1 Department of Epidemiology and Center for Molecular and Clinical Epidemiology of Infectious Diseases, University of Michigan School of Public Health, Ann Arbor, MI.
2 Center for Statistical Consultation and Research, University of Michigan, Ann Arbor, MI.
3 University of Nebraska Medical Center, Omaha, NB.
4 Department of Family Medicine, Wayne State University, Detroit, MI.
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
breast feeding; human body; lactation; mastitis; risk factors
Abbreviations: CI, confidence interval; OR, odds ratio; RR, rate ratio
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The signs and symptoms include fever of 38.5°C or greater; flu-like aches and chills; and a red, tender, hot, swollen, wedge-shaped area of the breast (3). In prospective studies of breastfeeding women, the incidence of lactation mastitis per infant nursed has been reported to be as low as 2.5 percent among United States women (1
), 24 percent among Finnish women (4
), and 27.1 percent among Australian women (5
). In a population-based study conducted in Scotland in the pre-antibiotic era, the incidence was 8.9 percent (6
).
The agents most frequently cultured are Staphylococcus aureus or coagulase-negative staphylococci (3, 7
). Several investigators (2
, 3
, 8
) suggest a fissure in the nipple as a route of infection. Other proposed routes include through the lactiferous ducts into a lobule or by hematogenous spread (2
, 3
).
As there have been few formal studies of lactation mastitis, most hypothesized risk factors are based on clinical impression. Risk factors fall into two general categories: poor breastfeeding technique (13
, 8
, 9
) and lowered immune status secondary to stress and sleep deprivation (10
). Poor breastfeeding technique may lead to poor drainage of a duct, insufficient emptying of the breast, milk stasis, and cracks or fissures of the nipple. In the one study that formally examined these factors, however, they occurred relatively infrequently: Milk stasis due to weaning or missed feeding preceded nine of the 65 infections, and in eight, the nipple of the involved breast was fissured (1
). The number of nipple fissures in the unaffected breast and in women without mastitis was not reported nor was the incidence of milk stasis in women without mastitis.
Increased stress and sleep deprivation, which are often complaints of new mothers, have also been reported as risk factors (3). In a retrospective study among participants at a breastfeeding conference, women with a history of mastitis associated their mastitis with fatigue, stress, a plugged duct, change in number of feedings, engorgement/stasis, an infection in the family, breast trauma, and poor diet (11
). These findings are suggestive only; the study was retrospective, the occurrence of these factors among women without mastitis was not reported, and the population had a strong commitment to breastfeeding.
We followed 946 breastfeeding women for the first 3 months postpartum or until they stopped breastfeeding to describe incidence of mastitis, mastitis treatment, and any associations between mastitis occurrence and hypothesized host characteristics and behaviors.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Most potential participants were contacted by telephone at 3 weeks postpartum, and their verbal consent was obtained at the time of the interview. Participants were interviewed at 3, 6, 9 and 12 weeks postpartum or until they ceased breastfeeding. After each interview, they were sent a small incentive, such as video coupons. Fifty-six women missed the first interview and were enrolled at week 6.
Interviews were conducted by using computer-assisted technology that enforced skip patterns, checked answers for appropriate ranges, and allowed for textual comments. Interviewers received 4 hours of training, were monitored periodically for adherence to the study protocol, and were retrained annually. Each participant had the same female interviewer for all four interviews. The study protocol was approved by the institutional review boards of all participating institutions.
Measuring instruments
All interviews included questions on breastfeeding habits, breast-related symptoms, and breast care, including use of breast pumps, bottle-feeding, mastitis, sleeping habits, depression, smoking and drinking habits, and household duties. Questions about the infant's birth; history of mastitis with a previous child, including date of diagnosis; other medical history; and sociodemographic information were asked at enrollment. As much as possible, subjects were asked to recall breastfeeding information on a week-by-week basis.
Case definition
Mastitis was defined by self-report of mastitis diagnosed by a health care provider. The use of a more specific definition, which included antibiotic prescription and symptoms, did not change the results, and thus, we opted to use the more sensitive definition. Because of an error in the computer-assisted interviewing program, 44 women who ceased breastfeeding sometime during the 3 weeks prior to a study interview were not asked about the occurrence of mastitis. Once the error was discovered, a validation study was conducted; this study is described in detail elsewhere (12) (Gillespie et al., University of Michigan, manuscript submitted for publication). Briefly, we recontacted all women whom the error impacted, all known mastitis cases, and a sample of the remaining participants. Recall of mastitis was found to be quite reliable and was unrelated to age, race, income, parity, history of mastitis, and study site. We detected one additional mastitis case among those in whom the error occurred. This case was included in the analysis to the extent possible.
