Fort Dodge Animal Health has recently launched Cydectin® Long Acting Injection for Sheep (Cydectin LA Injection). This new formulation of Cydectin kills resident worms and protects sheep against new infection by small brown stomach worm and barber's pole worm for at least 91 days and by black scour worm for up to 49 days. Cydectin LA Injection offers unique opportunities for worm control in sheep in the winter rainfall regions of Australia. In these regions, the major pathogenic worm species are small brown stomach worm and black scour worm, with barber's pole only a problem in years with wet summers. The greatly extended persistency over traditional Cydectin Oral drench also has implications for the impact of Cydectin LA Injection on the evolution of macrocyclic lactone (ML) resistance in worms.
Worm control
Cydectin LA Injection prevents re infection with small brown stomach and barber's pole worms for at least 91 days and prevents re infection with black scour worms for up to 49 days after treatment. This means that treated sheep will not contaminate pastures with eggs of the first two worms for at least 107 days, or with black scour worm eggs for up to 70 days after treatment. In contrast, short-acting drenches allow contamination of pastures as little as 18 to 21 days after treatment, even if the drench is fully effective. The worm control impact of just one Cydectin LA Injection treatment is therefore equivalent to around three equally spaced treatments with an effective short-acting drench for black scour worms, and around five spaced treatments with an effective short acting drench for small brown stomach or barber's pole worms.
The graph* above shows the concentration of infective larvae of small brown stomach worm and black scour worm on sheep pasture in a typical year. Sheep pick up large numbers of these larvae in winter and spring, and this is when outbreaks of worm disease are most likely to occur. Larvae on pasture in spring mainly arise from eggs deposited in the preceding autumn and winter. If egg deposition is prevented or severely reduced during this critical period, the worm population on pasture can be minimised for the rest of the year. Spring lambing (July-August), with weaning in November-December is common in winter rainfall regions, as this timing makes best use of feed availability. A single treatment at weaning with Cydectin LA Injection will largely prevent the seasonal autumn and winter increase in small brown stomach and black scour worms and will probably be the only worm control treatment required for the rest of the weaner year.
ML resistance selection by Cydectin LA Injection
Selection of ML resistant worms resident in the sheep at the time of treatment ("head" selection) will be reduced by the use of Cydectin LA Injection when compared with Cydectin Oral, as the LA product is given at five times the dose rate of the oral product (1 mg/kg vs 0.2 mg/kg). Because of Cydectin's superior potency, the advantage of Cydectin LA Injection over other oral or injectable MLs will be even greater. Many fewer resistant worms will survive treatment with Cydectin LA Injection than will survive treatment with other MLs. Moreover, the rate of elimination of Cydectin LA Injection from the sheep's body is very similar to the rate of elimination of Cydectin Oral. This means that the blood concentration of the LA product will only fall into the range where it selects in favour of resistant incoming larvae ("tail" selection) for the same total period as will Cydectin Oral, despite its much greater persistency. In summary, one dose of Cydectin LA Injection offers equivalent worm control to three to five spaced doses of a short-acting drench, but incurs a resistance selection cost of less than that incurred by one dose of Cydectin Oral or, for that matter, any other ML.
Summary
- Cydectin LA Injection prevents contamination of pastures with small brown stomach or barber's pole worm eggs for over 3 months following treatment. It also prevents black scour worm contamination for around 2 months.
- During these periods, sheep are effectively worm-free, and able to produce to the maximum capacity allowed by their genetics and nutrition.
- The impact of preventing worm egg production for 2 or 3 months is enormous if it is done over summer and autumn, when deposited eggs can generate large concentrations of worm larvae on spring pastures.
- In all but exceptional years, a single treatment with Cydectin LA Injection at weaning should be all that is required for effective worm control in weaners in winter rainfall zones.
- The effect of a single treatment with Cydectin LA Injection on selection for ML drench resistance should be less than that of Cydectin Oral or any short-acting ML drench.
- As an added bonus, weaners given Cydectin LA Injection will be fully protected against the devastating impact of barber's pole worms in wet summers.
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