In-water antibiotic medication for the treatment of infectious ovine keratoconjunctivitis (IOK)

Summary

Within the live export industry infectious ovine keratoconjunctivitis (IOK) remains an ongoing challenge. With increasing scrutiny on the health and welfare of animals within the industry it is important to address persistent issues. With an estimated 0.5% of sheep rejected from the export trade annually and additional costs related to holding sheep back from shipments for treatment, this work was undertaken to determine a treatment that was effective and easily administered to the numbers of sheep involved. As IOK is a progressive disease it is important to establish the benefits of treating cases at different stages of the disease to ensure the best outcome for the sheep. IOK is a multifactorial disease, in addition to the bacterial agents involved a number of environmental factors play a role in the development of disease. Attempts were made to identify some of these factors with the hope of finding ways to reduce them.

Experimental work focused on treating sheep with oxytetracycline (OTC), a broad spectrum antibiotic which is widely used within the animal health industry. Previous work by Chapman et al (2010) highlighted the efficacy of injectable OTC, however given the numbers of sheep involved, the researchers and the industry were keen to find a solution that would be easier to administer on a larger scale. Initial work looked at the efficacy of in-water OTC as this is a relatively easy method of medicating large numbers of sheep; following on from this in-feed OTC was investigated.

In-water OTC was found to cause a significant decrease in both feed and water intake during treatment; this effect did not appear to be dose dependant. Although a drop in feed intake can be acceptable with some treatments, the decreases seen with this treatment were such that sheep would lose weight. The decreased water intake resulted in low intakes of OTC and therefore sheep were receiving a sub-therapeutic dose that was considered unlikely to treat the disease and could potentially lead to the development of antibiotic resistance. Initially it was postulated that this decrease in water intake was due to an unpleasant taste, however work looking at mitigating the taste using a dextrose additive showed no improvement. Studies on the effect of in-water OTC on the rumen microflora indicated a significant change in the bacterial, archael and fungal populations. This, coupled with the decrease in feed intake and the poor water intake, rendered this treatment an unsuitable option for the treatment of IOK in the pre-export feedlot.

A small pilot experiment was carried out to establish the clinical effectiveness of OTC medicated feed and also to determine whether any negative effects on feed and water intake were observed as with in-water OTC. In-feed OTC at a dose of 20 mg/kg daily for 5 days resulted in a significant improvement of clinical IOK. Although feed intakes were reduced during the treatment phase, these reductions were not to the degree seen with in-water OTC and quantities consumed still remained above that required for maintenance.

Following on from the success of the pilot experiments, a large scale feedlot experiment was carried out to compare the two treatments known to be effective: in-feed OTC and two injections of OTC (20mg/kg) 4 days apart. These were given intramuscular into the neck. This large scale experiment included animals with varying grades of IOK to establish cut-offs for when to treat and what treatment to use. Both the injectable OTC and in-feed OTC proved to be effective treatments for sheep with IOK up to and including grade 3 and injectable OTC also resulted in good clinical improvement in sheep with IOK of grade 4 and 5.

Attempts to identify risk factors and to quantify numbers of cases arriving and leaving the feedlot were not successful. Historical data was obtained from one exporter that highlighted a seasonal pattern; cases increase over summer. Reports in the literature would support the increase in cases during the summer months due to the hot, dry, dusty conditions experienced during a Western Australian summer.

From this work it can be concluded that IOK is a treatable disease. Sheep in pre-export feedlots with IOK up to and including grade 3 should be treated with in-feed OTC (20mg/kg) for a minimum of 5 days; those with IOK grade 4 or 5 should be treated with 2 injections of OTC (20mg/kg) 4 days apart, the interval between injections may vary with proprietary product used. Given the increase in cases seen during the summer months, inspectors should pay particular attention to eye inspections during these months in order to identify cases early allowing for prompt treatment. As with many diseases, early treatment is beneficial in ensuring a successful outcome and minimising the impact of the disease on animal welfare.

Although this work was focused on sheep within a pre-export feedlot, the researchers believe that these treatment protocols could be used across the industry: commercial lamb feedlots; commercial flocks; potentially onboard livestock vessels. Given the limited access for administering injections to sheep on board vessels, medicated feed would be a suitable option. The limitations of medicating feed onboard will be isolating individual feeders and the ability of the vessel to carry medicated feed separately to non-medicated feed.

At present, the in-feed product used is registered for use in sheep however there is no label claim for the treatment of IOK. Additionally, to the researchers' knowledge there is no export slaughter interval (ESI) established in sheep for this medication. An ESI of 90 days has been established for this product in cattle. Since January 2012, Russia has imposed a 90 day ESI on all OTC products therefore use of this product within 90 days of export to Russia would exclude animals from the Russian market. This, and any concerns regarding withdrawal periods would need to be addressed before blanket use of the in-feed treatment could be recommended.