Ocular disease investigation in sheep

_{Outbreaks of ovine infectious keratoconjunctivitis can be very frustrating to control in sheep flocks.}

SRUC Veterinary Services usually receives more ocular swabs for the investigation of ovine eye disease around this time of year. When investigating outbreaks, it is important to consider the main pathogens likely to be involved and which tests would be the most useful.

Mycoplasma conjunctivae is considered the major primary pathogen for ovine infectious keratoconjunctivitis (OIKC) or ‘pink eye’.

Moraxella ovis and Chlamydophila psittaci have also been implicated in OIKC cases and we identify M. ovis regularly from swabs. Staphylococcus aureus, Listeria monocytogenes and Mycoplasma agalactiae have also occasionally been reported from cases of OIKC.

• Routine bacteriology will not identify M. conjunctivae but swabs can be sent for molecular testing, with a specific M. conjunctivae PCR priced at £34.50 and a broader ovine mycoplasma DGGE priced at £95.70
• Swabs for these tests should ideally be in mycoplasma transport medium, or plain, but not in charcoal
• Most of the other bacteria can be detected on routine cultures and antibiotic sensitivity testing can be performed (£35.35 per charcoal swab)

Clinical and farmer experience suggests that outbreaks of ovine infectious keratoconjunctivitis caused by M. conjunctivae are very frustrating to control in sheep flocks. Antibiotics (topical or systemic) do not always eliminate the organism and repeat treatments are often given due to recurrence of clinical signs. Sub-clinically infected, carrier animals of M. conjunctivae appear to be the cause of repeated outbreaks in flocks, and studies suggest persistence in the conjunctival sac for several months in some sheep (Baker and others, 2001). The in vitro sensitivity of M. conjunctivae to antibiotic may not be the limiting factor in many cases, but rather whether the minimum inhibitory concentration is maintained in the lacrimal fluid for long enough.

Outbreaks may be triggered by tight stocking and close head-to-head contact between sheep, such as occurs at feed troughs, during housing or gathering for handling. Indirect spread, for example on the hands of shepherds or vets, can also cause cases to increase. Outbreaks will be prolonged if different groups of sheep are frequently mixed together, and this should be avoided if possible.

References:

Baker SE, Bashiruddin JB, Ayling RD, et al. Molecular detection of Mycoplasma conjunctivae in English sheep affected by infectious keratoconjunctivitis. Vet Rec 2001; 148: 240– 1

Williams HJ, Duncan JS, Fisher SN, et al. Ovine infectious keratoconjunctivitis in sheep: the farmer's perspective. Vet Rec Open 2019; 6

Posted by SRUC Veterinary Services on 13/01/2023