Mycoplasma conjunctivae is the most commonly reported organism from outbreaks of keratoconjunctivitis in the UK. Morbidity of the disease is higher in lambs and hoggs, but severity greater in ewes (Jones, 1991). It may affect one or both eyes. Early signs include excessive lacrimation and conjunctival hyperaemia, followed by follicle formation, corneal vascularisation and iritis. Corneal ulcers do not occur in experimental infections, although ulceration has been described in severe naturally occurring cases (Jones, 1976). Infections are usually transient and permanent blindness is rare. Apart from the cost of treatment, any economic loss from the disease is caused by drowning or other misadventure due to bilateral blindness.
Experimental infections (Hosie et al., 1995) with Mycoplasma conjunctivae as a sole agent did not appear to produce the severe form of ovine keratoconjunctivitis observed in some field infections. This severe form of the disease may be attributable to the presence of other predisposing factors, pyogenic bacteria capable of establishing a secondary super-infection or variations in the sheep's resistance or the virulence of Mycoplasma conjunctivae.
Jones et al. (1976) and Hosie (1988) reported that development of immunity to M. conjunctivae appeared to be poor and demonstrated that mycoplasma may persist in the eyes of sheep many months after the remission of symptoms. Sheep can also be repeatedly infected with M. conjunctivae and exhibit clinical signs if the organism is eliminated by chemotherapeutic agents before each subsequent infection. Thus clinically normal sheep may act as a source of infection and re-infect sheep previously treated for the problem (Jones, 1991).
Hosie (1988) reported that frequent close contact between animals appears to be essential for the development of the disease; therefore any handling such as gathering for clipping, foot trimming or contact at troughs may precipitate an outbreak.
Much controversy surrounds the bacteria responsible for the secondary super-infection. Egwu et al. (1989) suggested that Staphylococcus aureus played an important role in converting mildly affected cases to severe cases, since it was found significantly more often in severe cases. The role of Branhamella (Neisseria) ovis, the most frequently isolated bacteria from healthy and affected eyes (Spradbrow, 1968; Egwu et al., 1989) remains controversial; Egwu et al. (1989) found that, although it was isolated more frequently in affected than unaffected eyes, it was found less often in severe cases. In an experimental mixed infection with an isolate of Mycoplasma conjunctivae, there was evidence that one isolate of B. ovis contributed to the severity of the keratoconjunctivis (Dagnall, 1994). Thus some isolates of B. ovis can play a role in inflammatory conditions of the conjunctiva of sheep.
Chlamydia psittaci as a cause of infectious keratoconjunctivitis has been reported mainly outside the United Kingdom. Individual animals may also exhibit signs similar to infectious keratoconjunctivitis, and should be carefully examined to differentiate these cases from problems that will spread rapidly within a flock. They include: