Warm weather can mean a lot of things to farmers, but to sheep farmers it brings with it the risk of external parasites. This year has seen some changes to the products that are available for dipping sheep. In this article, we look at the options around blowfly and tick control, we refer to these two as semi-permeant parasites, spending only some of their life cycle on the sheep. This is important because the other major part of their life cycle is in the environment. Both of these parasites like certain conditions and knowing what they are allows us to plan control measures in our flocks.

The other common external skin parasites are mites (scab) and lice.

Blow fly

This is called cutaneous myiasis, which is basically when flies lay their eggs on the skin of sheep, with the emerging larvae (maggots) causing the clinical symptoms. The most common fly involved is the Lucilla species sericata, or more commonly known as the blowfly.

These flies target soiled areas of skin around the back end of sheep and also any cuts or wounds. They lay their eggs, which hatch into maggots and attack the tissue, causing damage and pain to the sheep

Growing lambs can be particularly affected during periods of hot, humid weather. Infestations can be small to quite widespread, and often depend on how early they are spotted. Anywhere there is faecal material or wounds, the flies will be attracted to those areas.

Affected sheep will often turn their heads to nibble at the infected areas, while at the start it causes irritation it can progress to sheep going completely off their feed.

Many farmers will also be familiar with the putrid smell from the dead and rotting tissue. When these wounds are treated and heal they sometimes grow black wool.

Anything that makes dung loose or could cause diarrhoea in the summer months increases the risk of maggots

When these are examined, we can see the maggots in the skin. Individual sheep should have the wool cleaned away and a topical antibiotic spray applied. With more severely infected sheep, it is worth considering injectable antibiotics and pain killers. All affected sheep can have dip applied to the area, but it is a clear signal perhaps that the flock may need treatment. When we know that soiled areas attract blowflies we should look at ways to reduce faecal staining.

Anything that makes dung loose or could cause diarrhoea in the summer months increases the risk of maggots. Always ask why lambs are scoury. Is it worms, coccidiosis or a dietary issue? Dagging lambs is also good practice when they are being collected for worming or dosing. Shearing adult sheep around now may also reduce the risk of fly strike.

Remember, any dead animals in fields can attract flies and should be removed as quickly as possible.

Treatment options

Organophosphate plunge dips offer a good way of controlling blowfly within six weeks of activity. However, products like Clik offering 16 week’s cover continue to be the main choice of farmers.

When looking at using dips, by far the best option is plunge dipping for better coverage.

Ticks

There are many species of ticks and they can go through three stages of development from larvae to nymph to adult.

Each time they develop between stages they need to have a blood feed on a host like sheep.

At this time they can spread disease to sheep and typically feed for 4-14 days. With heavy tick infestations in lambs, you can even see anaemia occur or blood loss occasionally.

Sheep farmers don’t want to see ticks and most farmers will know the risk from their own farms and areas.

A common symptom is tick-borne fever, where affected sheep are unwell, not eating, stiff and are running a high temperature

Certain ground which is rougher grazing and particularly in the hills is the habitat you will see ticks. They also like mild and damp weather. They can survive for up to a year, waiting for a blood feed if these conditions are right.

The symptoms for tick-borne diseases can vary from mild to very severe.

A common symptom is tick-borne fever, where affected sheep are unwell, not eating, stiff and are running a high temperature.

Other tick-borne diseases can cause neurological or nervous signs, like louping ill.

Scab and lice

These can occur by more direct contact and can spread sheep-to-sheep. Both of these can cause severe scratching. When picking products to use, as shown in Table 1, it is important to remember what products cover what problems.

Look at how long each product is active for and what is the meat withdrawals, this is particularly important when we talk about lambs.

Control options

The pour-on method offers a way of controlling some of the external parasites and are either insect growth regulators (IGRs) or pyrethroids. Clik (dicyclanil) is a good example of an IGR and lasts for 16 weeks. These tend to be a little more expensive and have longer meat withdrawals. Then you have products like ectofly, which are pyrethroids.

It is so important when applying these that the fleece is dry and it doesn’t rain for at least 6-12 hours after application.

One of the main dips used cyper guard is currently unavailable, so farmers will be looking at organophosphate dips

Where sheep are dirty, the ability of these products to work is reduced, and each product will have a specific area to cover and sometimes a special gun or applicator.

Some of these products can also be used directly to treat blowfly infections.

One of the main dips used cyper guard is currently unavailable, so farmers will be looking at organophosphate dips.

The plunge dip is still by far the most effective because of emersion and contact time with the skin. Sheep should be submerged for 60 seconds, and dunked or submerged 2-3 times within that minute.

Injectables

With many Ivermectin-based injectables used on farms for controlling worms, remember they only treat or prevent sheep scab and are ineffective against the other external parasites.