Introduction

This document outlines the science behind the recommendations and guidance contained within ‘Protect your herd from TB’.

Bovine TB (bTB) is an infectious disease of cattle caused by the bacterium Mycobacterium bovis. Control is difficult for several reasons, including the ability of M. bovis to ‘hide in the immune system of the host, the limited sensitivity of available diagnostic tests and because the pathogen also occurs in wildlife, especially badgers [1].

Risk factors will vary across regions due to different farm structures and farm management practices, including the use of more than one premises or site. Other risk factors include the use of manure, artificial fertiliser, covered housing for animals, cattle buying practices and the prevalence of TB in local wildlife (Bourne 2007). As a result, there is no universal solution to reduce the risk of a TB breakdown. Changes to farm management practices could potentially reduce cattle-to-cattle transmission (for example, strict isolation of reactors and double-fencing to keep herds separate), and this paper offers a summary of available evidence base. However, several reviews encourage the implementation of practical and precautionary biosecurity measures [1, 2, 3].

Scientific studies have endorsed the provision of evidence-based information to promote the adoption of biosecurity ‘best practice’ amongst the farming community [4]. This is what ‘Protect your herd from TB’ aims to do.

 

Background

In order to appreciate the evidence base on the biosecurity measures discussed in this paper, it is important to be able to relate these measures to the common ways in which cattle and badgers, the main hosts of M. bovis in GB, acquire the infection.

Based on the distribution of lesions found in TB reactor cattle, infection of cattle is mainly by the respiratory route. This is supported by experimental evidence when it was found that the minimum infectious dose to establish infection by the oral route was 1,000 times that of the respiratory route. Respiratory excretion and inhalation of M. bovis is considered to be the main route for cattle to cattle transmission [31].

Indirect transmission via the respiratory route could potentially occur from contaminated aerosols generated during spreading of infective material e.g. slurry or inhalation of contaminated dust particles, droplets of contaminated water or aerosols generated during eructation when on contaminated pastures but supporting data is sparse [31].

M. bovis was first isolated from badgers in England in 1971 during an investigation in to an outbreak of TB in cattle in Gloucestershire. Since then infected badgers have been found throughout SW England and parts of Wales and are considered to be a source of TB infection for cattle [42].

The first site of TB infection in badgers is generally the respiratory system following aerosol infection with slow progression of disease thereafter. Open lung lesions can result in infected sputum and infected faeces through swallowed sputum. These are the most common sources of the TB bacillus in infected badgers but openly  draining  lymph nodes and infected urine (due to spread of infection to the kidneys) are other sources. As well as aerosol infection, it is likely that infection is spread between badgers through bite wounds [42].

 

1. Restrict contact between badgers & cattle

Ward et al. (2009) suggest that cattle exposure to direct or indirect contact with badgers will vary between farms, depending on the farm management practices.

Find out if badgers visit your farm

A study on the effectiveness of biosecurity measures in preventing badger visits to farm buildings [5] concluded that badgers:

  • Enter farm buildings when gates are left open or when a new potential entrance point in the building is not repaired
  • Are attracted to some farms more than others, for reasons that have not been determined. So, the benefits of measures to exclude badgers will vary
  • Are more likely to access your farm if your food is easily accessible

Direct contact between badgers and cattle at pasture appears to be relatively infrequent but indirect contact e.g. via the same feeding area can be common [6] [Woodroffe et al; in press]; Badgers infected with TB can exhibit different ranging behaviours. These include larger home ranges, ranging over a greater proportion of their own territory, an increased proportion of their home range extending into neighbouring territories and foraging further away from their main sett. These behaviours may increase the likelihood of direct and indirect contact with cattle [7,8].