Statistical analysis
We describe crude associations between exposures and rate of mastitis with rate ratios and cumulative incidence (risk) ratios and their approximate 95 percent confidence intervals (13). Rate ratios were calculated for time-dependent covariates, for example, breastfeeding habits, and cumulative incidence ratios for covariates that did not change, for example, marital status. Relative risks and confidence intervals were calculated using dEPID software (14
). We tested differences between groups for categorical variables with chi-square tests and for continuous variables with Student's t tests, with a two-sided significance level of
= 0.05.
We modeled the time to first mastitis as a function of baseline and time-varying covariates. Discrete-time survival analysis was used, with a time-dependent logistic regression model (15) that included site and breastfeeding variables. We estimated the association between hypothesized risk factors in the week of mastitis with mastitis risk by using rate ratios. In cases in which the hypothesized risk factor could have resulted from the mastitis or early signs of mastitis (e.g., cracks and nipple sores), we also calculated (by using the same model) the association of mastitis with the hypothesized variables in the week prior to mastitis. Because women could enter the study more than once, we analyzed the data both considering all births by a single mother as independent and taking into account clustering utilizing generalized estimating equations using the SAS program, Proc Genmod (16
). The results were very similar in both analyses, so we present them taking clustering into account.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The majority (n = 711 (75.2 percent)) of study participants were enrolled from Michigan. Michigan participants tended to be slightly older, to have more education, and to have a slightly higher income than Nebraska participants (table 1). The majority of participants at both sites were White and living with their spouse or partner. Participants from Nebraska were more likely to be first-time mothers and, if they had children, to have fewer children living with them.
|
|
|
The most common mastitis symptoms were breast tenderness (98 percent), fever (82 percent), malaise (87 percent), chills (78 percent), redness (78 percent), and a hot spot (a localized area of warmth and tenderness on the affected breast) (62 percent). Women averaged 4.9 days when they had one or more symptoms (range, 121 days). These symptoms were severe enough to cause the vast majority of women to restrict their activities at least somewhat (77 percent) and to spend at least 1 day in bed (72 percent). Almost half of the women changed their breastfeeding habits, either feeding more often (36 percent), less often (11 percent), feeding more often (49 percent) or less often (8 percent) on the affected breast, changing nursing holds more frequently (33 percent), trying a new nursing hold (12 percent), or making some other change (19 percent).
Most women with mastitis (88 percent) were prescribed one or more medications, either antibiotics (86 percent) and/or analgesics (17 percent); two woman reported receiving an antifungal medication, and another did not know the name of the drug prescribed. The most commonly prescribed antibiotics were cephalexin (46 percent), amoxicillin (7 percent), ampicillin (7 percent), and augmentin (7 percent). No cultures were performed. In addition, health care providers suggested applying hot compresses (83 percent), fully emptying the affected breast (65 percent), feeding more frequently (74 percent), and changing feeding positions often (48 percent). We did not ask whether women were advised to stop breastfeeding.
Associations of risk factors with cumulative incidence of mastitis and cracks and nipple sores
The overall cumulative incidence (risk) of mastitis among those who had never breastfed previously was 7.3 percent compared with 10.8 percent among those who had breastfed previously. For women with a history of mastitis, the risk was 23.9 percent compared with 8.3 percent among first-time mothers (rate ratio (RR) = 2.9, 95 percent confidence interval (CI): 1.79, 4.69) (table 2). Women with a history of mastitis were also more likely to report cracks and nipple sores (RR = 1.3, 95 percent CI: 1.04, 1.50). Neither the cumulative incidence of mastitis nor that of cracks and nipple sores was associated with age, income, returning to work, or participating in a breastfeeding support group (table 2). Women who reported having outside help were less likely to report cracks or nipple sores (odds ratio (OR) = 0.8, 95 percent CI: 0.71, 0.97), but having outside help was not associated with mastitis.
