||Mycoplasma spp. PCR1||Chlamydia spp. PCR1|
|1||1||Sparse mixed growth||D3 (3 swabs pooled) M. conjunctivae||ND (3 swabs pooled)|
|2||Sparse mixed growth|
|3||Sparse mixed growth including Moraxella ovis2|
|2||1||Profuse mixed growth||D3 M. conjunctivae||ND|
|2||Moderate mixed growth||D3 M. conjunctivae||ND|
|3||Sparse mixed growth||D3 M. conjunctivae||ND|
|4||No growth||D3 M. conjunctivae||ND|
|3||1||Not performed||D3 (5 swabs pooled) M. conjunctivae||ND (5 swabs pooled)|
|4||1||Sparse mixed growth including Moraxella sp.4||D3 M. conjunctivae||ND (3 swabs pooled)|
|2||Moderate mixed growth including Moraxella sp.4||D3 M. conjunctivae|
|3||Sparse pure growth of Moraxella ovis2||D5|
M. conjunctivae was detected by PCR from eye swabs from all four cases. M. conjunctivae is a common isolate associated with contagious ophthalmia (Abbott, 2018). Chlamydia spp. were not detected via PCR in any of the four cases.
The concurrent isolation of Moraxella ovis in Cases 1 and 4 may be due to its presence as part of normal conjunctival flora, or as a consequence of colonisation following infection with M. conjunctivae (Bankemper, et al.., 1990 and Dagnall, 1994). There are some reports that suggest concurrent infection with Moraxella ovis and M. conjunctivae may be associated with more severe clinical signs; however, severity may depend on the pathogenicity of the strain of Moraxella ovis (Dagnall, 1994).
In this case series, most lambs were not treated; however, severe cases such as shown in Figure 3 were treated with injectible long-acting tetracyclines. In our experience, severe cases respond to oxytetracyclines rather than antimicrobials that target cell wall synthesis (as expected given Mycoplasmas lack a cell wall).
Almost all of the affected sheep resolved within two weeks, which is consistent with the observations of Abbott, 2018. However, in our experience, severe cases may be refractory to treatment with permanent eye damage. This finding has been observed by others (Motha, et al.., 2003 and Naglić, et al.., 2000). Repeated yarding may exacerbate clinical signs due to increased stress and dust and increased transmission to naïve animals (Abbott, 2018). Animals with severely compromised vision may benefit from being separated to an area with adequate shade and easy access to food and water.
Flies and dust are believed to play a role in the transmission of infection (Toop, 1964, Abbott, 2018). However, on the Central Tablelands this disease often occurs in the colder seasons without predisposing factors such as flies, dust or other causes of eye damage such as grass seeds. While these mostly mild, but occasionally severe, cases all involved young sheep, contagious ophthalmia also occurs sporadically in flocks of mature ewes without obvious predisposing factors. Disease prevalence may be affected by host susceptibility and virulence of the organisms involved (Bankemper, et al.., 1990). Recovery from contagious ophthalmia results in short-term immunity for approximately one year before the sheep become susceptible if re-exposed to the organisms (Toop, 1964).
We thank NSW DPI Veterinary Bacteriology and Molecular Biology Diagnostic teams for performing the bacterial cultures and molecular testing.