15. Fly strike
Drenthe heath sheep (ram)
Maggots are fly larvae. The maggots of the blue-green fly feed on cadavers or fresh meat. But these parasitic flies also lay eggs in other places: the wool of sheep, for example. The eggs hatch after a few days, after which the maggots bury themselves in the sheep’s wool and then under its skin, penetrating the body. They feed off the sheep’s flesh.
Under the climate conditions in the Netherlands, these flies emerge in early spring and are active until the end of October. They lay their eggs in rotting flesh remains and in the wool of sheep, particularly where the wool is soiled with manure: on and around the tail and in the wool of the buttocks. As long as the sheep produces hard droppings, the wool in this area does not become soiled and the risk of getting maggots remains small. However, the consumption of tender grass and the presence of intestinal parasites (worms and coccidia) may cause diarrhoea. This cakes onto the tail and the surrounding wool. The risk of watery manure is high in the spring: fresh, tender grass begins to grow and huge numbers of worms become active in the gastrointestinal tract of sheep. Which is precisely when the blue-green flies come into action.
Maggots on and under the skin of the sheep cause itching and pain. If a sheep becomes contaminated with hundreds of maggots and is not timely treated, it will die. There are effective insecticides that can be poured or sprayed on the back and tail of the contaminated sheep, killing the maggots in minutes. But the use of these was prohibited in the Netherlands years ago because these agents are hazardous to the environment: fish die as a result of very low concentrations in water. Cows, pigs and sheep could not be properly treated as a result of the ban on the use and they (sometimes literally) died of itching caused by lice, maggots or scab. But since the invasion of midge caused an outbreak of bluetongue disease among cows and sheep in 2006 and 2007, a few of these insecticides are once again available for use in the livestock industry.
In order to prevent sheep from becoming contaminated with maggots, you have to ensure that the flies do not lay eggs in the wool. A simple measure to that end is to shear the sheep, because the flies will not lay eggs on bald skin. Once the wool starts to grow back, it must stay clean. The sheep must therefore produce hard droppings. And so it is essential to prevent a severe contamination with gastrointestinal worms. To force back the number of infectious larvae of intestinal worms on the meadow grass, you have to treat the sheep ‘strategically’: at the right time, using an effective anti-worm agent on every sheep, dosed according to weight. And the animals should not be allowed to graze the same pasture for too long: after a certain time, they should be moved to a clean plot where the grass is not or only slightly contaminated with gastrointestinal worms.
But how can you ensure that a pasture is not or only slightly contaminated? Mowing the grass reduces the contamination with worms. You can also have horses graze the pasture: with no disadvantage to themselves, they consume the worm larvae that are contagious to sheep along with the grass. And the same goes the other way round: sheep digest the worm larvae of horses along with the grass without any inconvenience. And so alternating sheep and horses on pastures will reduce the contamination of the grass. This will require fairly large pasture plots and not too many horses and sheep. I divided my own 1½ hectare pasture into three separate plots. A riding horse and a pony graze one of these, eight Drenthe heath sheep graze the
second plot. The third is kept empty. Once the last ewe has lambed, all of the sheep are dewormed and sheared. Usually in April. This is also when the animals are moved. The horses are relocated to the empty plot and the sheep are moved to the horses’ plot. They alternate a second time in July and then again at the end of September.
The spring of 2006 is warm and I have to wait three weeks before my sheep can be sheared. Which means that the deworming and relocating to another pasture must be postponed as well. When the shearer arrives, the animals are no longer producing hard droppings, but rather thick turds. One ewe has caked manure on its tail. And wouldn’t you know: wriggling dull white maggots are visible. The tail is swollen and inflamed. Fortunately, an injection of ivermectin not only kills intestinal worms, but also the maggots that have penetrated the sheep’s body through the skin. I treat all of the ewes with the same medication. Any maggots still present in the wool of the sheep are removed when the fleece is sheared. No maggots are found on the other sheep.
They are then transported to the (from their perspective, clean) horses’ pasture. The contaminated ewe quickly recovered after this and no new infections with maggots presented themselves.
Drenthe heath sheep (ewes)
© Leo Rogier Verberne