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Review article
Selection for mastitis resistance in dairy cattle: a review with
focus on the situation in the Nordic countries
*
Bjørg Heringstad , Gunnar Klemetsdal, John Ruane
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Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 As, Norway
Received 22 April 1998; received in revised form 2 March 1999; accepted 15 June 1999
Abstract
The literature concerning selection for mastitis resistance in dairy cattle is reviewed and the reasons for including mastitis resistance in dairy cattle breeding programs are described. The current situation in Denmark, Finland, Norway and Sweden is described with emphasis on the data recording schemes and the choice of models used for breeding value estimation. The use of clinical mastitis data and somatic cell counts in selection for mastitis resistance as well as implications and prospects for the future are discussed. 2000 Elsevier Science B.V. All rights reserved.
Keywords: Dairy cattle; Selection; Mastitis
1. Introduction death, leading to large economic losses. The minor
pathogens are coagulase-negative staphylococci and Mastitis is a complex disease which can be simply Corynebacterium bovis. Infections with such patho-defined as an inflammation of the mammary gland gens can lead to moderate inflammation of the resulting from the introduction and multiplication of mammary gland and a slightly increased SCC and pathogenic micro-organisms in the mammary gland. they only rarely lead to changes in milk composition, The causative bacteria can be classed as major or greatly reduced milk yield or clinical mastitis (Har-minor pathogens (Harmon, 1994). The major patho- mon, 1994).
gens include Staphylococcus aureus and Streptococ- There is evidence that the spectrum and
pro-cus agalactiae (both of which are contagious) and portions of mastitis causing bacteria are changing
coliforms, streptococci and enterococci (all of which over time (e.g. Myllys et al., 1998) and also that come from the cow’s environment, i.e. bedding, geographic differences exist. For example, pre-manure and soil). The major pathogens can cause dominating types of S. aureus seem to be specific clinical disease, with changes in milk composition, to each of the Nordic countries (Aarestrup et al., an increase in somatic cell counts (SCC) and even 1997).
According to Harmon (1994) clinical mastitis is characterised by swelling or pain in the udder, milk *Corresponding author. Tel.:147-6494-8000; fax: 1
47-6494-with an abnormal appearance and, in some cases, 7960.
E-mail address: [email protected] (B. Heringstad) increased rectal temperature, lethargy, anorexia and
even death. In addition, bacteria are present in the 2. Reasons to improve mastitis through milk, the milk yield is much reduced, and the milk breeding
content is altered considerably. Subclinical mastitis
does not lead to visible changes in the milk or udder. 2.1. High frequency It is characterised by reduced milk yield, altered milk
composition and the presence of inflammatory com- Mastitis is one of the most frequent diseases ponents and bacteria in milk. affecting dairy cattle. Generally, the incidence of Schukken et al. (1997) describe infection patterns clinical mastitis per cow-year varies between 20 and of the major mastitis-causing pathogens by using an 40%. In 1993, the number of cases of clinical example with E. coli, S. aureus and non-agalactiae mastitis (defined as treatment by a veterinarian of streptococci. They claim that E. coli infections one cow with clinical mastitis) per 100 cow years mainly cause clinical mastitis, that S. aureus in- was 56, 32, 30 and 21 in Denmark, Finland, Norway fections mainly cause sub-clinical mastitis and that and Sweden respectively (Forshell et al., 1995). In non-agalactiae streptococci have an infection pattern the Nordic countries, antibiotics are administered with both subclinical and clinical appearances. Ac- only by veterinarians; therefore, disease recording is cordingly, most E. coli infections should be captured quite reliable. It should be noted, however, that for by clinical mastitis records, while S. aureus in- many reasons (e.g. the trait can be defined in fections should also be reflected in changes of SCC, different ways, and mastitis frequency can be in-which increases as a result of infections with major fluenced by the type of feeding and management pathogens (e.g. Reneau, 1986). systems used) it is difficult to compare mastitis This shows that mastitis is caused by many incidences across countries or even across farms different micro organisms changing over time and within country.
location. Mastitis is a disease which shows differ- Comparable results in the literature from other ent infection patterns; from subclinical mastitis countries are not easily available because they do not with no clinical signs to peracute clinical mastitis have a system for registering health data. In surveys that may cause death of the animal. The duration from California, Michigan and Ohio incidences of of a mastitis case varies from a few days to long mastitis were found to be 30, 33 and 37 cases per duration chronic or subclinical infections which can 100 cows per year respectively (Gardner et al., 1990; last for weeks or months. Thus, udder health prob- Kaneene and Hurd, 1990a; Miller and Dorn, 1990). lems are not easily expressed as one single trait, These estimates include both veterinary-treated and but observationally clinical mastitis has the advan- owner-reported mastitis.
tage of being possible to record in national health
recording systems, and by utilising records of treat- 2.2. High costs ment of clinical mastitis in genetic evaluation, one
is selecting for all traits involved in the cows Mastitis is the most costly dairy cattle disease immunological performance. (Waage, 1989; Kaneene and Hurd, 1990b; Miller and Denmark, Finland, Norway and Sweden are the Dorn, 1990). Costs due to clinical mastitis include only countries with well-established, national re- veterinary and treatment costs, reduced milk pro-cording systems for health data in dairy cattle and duction during the remaining part of the lactation, the only countries which include clinical mastitis the loss of milk that has to be discarded due to directly in their breeding programs. Thus, the main contamination with antibiotics, early culling, extra goal was to review developments and status of labour, decreased milk quality and increased disease breeding for increased mastitis resistance in the risk in the future.
Nordic countries with main emphasis on traits, data Estimates of mastitis costs vary depending on recording and genetic evaluation. Another objective assumptions and country. According to Steine was to identify shortcomings as a basis for future (1996a) the estimated costs per case of clinical
on all the costs mentioned above. Sender et al. is expected to negatively affect mastitis resistance. (1992) estimated the economic losses in Finland due Thus, Strandberg and Shook (1989) showed that to one case of clinical mastitis to be 1000 FIM (215 breeding for increased production under a traditional US$), based on the value of discarded milk, vet- progeny testing programme, without selection for erinary costs, medicine costs and extra labour costs. mastitis, results in a genetic increase of 0.02 cases of Costs of clinical mastitis reported by US farmers mastitis per cow per year, assuming a genetic vary from 108 to 122 US$ per case, based on drugs correlation between mastitis and milk yield of 0.30. and veterinarian costs, preventive costs, costs of The rate of change in mastitis may seem low, but the extra labour, culling and milk loss (Kaneene and increase is alarming from a long-term perspective. Hurd, 1990b; Miller and Dorn, 1990). Including disease resistance in breeding programmes It is expensive to replace a diseased animal. is therefore needed to counteract the undesirable Mastitis increases culling rates and replacement correlated responses resulting from selection for milk costs. Udder health problems are a major reason for production alone.
culling dairy cattle. For example, in Finland, Norway
and Sweden, udder health problems were the reason 2.4. Total economic merit for culling in 35, 19 and 22%, respectively, of cases
in milk-recorded herds. It was the main reason in Selection for increased mastitis resistance contri-Finland and the second most important reason for butes to reduced production costs and is consistent culling in Norway and Sweden (Maaseutukeskusten with the goal of maximising genetic improvement for Liitto, 1997; SHS, 1995; NML, 1997). In the US, total economic merit.
mastitis is the third most important reason for In Norway it is more profitable to use broad premature culling of dairy cattle (Shook, 1989). breeding goals than breeding goals including pro-duction traits only (Steine, T., unpublished results). 2.3. Genetic correlation to milk production Simulation studies have shown similar results, with higher genetic gain for the overall economic value It is generally accepted that a positive genetic when selection for mastitis resistance is included in correlation exists between mastitis susceptibility and the breeding scheme than when selecting for milk milk yield. Estimates of the genetic correlation based yield only (Strandberg and Shook, 1989; Sender et on Nordic data range from 0.24 to 0.55 (Simianer et al., 1992; Colleau and le Bihan-Duval, 1995). al., 1991; Lund and Jensen, 1996; Sander Nielsen et
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al., 1996; Poso and Mantysaari, 1996; Luttinen and 2.5. Antibiotics and vaccination Juga, 1997), with an average of 0.43. All these
more important. In addition, few antibiotics are (Elleby and Veirup, 1977). The current health-re-effective against Gram-negative bacteria. For exam- cording system in Denmark started in 1986 and was ple, coliform mastitis appears to be unaffected by introduced nation-wide in 1990. Recordings are
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antibiotic treatment (Pyorala et al., 1994). Mastitis primarily made by veterinarians.
vaccination can be viewed as an alternative strategy In Finland, a health-recording system was started ¨ to genetic improvement (Rinehart et al., 1996). for all dairy farms in the spring of 1982 (Grohn et However, problems remain in verifying its short- and al., 1986). The veterinarian records the date, diag-long-term effects. In view of the adaptive nature of nosis, treatment and medicine used on the health bacteria, it is questionable whether a single vaccina- card. Cases handled over the telephone may also be
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tion would offer prolonged protection. Even if recorded by the dairy farmer (Grohn et al., 1986). effective vaccination is available at low costs which In Norway, a disease recording system was tested may be more cost effective in the short run, genetic in one part of the country starting in 1970. The improvement has more advantage in the long run. «health card system» was introduced on a national basis in 1975. Since then, diseases have been re-corded in most milk-rere-corded herds in Norway 2.6. Ethics and animal welfare
(Solbu, 1983). In 1996, 98% of cows in the milk-recording system were included in the disease-re-The ethical aspects of disease are related to animal
cording system (NML, 1997). Each cow has her own welfare considerations and consumer interests.
Con-health card, and only veterinarians record data on sumer interest in production methods and concern
this card. about animal welfare are growing. In general,
con-In Sweden, the recording of disease treatments by sumers want products produced by healthy animals
veterinarians started in a single province in 1971 with as little use of antibiotics or other drugs as
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(Lindhe et al., 1978), and the system was introduced possible. Even if economic losses due to mastitis
on a nation-wide scale in 1984 (Eriksson and Wre-could be offset by additional production, ethical
tler, 1987). The veterinarian records the disease considerations might not allow us to ignore the
diagnosis and the ID number of treated animals on a impact of selection for increased production on the
special form and then sends the data to the central health status and general welfare of cows (Shook,
data base. 1989; Solbu and Lie, 1990).
3.2. Data and models used for breeding value estimation of mastitis resistance
3. Situation in the Nordic countries
Mastitis resistance is taken into account in Nordic
3.1. Data recording breeding programmes by including a breeding value
for mastitis in the total merit bull index. In Norway, The Nordic health-recording systems, which have estimated breeding values for mastitis were first been established over the last 20 years, combine data calculated in 1978, whereas in Sweden, Finland and from three different sources; veterinary, milk record- Denmark breeding values were first published in ing and AI records. Each case of a disease treated by 1984, 1986 and 1992 respectively. In the evaluation, a veterinarian is recorded on a health card (Denmark, Denmark uses data from 1990 onwards; Finland and Finland and Norway) or on a special form (Sweden). Sweden use data from 1983 onwards, and Norway The information is then matched against other data in currently uses data only from the most recent batch the national milk recording system and stored on an of progeny tested bulls.
based on a sire model using field records of vet- data, but do no create systematic differences between erinary-treated cases of clinical mastitis, which is progeny groups.
considered as an all-or-none trait. Cows reported to To summarise, the models and data used for have had mastitis within a defined period of the breeding value estimation vary slightly between the lactation, from a few days before calving to about Nordic countries, but they nevertheless have far the middle of the lactation, are treated as diseased, more in common than they have differences. and the remainder are considered to be healthy. The
period used varies between countries, stretching from 3.3. Nordic breeding programmes 10 days before to 180 days after calving in Denmark,
7 days before to 150 days after calving in Finland, Dairy cattle breeding in the Nordic countries 15 days before to 120 days after calving in Norway, includes a breeding goal with functional traits of low and 10 days before to 150 days after calving in heritability, such as health and fertility, and is based Sweden. The main reason for using only a short on progeny testing of large daughter groups. For period of the lactation is to avoid bias due to culling example, the average size of daughter groups for AI of cows. In the first part of the lactation the culling bulls in 1992 was 90, 220, 250 and 140 in Denmark,
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rate is low and, according to Syvajarvi et al. (1986), Finland, Norway and Sweden respectively (Lindhe, two thirds of all mastitis treatments occur within two 1995). Participation in the milk and health-recording
months after calving. system is high. For example, in 1996, 90% of the
In Finland and Sweden, the reason for culling dairy cows in Norway were included in the milk-reported by the farmers is used as an additional recording system (NML, 1997). For the Nordic source of information about mastitis. Cows culled countries, an average of 45% of the milk-recorded
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due to udder health problems before 150 days after cows are bred using young bulls (Lindhe, 1995). calving are treated as diseased, even if they were not Hence, most of the dairy cattle population is active recorded as having mastitis. Koenen et al. (1994) in the breeding programme. As a consequence, every found that heritability estimates for mastitis in year a relatively large number (450) of young bulls Swedish data were significantly higher when in- of red breeds can be progeny tested with large ´ formation on culling was included. In Denmark and daughter groups in the Nordic countries (Lindhe, Norway, the reason for culling is not used in the 1995). Another characteristic of the Nordic breeding genetic evaluation. systems is the co-operative cattle breeding organisa-In Denmark and Sweden, records are used from tions, which are owned by the dairy farmers, and first-lactation cows, whereas in Finland records are which thus take a longer-term view of cattle breeding used from the first three lactations. In Norway the and so include non-production traits in the breeding first lactation is analysed separately, although «re- goal.
peated evaluations» are carried out based on the second- and third-lactation records of daughters of
potential bull sires. Other minor differences with 4. Selection for mastitis resistance respect to the models used for genetic evaluation also
exist and are discussed by Ruane and Klemetsdal Breeding for increased resistance to mastitis can
(1996). be performed by direct selection using clinical
measure of mastitis resistance (De Jong and Lan- estimates of clinical mastitis in agreement with the sbergen, 1996). Detection of putative Quantitative Nordic studies.
Trait Locus (QTL’s) for mastitis resistance may in It should be noted that heritability estimates of the future make marker assisted selection an alter- all-or-none traits are functions of incidence and native or supplementary selection strategy for im- differences in estimates between different studies proving mastitis resistance in dairy cattle. QTL’s for may be caused by real differences between popula-SCC have been reported (e.g. Reinsch et al., 1998) tions and countries, but also be due to somewhat and mapping of QTL’s for clinical mastitis has different definitions of mastitis traits. Therefore started in Norway. Among others, BoLA alleles may parameters should be estimated in the population and be potential candidate genes (e.g. Sharif et al., 1998). country where they are going to be used, and possibly multiple trait evaluation e.g. for clinical 4.1. Direct selection for clinical mastitis mastitis and culling should be done rather than lumping all data together in one mastitis variable. The most common approach when utilising mas- Mastitis defined as veterinary treatments of clini-titis data in genetic evaluation has been to consider cal cases is a trait that may change over time. mastitis as an all-or-none trait and apply linear Introduction of somatic cell count as a quality models, which assume normal distribution of the criteria for milk price, and the gradually strengthened data. An alternative is the threshold model which quality over time have increased the attention paid to takes into account the binary nature of the data mastitis by the farmers, and their criteria for calling a which can be advantageous for variance component veterinarian may have been changed.
and breeding value estimation (Gianola and Foulley, The definition of mastitis as an all-or-none trait
1983). does not fully utilise all information in the data,
The heritability of clinical mastitis has been since some cows have more than one case of mastitis estimated from several studies based on data from and also the date of treatment is known. Therefore the Nordic health-recording systems. Estimates from alternative modelling of mastitis data is an important analyses with traditional linear methods on the area of research, and development of test-day models observable scale range from 0.001 to 0.06, with most for longitudinal binary response (Rekaya et al.,
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values in the interval 0.02–0.03 (Lindstrom and 1998) is one alternative that may be used to improve ¨ ¨
Syvajarvi, 1978; Philipsson et al., 1980; Solbu, 1984; modelling of field records of clinical mastitis. ¨ ¨
Jensen et al., 1985; Syvajarvi et al., 1986; Madsen et Another development is the joint analysis of data al., 1987; Emanuelsson et al., 1988; Koenen et al., across Nordic countries which should reveal the
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1994; Lund et al., 1994; Poso and Mantysaari, 1996; genetic correlation between countries and thus to Sander Nielsen et al., 1996; Heringstad et al., 1997; what degree clinical mastitis is the same trait in Luttinen and Juga, 1997). Heritability estimates of different Nordic countries (Ruane and Klemetsdal, clinical mastitis from analyses with threshold models 1996). A smaller correlation than for milk yield on the underlying scale are higher, ranging from 0.06 would be a first indication that such traits, genetic-to 0.12 (Simianer et al., 1991; Lund and Jensen, ally are more different between countries than milk 1996; Heringstad et al., 1997). traits are.
and 0.83 with progeny groups of 100, 200 and 300 Swanson (1996) found a weighted average heritabili-respectively. In addition, the genetic standard devia- ty for first lactation SCC of 0.11 (1/2 0.04). tion of mastitis resistance is reasonably large. For Recent heritability estimates of SCC range from 0.08 example, in Norway, daughters of the three bulls to 0.19 (Lund and Jensen, 1996; Sander Nielsen et
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with the worst index values for mastitis resistance al., 1996; Poso and Mantysaari, 1996; Boettcher et after progeny testing in 1995 had twice the mastitis al., 1997; Boichard and Rupp, 1997; Luttinen and
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frequency (35%) of daughters of the three bulls with Juga, 1997; Pryce et al., 1997a; Poso et al., 1997). the best index values (18%) (Steine, 1996b). The efficiency of SCC as a selection criterion to
Thus, effective direct selection for mastitis resist- reduce the frequency of clinical mastitis depends on ance can be expected as long as proper recording and the genetic correlation between the two traits, and a sufficiently large daughter groups are used for wide range of values have been cited in the litera-progeny testing. This has recently been demonstrated ture. Estimates vary from close to zero (Coffey et al., by Steine (1998), who observed a 5% reduction in 1986) to close to unity (Lund et al., 1994). Other mastitis frequency among daughters of bulls with the estimates based on Nordic field data vary between best estimated breeding values for mastitis compared 0.3 and 0.8, with an average of 0.60 (Madsen et al., to daughters of bulls with the best estimated breeding 1987; Emanuelsson et al., 1988; Philipsson et al.,
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values for milk yield. 1995; Lund and Jensen, 1996; Poso and Mantysaari, 1996; Sander Nielsen et al., 1996; Luttinen and Juga, 4.2. Indirect selection using somatic cell counts 1997). Mrode and Swanson (1996) concluded that the average genetic correlation between clinical Somatic cells consist of many types of cells, mastitis and SCC, based on values from the litera-including neutrophile leukocytes, macrophages, lym- ture, was roughly 0.7.
use of simple selection index theory shows that the as a selection criterion based on the role certain genetic correlation between the two traits has to be somatic cells have in defence against udder patho-greater than 0.70, 0.77, 0.82, 0.85 and 0.89 with a gens. Coffey et al. (1986) refer to results suggesting progeny-group size of 50, 100, 150, 200 and 300 that selection for decreased SCC may reduce the respectively for indirect selection on SCC to be more cows’ ability to respond to infection. For cows with effective than direct selection on clinical mastitis. a low SCC it is not clear that reducing SCC further Thus, selection on SCC alone appears to be less will reduce mastitis (Kehrli and Shuster, 1994). effective than selection directly on clinical mastitis. Some observational results (Erskine et al., 1988) A more thorough analysis would require stochastic show that herds with SCC ,150 000 had more simulation as there are some problems with applying clinical mastitis than herds with SCC.700 000. simplistic formulae for predicting genetic response Miltenburg et al. (1996) also found a significant with binary traits (Foulley, 1992). higher incidence of clinical mastitis in herds with Currently, Denmark uses SCC as an additional low SCC (,150 000) than in herds with SCC. source of information in a multi-trait model to 250 000.
increase the accuracy of breeding value estimation Even though linear relationships have been found for mastitis (Interbull, 1996). In Finland and between sire evaluations for SCC and clinical mas-Sweden, single trait evaluation is carried out for both titis (Philipsson et al., 1995, Rogers et al., 1998), by SCC and clinical mastitis and both traits are then a method with low power of detecting non-linear weighted in the total merit bull index (Eriksson, relationships, there is still uncertainty about the
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1991; Interbull, 1996; Poso and Mantysaari, 1996). effect of further reductions in SCC already at low In Norway, calculations using selection index theory levels. The relationship between sire evaluations for have shown that including SCC will not improve the the two traits, presented by Rogers et al. (1998), accuracy of breeding value estimation for mastitis covers only the range in which most of the sires resistance because daughter-group size are larger in occur, and the authors point out that extrapolation Norway than in the other Nordic countries (Steine, beyond this range should be done cautiously. A T., unpublished results), and SCC has thus not been further reduction of a low SCC by genetic selection used for genetic evaluation, although it is recorded. may impair the cows’ innate immune system, as In some non-Nordic countries where direct selection Schukken et al. (1994) found that cows that resisted on mastitis resistance is not an option, SCC is infection had a higher SCC prior to S. aureus included in the sire evaluation procedures as an challenge than cows that became infected. This may indirect measure of mastitis resistance (Interbull, indicate that a very low SCC is not optimal and that
1996). optimal udder health will not necessarily occur at the
of the same size as mastitis from longer sampling dairy cattle population (Ruane and Klemetsdal, periods (Heringstad et al., 1999). 1996).
The national health recording systems in the Nordic countries are valuable data banks that can be 5. Conclusions and prospects for the future used for further investigating the genetic background of mastitis, and are of importance to both the Nordic Although the four Nordic countries (Denmark, and international communities alike (Ruane et al., Finland, Norway and Sweden) record information on 1997). Continued research is necessary to make both clinical mastitis and SCC, different approaches selection for mastitis resistance more effective. are used in breeding for increased mastitis resistance. Documentation of genetic trend for clinical mas-Norway, with large daughter groups, only includes titis in populations under long-term selection and the information on clinical mastitis to improve mastitis relationship between clinical mastitis and other im-resistance. Denmark utilises SCC as an additional portant traits are needed. Improved modelling of source of information for estimating breeding values clinical mastitis data, to make better use of in-for mastitis, while Finland and Sweden consider both formation available in the data, can contribute to traits in the breeding goal. more effective selection for mastitis resistance.
When only utilising information on clinical mas- Detection of possible QTLs for mastitis resistance titis in breeding, one is selecting for the resultant of may make marker assisted selection a complemen-all biological processes that improve mastitis resist- tary or alternative selection strategy.
ance. With SCC, the situation is different as a high value is indicative of a diseased udder while a low value is not necessarily an indicator of a healthy
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