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1. If you suspect that your sheep are carrying a chronic barber’s pole infection, would you expect to see hard dry faecal pellets or large soft ones?
2. Why would you miss spotting a low to moderate worm burden if you were using visible signs to assess sheep and to inform a drench decision?
3. Which lungworms of sheep and goats have a developmental phase in slugs or snails?
4. Name a primary and secondary type of blowfly maggot you may see in a week old flystrike.
When sheep are on a poor plane of nutrition and carrying an ongoing low-level barber’s pole infection, blood loss into the GIT results in dehydration, and most affected sheep pass hard, dry pellets (faecal consistency score (FCS) of 0.5) characteristic of constipation.
During the dry season, weight loss may occur even though larval uptake is negligible as the available nutrition cannot adequately compensate for the loss of minerals via the blood. This loss of iron, in particular, is thought to be responsible for the ‘break in the wool’.
On the other hand, very watery faeces (scouring, FCS of 3.5) may indicate a severe infection with scour worms. Diarrhoea is not a feature of a barber’s pole infection although it is sometimes seen in a mixed infection with scour worms.
It is just not possible to see these changes at this level of infection if sheep are on a reasonable level of nutrition.
A low to moderate worm burden typically causes a mild anaemia if the infection is predominately barber’s pole, some weight loss in a scour worm infection, or both slight anaemia and some scouring in a mixed worm infection.
So would you drench? No, because at this stage, the cost of mustering and drenching is more costly than the productivity loss.
However, as the burden increases to a higher level and the degree of anaemia and or weight loss worsens, the scenario changes. The cost of production losses would now definitely outweigh the costs of regular testing, mustering and drenching costs. At this stage, you are unlikely to see bottle jaw or severe scouring.
But, only when the burden is high, are signs apparent, and drenching imperative.
There are two lungworms, the small lungworms that have developmental phases in slugs and snails. They are Muellerius capillaris and Protostrongylus rufescens
Eggs laid by the adult worms in the nodules of the lung tissue (M. capillaris) or the airways (P. rufescens) hatch into first-stage larvae. These then wriggle up into the bronchi, are coughed up, swallowed and passed into the intestine for passage out onto the pasture. Here the first stage larvae migrate into slugs and snails (this is an indirect life cycle). Infection occurs when the sheep or goat eats an infected slug or snail. Larvae are released in the intestinal tract, force their way through the intestinal wall and migrate to the lungs.
In contrast, the large lungworm, Dictyocaulus filaria has a direct life cycle, and the infective third-stage larvae are swallowed directly with the graze.
NB. Memory hook: ‘D’ for Dictyocaulus and ‘D’ for direct!
The primary strike fly is usually the green blowfly, Lucilia cuprina, and the green hairy-maggot blowfly Chrysomya rufifacies is often the secondary strike fly. Some species of Calliphora, the brown blowflies, are also common primary strike flies.
Most blowfly maggots feed on carcasses. Lucilia maggots are poor competitors in the race for territory compared to those of the native brown blowflies and so have evolved to become obligate parasites of sheep, that is, they breed almost exclusively on sheep and are, therefore, first on the scene. By the time other species are attracted to a strike, the Lucilia maggots are well established.
Chrysomya rufifacies, the green hairy-maggot blowfly, is a secondary strike species. It usually does not strike sheep or blow carcasses until the primary maggots are already feeding.
The ‘hairy maggots’ of Chrysoma appear dark in colour and have sharp spines over much of their body. They are up to 14 mm long. The 'smooth maggots' of Lucilia, in contrast, are smooth-skinned and cream in colour and easily distinguished from other maggots.
By the time primary and secondary maggots reach full size on sheep, the animal has been struck for more than a week.