The sheep scab mite, Psoroptes ovis, is small (less than 1mm in length) and as a result is virtually undetectable, even on close examination. The disease is highly infectious, and can be transmitted by just one female mite. Early clinical signs can be difficult to spot, but can include restlessness, scratching and stained fleece. These early symptoms can be similar to those of lice infection, and it is important to distinguish between them because treatment may be difficult for each parasite. The skin lesions themselves can appear two to three weeks after initial infection, although it can take longer (>50 days) for these to become apparent. Although the majority of the mites reside under the scab at the outer edges of the lesion, small numbers often 'hide' in the cryptic sites such as the eye fossae, ear canal and perianal region.

There is a huge choice of products available to both treat and prevent sheep scab and lice infestations. They all have their place but for incoming sheep the choice must he narrowed to ensure that treatment is 100% effective.

Both sheep scab and lice are becoming resistant to the synthetic pyrethroids (SPs), but plunge dipping with organophosphates (OPs) is generally effective for both parasites. Health and safety issues surrounding the use and handling of OPs should be fully appraised. For scab the method of application of these plunge chemicals is important, and it is worth noting that no shower or jetting races have been appi oved for the treatment of sheep scab.

The injectable compounds are effective against scab but are generally ineffective against lice (see below). Some endectocides require two doses (Ivermectin and Moxidectin) to treat clinically affected animals, whereas others (Doramectin) are effective after one injection. These drugs will also worm the sheep and are effective against the benzimidazole and levamisole resistant worms. Further information on this disease can be found in Moredun Foundation newsheet Volume 3 Number 3.

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Lice are becoming far more common and are an increasing problem not least because a lousy flock can easily be mistaken for a scab infested flock. Moreover, none of the endectocides are directly effective in controlling lice, and it is now increasingly common to have sheep simultaneously infected with scab and lice. Prolonged irritation after treatment for scab with endectocides can occur because lice can feed on the scab lesion. and after removal of the mites the lice population increases rapidly to prolong the irritation.

Lice, as for the sheep scab mite, are also becoming increasing resistant to the synthetic pyrethroid acancides (eg flumethrin or high cis cypermethrin), but the OP plunge dips containing diazinon remain effective. Amitraz, which is an amidine, has also shown to be effective against lice and is available as either spray or dip. No injectable treats lice.

Of the two parasites sheep scab is potentially the most devastating and the major effort must be directed against it.

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Prevalence of white drench (benzimidazole) resistance, the most recent survey in Scotland showing that 6 out of 10 farms harboured worms resistant to these drugs. Resistance to the clear (levamisole) drenches has also been recorded and very recently the first cases of ivermectin resistance have also been detected in sheep in the UK. The avermectins and milbemyeins remain the preferred choice for quarantine treatments aimed at preventing the introduction of resistant worms onto a farm by newly purchased stock. These treatments should also take into account strategies to limit the introduction of ectoparasites such as lice and scab. Two approaches can be made:

1. Use an OP dip to eliminate scab and lice and orally dose with any of the avermectin/milbemyein wormers or
2. use an injectable avermectin/milbemycin, preferably one that offers a high efficacy using a single dose, to treat for both scab and worms. If lice also require treatment then an SP pour on or amitraz may be used bearing in mind that the lice may be SP resistant.

Following anthelmintic treatments animals should ideally be yarded for 24-48 hours to ensure that no viable eggs are passed onto pasture by the treated animals. Put newly introduced and treated stock onto dirty pasture so that any larvae that are derived from drug resistant worms that have survived treatment will be diluted by the residual drug susceptible larval population.

It is particularly important not to put newly introduced treated animals onto 'clean' pasture carrying very low numbers of nematode larvae since this has the potential to rapidly produce pastures contaminated only with drug resistant larvae.

Although under these circumstances the initial contamination with resistant parasites may be low, sheep grazing these 'clean' pastures will eventually multiply the resistant population. In this way initially 'clean' pastures may serve as a springboard for the spread of resistant worm populations throughout the farm.

Purely on cost, the first treatment strategy is the most economic costing about half that of the second strategy. However if a resistant nematode worm population or one of the ectoparasites is introduced and spreads to the whole flock then the costs of treating the whole flock will be many times that to treat all the incomers on arrival. Further information on this disease can be found in Moredun Foundation newsheet Volume 3 Number 2.

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Footrot can be carried by apparently sound sheep particularly in dry weather. The bacteria (Dichelobacter nodosus) can survive in small numbers in the cracks and crannies in the horn. All sheep should be footbathed at least twice in the 21 day quarantine period. Either in 10% zinc sulphate in which they must be held for at least 5 minutes or walked through a 3% formalin, no stronger, footbath. There is also good evidence that a single dose of footrot vaccine given at this time will prevent re-infection in the case of replacements. In the case of total re-stocking it should ensure that a truly footrot free flock is established. Despite two footbathings a few bacteria will remain deep in some cracks and will eventually break out to cause footrot when optimal environmental conditions occur. Always remember that footrot is a highly infectious disease and that even one affected animal can he responsible for causing a serious outbreak. Sheep owners should also be aware of contagious digital dermatitis (CODD) responsible for causing severe lameness involving the loss of hooves on affected feet. If this disease is suspected then veterinary advice should be sought without delay.

Remember footrot vaccinated flocks cannot be treated with injectable moxidectin (Cydectin) Further information on this disease can be found in Moredun Foundation newsheet Volume 3 Number 1.

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Clostridial diseases include pulpy kidney (Clostridium perfringens Type D), lamb dysentery (C. perfringens Type B), black disease (C. Piovyi ), braxy (C. septicum), tetanus (C. tetani) all of which can produce horrific losses over a very short period of time. Other clostridial diseases which are more sporadic include post parturient gangrene and cardiac failure (C. chauvoie), bacillary redwater (C. haemolyticum). Others found locally include struck (C. perfringens Type C) and the newly described septicaemia and abomasitis caused by C. sordellii.

Clostridial bacteria are ubiquitous both in the environment and in the sheep themselves. With the bacteria being ever present it only needs one of a number of trigger factors to stimulate their rapid multiplication and the production of powerful lethal toxins.

For the last 30 or so years clostridial diseases have been rarely seen due to the availability of very effective multicomponent vaccines. Recently their incidence has risen, in particular pulpy kidney and lamb dysentery. The view is that failure to vaccinate or vaccinate correctly is responsible, and the reasons might be a combination of complacency, false economy or simple lack of awareness.

It is essential that any incoming sheep are currently in a vaccination programme. Check carefully if shearlings have had a booster since their primary lamb doses. If not boost them in the 21 day quarantine period. If there is any doubt as to their vaccinal status assume that they require a full course. To be effective clostridial vaccines need a primary course of 2 doses 4-6 weeks apart followed by an annual booster. If any ewes miss the annual booster they will require two doses of vaccine pre-lambing to offer full colostral protection to their offspring. If riot, you risk the possibility of either lamb dysentery or pulpy kidney occurring. At the moment none of the vaccines available include protection against C. sordellii.

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The vast majority of sheep harbour in their tonsils or back of the throat, the two bacteria Mannheimia haemolytica and Pasteurella trehalosi, which cause the various manifestations of pasteurellosis. As with the clostridial diseases, trigger factors such as stress have a significant role in the development of pasteurellosis. Stress can be caused by transportation, mixing groups and other management factors. M. haemolytica infection results in either septicaemia and death or severe pneumonia in young lambs. In older sheep the main manifestation is severe pneumonia which is often rapidly fatal. Losses can be considerable and although ill sheep can be treated if caught early enough they are often left with permanently damaged lungs. P. trehalosi is almost exclusively associated with acute septicaemia often involving the lungs in weaned lambs. Sudden death in a group of lambs is the usual picture. It is rare that animals are seen early enough to initiate treatment and as a result, losses can be very severe.

Research by Moredun scientists developed successful Iron Regulated Protein (IRP) vaccines against the infections caused by both M. haemolytica and P. trehalosi. IRPs essentially enhance the efficiency by adding another degree of immunity by preventing rapid multiplication of the bacteria. Vaccines are available offering either combined clostridial/pasteurella protection or pasteurella alone. The risk to the flock should be assessed and appropriate vaccination programs must be adopted to provide maximum protection. Just like the clostridial vaccines, 2 doses of vaccine given 4-6 weeks apart are necessary as a primary course followed by annual boosters. Where natural challenge is very high, extra booster doses may at times be necessary under veterinary guidance. Further information on this disease can be found in Moredun Foundation newsheet Volume 2 Number 17.

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Toxoplasmosis, causing barrenness, abortion and weakly lambs, is contracted from the environment or feed which has been contaminated by Toxoplasma ooeysts (eggs) in cat faeces. The age at which pregnant sheep are exposed to the oocysts (eggs) determines the outcome. Younger sheep are more likely to abort or produce weakly lambs, since once exposed the animal develops a strong immunity and will breed satisfactorily in the future. Vaccination is highly effective at preventing losses due to toxoplasmosis. Normally a single dose of vaccine affords life long immunity, although the data sheet recommends biannual boosters. The incidence of toxoplasmosis is so widespread that routine flock vaccination should become the norm. Further information on this disease can be found in Moredun Foundation newsheet Volume 2 Number 18.

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