The potential losses associated with herd infection with BVD and Johne’s Disease and an estimate of the cost of measures to prevent herd infection for Irish Dairy Farms
Veterinary Services, Scottish Agricultural College, St Boswells TD6 0EQ, email@example.com
Dairy Production Research Centre, Teagasc, Moorepark.
Biosecurity systems are essential in order to prevent the losses that follow the introduction of infectious disease to dairy herds. For such systems to be adopted farmers require information on the losses that arise from infectious disease and on the cost of biosecurity systems. Using basic assumptions the losses due to BVD and Johne’s disease were estimated for a 100 cow dairy herd. Using a model of the production in a seasonally calving herd in Ireland these losses were translated into an estimate of the financial loss. The cost of biosecurity systems was then calculated. For effective disease prevention there is a need for a pool of herds that are free of infection. The costs of an accreditation procedure or health scheme were also calculated for both BVD and Johne’s disease.
Animal disease causes direct losses to producers in terms of reduced output, increased replacement costs, increased veterinary costs and increased labour requirements. Many of the diseases responsible for these losses in dairy cattle are the so-called multi-factorial diseases where the causal organisms are part of the normal flora of the animal. These diseases are manifest through an interaction between the husbandry of the animals, the environment and the animals themselves. Hence by refining management and vaccination programmes most types of mastitis, lameness, calf pneumonia and enteritis may be controlled to some degree. However there are a significant number of infectious diseases where the causal organism is not part of the normal microbial flora of the animal nor of its environment and where herd infection with these organisms usually results in the expression of disease and associated losses, irrespective of the quality of the husbandry and the environment that the animals are exposed to. These diseases are therefore amenable to control by adopting management procedures that reduce the risk of infection for the herd. Vaccination also has a role in this approach where the risk of infection is considered to be exceptionally high and the consequences of infection severe.
Biosecurity is the term used to refer to the management systems that are designed to reduce the risk of herd infection. At the highest level biosecurity is the closed herd where no animal is introduced to the herd and new genetic material is brought into the herd through semen and embryos from suitably controlled sources. The next best option is the control of added animals through test and quarantine regimes and sourcing replacement from herds of a high health status. In addition boundaries must be stock proof and prevent nose-to-nose contact with stock on other farms. Water and food sources must be clean and free from infection and the risk of infection being introduced on the clothes of people working with other cattle also requires to be addressed.
In an American study the majority of herds that were undergoing expansion failed to employ comprehensive biosecurity programmes. Losses from BVD and digital dermatitis were notable (Faust, Kinsel et al. 2001). In Canada failure to implement biosecurity was also identified and farms purchasing replacements were more likely to be infected with Johne’s disease (Chi, VanLeeuwen et al. 2002). For biosecurity to be an attractive disease control objective for dairy farmers it must be demonstrated to offer a positive cost benefit (Stott, Lloyd et al. 2003). In the Netherlands dairy herds adopting a closed herd approach to biosecurity achieved a 5% increase in net return compared to herds that purchased cattle or shared pasture (van Schaik, Dijkhuizen et al. 1998). However there have been few studies to show that the implementation of biosecurity does result in a positive cost benefit. Furthermore there have been very few studies carried out to demonstrate either the cost benefit of disease control of the diseases that are amenable to control through biosecurity or to demonstrate the levels of losses that can be expected to arise commonly in herds endemically infected with these diseases. To tackle this deficiency many workers have turned to modelling disease at the herd level. These models are valuable exploratory frameworks, but are complex and lack transparency for those unfamiliar with the theory or techniques of modelling. Perhaps more importantly they lack precision because they are largely based on assumptions generated by experts rather than data collected through observational studies. The high cost of such observational studies means that this impasse is likely to remain. In order to overcome these barriers we have adopted a simple approach to estimating the physical and financial losses that can be expected with two of the diseases. We have then estimated the important costs of implementing a biosecurity programme in the Irish dairy herd.
The diseases of major importance are those that are endemic in the country and therefore the risk of infection is not merely theoretical. For most of Europe these diseases are Johne’s disease, infectious bovine rhinotracheitis (IBR), leptospirosis, venereal campylobacter infection, bovine virus diarrhoea (BVD), salmonellosis, neospora infection, and digital dermatitis. They are assumed to be of significant financial importance through reducing performance. The first four feature as OIE list B diseases (OIE 2003), underlining their significance. The need for prevention and control of these diseases at the herd or national level has resulted in national control programmes in many countries throughout the world for all of the above with the exception of neospora and digital dermatitis. These last two diseases are emerging diseases and the formulation of control programmes has been limited by a lack of consensus on how this should be achieved or the lack of a suitable diagnostic test.
For the purposes of the present discussion the paper will be restricted to examining the losses associated with BVD and Johne’s disease.
BVD: bovine virus diarrhoea infection results in losses through its effect on reproduction, suppression of disease resistance particularly in rearing cattle and through the fatal condition known as mucosal disease. It is prevalent in most countries where cattle are reared. In England and Wales a prevalence study using milk antibody found an exposure prevalence of 95% of herds with a high concentration of antibody suggesting recent active infection in 65% of herds (Paton, Christiansen et al. 1998). Herd infection usually arises through exposure to a purchased animal that is persistently infected with the virus, although contact with such animals over the fence or the spread of the virus on personnel or equipment may also occur. Infection in breeding animals that have never encountered the infection before (naïve) can result in transient male infertility, depressed ovarian function, a failure in conception, embryonic or foetal death, abortion and the birth of calves with birth defects or persistently infected with the virus (Fray, Paton et al. 2000). Persistence of infection in the herd is considered to be more often the result of the birth of persistently infected calves, rather than the survival of a persistently infected adult in the herd. Persistently infected adults are rare, because persistently infected calves have a high mortality rate as they mature and ultimately have poor reproductive performance so those surviving into adult hood are likely to be culled because they are infertile. In small to moderate sized herds a high prevalence of immunity often follows after BVD infection through one or two reproductive seasons in the herd and the infection may then disappear until it is introduced to the herd again several years later. In larger herds infection is likely to persist as there will continue to be a sufficient number of naïve breeding animals added to the herd each year to result in the birth of a new generation of persistently infected calves.
The highest losses will arise in naïve herds where infection is introduced to the herd during or immediately after the breeding season. In these situations conception rate can be significantly depressed with abortions observed before the birth of persistently infected calves. In large endemically infected herds losses are considerably lower. While depression in conception rates of 44% and abortion rates as high as 40% have been documented (Fray, Paton et al. 2000), these are unlikely to occur in the field. However outbreaks where up to 40% of calves are born persistently infected are observed. The losses that can be expected in an endemically infected herd are more difficult to arrive at (Fray, Paton et al. 2000), (Brownlie, Thompson et al. 2000) and for the purposes of this study conservative estimates of 10% lost conceptions and 2% abortions are offered (table 1).
The birth of persistently infected calves results in a source of virus for the rest of the calf crop and infection will result in suppression of the immune system and exacerbation of calf enteritis or pneumonia, impacting significantly on mortality rates and growth rates (table 1). Persistently infected heifers are also likely to die from mucosal disease before they enter the herd. Indeed in countries that have succeeded in eradicating BVD or bringing it under control the major benefits claimed are in the area of young stock health (Lindberg, in press).
Johne’s Disease: this is a chronic disease where infection in early life will result in the development of clinical disease most commonly at three to five years of life. Infected heifers or young bulls can be expected to appear healthy at the time of purchase only to develop the disease some time later. It is through the purchase of added animals that disease is introduced to the herd. Other species of animal, particularly goats and sheep can be infected and develop the disease, but such infections are not considered to be of significance in terms of introducing infection to cattle herds. It has also been shown that non-ruminant wildlife may also be infected with the causal organism of Johne’s disease (Greig, Stevenson et al. 1999), although the significance of this to disease control has yet to be established.
The disease develops in the intestinal tract and results in a progressive failure in digestion and absorption. In the subclinical stage of the disease this may affect the energy balance and so impact on fertility. In the lactation before clinical signs become apparent cows fail to reach their milk yield potential. Infection is associated with a higher probability for being culled for reasons such as mastitis, infertility and low yield. These animals frequently go unrecognised as Johne’s disease cases and lead to a failure to recognise the true extent of the disease. Indeed as many as 70% of animals that are positive by faecal culture may be culled for reasons other then clinical Johne’s disease.
For the purposes of modelling losses from Johne’s disease in the British dairy herd the following estimates were taken from the literature (Stott, Lloyd et al. 2001). In the year before disease becomes apparent milk yields are reduced by 10% and such cows had an increased chance of being culled of 11%. Cows showing clinical signs were certain to be culled and had a reduced yield of 20% (Benedictus, Dijkhuizen et al. 1987). Following the introduction of infection to a herd it may be several years before clinical disease is manifest. The prevalence of disease in the herd is then the result of herd size, degree of faecal contamination of the environment and other management practices (see below). This means that the prevalence in affected herds can vary enormously, but by the time the problem is recognised as being significant annual culling rates of 5% can be expected and these may be split 2% in lactation two and 3% in lactation three (table 2). In some herds the annual losses may be half this, but herds exist where the annual culling for clinical Johne’s disease is 10%.
