Economic impact of staphylococcal and mammaliicoccal subclinical mastitis in dairy herds from Northeast Brazil

Abstract

This study aimed to assess the impact of S. aureus as well as Non-aureus Staphylococci and Mammaliicocci (NASM) subclinical mastitis-causing in the economic return in dairy herds. Data were gathered from the databases of five dairy herds located in the Agreste region of Pernambuco state, Northeastern Brazil, over a period of three consecutive months. A total of 155 mammary quarters from 155 healthy cows were categorized into the healthy group. Meanwhile, 257 mammary quarters from 187 cows that tested positive for either S. aureus or NASM were categorized into the infected group. The effect of mammary quarter infection in economic return was estimated using milk payment criteria on milk samples from healthy vs. infected cows based on a linear mixed model. Milk yield and milk price influenced the economic return, and both varied according to factors like herd, parity, stage of lactation, period of analysis, and the type of pathogen causing subclinical mastitis. There was a reduction in the average economic return caused by NASM infection (by 0.41 to 0.65 US$/cow/day) and S. aureus infection (by 0.25 to 0.36 US$/cow/day), when considering the effect of the pathogen over time for 1 and ≥ 2 infected quarters. Although no significant differences were observed in economic return between healthy cows and those infected with NASM or S. aureus, it is important to collect data on these pathogens for a more precise assessment of the economic impact of subclinical mastitis and for developing enhanced approaches for prevention and control.

Introduction

Globally, mastitis is widely acknowledged as the primary cause of financial losses in dairy herds due to its costly nature [1]. Subclinical mastitis adversely impacts farm profitability by reducing milk production and quality [2, 3]. Among the genus Staphylococcus, Staphylococcus (S.) aureus is the most frequent species related to bovine subclinical mastitis [4] and it leads to an increase in milk somatic cell count (SCC) [5], and a decrease in milk yield and economic return [6–8]. In the state of Pernambuco, Brazil, 29% of bovine subclinical mastitis cases have been associated with S. aureus [9]. However, the occurrence and potential effects of other Staphylococci species remain uncertain.

In the past decade, Non-aureus Staphylococci and Mammaliicocci (NASM) [10], including S. chromogenes [11] and M. sciuri [12], have increasingly been recognized as important bovine subclinical mastitis pathogens [13]. For instance, previous studies have already established associations between NASM subclinical mastitis-causing and a potential to increase SCC [14], to reduce milk yield [15, 16] or milk quality [11].

Different methods have been used to evaluate the financial impacts of subclinical mastitis on dairy herds [2, 17–20]. The predominant approach involves the analysis of somatic cell count (SCC) to estimate the reduction in milk production associated with subclinical mastitis. There have been few studies assessing the impact of specific pathogens in economic return. Combining SCC data with pathogen-specific data would provide the best approach to determining the effect of specific pathogens on milk quality, production, and economic return. By using these loss estimates, dairy producers can be stimulated to improve mastitis control measures and optimize mammary gland health and milk production.

It is noteworthy that the economic effects of bovine mastitis should be calculated at the farm or herd level, considering local, regional, epidemiological, and economic factors [2]. Subclinical mastitis is highly prevalent in the state of Pernambuco, Brazil [21]. This is a concern given that most dairy farms in Pernambuco produce milk primarily for family subsistence, compromising milk quality and production. Nonetheless, there is a clear lack of studies estimating the effects of mastitis on milk quality and its economic impact in the state of Pernambuco.Therefore, this study aimed to estimate the impact of staphylococcal and mammaliicoccal subclinical mastitis in the economic return in dairy herds located in Pernambuco state, Northeastern Brazil.

Materials and methods

Ethical approval

This is a descriptive study conducted in agreement with farm owners and approved by the Ethics Committee on the Use of Animals (CEUA) of Federal Rural University of Pernambuco (UFRPE), Recife, Brazil (Protocol No. 5100110120), and the Research Ethics Committee of the University of Pernambuco (UPE), Recife, Brazil (CAAE number: 27859120.8.0000.5207).

Herds and cows

A non-probabilistic convenience sampling approach was used to select herds for this study, focusing on those located within a convenient driving distance from the laboratory to facilitate milk analysis. To be eligible for inclusion, herds had to meet specific criteria: they needed to have permanent cow identification and data recording systems in place, and they were required to implement a mastitis control program consistent with guidelines from the National Mastitis Council (NMC; http://www.nmconline.org) [22]. This program included consistent use of pre- and post- milking teat dipping, use of dry cow therapy, routine maintenance of milking machines, and adherence to proper milking and intramammary treatment procedures.

Data from each dairy herd database from five dairy herds were collected for three consecutive months located in the Agreste region of Pernambuco State, Northeastern Brazil. The farm owners were contacted and interviewed to assess their farm management practices and gather information about the lactating animals, such as udder health, daily milk yield and SCC.

Database of this longitudinal study, conducted from November 2020 until February 2021, included information on daily milk yield and daily SCC on all lactating dairy cows (n = 611). Prior to inclusion, all selected lactating dairy cows were confirmed to be healthy, showing no signs of clinical or subclinical mastitis. All cows were crossbred Holstein-Gir cows (Bos taurus taurus-Bos taurus indicus), housed in free-stall barn facilities, and milked twice a day in parlours.

Microbiological analysis

Sterile milk samples collection for S. aureus and NASM isolation was conducted in the second month of the study, in accordance with the National Mastitis Council guidelines, with minor adaptations [22]. Bacterial isolation was performed using Mannitol Salt Agar (Difco Laboratories Inc., Detroit, USA) enriched with 5% v/v egg yolk emulsion (HiMedia, Mumbai, India) and incubated aerobically at 37ºC (± 1 °C) with readings after 24, and 48 h to identify colony morphology. All colonies of S. aureus and NASM were cultured again using mannitol salt agar to obtain a greater number of bacteria for bacterial species identification using MALDI-TOF MS. Based on microbiological culture and pathogen identification, cows were grouped into three categories: healthy, subclinical mastitis caused by NASM or S. aureus.

It should be pointed out that certain cows (n = 269) were excluded from the analysis based on the following criteria: cows that developed clinical mastitis during the experimental period (n = 127 cows), cows with at least one non-functional quarter (n = 59 cows), cows with negative bacterial culture but SCC of ≤ 200 × 10³ cells/mL over the duration of the experiment (n = 48 cows), cows with incomplete or missing data (n = 19), and infections in multiple quarters of a cow’s udder, each caused by different Staphylococcus or Mammaliicoccus species (n = 16 cows; e.g., one quarter infected by S. aureus and another quarter infected by NASM, or quarters infected by different NASM species) were not included in the analysis. These exclusions were made to ensure the integrity and specificity of the analysis.

Bacterial species identification using MALDI-TOF MS

For bacterial species identification using MALDI-TOF MS, a single colony was applied to the MALDI-TOF steel plate spot with a disposable loop, as described by Barcelos et al. [23]. A volume of 1.0 µl of formic acid (70%) was applied to the spot and allowed to dry at room temperature. After drying, 1.0 µl of α-cyano-4-hydroxycinnamic acid (HCCA) matrix solution was applied, and again left to dry at room temperature for 5 to 10 min. A standard protein solution (Bacterial Test Standard, BTS; Bruker) was used for calibration. The analysis employing the MALDI-TOF mass spectrometry methodology was performed in FlexControl 3.4 software (Bruker Daltonik, Bremen, Germany). The spectral data processing was done using the MALDI Biotyper 4.1.70 (Bruker Daltonik, Bremen, Germany) computer software for microorganism identification (MBT version 7311 MPS library) and a score of ≥ 2.0 indicated a species-level identification.

Economic calculations

The economic calculation of milk price and returns was based on Gonçalves et al. [8]. The milk price (MP) per liter was simulated using payment programs (DPA/Nestlé - Fonterra^®). The Brazilian base milk price was set at US$ 0.33/L (R$ 1.54/L ± 0.42), considering price data from the previous 5 years, 2018 to 2022 [24]. After these preliminary calculations, the final milk price (MPfi), considering the quality payment based on SCC (x10³ cells/mL) at the cow level, was calculated by adding the Brazilian base milk price to the adjustments for quality premiums or penalties. Data collection for milk yield (MYi) and SCC was conducted over a period of three consecutive months, with a 30-day interval between each collection. Daily economic return (Ri, expressed as R$/cow/day) at the cow level was calculated as Ri = MPfi × MYi, where: MPfi is the final milk price per liter (R$/L) and MYi is daily milk yield (L/cow/day) from the cow i (Gonçalves et al. [8]). Daily economic returns were used to calculate economic returns for each month over three consecutive months (Ri₁, Ri₂, Ri₃, expressed as R$/cow/month). The Ri was converted to US$ dollar (1 US$ ≡ R$ 4.66) and is referred to as US$/cow/day.

Database

A subset database was created to store the records of cows including the following information: herd (1, 2, 3, 4, and 5); stage of lactation (early: < 100 days, mid: 100–200 days; and late: > 200 days); parity (1: primiparous; and ≥ 2: multiparous); milk yield (L/cow/day); SCC at the cow level over three consecutive months (low: ≤ 200 × 10³cells/mL; or high: > 200 × 10³cells/mL); economic return (US$/cow/day); category (healthy cow: no pathogen detected by culture and three consecutive SCC ≤ 200 × 10³cells/mL; NASM: a cow that had at least one mammary quarter with NASM isolation, same species, without the presence of any other concurrent Staphylococcus or Mammaliicoccus species in the same quarter or other quarters of the udder; or S. aureus: a cow that had at least one mammary quarter with S. aureus isolation, without the presence of any other concurrent Staphylococcus or Mammaliicoccus species in the same quarter or other quarters of the udder); and number of infected quarters by category within cow udder (0: healthy; 1: a cow of a specific category with only one quarter infected; and ≥ 2: a cow of a specific category with more than one quarter infected).

Statistical analysis

The impact of subclinical mastitis caused by S. aureus or NASM in the economic return was evaluated using a linear mixed model (PROC MIXED) using SAS version 9.4 (SAS Institute, Cary, NC). For all statistical analyses, significance was declared at P ≤ 0.05 and trends at P ≤ 0.1. The following statistical model was used:

where Return_jklmn was the dependent variable (US$/cow/day); µ was the overall mean; H was the herd (i = 1 to 5) that was considered as random effect; SL was the stage of lactation (early, mid, late; j = 1 to 3) as a classification effect in the model; P was the parity (primiparous, multiparous; k = 1 to 2) as a classification effect in the model; category was the state of subclinical mastitis (healthy, NASM, S. aureus; l = 1 to 3); Period_m was the three consecutive months (m = 1 to 3); Period_m × Category_l was the interaction between the fixed effects of the analysis period (e.g. data from each of the three months) and category; C_n(H_iSL_jCategory_l) was the fixed effect of cow (C_n) nested within herd, stage of lactation and category; and e_ijklmn was the random error term.

Our model allowed us to evaluate the impact of subclinical mastitis in economic return over the three months (R₁, R₂, and R₃) using two different approaches: (1) comparisons of economic returns within category over time (e.g., the economic return for individual cows infected by S. aureus was compared across the three months) as well as (2) comparison of economic returns between category was conducted only for the second month i.e., the month in which microbiological culture was conducted (e.g., R₂ of healthy cows versus R₂ of cows infected by S. aureus). In approach 2, we compared the mean differences of R₂ between two sets of data (Set A–Set B): (A) 155 healthy cows and (B) 187 cows infected by S. aureus (n = 118) or NASM (n = 69). The mean differences between these two sets were referred to as deltas (Δ). The deltas were calculated using the same dataset and linear mixed models as described previously, providing similar results.

