Serum micronutrient concentrations and decline in physical function among older persons

CONTEXT

Maintaining independence of older persons is a public health priority, and identifying the factors that contribute to decline in physical function is needed to prevent or postpone the disablement process. The potential deleterious effect of poor nutrition on decline in physical function in older persons is unclear.

OBJECTIVE

To determine whether a low serum concentration of micronutrients is associated with subsequent decline in physical function among older men and women living in the community.

DESIGN, SETTING, AND PARTICIPANTS

Longitudinal study of 698 community-living persons 65 years or older who were randomly selected from a population registry in Tuscany, Italy. Participants completed the baseline examination from November 1, 1998, through May 28, 2000, and the 3-year follow-up assessments from November 1, 2001, through March 30, 2003.

MAIN OUTCOME MEASURE

Decline in physical function was defined as a loss of at least 1 point in the Short Physical Performance Battery during the 3-year follow-up. Odds ratios (ORs) were calculated for the lowest quartile of each nutrient using the other 3 quartiles combined as the reference group. Two additional and complementary analytical approaches were used to confirm the validity of the results.

RESULTS

The mean decline in the Short Physical Performance Battery score was 1.1 point. In a logistic regression analysis that was adjusted for potential confounders, only a low concentration of vitamin E (<1.1 microg/mL [<24.9 micromol/L]) was significantly associated with subsequent decline in physical function (OR, 1.62; 95% confidence interval, 1.11-2.36; P = .01 for association of lowest alpha-tocopherol quartile with at least a 1-point decline in physical function). In a general linear model, the concentration of vitamin E at baseline, when analyzed as a continuous measure, was significantly associated with the Short Physical Performance Battery score at follow-up after adjustment for potential confounders and Short Physical Performance Battery score at baseline (beta = .023; P = .01). In a classification and regression tree analysis, age older than 81 years and vitamin E (in participants aged 70-80 years) were identified as the strongest determinants of decline in physical function (physical decline in 84% and 60%, respectively; misclassification error rate, 0.33).

CONCLUSIONS

These results provide empirical evidence that a low serum concentration of vitamin E is associated with subsequent decline in physical function among community-living older adults. Clinical trials may be warranted to determine whether an optimal concentration of vitamin E reduces functional decline and the onset of disability in older persons.

Subtypes of disability in older persons over the course of nearly 8 years

OBJECTIVES

To characterize distinct and clinically meaningful subtypes of disability, defined based on the number and duration of disability episodes, and to determine whether the incidence of these disability subtypes differ according to age, sex, or physical frailty.

DESIGN

Prospective cohort study.

SETTING

Greater New Haven, Connecticut.

PARTICIPANTS

Seven hundred fifty-four community-living residents aged 70 and older and initially nondisabled in four essential activities of daily living.

MEASUREMENTS

Disability was assessed during monthly telephone interviews for nearly 8 years; physical frailty was assessed during comprehensive home-based assessments at 18-month intervals. The incidence of five disability subtypes was determined within the context of the 18-month intervals in participants who were nondisabled at the start of the interval: transient, short-term, long-term, recurrent, and unstable.

RESULTS

Incident disability was observed in 29.8% of the 18-month intervals. The most common subtypes were transient disability (9.7% of all intervals), defined as a single disability episode lasting only 1 month, and long-term disability (6.9%), defined as one or more disability episodes, with at least one lasting 6 or more months. Approximately one-quarter (24.7%) of all participants had two or more intervals with an incident disability subtype. Although there were no sex differences in the incidence rates for any of the subtypes, differences in rates were observed for each subtype according to age and physical frailty, with only one exception, and were especially large for long-term disability.

CONCLUSION

The mechanisms underlying the different disability subtypes may differ. Additional research is warranted to evaluate the natural history, risk factors, and prognosis of the five disability subtypes.

Modeling repeated time-to-event health conditions with discontinuous risk intervals. An example of a longitudinal study of functional disability among older persons

OBJECTIVES

Researchers have often used rather simple approaches to analyze repeated time-to-event health conditions that either examine time to the first event or treat multiple events as independent. More sophisticated models have been developed, although previous applications have focused largely on such outcomes having continuous risk intervals. Limitations of applying these models include their difficulty in implementation without careful attention to forming the data structures.

METHODS

We first review time-to-event models for repeated events that are extensions of the Cox model and frailty models. Next, we develop a way to efficiently set up the data structures with discontinuous risk intervals for such models, which are more appropriate for many applications than the continuous alternatives. Finally, we apply these models to a real dataset to investigate the effect of gender on functional disability in a cohort of older persons. For comparison, we demonstrate modeling time to the first event.

RESULTS

The GEE Poisson, the Cox counting process, and the frailty models provided similar parameter estimates of gender effect on functional disability, that is, women had increased risk of bathing disability and other disability (disability in walking, dressing, or transferring) as compared to men. These results, especially for other disabilities, were quite different from those provided by an analysis of the first-event outcomes. However, the effect of gender was no longer significant in the counting process model fully adjusted for covariates.

CONCLUSION

Modeling time to only the first event may not be adequate. After properly setting up the data structures, repeated event models that account for the correlation between multiple events within subjects can be easily implemented with common statistical software packages.

Hierarchical models to evaluate translational research: Connecticut collaboration for fall prevention

BACKGROUND AND OBJECTIVE

Evidence-based second stage translational studies are necessary and difficult to evaluate. A quasi-experimental design is used to compare the rate of fall-related health care utilization of two geographically disparate areas in Connecticut, a small state in the northeastern United States, to evaluate an intervention designed to reduce fall-related injuries among older persons. This evaluation examines the two years immediately prior to intervention.

METHODS

The experimental units are postal (i.e., zip) code tabulation areas (ZCTAs) in which counts of fall-related health care utilization and demographic characteristics can be gathered from local and federal public health sources. We employ hierarchical modeling to determine whether there was a difference in fall-related health care utilization between the study arms prior to initiating the intervention. Geographic information systems are used to characterize neighboring ZCTAs and to graph model-adjusted rates of fall-related utilization.

RESULTS

After adjustment for covariates and spatial variation, we observed no significant difference between rates or temporal trends of fall-related health care utilization in the study arms over the two year pre-intervention period.

CONCLUSION

The study arms of the Connecticut Collaboration for Falls Prevention have equivalent rates and temporal trends of fall-related utilization over the two year pre-intervention period.

Bath aids and the subsequent development of bathing disability in community-living older persons

OBJECTIVES

To determine whether the availability of bath aids may forestall the subsequent development of bathing disability.

DESIGN

Prospective cohort study.

SETTING

Greater New Haven, Connecticut.

PARTICIPANTS

Five hundred one nondisabled (required no personal assistance in bathing) community-living residents aged 73 and older.

MEASUREMENTS

The availability of five bath aids (grab bars, bath seat, nonskid mat or abrasive strips, handheld shower spray, and long-handled brush or sponge) was ascertained during a comprehensive home-based assessment. Subsequently, participants were followed with monthly telephone interviews to determine the onset of persistent (i.e., present for at least 2 consecutive months) disability in bathing and were evaluated for disability in three bathing subtasks (bathing transfers, washing whole body, and drying whole body) during the next home-based assessment, which was completed 18 months after the initial assessment.

RESULTS

The presence of a bath seat was associated with a greater likelihood of developing persistent disability in bathing and disability in each of the three bathing subtasks, although these associations were not statistically significant after adjustment for potential confounders. Nonsignificantly greater risk was also observed for grab bars, handheld shower spray, and long-handled brush or sponge. In the adjusted analysis, the presence of nonskid mats or abrasive strips was associated with a 23% lower risk (nonsignificant) of persistent bathing disability and a lower likelihood of developing disability in washing and drying one's whole body, with corresponding odds ratios of 0.28 (P=.003) and 0.38 (P=.030), respectively.

CONCLUSION

In this longitudinal study, the presence of bath aids, with the exception of nonskid mats or abrasive strips, did not forestall the subsequent development of bathing disability. Because it may not be possible to fully account for the effects of self-selection, clinical trials may be necessary to demonstrate the potential value of bath aids in community-living older persons.