|
Cracks and nipple sores that occurred during the same week as mastitis were associated with an almost sixfold increase in mastitis rate for women with no history of mastitis and a threefold increase in mastitis rate for women with such a history (table 3). Women without cracks and nipple sores in the same week as mastitis had similar rates of mastitis in the right (8.8/1,000 person-weeks) as in the left (8.5/1,000 person-weeks) breast and lower rates of mastitis in both breasts at the same time (1.5/1,000 person-weeks). Women with cracks and nipple sores in one breast in the same week as mastitis had higher rates of mastitis in the breast with cracks or nipple sores (48.9/1,000 person-weeks) than in the breast without them (13.3/1,000 person-weeks). Women with cracks or nipple sores in both breasts in the same week as mastitis experienced the highest mastitis rate (88.7/1,000 person-weeks).
|
Average frequency of feeding per day, but not duration of feeding, was associated with mastitis rate; this was true whether or not the feeding habits were in the same week or the week before mastitis. Women without a history of mastitis who fed six or fewer times a day had a rate of mastitis five times lower than those who fed 10 or more times a day (same week as mastitis: RR = 0.2, 95 percent CI: 0.06, 0.46; week before mastitis: RR = 0.2, 95 percent CI: 0.07, 0.55); for women with a history of mastitis, the rate was 2.5 times lower (same week as mastitis: RR = 0.4, 95 percent CI: 0.15, 1.11; week before mastitis: RR = 0.4, 95 percent CI: 0.14, 1.19) (figure 3).
|
Among women with no history of mastitis, use of a breast pump, particularly a manual pump, resulted in an increased rate of mastitis. A history of using a breast pump for the previous week was also associated with mastitis, but the rate ratio was reduced and was no longer statistically significant; for other breast pumps, use of a pump during the previous week also noticeably reduced the association. For women with a history of mastitis, using a pump had no association with mastitis rate, with the point estimates for manual pumping suggesting a protective effect for both the same and the previous week. Other breastfeeding behaviors, including bottle-feeding, engorgement, washing breasts before or after feeding, using nursing pads, and wearing a nursing bra had little association with mastitis rate; none of the observed associations were statistically significant. Women who reported taking daytime naps had higher rates of mastitis, but, again, this result could be an effect rather than a cause of mastitis. When information from the previous week was used, there was still a modest increase in the rate for women with no history of mastitis, but it was no longer statistically significant.
Discrete survival regression analysis
To explore further the relations between variables identified in the bivariate and stratified analyses, we fit a time-dependent logistic regression model (table 4), including mastitis history, weeks since birth, and the following variables measured in the same week as mastitis: nipple sores or cracks, antifungal nipple cream, frequency of feeding, and use of a manual breast pump. Duration of feeding and daytime naps were not associated with risk of mastitis after adjustment for other variables and, thus, were not included in the model. After adjustment for covariates and clustering between responses for women who participated more than once, mastitis history, cracks and nipple sores in the same week as mastitis, and use of antifungal nipple cream in the same 3-week interval as mastitis remained positively associated with mastitis, increasing risk more than threefold. Those who fed fewer than 10 times per day in the same week as mastitis were at significantly decreased risk of mastitis, with those feeding fewer than six times per day having a risk 2.5 times lower than women feeding 10 or more times a day. Use of a manual breast pump in women without mastitis history was also significantly associated with mastitis rate. The interaction with mastitis history (p = 0.02) indicates that those with mastitis history do not share the increased risk with manual breast pump use: Using a manual breast pump increased risk of mastitis 2.1 times (95 percent CI: 1.09, 3.86) among women with no mastitis history, but had no association with mastitis among women who had experienced mastitis with a previous baby. When the model was fit using covariates from the previous week, the results were similar, although cracks and nipple sores were only marginally statistically significant (p = 0.06). The interaction between mastitis history and use of a manual breast pump was also no longer statistically significant. In both models, the time variables showed mastitis rates to be significantly higher in weeks 16 relative to weeks 1012.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The incidence of mastitis reported here is approximately half that reported from prospective cohorts in Australia (27 percent, 95 percent CI: 22, 32 percent, n = 306 (5)), New South Wales (20 percent, 95 percent CI: 18, 22 percent, n = 1,075 (17
)), or southwest Finland (24 percent, 95 percent CI: 21, 27 percent, n = 664) (4
)), but was more than three times the incidence reported in a United States cohort (2.5 percent, 95 percent CI: 1.83.0 percent, n = 2,534) (1
) . This may be explained by differences in case definition, population, temporal variation, selection biases, or other unmeasured factors. Nevertheless, there is general agreement that mastitis is a common bacterial infection among breastfeeding women.