The disease is maintained in the herd through the vertical transmission of infection, the presence of the organism in colostrum, in milk and the faecal contamination of these products and the contamination of the environment of immature cattle and neonatal calves in particular with faeces from infected adults. The practice of pooling colostrum and feeding waste milk to calves is considered to be extremely effective in creating new infections and resulting in a high prevalence of Johne’s disease in the herd.
The financial cost of lost production was calculated using a model of pasture-based dairy systems in Ireland (Veerkamp, Dillon et al. 2002) and updated for 2003 costs. The losses caused by a failure to conceive at a mating or similarly where early embryonic death occurs is not the same as occurs where breeding takes place throughout the year. For the Irish situation it has been estimated that for each day beyond a calving interval of 365 days there is a production loss of €2.07 per day. In a seasonal calving herd where a defined breeding season is adhered to rigorously the costs in cows being discarded from the herd through failure to conceive may be high. In practice it is likely that where the herd manager is aware of the infertility he will extend the breeding season to ensure sufficient cows conceive in order to achieve quota. Therefore both these options are considered. For all examples a standard herd size of 100 cows has been used. The losses associated with reduced milk yield, culling, poor growth rate, treatment and mortality are given in tables 3 and 4.
BVD: Using these conservative estimates for losses in an endemically infected herd of 100 milking cows we arrive at an annual cost of infertility of €7553 where cows that do not conceive within the set breeding season are culled and €2748 when cows continue to be bred. In the endemic herd it is assumed that the first lactation cows are most likely to be naïve and so susceptible to infection. The costs in terms of calf health are €2997 (table 5). An estimate of the total cost of BVD infection in an endemically infected, seasonally calving herd in Ireland is therefore €10550 or €5745 depending on whether or not a strict breeding season is adhered to. Losses in a naïve herd that is infected are significantly higher at €21170 and €6753 for fertility and €6465 for calf losses, giving a total of €27635 or €13218, depending as above on the decision taken on culling or continuing to breed (table 6). No effort has been made to estimate the subsequent annual losses that will result from the failure to maintain a tight calving season.
Johne’s disease: Losses in a herd endemically infected with Johne’s herd do not have an easily demonstrable impact on fertility and these “hidden” losses have not been estimated, leading to a significant under estimate of the losses attributable to this infection. Johne’s disease does not have a significant impact on calf health. The breakdown on losses is given in table 7. The total loss in a herd with moderate disease prevalence as described above would be €2943. The cost of a control programme relates to the management of periparturient cows and neonatal calves and the feeding of milk replacer only to calves. These costs are not given in this paper.
Disease control programmes and Biosecurity
Biosecurity has been defined as a strategy of management practices to control and prevent the introduction of disease and to control spread of disease within the operation (Wells 2000). Risk assessment is central to such a strategy and implies individual programmes defined for the herd by the veterinarian. There are therefore costs for professional consultancy in order to design an appropriate programme. Decisions on whether or not to close the herd and to resort to artificial breeding techniques alone also require to be informed by dairy production specialists with a knowledge of the genetics and production potential of the herd so adding a further level of professional consultancy into the costs.
BVD: The minimum effective standard for biosecurity for viral infections such as BVD and IBR are given in the table. It is essential to prevent nose-to-nose contact across boundary fences and to have an effective test and quarantine programme for all added animals. Electric scare wires either side of a boundary fence are the cheapest solution to the problem, but the degree of protection conferred by boundary barrier increases with distance. Therefore the use of boundary tree plantation strips should also be considered as long-term solutions. These biosecurity measures have not been estimated in this paper as the possible solutions for the individual herd are extremely variable.
It has been suggested that in areas of high stocking density the practice of maintaining herds that are seronegative to BVD (and by inference to IBR) is a high risk strategy and the use of vaccination should also be considered (Fray, Paton et al. 2000). This appears a sensible precaution as the cost of an outbreak of BVD for a previously naïve herd can be so great. The effective control of BVD in the dairying regions of Scandinavia do indicate that control can be achieved with effective biosecurity, providing all the herds in the region adopt the control programme. In the Irish situation at this time the insurance provided by a herd BVD vaccination programme should supplement a biosecurity system. There is little information on the efficacy of the available vaccines in the field and apparent vaccine failures have been recorded in the UK, however epidemiological information from Canada suggested that the use of vaccine was associated with a low prevalence of infection of BVD infection in young stock (Chi, VanLeeuwen et al. 2002). The current cost of a primary course of vaccination for a 100 cow herd is in the region of €800 to €1000 with annual costs of maintaining the programme estimated at €400 to €500.
Test and quarantine of added animals is required for BVD control and is recommended whether or not the herd is vaccinated. The requirements for buildings or paddocks used for quarantine and a suitable disease screening protocol has been described (Crawshaw, Caldow et al. 2002). For a herd that is purchasing all its replacements a screening and quarantine programme will cost around €400 each year. Reliance on the quarantine and screening of individual animals for BVD carries some risk as no laboratory test is totally sensitive and there is therefore a theoretical risk of missing virus carriers. Screening and accrediting high genetic merit herds as free of BVD infection would overcome this by providing breeding stock for sale that are certified free of BVD. The annual cost of demonstrating that a high genetic merit seasonally calving herd is free from active infection is relatively small as only five to ten animals per management group require to be screened. A minimum standard for herds selling breeding stock should be herd BVD vaccination of the breeding stock and annual screening of animals over 9 months of age prior to vaccination for antibody to BVD. Absence of antibody would indicate satisfactory control. The annual cost of accreditation for BVD for a herd that is already free of active infection is €409.
Finally it should be recognised that pregnant animals may be negative for the virus, positive for exposure to infection (antibody) and carrying a foetus that is persistently infected. Therefore pregnant animals should only be accepted into the herd if they are seronegative at the beginning and end of a four-week quarantine period and virus negative.
For a herd buying in replacements the costs for a BVD biosecurity and vaccination programme are therefore around €1400 year one and €1000 in subsequent years screening replacements and maintaining vaccination of the herd against BVD. For a herd that is closed this falls to the cost of the vaccine alone.
Johne’s disease: Biosecurity is easier to achieve for Johne’s disease. A herd that is closed and neither brings in slurry nor dung from other units will be at negligible risk of becoming infected with the disease. Other factors which are believed to offer a theoretical but negligible risk are sharing pastures with sheep and allowing cattle access to water courses that have flowed through other farms. The problem of Johne’s disease arises for those herds that have to purchase breeding replacements. Quarantine and screening for infection is of almost no value for Johne’s disease as most young animals that are infected will not test positive until some time in their adult life. Therefore the only way to reduce the risk of purchasing infected stock is to purchase from herds where all adults are tested on an annual basis without any evidence of infection being found. The annual cost of demonstrating herd freedom from infection with Johne’s disease according to the British Cattle Health Certification Standards (CHeCS) programme is €1157 for a 100 cow herd (table 9).
As there are currently no herds in Ireland that are certified free of Johne’s disease maintaining a closed herd offers the best means of prevention.
General Biosecurity Practices: Many infections can be introduced to a herd through indirect contact with other cattle. Therefore the risk that personnel and vehicles pose to the health of the herd must be considered. Disinfection practices and the provision of protective clothing to ensure biosecurity have been detailed elsewhere (Crawshaw, Caldow et al. 2002). The financial costs of these controls are related to management and staff time and are not estimated in this paper.
The principles of the biosecurity programme for BVD and Johne’s disease also offers safeguards against the introduction of many other diseases. The implementation of such programmes therefore have a further cost benefit beyond those considered for the two diseases discussed here.
Where there remains a need for the introduction of high genetic merit replacements to boost the potential of the herd or to provide bulls for natural mating then there is a need for high genetic merit herds of high health status. The major conditions to be controlled in these herds are those that are listed above, but of greatest importance are the diseases that cannot be simply screened for in the individual animal. Of those above salmonellosis, digital dermatitis and Johne’s disease fall into this category. The industry should be aware of the value of such herds and seek to promote the achievement of high health status and support the concept of a premium on the price of such animals. Using the above methodology it is possible to estimate the value of such disease free animals to herds that are currently free of the diseases in question.
There are several diseases that cause significant losses to the Irish dairy herd. Little has been done to establish the true costs of these diseases or the cost benefit of control options. The estimates given in this paper are based on simple assumptions that for both BVD and Johne’s disease lead to an underestimate of associated losses. Stott and others (Stott, Lloyd et al. 2001) have created sophisticated disease models for Johne’s disease in the British dairy herd that have taken into consideration more of the associated losses of the disease and reached a figure of €3744 for a 100 cow herd with only moderate numbers of clinical cases per year. This is considerably higher than the estimate arrived at in this paper. In order to improve the estimates given here observational studies on disease losses in endemically infected herds in Ireland are required. Disease modelling can then be used to explore the cost benefit of control options.