Motor deficits and altered striatal gene expression in aphakia (ak) mice

Like humans with Parkinson's disease (PD), the ak mouse lacks the majority of the substantia nigra pars compacta (SNc) and experiences striatal denervation. The purpose of this study was to test whether motor abnormalities in the ak mouse progress over time, and whether motor function could be associated with temporal alterations in the striatal transcriptome. Ak and wt mice (28 to 180 days old) were tested using paradigms sensitive to nigrostriatal dysfunction. Results were analyzed using a linear mixed model. Ak mice significantly underperformed wt controls in rotarod, balance beam, string test, pole test and cotton shred tests at all ages examined. Motor performance in ak mice remained constant over the first 6 months of life, with the exception of the cotton shred test, in which ak mice exhibited marginal decline in performance. Dorsal striatal semi-quantitative RT-PCR for 19 dopaminergic, cholinergic, glutaminergic and catabolic genes was performed in 1- and 6-month-old groups of ak and wt mice. Preproenkephalin levels in ak mice were elevated in both age groups. Drd1, 3 and 4 levels declined over time, in contrast to increasing Drd2 expression. Additional findings included decreased Chrnalpha6 expression and elevated VGluT1 expression at both time points in ak mice and elevated AchE expression in young ak mice only. Results confirm that motor ability does not decline significantly for the first 6 months of life in ak mice. Their striatal gene expression patterns are consistent with dopaminergic denervation, and change over time, despite relatively unaltered motor performance.

The use of missingness screens in clinical epidemiologic research has implications for regression modeling

OBJECTIVE

Properly handling missing data is a challenge, especially when working with older populations that have high levels of morbidity and mortality. We illustrate methods for understanding whether missing values are ignorable and describe implications of their use in regression modeling.

STUDY DESIGN AND SETTING

The use of missingness screens such as Little's missing completely at random "MCAR test" (1988) and the "Index of Sensitivity to Nonignorability (ISNI)" by Troxel and colleagues (2004)introduces complications for regression modeling, and, particularly, for risk factor selection. In a case study of older patients with simulated missing values for a delirium outcome set in a 14-bed medical intensive care unit, we outline a model fitting process that incorporates the use of missingness screens, controls for collinearity, and selects variables based on model fit.

RESULTS

The proposed model fitting process identifies more actual risk factors for ICU delirium than does a complete case analysis.

CONCLUSION

Use of imputation and other methods for handling missing data assist in the identification of risk factors. They do so accurately only when correct assumptions are made about the nature of missing data. Missingness screens enable researchers to investigate these assumptions.

Predisposing factors and precipitants for bathing disability in older persons

OBJECTIVES

To identify potential predisposing factors and precipitants for bathing disability.

DESIGN

Prospective cohort study.

SETTING

Greater New Haven, Connecticut.

PARTICIPANTS

Seven hundred fifty-four community-living residents aged 70 and older.

MEASUREMENTS

Potential predisposing factors were measured every 18 months for 6 years during comprehensive home-based assessments. Participants were followed with monthly telephone interviews for a median of 61 months to ascertain exposure to potential precipitants, which included illnesses and injuries leading to hospitalization or restricted activity, and to determine the occurrence of persistent disability in bathing.

RESULTS

Over the course of nearly 8 years, 333 (44.2%) participants developed persistent disability in bathing. After accounting for age, sex, and race, nine predisposing factors were independently associated with persistent bathing disability. The strongest associations were found for inability to rise from a chair, low bathing self-efficacy, and low physical activity, each of which more than doubled the risk of persistent bathing disability. After the potential precipitants were added to the final model, the associations between the predisposing factors and persistent bathing disability were only modestly diminished. The effects of the precipitants on persistent bathing disability were large, with multivariable hazard ratios of 24.6 for hospitalization and 3.01 for restricted activity only. The relative contributions of the predisposing factors and precipitants to the development of persistent bathing disability were 47.7% and 27.1%, respectively.

CONCLUSION

Disability in bathing is attributable to a combination of predisposing factors that make community-living older persons vulnerable and intervening illnesses or injuries that act as precipitants. These factors and precipitants may be suitable targets for the prevention of bathing disability.

Access to Primary Care for Medicare Beneficiaries

OBJECTIVES

To determine the acceptance rate of new Medicare patients by all primary care physicians. Among primary care physicians accepting new patients, to determine whether demographic and geographic factors are associated with the likelihood of accepting new Medicare patients.

DESIGN

Cross-sectional.

SETTING

Primary care physicians drawn from a national sample.

PARTICIPANTS

Eight hundred forty-eight primary care physicians.

MEASUREMENTS

Percentage of physicians accepting, not accepting, or conditionally accepting new Medicare patients.

RESULTS

Of the 848 primary care physicians contacted, only 58% unconditionally accepted all new Medicare patients; 20% accepted new patients but restricted new Medicare patients using policies of nonacceptance or conditional acceptance. Of the 665 physicians accepting new patients, those in the south and west were more likely not to accept new Medicare patients than those in the northeast, with multivariable odds ratios (ORs) of 2.79 (95% confidence interval (CI)=1.34-5.78) and 3.14 (95% CI=1.35-7.33), respectively. Similarly, family physicians were more likely than internists not to accept new Medicare patients (OR=2.36, 95% CI=1.39-3.99). Primary care physicians in the Midwest were more likely to conditionally accept new Medicare patients than those in the northeast (OR=4.84, 95% CI=1.32-17.76), and primary care physicians in metropolitan areas with a population less than 100,000 were more likely to conditionally accept new Medicare patients than those in areas with a population greater than 100,000 (OR=2.39, 95% CI=1.18-4.84).

CONCLUSION

Medicare beneficiaries' access to primary care is limited and varies significantly by region, population size, and type of provider.

Prevaccine determination of the expression of costimulatory B7 molecules in activated monocytes predicts influenza vaccine responses in young and older adults

BACKGROUND

Innate immunity, including Toll-like receptor (TLR)-mediated expression of the B7 costimulatory molecules CD80 and CD86, is critical for vaccine immunity. We examined whether CD80 and CD86 expression vary with aging and predict response to the trivalent inactivated influenza vaccine.

METHODS

One hundred sixty-two subjects between 21 and 30 years of age (the young group) or > or =65 years of age (the older group) enrolled before vaccination. We determined TLR-induced monocyte CD80/CD86 expression by flow cytometry and vaccine antibody responses by hemagglutination inhibition.

RESULTS

The mean increase in TLR-induced CD80(+) monocytes was reduced in older, compared with young, adults by 68% (P=.0002), and each decile increase of CD80(+) cells was associated with an 8.5% increase in mean number of vaccine strains with a > or =4-fold titer increase (P=.01) and a 3.8% increase in mean number of strains with a postvaccine titer > or =1 : 64 (P=.037). Each decile decrease of CD86(+) cells was associated with an 11% increase in the mean number of strains with a 4-fold increase (P=.002) and a 3.9% increase in the mean number of strains with a postvaccine titer > or =1 : 64 (P=.07).

CONCLUSIONS

CD80 and CD86 expression on activated monocytes is highly associated with influenza vaccine response. This approach prospectively identifies adults unlikely to respond to immunization who may benefit from alternative vaccines or antiviral prophylaxis during influenza outbreaks.

Age-associated defect in human TLR-1/2 function

The effects of aging on human TLR function remain incompletely understood. We assessed TLR function and expression in peripheral blood monocytes from 159 subjects in 2 age categories, 21-30 and >65 years of age, using a multivariable mixed effect model. Using flow cytometry to assess TLR-induced cytokine production, we observed a substantial, highly significant defect in TLR1/2-induced TNF-alpha (p = 0.0003) and IL-6 (p < 0.0001) production, in older adults compared with young controls. In contrast to findings in aged mice, other TLR (including TLR2/6)-induced cytokine production appeared largely intact. These differences were highly significant even after correcting for covariates including gender, race, medications, and comorbidities. This defect in TLR1/2 signaling may result from alterations in baseline TLR1 surface expression, which was decreased by 36% in older adults (p < 0.0001), whereas TLR2 surface expression was unaffected by aging. Production of IL-6 (p < 0.0001) and TNF-alpha (p = 0.003) after stimulation by N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-Cys-[S]-Ser1-[S]-Lys(4) trihydrochloride was strongly associated with TLR1 surface expression. Diminished TLR1/2 signaling may contribute to the increased infection-related morbidity and mortality and the impaired vaccine responses observed in aging humans.