Results

Frequency of S. Aureus and NASM isolation at the mammary quarter level

A total of 155 cows were categorized as healthy and assigned to the negative control group. Meanwhile, 257 mammary quarters from 187 cows, which showed isolation of either S. aureus or NASM, were categorized as the infected group (Table 1). It’s important to note that 269 cows were excluded from the analysis based on the exclusion criteria as described in methods - Microbiological analysis.

Table 1.

Herd wise distribution of healthy and infected lactating cows enrolled in the study

Herd	Location (city)	No. lactating cows	Healthy	Infected
1	Garanhuns	229	77	68
2	Aguas Belas	98	25	16
3	Tupanatinga	56	13	12
4	Canhotinho	121	18	49
5	Canhotinho	107	22	42
	Total	611	155	187

A total of 412 mammary quarters were used in the analysis, consisting of 155 quarters from 155 healthy lactating cows and 257 quarters from 187 infected lactating cows. Of all mammary quarters analysed (n = 412), the most frequently isolated bacteria were, S. aureus (42.72%; n = 176), followed by S. chromogenes (13.59%; n = 56), S. xylosus (2.18%; n = 9), S. haemolyticus (0.97%; n = 4), S. gallinarum (0.73%; n = 3), M. sciuri (0.73%; n = 3), S. hyicus (0.49%; n = 2), S. saprophyticus (0.49%; n = 2), S. epidermidis (0.24%; n = 1), and S. simulans (0.24%; n = 1).

The data revealed varying patterns of S. aureus and NASM infection within cow udder, including cases where one or multiple quarters were infected. Interestingly, more than 50% of subclinically infected cows had a single mammary quarter infected by S. aureus (74 quarters) and NASM (55 quarters). Among cows that had more than one quarter infected, 34.3% had two infected quarters (66 quarters by S.aureus; 22 quarters by NASM) while the remaining 9.4% had three infected quarters (24 quarters by S. aureus) and only 6.3% had all quarters infected by the same pathogen (12 quarters by S. aureus and four quarters by NASM).

As shown in Table 2, among cows that had only one quarter infected by S. aureus, 27% were primiparous which 10.8% occurred in early lactation, 9.5% in mid and 6.7% in late while 73% were multiparous cows, being 23% in early lactation, 23% in mid and 27% in late. When primiparous cows had ≥ 2 quarters infected by S. aureus, late lactation represented 21.5%, 6.9% in mid and 13.8% in early whereas 27.4% of multiparous cows had infection in early lactation, 13.8% in mid and 16.6% in late.

Table 2.

Proportion of cows infected with S. Aureus and NASM among primiparous and multiparous cows based on stage of lactation (early, mid, or late) and number of infected mammary quarters

Variables	Primiparous (%)		Multiparous (%)
	Single quarter	≥ 2 Quarters	Single quarter	≥ 2 Quarters
S. aureus Infection
early lactation	10.8	13.8	23	27.4
mid lactation	9.5	6.9	23	13.8
late lactation	6.7	21.5	27	16.6
%	27	42.2	73	57.8
NASM Infection
early lactation	1.8	15.3	23.6	23.1
mid lactation	20	7.7	20	15.4
late lactation	21.8	30.8	12.8	7.7
%	43.6	53.8	56.4	46.2

% = proportion of cows falling into each category

As further illustrated in Table 2, among primiparous cows with only one quarter infected by NASM, 1.8% were in early lactation, 20% in mid and 21.8% in late, accounting for 43.6% of all NASM infections. The remaining of 56.4% were multiparous cows which the frequency of NASM infection was higher in early lactation (23.6%) followed by mid (20%) and late (12.8%). The frequency of cows with ≥ 2 quarters infected by NASM was 53.8% in primiparous and 46.2% in multiparous. Higher frequency of NASM infections were observed in primiparous late lactation cows (30.8%) than mid (7.7%) and early (15.3%). However, the frequency of multiparous cows that had ≥ 2 quarters infected by NASM was 23.1% in early lactation, 15.4% in mid and 7.7% in late.