One of the enduring myths of mastitis risk is that it results from inexperience with breastfeeding. However, in our study, women with a history of mastitisby definition, previous breastfeedershad a greater risk. Adjustment for the observed breastfeeding practices did not remove the significant effect of mastitis history. Standard breastfeeding literature recommends frequent, short feedings to build milk supply (18). While each additional 10 minutes per feeding increased risk of developing nipple fissures, cracks, or sores, longer feedings were not associated with increased incidence of mastitis. More frequent, rather than longer, feedings were associated with nipple fissures, cracks, or sores and mastitis incidence. Nipple trauma may result from latching the baby onto and removing the baby from the nipple. It is also possible that the act of latching on or removing the baby from the nipple results in increased exposure to potential mastitis pathogens on the mother's hands. Interestingly, washing the nipple either before or after feeding was not associated with mastitis incidence. We did not evaluate hand-washing practices. Nipple cracks and sores have been identified previously as mastitis risk factors (1
, 4
); cracks and sores enhance the entry of bacteria into breast tissue. Nipple cracks and sores were reported by more than one third of all participants in the first week postpartum.
The observed threefold increase in mastitis incidence with mastitis history is similar to the threefold increase found among 664 women in southwest Finland who were also followed for 12 weeks. Women with a history of mastitis may have a breastfeeding style that puts them at greater risk of mastitis; indeed, they had significantly different breastfeeding practices than do women without mastitis history. Alternatively, the increase in risk with mastitis history may reflect persistent colonization with a potential pathogen. Breast anatomy may also predispose to mastitis or skin type prone to nipple cracks or sores. Siblings may nurse with similar frequency either due to genetic predisposition or to learned behaviors on the part of the mother.
Mastitis caused considerable pain, worry, days in bed, and physician visits. Further, mastitis can lead to early weaning (5, 19
). It was frequently diagnosed and treated over the telephone. As antibiotic resistance is an increasing concern, better algorithms to distinguish between milk stasis and mastitis without the benefit of physical examination are required. Moreover, the field would benefit from controlled studies of mastitis treatment; such studies are virtually nonexistent in the general medical literature.
Our sample size was large, with very stable estimates. We had frequent, regular contact (every 3 weeks) with study participants to assess breastfeeding practices and health behaviors. We had a high initial response (99 percent) and follow-up rate (89 percent). Study participants were recruited from two sites. While Michigan women had much higher rates of breastfeeding and tended to breastfeed longer, there was no difference in mastitis incidence rate or the incidence of nipple cracks, fissures, or sores by site. Although the women in our study are not representative of all breastfeeding women because they are primarily White and middle class, the associations should be broadly generalizable, especially since we did not find any difference between sites in mastitis incidence or any associations with sociodemographic characteristics.
Mastitis is a multiagent syndrome. To the best of our knowledge, the role of specific agents has yet to be described. The most common etiologic agents, S. aureus, Staphylococcus epidermidis, and Streptococcus species, inhabit either the skin or nasopharynx but are not uniformly present among all persons. For example, an estimated 20 percent of the general population are chronic carriers of S. aureus (20). Given that risk of mastitis is independent of sociodemographic variables and site, that mastitis history is a strong predictor, and that nipple sores occur frequently in the absence of infection, agent characteristics must be an important determinant of mastitis risk. Unfortunately, this study was not designed to examine the etiologic agents. Published articles on the bacteria-causing lactation mastitis are few, and most are at least 10 years old. The frequency of these bacteria in mothers' milk and on their breasts and in the nasopharynx among women with and those without mastitis deserves attention.
Mastitis is common but poorly understood. Our study highlights how little is known about the epidemiology and pathogenesis of this common condition. The popularly reported risk factors explain very little. Future studies should describe the role of nasal and skin flora on mastitis risk.
![]() |
ACKNOWLEDGMENTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The authors thank the Providence Family Birthing Center and the Mutual of Omaha for allowing us to identify study participants. Telephone interviews were conducted by the Survey Unit at the Institute of Social Research.
![]() |
NOTES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Reprint requests to Dr. Betsy Foxman, Department of Epidemiology, University of Michigan School of Public Health, 109 Observatory Street, Ann Arbor, MI 48109-2029 (e-mail: Bfoxman{at}Umich.edu).
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Related articles in Am. J. Epidemiol.: