An outbreak of iritis and uveitis in dairy cattle at pasture associated with the supplementary feeding of baleage

Abstract

Ocular disease associated with infection by Listeria monocytogenes has been reported in the veterinary literature for over 30 years (Kummeneje and Mikkelsen 1975), however it remains a poorly-defined condition (Morin 2004). Ocular listeriosis has generally been reported as having a similar disease pattern to that associated with listerial abortion or meningoencephalitis, i.e. the disease is primarily sporadic in occurrence although outbreaks can occur (Morgan 1977; Bocklisch et al 1991; Khodakaram-Tafti and Ikede 2005). However, Watson (1989) reported an outbreak of iritis and anterior uveitis in housed cattle linked to ocular infection with L. monocytogenes and associated with the feeding of silage. Since then, bovine iritis, commonly referred to as ‘silage eye’, has been increasingly recognised as a syndrome in dairy cattle in the United Kingdom (UK). Indeed, outbreaks of iritis are now more commonly reported by veterinarians in the UK than infectious bovine keratoconjunctivitis (Watson 2004), and a recent postal survey of dairy farmers in the UK found that bovine iritis accounted for over 80% of cases associated with L. monocytogenes infection, while nervous signs accounted for <5% and abortion for <3% (Erdogan et al 2001).

Outbreaks of L. monocytogenes-associated bovine iritis have almost exclusively been associated with the feeding of silage. In such cases, the source of the bacterium is primarily soil contamination of the grass during cutting and/or wilting. In addition, bovine iritis is more commonly associated with the feeding of silage preserved in bales (baleage) rather than in stacks or pits. This is because in order to produce clinical disease the bacterium needs to survive the fermentation process, and one of the main factors determining its survival is the pH of the silage (Pauly and Tham 2003). The fermentation of baleage is less extensive and slower than that of pit silage (Ohlsson 1998). This means that its pH is generally higher than that of pit silage, often remaining >5. Consequently, Listeria spp are more likely to survive in baleage than in pit silage.

Despite the widespread use of baleage in other countries, ocular infections due to L. monocytogenes have been primarily reported in the UK. However, it is likely that such infections are present worldwide and that the lack of reports indicates a lack of investigation rather than absence of disease (Evans et al 2004). We describe here an outbreak of bovine iritis in a herd of dairy cattle in New Zealand fed baleage while at pasture.

In November 2005, in response to reduced grass growth, a herd of 650 dairy cows on a farm in the Horowhenua district of the North Island of New Zealand was supplemented with silage to maintain milk production. One half of the milking herd was supplemented with pit silage, the other half with baleage; both feeds were fed on the ground in heaps. Both types of silage looked grossly normal and there was no evidence of rejection by the cattle. However, within 7 days of the start of feeding the supplementary forage, cattle in the group being fed the baleage started developing ocular lesions; eight cows were affected when veterinary involvement was first sought. By December 2005, a total of 17 cattle had developed visible ocular lesions; cases continued to be seen until 7 weeks after feeding the baleage had ceased. In January 2006, feeding of baleage recommenced and clinical ocular disease reappeared 3 weeks later. By the end of February 2006, a further 12 cases had occurred, making a total of 29 cases, i.e. 9% of cattle were affected.

The initial clinical signs reported by the farmer were marked lachrymation and ptosis with blepharospasm. In the majority of cases, the signs were unilateral. Ocular examination identified an anterior uveitis accompanied by miosis. In later cases, there were more severe inflammatory changes, characterised by folding of the iris and the development of a bluish opacity within the cornea. In a few cases, this opacity was confined to the limbic border between the cornea and the sclera, but in most cases it encompassed the whole of the visible cornea. Additionally in later cases, focal aggregations of fibrin accumulated in the anterior chamber; these were visible as white foci beneath the surface of the cornea. No conjunctivitis was apparent in any case, nor was there any evidence of corneal ulceration. All affected cattle showed severe ocular discomfort.

The primary treatment used was 250 mg cloxacillin (Orbenin Eye Ointment; Pfizer, Auckland, NZ), administered topically every 48 h until resolution. The majority of cases required two or more treatments before lesions disappeared. More severe cases were treated with a single subconjunctival injection of 5 mg dexameth-asone sodium phosphate (Dexone-5; Bomac Laboratories Ltd, Auckland NZ), in addition to the cloxacillin. The farmer reported no significant improvement in cattle treated with dexamethasone and cloxacillin compared to those treated with cloxacillin alone.

During the initial outbreak no bacteriology was undertaken as most of the cases had been treated with antibiotics prior to veterinary involvement. However, when the condition recurred the farmer was supplied with sterile swabs and shown how to sample the conjunctiva/cornea. Three swabs were submitted for bacteriology. No significant bacteria were isolated; neither Moraxella bovis nor L. monocytogenes was isolated from any sample.

Based on the clinical signs, and the absence of systemic disease, a diagnosis of bovine iritis was made. The presentation was distinctly different from that of infectious bovine keratoconjunctivitis, as both conjunctivitis and corneal ulceration were absent. Other causes of uveitis, such as malignant catarrhal fever, tuberculosis, toxoplasmosis and bacterial septicaemia, were ruled out because of the absence of systemic disease and resolution of the uveitis in response to topical treatment only. The presumption was made that, because clinical signs were seen only in cattle that had been fed baleage, the primary cause was likely to have been L. monocytogenes.

This is the first report of silage-associated iritis and uveitis in dairy cattle in New Zealand. Similar clinical signs had not been seen on this farm before and none of the members of the local farmer discussion group reported seeing similar signs. However, it is unlikely that this is the first occurrence of bovine iritis in New Zealand as similar lesions have been seen on farms in several areas of the country (KR Lawrence, pers. obs.). As the epidemiology and clinical signs closely matched those seen in dairy cattle in the UK (Watson 1989), ocular infection with L. monocytogenes was thought to be the primary cause of the outbreak. However, as this assumption was not confirmed by bacteriological culture it remains unproven.

Cloxacillin eye ointment was used in this case as the primary treatment. Resolution after treatment was relatively rapid, and most cases resolved within 96 hours of the initiation of treatment. However, as bovine iritis is a self-limiting disease (Watson 2004), particularly when access to the initiating cause is removed, it is unclear whether this was a response to treatment. Similarly, the apparent lack of response to dexamethasone does not preclude its effectiveness, particularly as it was given only to the most severely affected cattle. Indeed, Watson (2004) stated that subconjunctival administration of dexamethasone was the most advantageous treatment. Subconjunctival antibiotics were not used in the outbreak described here as neither author has found a significant benefit in their use to treat bovine iritis, which concurs with the conclusions made by Watson (2004) that treatment with antibiotics was ineffective and unnecessary

In the outbreak presented here, the baleage was fed on the ground, which was one of the recommendations made by Watson (2004) to reduce the risk of bovine iritis. Other risk factors mentioned by that author included avoiding feeding in windswept areas, which can be difficult to achieve on the Horowhenua coast, and not making silage from mature grassland containing large numbers of seedheads, which can irritate and damage the cornea, allowing L. monocytogenes to penetrate it.

One of the most interesting features of this case was the long incubation period of up to 7 weeks between feeding of the silage and the disappearance of clinical signs, which has not been reported before. However, in the UK most cattle would continue to be fed silage from a different source, and in such a situation it would be impossible to exclude the second silage as a source of infection.

Further research is required to establish how common bovine iritis is in New Zealand, whether it is caused by L. monocytogenes in this country as it is in the UK, and what the main risk factors for its development are. As such, we would be very interested to hear from colleagues who have seen signs similar to those reported here.