Table 2 shows the distribution of infections based on parity (primiparous vs. multiparous), the number of infected mammary quarters (single quarter vs. multiple quarters), and the stage of lactation (early, mid, or late).

In summary, primiparous cows tended to experience more severe infections (involving more quarters) as lactation progressed, especially with NASM. In contrast, multiparous cows generally had higher initial infection rates with S. aureus, especially in cases where multiple quarters were infected during early lactation. However, the severity of infections (number of affected quarters) typically decreased as lactation advanced. For NASM infections in multiparous cows, the infection rate in single quarters remained more consistent, while multiple quarter infections decreased significantly by late lactation.

The impact of S. Aureus and NASM in economic return

Milk yield and SCC within categories.

Healthy cows produced in average 18.3 L (± 1.2), NASM-infected cows 17.7 L (± 1.9) and S. aureus-infected cows 17.1 L (± 1.4). Our control group of healthy cows had 75.5 × 10³ cells/mL (± 8.7) while cows infected by NASM had 523.6 × 10³ cells/mL (± 272.1), and those with S. aureus had 1,065.9 × 10³ cells/mL (± 457.8). As expected, milk yield varied across stages of lactation, being higher in multiparous cows than in primiparous ones, and was lower in infected cows compared to healthy ones. Only primiparous cows in the mid-lactation, subclinically infected by NASM and S. aureus, had SCC < 250 × 10³ cells/mL (Fig. 1 - A to C).

Fig. 1.

Milk yield (L/cow/day) and SCC (×10³ cells/mL) of primiparous and multiparous cows by stages of lactation from three categories included in the analyses: (A) healthy, (B) NASM, and (C) S. aureus

Approach-1: comparisons among R1, R2, and R3 within specific category over time

This approach enabled the comparison of the economic returns of the same cows within specific category (healthy cow, NASM subclinical mastitis, S. aureus subclinical mastitis) over three months. In our definition of subclinical mastitis, a cow was considered to have an infection starting from the moment the pathogen was detected which occurred in the second month. Consequently, it was expected that R₁ (the first month’s return) would be greater than R₂ as well as R₃ and R₂ to be greater than R₃, due to the pathogen’s adverse effect on the mammary tissue over time. In this study, NASM infections reduced the average economic return by 0.41 to 0.65 US$/cow/day, while S. aureus infections led to a reduction of 0.25 to 0.36 US$/cow/day. Notably, the decrease in economic return was more significant in cows with infections in two or more quarters, as detailed in Table 3.

Table 3.

Comparisons among economic returns within specific categories over time

Variables	NASM			S. aureus
Economic return and differences over the periods	All cases	Cows with only 1 quarter infected	≥ 2 infected quarters	All cases	Cows with only 1 quarter infected	≥ 2 infected quarters
No. of quarters	81	55	26	176	74	102
R1	6.06 (± 0.2)a	5.82 (± 0.3)a	6.47 (± 0.5)a	5.64 (± 0.2)a	5.54 (± 0.2)a	5.77 (± 0.3)a, b
R2	5.41 (± 0.2)b	5.42 (± 0.3)b	5.28 (± 0.5)b	5.76 (± 0.2)a	5.56 (± 0.2)a	5.96 (± 0.3)a
R3	5.65 (± 0.2)b	5.61 (± 0.3)b	5.65 (± 0.5)b	5.40 (± 0.2)b	5.29 (± 0.2)a	5.53 (± 0.3)b
Δ1 (R1-R2)	0.65*	0.40*	1.19*	-0.11	-0.02	-0.18
Δ2 (R1-R3)	0.41*	0.21*	0.81*	0.25*	0.25	0.25
Δ3 (R2-R3)	-0.25	-0.18	-0.38	0.36*	0.27	0.43*

^a Variables were represented in average and standard error mean ( ± )

^b Values per variable within a column with different lowercase letters differ significantly at P < 0.05

* Represents the difference amongst Rs statistically significant

NASM = non-aureus Staphylococci and Mammaliicocci

Δ represents the pairwise comparison between economic returns (R) over three consecutive months (R1, R2 and R3)

Confounders included in the model and their respective significance levels were as follows: herd (P < 0.0001), stage of lactation (P = 0.036), parity (P < 0.0001), and the three-month analysis period (P < 0.0001)

Approach-2: comparisons of R2 among categories

This approach enabled the comparison of economic returns, specifically in the month when subclinical mastitis was detected (R₂), between cows categorized as healthy and those categorized as infected. No significant differences were observed in the economic returns when comparing healthy cows with those infected by NASM or with cows infected by S. aureus. However, there was a trend (P < 0.10) indicating that infected cows had a reduction of 0.35 US$/cow/day in economic returns compared to healthy ones (Table 4).

Table 4.

Comparisons of economic returns, as measured in the month when subclinical mastitis was detected (R₂), among different categories

Variables	Dataset	Healthy	NASM	S. aureus	P-value
No. of quarters		155	81	176
R2	All cases	5.69a	5.41a	5.76a	0.15
ΔH−I			0.29	-0.06
R2	1 quarter	5.66 a	5.42a	5.56a	0.12
ΔH−I			0.23	0.10
R2	≥ 2 quarters	5.64a	5.28a	5.96a	0.78
ΔH−I			0.36	-0.32
Variables	Dataset	Healthy	Infected (NASM & S.aureus )		P-value
No. of quarters		155	257
R2	All cases	6.00a	5.65a
ΔH−I			0.35		0.09

^a Variables were represented in average and standard error mean ( ± )

NASM = non-aureus Staphylococci and Mammaliicocci

Δ represents the adjust values of healthy quarter minus infected; H = represents the healthy quarters; I = represents the infected quarters

Confounders included in the model and their respective significance levels were as follows: herd (P < 0.0001), stage of lactation (P = 0.036), parity (P < 0.0001), and the three-month analysis period (P < 0.0001)

Discussion

Analyzing the prevalence of Staphylococcal and Mammaliicoccal infections and quantifying the associated losses in each dairy herd is an essential step to assess the economic impact of subclinical mastitis [16, 18]. By comparing mammary quarters of healthy and infected cows, as well as quarters of cows infected by S. aureus or NASM species, we were able to evaluate the impact of subclinical mastitis in the economic return, calculated as cow milk yield × milk price.

Considering the three-month analysis period in each herd, mammary quarters of multiparous and primiparous cows infected by S. aureus or NASM, mainly in early and late stages of lactation, decreased milk yield and increased SCC when compared with quarters of healthy cows. The findings reported here suggest that the economic return is likely influenced by particularities of the pathogens (Staphylococcus or Mammaliicoccus species) identified during the research period [25–27].

Our study primarily focused on identifying subclinical Staphylococcal and Mammaliicoccal infections for economic calculations. However, it’s important to acknowledge that cows categorized as infected may also harbor other mastitis pathogens. Taking this broader perspective into account, we recognize that the observed increase in SCC and decrease in milk production could be influenced by the presence of unidentified pathogens beyond our primary focus. This might have led to an overestimation of the effect of S. aureus and NASM.

In a study conducted Gonçalves et al. [8], it was observed that the economic return decreased when comparing contralateral healthy vs. infected mammary quarters. Overall, our study found no statistically significant difference in the economic return measured in the second month among the three categories of cows: healthy quarters, S. aureus infected quarters, and NASM infected quarters.

S. aureus was the most predominant pathogen, a trend consistent with findings from other Brazilian dairy herds [6, 28, 29]. This is concerning because quarters affected by S. aureus subclinical mastitis usually lead to economic losses associated with decreased milk yield [30].

Although NASM species are not as common as S. aureus, they still have a significant economic impact [26]. Historically, this group of pathogens has been managed as a minor group [16]. The effect of NASM subclinical mastitis on milk yield remains inconclusive [31]. Nevertheless, certain studies have suggested a potential negative impact of NASM mastitis on milk yield [32, 33]. In our study, we found that cows infected with NASM had lower milk yield compared to healthy cows, along with higher SCC. This finding underscores the significance of recognizing and addressing the impact of NASM in the dairy industry.

The present study raises the possibility that the predominance of S. aureus and NASM species in cases with only one infected quarter could be attributed to the specific characteristics and virulence of these bacteria [25, 26]. Factors such as teat condition, milking hygiene, and the cow’s immune response also play a significant role in the likelihood of infection [34]. Additionally, in the study by Gonçalves et al. [8], economic losses were found to be high in cases of subclinical mastitis caused by S. aureus at the mammary quarter level (US$ 0.26 per quarter per milking), highlighting its economic impact. Furthermore, it is important to note that our results, at the cow level, suggest a more noticeable decrease in economic return in cows with two or more infected quarters. This emphasizes the economic impact of Staphylococcal and Mammaliicoccal subclinical mastitis in dairy cows and underscores the critical need for effective management strategies to minimize losses.

Surprisingly, we found that NASM infection in primiparous cows occurred primarily in late lactation. This contrasts with the suggestion by De Vliegher et al. [35] that mastitis in primiparous cows is more common in early lactation. There are several possible explanations for our results. These may include higher stress levels experienced by cows during their first lactation, increased exposure to pathogens over the course of lactation, or significant metabolic and physiological changes during the transition period [34, 36]. These observations highlight the different pathogen profiles and infection patterns in subclinical mastitis based on the parity of the cows. Furthermore, accurate interpretation of milk production and associated impact in economic return requires the consideration of both parity and stage of lactation of each animal.

The SCC values are commonly employed for diagnosis of subclinical mastitis and establishment of milk quality standards [37]. Gonçalves et al. [18], reported a significant increase in milk loss as the average SCC increased during lactation. According to Martins et al. [7], lactating cows with subclinical mastitis have higher SCC, lower milk yield and altered milk composition. As described by Taponen and Pyörälä [38], S. aureus usually remains persistent in the mammary gland and increases SCC, whereas NASM tends to cause only a moderate increase in SCC. It appears contradictory that S. aureus, despite leading to a higher SCC, results in lower economic loss compared to NASM in the current study. However, we posit that using data from three consecutive months may underestimate the impact caused by S. aureus, as this pathogen tends to persist and cause more damage to the mammary tissue over time. This could be acknowledged as a limitation of our study.

Overall, the economic return was dependent on milk yield and milk price, which varied across different factors including herd, parity, stage of lactation, period of analysis, and the type of pathogen causing subclinical mastitis. S. aureus and NASM species showed variation across farms, parities, and stages of lactation. While there were no significant differences in economic returns between healthy cows and cows infected with either NASM or S. aureus, gathering data on these pathogens could enable a more precise assessment of the economic impact of subclinical mastitis and help in developing targeted strategies for prevention and control.

Authors contribution

Amanda Silva: Conceptualization, Methodology, Writing- Original draft. Juliano Gonçalves: Methodology and Writing- Reviewing and Editing. Marcos Veiga: Methodology and Writing- Reviewing and Editing. Rodolfo Peixoto: Writing- Reviewing and Editing. Roger Cue: Writing- Reviewing and Editing. Rinaldo Mota: Supervision, Writing- Reviewing and Editing, Visualization, and Investigation.

Funding

This work was supported by the Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE) [grant number IBPG-1809-5.05/19] and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) [Finance Code 001].

Declarations

Ethics approval

This study was approved by the Ethics Committee on the Use of Animals (CEUA) of Federal Rural University of Pernambuco (UFRPE), Recife, Brazil (Protocol No. 5100110120).

Competing interests

The authors have no competing interests to declare that are relevant to the content of this article.