Introduce barriers to prevent badgers accessing cattle

This study [5] also concluded that:

  • Exclusion measures can be 100% successful in reducing badger visits to farm buildings when properly maintained

Exclusion measures include:

  • Solid aluminium sheeted gates
  • Aluminium sheeting on rail fences
  • Retractable electric fencing – badgers are less likely to visit unfenced facilities on farms which use electric fencing around feed stores. This is known as the ‘deterrent effect’ [9].
  • Aluminium feed bin
  • Rail gate with adjustable galvanised aluminium panels

Limit access of cattle to badger latrines and setts

Cattle may potentially become infected with TB at pasture [10]. Infection can happen via the following ways:

  • Cattle may sniff or even move badger droppings/faeces to gain access to the grass below, which increases the risk of transmission [11]
  • Cattle may sniff badger urine, which is more difficult to detect and urine from infected badgers has been reported to contain up to 250,000 colony forming units per millilitre. In comparison, up to 75 colony forming units per gram of faeces have been observed [11].

Grazing regimes can potentially increase the chance of cattle accessing potentially infectious badger excreta (faeces or urine). For example:

  • Set-stocked cattle (a fixed number of cattle grazed over one area of pasture for a delayed period) has a higher probability that at least one animal will be exposed to potentially infectious material in a contaminated environment, or where infected badgers are present, than in other grazing regimes.
  • Cattle under rotational paddock grazing (where they are regularly moved between discrete pastures) are exposed to a particular area for relatively shorter period, and tend to remain within a more cohesive herd [12]; this may reduce the risk of contact with potentially infectious material.
  • Strip-grazing (keeping cattle within a narrow strip of pasture) tend to keep cattle in a cohesive herd, limiting the duration of potential exposure to badger excreta depending on the regularity of strip movement [12]. This reduces the amount of field edge available to cattle at any given time, which may reduce risks of contact with potentially infectious material.

Grazing will always carry a potential risk of infection when infectious badgers, and/or their excreta, are present [13].

It has been estimated that restricting cattle access to badger latrines could potentially reduce the number of cattle infected per year by 50%, assuming that cattle would come into contact with all infected excreta within their grazing area and that this was the only route of transmission [13]. Care must, however, be applied to how this is done, as fencing the latrine off close to the latrine site can lead to the badgers extending the latrine area to the other side of the fence [Woodroffe et al; in press]

Although badgers use latrines all-year-round, there is a peak in activity during spring and so efforts to exclude cattle from latrine sites might be most effective at this time of year [14].

Destruction and removal of latrines is unlikely to be effective in reducing the risk of TB infection to cattle, as badgers may continue to visit the original site and urinate elsewhere. This wider distribution of badger urine on pasture could potentially increase the risk of disease infection to cattle [15,4].

Grazed pastures provide a high abundance of earthworms, which are preferentially consumed by badgers [4].

 

2. Manage cattle feed and water

Badger-proof feed stores, troughs & mineral licks

Feed stores, water troughs, and mineral licks all provide opportunities for indirect TB infection between badgers and cattle [16].

If badgers and cattle can access the same feed and water, either when housed or at pasture, this increases the risk of TB breakdown and spread [17].

As a result, a study of bTB in County Down, Northern Ireland [18] suggests that farmers:

  • Make feed stores badger-proof
  • Prevent badger access to the building
  • Feed cattle from raised feed troughs

Don’t put feed on the ground at pasture and clean up spillages

Badgers have been observed to visit cattle housing and feed stores, attracted by food (mainly high energy cattle concentrates, but also maize and silage) and feed supplements [18]. A recent study found that ground feeding sites at pasture were the most likely site for indirect cattle and badger contact [Woodroffe et al; in press].

Badgers are, therefore, more likely to access your farm if food is easily accessible [6].

Use clean, fresh water and badger-proof water troughs

Several studies [19, 20, 21] identify infected wildlife accessing water sources as a risk factor. A review of cattle-to-cattle transmission, risk factors and susceptibility [22] also advises regular cleaning and disinfection of water troughs. Its research concludes that if there are signs of badger activity, it would be advisable to prevent badgers from accessing cattle water troughs by raising them to above 80cm.

Only feed waste milk to calves if it has been boiled or pasteurised

Milk from reactor animals and inconclusive reactors (including those awaiting slaughter) should not be fed to calves [22]. Milk contamination is a risk and this can occur before the animal tests positive via the skin test or before signs of infection are apparent [23,24].

 

3. Stop infected cattle entering the herds

Transmission occurs within cattle herds, and movement of undetected infected cattle can lead to transmission between herds [1].

In recent decades, the observed pattern of bTB breakdowns in areas of low incidence has been correlated with cattle movements, mainly from high-incidence areas [1]. A study of the introduction of bTB to North East England [25] investigated the source of TB in 31 herds in North East England (low risk area) that experienced confirmed breakdowns between January 2002 and June 2004. It identified that newly-brought cattle was the biggest factor behind new TB infections.

Outbreak data for 2004 were best explained by a model attributing 16% of herd infections directly to cattle movements [26]

Ask for TB history information before you buy new cattle

Cattle movements, especially movements from high-incidence areas, are associated with increased risk of the onward transmission of bTB [1, 27, 28]. Studies on herd-level risk factors in after the 2001 foot-and-mouth disease epidemic and risk factors of cattle herds in South West England identified that cattle trading or purchasing cattle from markets or herds in high-risk areas increases the risk of infecting the existing herd [17,29].

Post-movement test cattle entering the herd

The review of herd-level risk factors for bTB advises that detecting infections early by testing new or existing animals as soon as possible is the best possible way to interrupt TB transmission [3].

A report on cattle movements and bTB in Great Britain [30] found that movements consistently appear to be the main factor of TB disease occurrence and thus supports the introduction of more stringent cattle movement controls, as cattle movement outperforms other factors (environmental) in spreading TB disease.

The requirement for post-movement testing in Scotland has been shown to provide an incentive to farmers to purchase cattle from low-disease areas and so probably reduces the risk to the individual herds owned by these farmers, as well as limiting onward transmission [1]. A statutory requirement to post-movement test all cattle arriving to the Low Risk Area of England from the annually tested areas of GB was introduced in April 2016.

Isolate all higher-risk cattle before they enter a herd

The study of bTB in County Down, Northern Ireland recommended that farmers should maintain a closed herd and consider mitigating strategies such as pre and/or post movement bTB testing and isolation of purchased animals [18].

The review of cattle-to-cattle transmission, risk factors and susceptibility recommended that purchased animals should be isolated for 3-4 weeks and pre- or post-movement testing should be completed [22].

 

4. Reduce risk from neighbouring herds

Check local TB breakdown data online at www.ibtb.co.uk

Transmission occurs within cattle herds, and movement of undetected infected cattle can lead to transmission between herds [1].

Some short-distance cattle movements and other interactions between cattle on nearby or linked premises will be unrecorded and could result in cattle-to-cattle transmission of bTB, though the extent of this is not well quantified [1].

Put in place effective barriers between neighbouring herds

The study on herd-level risk factors after the 2001 foot-and-mouth disease epidemic reported that contacts with cattle from contiguous herds and sourcing cattle from herds with a recent bTB history were associated with an increased risk of bTB [17].

The review of cattle-to-cattle transmission, risk factors and susceptibility recommended that secure fencing and physical barriers between herds should be enforced [22]. Secure perimeter fencing and double-fencing of boundary fields (or other methods to prevent close contact between cattle herds) offers a cost-effective means of controlling numerous infectious cattle diseases [18,22].

Avoid sharing equipment or vehicles with other farms

Sharing farm equipment is not advised [18]. This includes the hiring and sharing of bulls, which should be discouraged [22].

Thoroughly clean and disinfect equipment used for slurry before moving off the farm to another property [31].

Avoid sharing cattle grazing with other herds

The studies on herd-level risk factors in after the 2001 foot-and-mouth disease epidemic and bTB in County Down, Northern Ireland, show that contacts with cattle from contiguous herds and sourcing cattle from herds with a recent bTB history were associated with an increased risk of bTB [17,18].

 

5. Minimise infection from cattle manure

Spreading TB via contaminated cattle slurry and manure was considered in a qualitative veterinary analysis of the risk of transmission of bTB through the disposal on farm land of cattle slurry and manure from TB breakdown herds [32]. It was proposed that bTB-infected slurry or manure could potentially spread bTB by the respiratory and ingestion routes. This would require that at least one bovine in the herd was infected, infectious and shedding bacteria in faeces, urine (unlikely), or milk that was disposed in slurry.

