Mammary Gland Health of Dairy Cattle

The global dairy industry has dramatically changed in the past quarter century. Volatile milk prices, increasing herd size and reliance on hired labor, more complex feeding and housing systems, and increased pressure for environmental stewardship have fostered greater diversity among dairy enterprises.

Economic Impact of Mastitis of Dairy Cattle Decreased milk production in affected mammary glands is the primary economic loss from mastitis. Inflammation causes pathologic changes in milk-secreting epithelial cells that decreases functional capacity. Depending on the type of pathogen, losses may continue into subsequent lactations. Most infections result in local inflammation or relatively mild clinical cases.^5,6 However, chronic infections can lead to agalactia in the affected quarter or, in the case of severe clinical cases, profound systemic involvement that occasionally results in death.

Somatic cell counts (SCCs) are indicators of leukocyte migration into milk that primarily occurs from infection of the mammary gland. Bulk tank somatic cell counts (BTSCCs) are strongly correlated with the proportion of infected cows within a dairy herd and are used as a measure of milk quality. Substantial economic loss may be incurred when regulatory or processor limits for BTSCCs are exceeded. Although the Pasteurized Milk Ordinance sets the BTSCC limit at 750,000 cells/mL for Grade A milk in the United States, in 2012 the U.S. Department of Agriculture (USDA) required that dairy exports to the European Union (EU) originate from farms with BTSCCs of 400,000 cells/ mL or less. Because of this mandate and to be competitive for global dairy markets, 400,000 cells/mL has become the de facto limit for many milk processors, who may refuse milk from dairy operations that do not meet this standard. In addition, many producers are paid monetary premiums for low BTSCCs or are penalized for high BTSCCs. This may help explain why in 2015, BTSCCs in 96.0% of milk produced in the United States were less than 400,000 cells/mL; 68.8% of producers shipped milk below this limit for the entire year.⁷ In addition, the percentage of total milk shipped with BTSCCs of less than 200,000 cells/mL during 2010 to 2016 increased from 40.6% to 52.5%.

Milk from cows with subclinical mastitis is not usually withheld from the bulk tank and thus contributes to increased BTSCCs and reduced milk quality. Proteolytic and other changes in milk due to subclinical mastitis adversely affect organoleptic qualities and the shelf life of milk and milk products. BTSCCs greater than 150,000 cells/mL are associ­ated with decreased productivity per cow, shorter shelf life of fluid milk, and reduced yield of products such as cheese.^8,9 Overall, reduced milk production and quality associated with BTSCCs of 200,000 cells/mL or greater has been estimated to result in an annual economic loss of $3.1 billion to consum- ers.¹⁰ Individual cow milk yield is estimated to decrease by

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SCCs of 100,000 cells/mL or greater, resulted in higher milk losses than cows with new infections.¹⁴ In addition, DHIA records from more than 166,000 cow lactations in the western United States found that cows with a first test date SCC of 200,000 cells/mL or greater produced 718 kg less milk than cows with a first test date SCC less than 200,000 cells/mL.¹⁵

High-BTSCC milk also has a greater risk of condemnation because of violative antimicrobial residues.^16-18 In a retrospec­tive study, the rate of antibiotic residue violations increased as herd SCC increased, and the relative risk of antibiotic residue violations (compared to herds with SCCs less than 250,000 cells/mL) was 2.4, 2.8, and 7.1 times greater for herds with SCCs of greater than 400,000, greater than 550,000, and greater than 700,000 cells/mL, respectively.¹⁶

Cows with chronically increased SCCs caused by intra­mammary infections (IMIs) are often culled prematurely as a consequence of decreased milk production. The cost of premature culling depends on the perceived value of the cow, the market prices for culled cows and replacement heifers, and the potential value of the replacement heifer.¹⁹ Cows with an average SCC greater than 700,000 cells/mL were twice as likely to be culled as cows with an average SCC under 250,000 cells/mL, especially in herds with low BTSCCs.²⁰ Cows that are not culled may be segregated or require the use of milking procedures that increase labor costs.

The incidence of clinical mastitis averages between 15% and 25% of lactations, although some herds may have rates greater than 45%.^21-25 Estimates of lost milk production from a case of clinical mastitis over an entire lactation range from 110 to 552 kg, depending on the parity and stage of lactation when the case occurred.^26-28 Further losses from clinical mastitis can result from subsequent repeated episodes during the same or later lactations. In a study of 7 New York dairies, milk losses in the first 50 days following onset of clinical mastitis were 304, 128, and 92 kg for cases caused by gram-negative, gram­positive, and other pathogens, respectively.²⁹ Clinical mastitis also results in additional economic losses beyond milk produc­tion. In a 1700-cow dairy, cows with clinical mastitis lost 341 kg of saleable milk, compared to projected yield, during the first 60 days after case onset.³⁰ Nearly 75% of the milk loss was discarded because of treatment, with the remainder accounted for by lost production. Cows in their second or greater lactation, or more than 150 days in milk, incurred greater losses, although the type of causative pathogen was not associated with differ­ences in losses.³⁰ The average total costs per case (US$) of gram-positive, gram-negative, and other pathogens were estimated to be $133.73, $211.03, and $95.31, respectively.³¹ Premature culling resulting from unresolved clinical mastitis cases or subsequent suboptimal milk yield of the affected cow may increase these costs to as much as $600.³²

The occurrence of both subclinical and clinical mastitis has been associated with reduced reproductive performance. In a preliminary field study, cows were almost two times more likely to have altered interestrus intervals following an episode of clinical coliform mastitis compared to herdmates without clinical mastitis.³³ Conception rates for cows with clinical mastitis between 3 weeks before and 5 weeks after insemination have been reported to be lower compared to cows without clinical mastitis.^34,35 Clinical mastitis due to gram-negative bacteria has a more detrimental effect on the probability of conception than clinical mastitis caused by gram-positive bacteria or other organisms.³⁶ In addition, the days to first service, services per conception, and days to conception were all increased in cows with clinical mastitis as compared to nonmastitic cows.³⁷ Cows with a case of clinical mastitis in the first 45 days of gestation are nearly three times more likely to abort within the next 90 days than nonmastitic cows.³⁸ As well, cows with increased SCCs (>150,000 cells/mL) have decreased conception rates as compared to cows with lower SCCs, and as the SCC increases, the odds of pregnancy decrease.^39-41 It is likely that inflammatory mediators released in response to IMI play a role in oocyte and embryonic degradation.

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in the first trimester of pregnancy.^40,42

The total cost of mastitis is impossible to quantify accurately, varies over time, and is herd dependent. Nationally, the cost of mastitis has been estimated at $200/cow/year, or $1.5 to $2 billion.¹⁹ Costs associated with implementation of mastitis control programs can be substantial. Return on investment in mastitis control is more important than the absolute cost and ranged from —$20 to $275/cow/year in a review of nine studies.^37,43 However, this return depends on the prevalence and type of mastitis in the herd, existing control measures, and producer compliance. In summary, although the costs incurred by producers to control mastitis are significant, the benefits of reducing mastitis include improved cow health and productivity, a superior raw milk supply for processors, better quality and safer dairy foods for consumers, and a more sustain­able market for dairy products globally.