17 Oct 2016
Lee-Anne Oliver considers specific diseases that can be brought on to the farm when introducing flock replacements, and what biosecurity principles should be implemented.
Lee-Anne Oliver
Job Title
Standardised biosecurity protocols are usually documented in a flock health plan, but how often are they discussed between the farmer and his or her vet?
The systems and health status on farm change continuously. In addition, advances in veterinary medicine require protocols to be reviewed and updated regularly. Industry guidelines on biosecurity principles – such as hygiene, quarantine and farm security – are consistent, but the implementation of this advice on farm is often poor.
The protocols for specific disease threats, such as anthelmintic resistance, and the diagnostic tests available to screen for disease, are continually changing. Without sound, general biosecurity principles being implemented we cannot be sure more complex quarantine treatment regimes and testing are carried out – so back to basics.
The biggest disease risk to a flock is purchased stock. Running a closed flock would omit this risk, but for units where purchasing animals is a necessity, accredited stock should be sourced where possible. The Premium Sheep and Goat Health Scheme (PSGHS) provides disease-free accreditation for maedi-visna, enzootic abortion and caseous lymphadenitis. A list of accredited and monitored flocks can be found at www.psghs.co.uk
Where sourcing accredited stock is not an option it is vital producers find out as much as possible about the health status of animals they are looking to purchase. Developing a customised checklist for farmers on what questions to ask can be beneficial. As always, the reliability of information given by the vendor should be considered – this may be particularly relevant when vaccines and treatments are to be administered pre-sale.
To ensure the efficacy of vaccinations and products you may advise treatments are repeated during the quarantine period. Returning stock should be subjected to the same quarantine procedures as purchased stock.
Neighbouring stock poses a risk, so every border to the farm should be accounted for in a biosecurity risk assessment. Arable land, major roads and forestry are considered low risk. Less confidence should be given to minor roads, watercourses and bridleways.
Keeping stock in, and keeping the neighbour’s stock out, relies on stock-proof fencing. If there is direct nose-to-nose contact with neighbouring stock, double fencing with a three-metre gap should be strongly advised. Alternatively, cooperation with neighbours can lead to a transparent health status and local disease initiatives such as regional sheep scab eradication.
Visitors include the farmer’s family and staff returning to the farm after being in contact with other livestock or, for example, a visit to the mart. High-risk visitors include vets, livestock hauliers and contractors – including shearers and scanners.
Simple policies should be introduced to minimise the risk of visitors introducing disease – for example, footbaths and disinfection points, parking vehicles outside the main livestock area and ensuring all equipment is clean and disinfected on arrival.
Collection of dead stock is inevitable on livestock farms, but measures can be taken to reduce the risk dead stock wagons pose to a unit. A dedicated collection point should be located away from the main livestock area, which can be cleaned and disinfected after prompt collection of dead stock. If the area can be made secure to avoid predation, that is also advantageous.
Consideration also needs to be given to risks from vermin and wildlife, slurry storage and disposal, feed sourcing and storage. Disinfectant products and dilution rates are critical as well as ensuring the quarantine facilities are fit for purpose.
The standard quarantine period has traditionally been three weeks, although control measures for specific diseases require at least six weeks. During this time anthelmintic quarantine treatments and flukicides should be administered. Diagnostic testing should also take place. Often, time is required for seroconversion to occur. A six-week quarantine period is, therefore, more appropriate – giving time for test results to be obtained and any retesting if required. Visual monitoring for diseases such as infectious keratoconjunctivitis, orf, contagious ovine digital dermatitis and foot rot is advised during this time.
Specific disease particulars are detailed in this article, although protocols should be tailored to the farm according to its present disease status, the diseases of concern to that unit and its budget.
Caseous lymphadenitis (CLA) is most frequently introduced into flocks from purchased animals. During the quarantine period superficial lymph nodes should be palpated to identify any swellings indicative of CLA (Fontaine, 2009). However, one in three infected animals may only have internal abscessation and no outward signs of disease. Serological testing should take place at least three weeks after arrival and before sheep are mixed with the home flock. The chronic nature of the disease means antibody levels rise and fall intermittently, so false negatives can occur. Vendors can isolate and test sheep a minimum of three weeks pre-sale, although it is advised purchasers retest after purchase (SRUC, 2016).
Vaccination is available under licence; however, this will cause false-positive results on serological tests. It has been suggested contract shearers moving between flocks are a considerable risk factor in the transmission of CLA. A cleaning and disinfection regime, therefore, should be carried out before shearers commence work (Wells, 2016).
To prevent the introduction of enzootic abortion into a flock, replacements should be sought from enzootic abortion-accredited PSGHS flocks.
The PSGHS detects freedom from disease through scheduled blood testing and the analysis of aborted material from member flocks. A single blood test from a sheep during the quarantine period from a non-accredited flock is certainly no guarantee it is not infected with Chlamydia (Sargison, 2008). Where the health status is unknown, purchased sheep should be run as a separate flock until after lambing and any abortions should be investigated.
To minimise the risk of introducing maedi-visna, replacements should be sourced from a maedi-visna-accredited PSGHS flock. Where sheep are sourced from a non-accredited flock, serological testing can be used to detect infected animals. It is advised sheep are tested three weeks into the quarantine period and again six months after arrival. Testing is only effective in animals more than a year old (Wells, 2016).
The virus is transmitted by close contact and shared air space, so specific consideration needs to be given to quarantine accommodation.
No commercially available blood tests exist for ovine pulmonary adenocarcinoma. Ultrasound scanning of the chest can be used to detect lesions greater than 2cm in diameter, but a negative ultrasound scan cannot be used as conformation an animal is free from disease.
Three scenarios can be considered when discussing quarantine strategies for liver fluke:
Two treatment options are available for fasciolosis:
Sheep should be grazed on a low-risk pasture where any surviving fluke will not be able to become established. Faecal egg counts should be used three weeks after the second treatment to ensure the quarantine treatments have been effective before moving the sheep from the low-risk pasture (Abbott et al, 2012).
When sourcing a large number of sheep it is difficult to achieve effective biosecurity against Johne’s disease. Unlike cattle, no accreditation scheme exists for sheep. Serology and faecal PCR testing is available, but variable antibody responses and intermittent shedding prohibit their use as a quarantine screen. Faeces from quarantined sheep can be pooled in groups of 10 for PCR, but a negative result is not a guarantee of freedom from disease.
A killed vaccine is available for sheep, but it has to be remembered that although it reduces clinical disease and shedding, it doesn’t stop animals from becoming infected and being a source of infection.
All sheep arriving on the farm – purchased or returning – should be treated with two broad spectrum wormers (from different wormer groups) that are most likely to remove both resistant and susceptible genotypes – either monepantel and moxidectin sequentially or a derquantel and abamectin combination product. Abamectin and moxidectin are recommended as opposed to other macrocyclic lactones (3-ML), such as doramectin and ivermectin, as fewer 3-ML-resistant parasites are likely to survive these treatments.
Once a quarantine drench has been administered sheep should be yarded with access to food and water for 48 hours until worm eggs in the guts have passed out in the faeces. Sheep should then be turned out on to contaminated pastures for two reasons:
This may not be the case where injectable moxidectin has been used as a quarantine treatment due to the persistent nature of the product (Abbott et al, 2012).
If an injectable moxidectin is used as part of the quarantine treatments for gastrointestinal parasites this can also prevent the introduction of sheep scab. A single injection of two per cent moxidectin is required. If one per cent moxidectin is used, two treatments are required 10 days apart. Treated sheep should remain in isolation for two weeks after treatment (Abbott et al, 2012). Care should be taken when advising the use of moxidectin one per cent as it is contraindicated if the sheep have been injected with Footvax.
Organophosphate plunge dips are an alternative quarantine treatment to prevent the introduction of sheep scab and lice. Psoroptes ovis can survive off the host for up to 17 days – this includes handling pens, trailers, shearers and so on. A serological test is commercially available for sheep scab and has the potential to be used to screen added animals when used appropriately.
Individual blood samples can be tested for border disease virus detection by PCR to ensure no persistently infected animals are being introduced.
Developing a farm-specific biosecurity plan that can be implemented requires a risk-based practical approach working with the flock manager. The protocols should be reviewed as regularly as necessary; often a good time to initiate the conversations is before the autumn sheep sales.