There are risks of creating aerosols by spreading slurry according to a report on bTB: an update [33]. Thus, research suggests managing the risks of spreading TB via slurry or manure. Recommendations include:

  • Cattle manure with low moisture levels and high straw content should be stored in a heap to heat and decompose for a minimum of 30 days before disposal on farm land, otherwise manure should be treated as for slurry;
  • Treat slurry with disinfectants and alkalis. This could be with ‘thick lime milk’, a mixture of calcium hydroxide and milk, which should inactivate TB bacteria.
  • Advice on suggested concentrations, applications, and other chemical compounds known to eradicate disease bacteria be found on the DEFRA approved list of disinfectants;
  • Dispose excreta on arable land, either by injection below the surface or by ploughing it in immediately after spreading;
  • Dispose excreta on land that is not going to be grazed by cattle or cut for forage for at least 2 months;
  • Dispose cattle excreta only on land within the affected farm which is not accessible to cattle from other herds;
  • Spread slurry and manure under conditions that minimise or eliminate chances of generating bacterial aerosols which can be breathed and/or digested by other cattle;
  • Avoid sharing spreading equipment.

It is difficult to estimate the levels of TB in cattle manure and slurry. But, indirect transmission of TB occurs through inhalation/ingestion of infectious aerosols produced during slurry spreading or inhalation/ingestion of the bacteria from contaminated pasture, soil and silage [31].

Store manure for a long period before spreading on your farm

Studies indicate that inadequate storage of slurry is associated with an increased risk of TB transmission, as TB bacteria can remain active in soil for about 6 months, which can be ingested by cattle [31]. Slurry storage for 6 months is supported by experimental studies on five farms in Co. Dublin [34].

The longer waste products are stored and the longer the land where they are spread is not grazed, the smaller the risk of TB infection. TB bacteria survive in moist, cool conditions rich in organic matter, especially when it is away from direct sunlight. Slurry can stay contaminated when stored for less than 6 months [35].

The review of cattle-to-cattle transmission, risk factors and susceptibility advises that Mycobacterium bovis may be excreted or secreted into soil from cattle faeces. The bacteria probably remain viable and pathogenic in soil for about 6 months [22]

A study on TB in cattle: reducing the risk of herd exposure, stated that solid manure (faeces) should be properly composted for many months before land-spreading [36].

Studies on temperatures in cattle manure recorded on five farms in Co. Dublin, cattle manure and the spread of bTB, and aerosol dispersal of cattle slurry on bTB-restricted holdings, concluded that the disease risks from spreading slurry with less than six months of storage were two-times higher in their research of Irish farms [34,37,38].

Only spread manure on arable land or pasture that is not going to be grazed by cattle for at least two months

If slurry is to be spread on grazing pasture, land should not be grazed for at least 2 months following spreading. Alternatively, slurry should be spread on arable land by injection or ploughing [31].

TB bacteria are able to survive in the environment for significant time periods [22].

The study on risk factors of cattle herds in South West England concluded that storing manure and slurry indoors or in a closed container, and spreading manure all year, are associated with increased risk of spreading TB as it allows TB to persist in the environment and thus has the potential to infect cattle [29].

Minimise aerosols and contamination of roadways when spreading

The likelihood of infection may be reduced by the dilution effect (of air, uninfected soil and uninfected slurry/manure) but there are risks of creating aerosols by spreading slurry [30]. Cattle excreta should be disposed on farm land not accessible to other herds. Spreading slurry can generate aerosols that potentially carry bacteria for considerable distances, including to neighbouring farms [31].

Manure should only be spread when conditions minimize aerosol production. Mixing and pumping slurry should be avoided while animals are present to prevent inhalation of aerosols that are generated from spreading slurry [31, 35]. Their recommendations are:

  • Slurry should be spread in calm weather (not windy);
  • The distance to neighbouring cattle should red;
  • A method which produces aerosols that travel long distances should be avoided;
  • Slurry application should be avoided outright when cattle are in neighbouring fields and when slurry risks drifting into neighbouring fields even when there are no cattle on them.

Don’t spread manure from other farms

Slurry and manure should only be spread when conditions minimise aerosol production and herd-keepers should avoid sharing spreading equipment [22].

 

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