Estimates of the cost of control are simpler to arrive at. In this way an indication of the theoretical cost benefit of biosecurity and disease control can be reached. For both Johne’s disease and BVD a significant cost benefit has been shown. It should be recognised that improvement in the control of several of the important diseases of cattle cannot be made without health assurance programmes for the herds that are selling breeding stock. The methodology on cost can be used to support the argument for the creation of such a system of health accreditation providing the participating herds are supported by improved prices for the breeding stock. The major candidate diseases for this approach are Johne’s disease, BVD, Salmonella Dublin and digital dermatitis.
Acknowledgments: The information on disease control has been collected by SAC’s team of veterinary surgeons working on the Premium Cattle Health Scheme. Dr Kevin O’Farrell provided information on the costs of veterinary intervention based on experience at Moorepark.
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|Disease||Reduced breeding success||Loss in milk output||Early culling||Loss of heifers/steers before adult due to mucosal disease.||Reduced growth rate and attributable calf disease where PI calves are present|
|BVD||Naïve herd: 50% reduction in conception rate, 5% of cows abort. Endemic herd: 10% reduced conception and 2% abortions||Nil||Nil||
Naïve herd: 10% of calf crop at ages 3 to 15 months of age.
Endemic herd: 2% of calf crop at ages 3 to 15 months of age.
In naïve and endemic herds: 10% calf treated for enteritis,
20% calves treated for respiratory disease,
10% reduced growth rate in treated calves.
|Disease||Loss in milk output||Early culling*|
10% reduction in lactation before clinical disease of 2% of cows in lactations 1 and 3% in lactation 2;
20% reduction in lactation with clinical disease of 2% of cows in lactations 2 and 3% in lactation 3
2% of cows culled in lactation 2,
3% of cows in lactation 3
*Culling of animals that are subclinically infected and culled for other reasons has not been estimated.
|10% reduction in yield||20% reduction in yield||Cow Mortality||Cull Cow|
|Mortality||10% Reduction in Growth Rate as result of pneumonia|
|Calf Mortality||1 Month||208||336||21||34|
|Endemic herd||Strict adherence to seasonal calving||Breeding continues to get cows in calf*|
|5 cows in lactation 1 fail to conceive and return at 21 days to conceive||217||217|
|5 cows in lactation 1fail to conceive and return at 42 days to conceive||Not applicable||435|
|5 cows in lactation 1 failing to conceive and removed from the herd||5240||Not applicable|
|2 cows aborting in lactation 1||2096||2096|
|2 calves dead, average age 9 months, half female and half male||867||867|
|10% calves treated for enteritis and 20% for respiratory disease.||1500||1500|
|Reduced growth rate in calves treated for respiratory disease||630||630|
*Losses due to the failure to maintain seasonal calving pattern in subsequent years are not included. Losses due to calf disease other than mucosal disease are the same as in naïve herd immediately after the introduction of infection.
|Naïve Herd||Strict adherence to seasonal calving||Breeding continues to get cows in calf*|
|25 cows of mixed ages do not conceive and return at 21 days to conceive||1085||1085|
|25 cows of mixed ages do not conceive and return at 42 days to conceive||Not applicable||2175|
|25 cows of mixed ages failing to conceive and removed from the herd||16592||Not applicable|
|5 cows aborting||3493||3493|
|10 calves dead, average age 9 months, half female and half male||4335||4335|
|10% calves treated for enteritis and 20% for respiratory disease.||1500||1500|
|Reduced growth rate in calves treated for respiratory disease||630||630|
Naïve herd: culls split 10:7:4:4 and abortions split 2:2:1:0 by lactations 1,2,3 and 4 respectively. Assumed that 2 calves are treated per visit. Assumed that half the pneumonia cases occur at one month of age and half at 6 months of age
*Losses due to the failure to maintain seasonal calving pattern in subsequent years are not included.
|Loss in lactation potential||Cost|
2 cows 10% reduction lactation 1
|3 cows 10% reduction lactation 2||156|
|2 cows 20% reduction lactation 2||186|
|3 cows 20% reduction lactation 3||412|
|2cows lactation 2|
|3 cows lactation 3||1047|
|Administration and veterinary consultation||151.2|
|Blood sampling 20 young stock at 9 to 18 months of age||
Veterinary fee to collect samples
|Laboratory testing for 20 young stock at 9 to 18 months of age||75.4|
|Associated vet consultancy and testing||72|
(Does not include farm labour at handling for testing.)
|Administration and veterinary consultation||151.2|
|Blood sampling 100 cows||
Veterinary fee to collect samples
|Laboratory testing for 100 cows||410||590.4|
|Associated vet consultancy and testing||50||72|
(Does not include farm labour at handling for testing.)