Multimorbidity and survival in older persons with colorectal cancer

OBJECTIVES

To ascertain the effect of common chronic conditions on mortality in older persons with colorectal cancer.

DESIGN

Retrospective cohort study.

SETTING

Population-based cancer registry.

PARTICIPANTS

Patients in the Surveillance Epidemiology and End Results-Medicare linked database who were aged 67 and older and had a primary diagnosis of Stage 1 to 3 colorectal cancer during 1993 through 1999.

MEASUREMENTS

Chronic conditions were identified using claims data, and vital status was determined from the Medicare enrollment files. After estimating the adjusted hazard ratios for mortality associated with each condition using a Cox model, the population attributable risk (PAR) was calculated for the full sample and by age subgroup.

RESULTS

The study sample consisted of 29,733 patients, 88% of whom were white and 55% were female. Approximately 9% of deaths were attributable to congestive heart failure (CHF; PAR = 9.4%, 95% confidence interval (CI) = 8.4-10.5%), more than 5% were attributable to chronic obstructive pulmonary disease (COPD; PAR = 5.3%, 95% CI = 4.7-6.6%), and nearly 4% were attributable to diabetes mellitus (PAR = 3.9%, 95% CI = 3.1-4.8%). The PAR associated with CHF increased with age, from 6.3% (95% CI = 4.4-8.8%) in patients aged 67 to 70 to 14.5% (95% CI = 12.0-17.5%) in patients aged 81 to 85. Multiple conditions were common. More than half of the patients who had CHF also had diabetes mellitus or COPD. The PAR associated with CHF alone (4.29%, 95% CI = 3.68-4.94%) was similar to the PAR for CHF in combination with diabetes mellitus (3.08, 95% CI = 2.60-3.61%) or COPD (3.93, 95% CI = 3.41-4.54%).

CONCLUSION

A substantial proportion of deaths in older persons with colorectal cancer can be attributed to CHF, diabetes mellitus, and COPD. Multimorbidity is common and exerts a substantial effect on colorectal cancer survival.

The Epidemiology of Bathing Disability in Older Persons

OBJECTIVES

To quantify the burden of bathing disability over time; to determine whether the burden of bathing disability differs according to age, sex, and physical frailty; and to evaluate the relationship between disability in bathing and disability in other essential activities of daily living (ADLs).

DESIGN

Prospective cohort study.

SETTING

General community in greater New Haven, Connecticut.

PARTICIPANTS

Seven hundred fifty-four community-living older persons aged 70 and older who were nondisabled (required no personal assistance) in four essential ADLs: bathing, dressing, transferring from a chair, and walking inside the house.

MEASUREMENTS

Bathing disability, defined as the inability to wash and dry one's whole body without personal assistance, was assessed every month for up to 6 years, along with disability in dressing, transferring, and walking.

RESULTS

Over the course of 6 years, 440 participants (58.4%) had at least one episode of bathing disability, and 266 (34.0%) had multiple episodes, with the duration of each episode averaging about 6 months. Whether assessed as number of episodes, duration of episodes, incidence rates, or number of months per 100 months, the burden of bathing disability was greatest in participants who were physically frail and was consistently higher in women than men and in participants who were aged 80 and older than those who were aged 70 to 79. Most episodes of bathing disability (86.1%) were not preceded in the prior month by disability in dressing, transferring, or walking, and nearly half (48.3%) were not accompanied at onset by disability in one or more of these other ADLs. In a multivariable model that included age, sex, and physical frailty, the onset of bathing disability increased the likelihood of developing disability in the other essential ADLs the following month fivefold (hazard ratio=5.1, 95% confidence interval=4.1-6.4).

CONCLUSION

Disability in bathing may serve as a sentinel event in the disabling process. Given the recurrent nature of bathing disability, programs designed to enhance independent bathing will need to focus not only on the prevention of bathing disability, but also on the restoration and maintenance of independent bathing in older persons who become disabled.

Improved self-confidence was a mechanism of action in two geriatric trials evaluating physical interventions

OBJECTIVE

To evaluate the mechanisms of action in two successful geriatric clinical trials that tested multicomponent physical conditioning programs and to determine whether the pathways for overall benefit were through improvement in physical ability and/or self-confidence.

STUDY DESIGN AND SETTING

PREHAB and DRIVER were conducted by the Yale Pepper Center. PREHAB participants received an individualized program that focused on the impairments present (standardly tailored design); DRIVER participants received the entire intervention (global design). PREHAB enrolled 188 community-living persons, aged 75 years or older, who were physically frail but ambulatory; DRIVER enrolled 178 drivers aged 70 years or older with physical impairments associated with poor driving performance. The primary outcome for PREHAB was a disability score and for DRIVER it was a driving score; potential mediators were measures of physical ability and self-confidence.

RESULTS

In PREHAB, pathways for the intervention were established through improvement in physical ability and self-confidence. In DRIVER, there was some evidence for a pathway through improved driving self-confidence but not through physical ability; however, the intervention effect was largely unexplained.

CONCLUSION

Multicomponent physical interventions may operate through psychological mechanisms, and these mechanisms should be anticipated in trial designs so that the component effects can be suitably evaluated.

Bathing disability and the risk of long-term admission to a nursing home

BACKGROUND

The prevalence of disability in bathing and the likelihood of a long-term nursing home admission increase substantially with age. We performed a prospective study to determine whether the occurrence of persistent disability in bathing is associated with the risk of a long-term nursing home admission, independent of potential confounders, including persistent disability in other essential activities of daily living.

METHODS

We studied 754 community-living persons, 70 years old or older, who were nondisabled in four essential activities of daily living. Participants were followed with monthly telephone interviews for a median of 75 months to determine the occurrence of persistent (i.e., present for at least 2 consecutive months) disability in bathing and the time to the first long-term nursing home admission, defined as longer than 3 months.

RESULTS

One hundred thirteen (15.0%) participants had a long-term nursing home admission. At least one episode of persistent bathing disability occurred among 59 (52.2%) participants with a long-term nursing home admission and 210 (32.8%) without a long-term admission (p <.001). In a proportional hazards model that was fully adjusted for potential confounders, the occurrence of persistent bathing disability increased the risk of a long-term nursing home admission by 77% (hazard ratio 1.77, 95% confidence interval 1.05 to 2.98), but had no effect on the risk of a short-term nursing home admission (hazard ratio 0.87, 95% confidence interval 0.51 to 1.49).

CONCLUSIONS

Among community-living older persons, the occurrence of persistent disability in bathing is independently associated with the risk of a long-term nursing home admission, but has no effect on short-term admissions. Interventions directed at the prevention and remediation of bathing disability have the potential to reduce the burden and expense of long-term care services.

Potentially Modifiable Resident Characteristics That Are Associated With Physical or Verbal Aggression Among Nursing Home Residents With Dementia

BACKGROUND

Physical aggression by nursing home residents is a burden to residents and staff. The identification of modifiable correlates would facilitate developing preventive strategies. The objectives of the study were to determine potentially modifiable resident characteristics that are associated with physical aggression and to correlate these characteristics with verbal aggression.

METHODS

This was a cross-sectional study of nursing home residents in 5 states who had at least 1 annual Minimum Data Set assessment completed during 2002. Case subjects were defined as nursing home residents 60 years and older with dementia who were reported to have been physically aggressive in the week before their assessment. Control subjects were all other residents 60 years and older with dementia. The main outcome measure was being physically aggressive during the past week.

RESULTS

A total of 103 344 residents met study criteria, of whom 7120 (6.9%) had been physically aggressive in the week before their annual Minimum Data Set assessment. After adjustment for potential confounders, including age, sex, severity of cognitive impairment, and dependence in activities of daily living, physical aggression was associated with depressive symptoms (adjusted odds ratio [AOR], 3.3; 99% confidence interval [CI], 3.0-3.6), delusions (AOR, 2.0; 99% CI, 1.7-2.4), hallucinations (AOR, 1.4; 99% CI, 1.1-1.8), and constipation (AOR, 1.3; 99% CI, 1.2-1.5). Urinary tract infections, respiratory tract infections, fevers, reported pain, and participation in recreational activities were not significantly associated with physical aggression in multivariate analyses (P >.01 for all). Except for constipation, the correlates of verbal aggression were similar to those of physical aggression.

CONCLUSION

If the associations we have estimated are causal, then treatment of depression, delusions, hallucinations, and constipation may reduce physical aggression among nursing home residents.

The effect of prior disability history on subsequent functional transitions

BACKGROUND

Many older persons experience multiple transitions between states of disability and independence, but little is known about the effect of prior disability history on subsequent functional transitions. Our objective was to determine the effect of prior disability on subsequent transitions between no disability, mild disability, severe disability, and death.

METHODS

For 60 months, 754 persons aged 70 or older underwent monthly assessments of disability in basic activities of daily living. We used a multistate extension of the proportional hazards model to determine the effects of amount, defined as cumulative duration, and distribution, defined as number of episodes, of prior disability on subsequent functional transitions, adjusted for age, gender, cognitive status, timed gait, and habitual physical activity.

RESULTS

For each additional month of prior disability, participants were more likely to make transitions representing new or worsening disability and were less likely to make transitions from disability to independence or from severe disability to death. Adjusting for the cumulative duration of prior disability, more episodes of prior disability were associated with a higher likelihood of most transitions, representing both increasing and decreasing disability, but had no effect on transitions to death.

CONCLUSIONS

Both the amount and distribution of prior disability are important determinants of the likelihood of subsequent functional transitions. Analytic methods that account for prior disability history should be used in studies of functional transitions, and new measures of disability burden are needed that incorporate distribution as well as amount of disability.

Transitions between frailty states among community-living older persons

BACKGROUND

Little is known about the natural course of frailty. We performed a prospective study to determine the transition rates between frailty states and to evaluate the effect of the preceding frailty state on subsequent frailty transitions.

METHODS

We studied 754 community-living persons, aged 70 years or older, who were nondisabled in 4 essential activities of daily living. Frailty, assessed every 18 months for 54 months, was defined on the basis of weight loss, exhaustion, low physical activity, muscle weakness, and slow walking speed. Participants were classified as frail if they met 3 or more of these criteria, as prefrail if they met 1 or 2 of the criteria, and as nonfrail if they met none of the criteria.

RESULTS

Of the 754 participants, 434 (57.6%) had at least 1 transition between any 2 of the 3 frailty states during 54 months. The rates were 36.8%, 21.5%, and 9.2% for 1, 2, and 3 transitions, respectively. During the 18-month intervals, transitions to states of greater frailty were more common (rates up to 43.3%) than transitions to states of lesser frailty (rates up to 23.0%), and the probability of transitioning from being frail to nonfrail was very low (rates, 0%-0.9%), even during an extended period. The likelihood of transitioning between frailty states was highly dependent on one's preceding frailty state.

CONCLUSIONS

Frailty among older persons is a dynamic process, characterized by frequent transitions between frailty states over time. Our findings suggest ample opportunity for the prevention and remediation of frailty.

CNTF induces dose-dependent alterations in retinal morphology in normal and rcd-1 canine retina

Ciliary neurotrophic factor (CNTF) provides morphologic preservation of rods in several animal models of retinitis pigmentosa (RP). However, CNTF may alter photoreceptor morphology and rod photoreceptor differentiation in vitro, as well as affecting normal retinal electrophysiology. In addition, the capacity of CNTF to support other cell types affected secondarily in RP (cones and ganglion cells) is unclear. The purposes of this study were to examine the effects of CNTF upon a canine model of RP, the rod-cone degeneration (rcd-1) dog. Archival tissue from a previous study assessing the capacity of CNTF to rescue photoreceptors in rcd-1 dogs was used. One eye was treated for 7 weeks before being explanted. The contralateral eye was untreated. A total of 23 rcd-1 dogs and seven control dogs (four untreated and three CNTF-treated) were used. Morphometric data describing outer and inner nuclear layer thickness, inner retinal thickness, cones and ganglion cells were collected at nine evenly spaced points along each retina and analysed using a mixed effects model. Immunohistochemistry was performed on a subset of 11 dogs for expression of rhodopsin, human cone arrestin (hCAR) and recoverin. CNTF protected the outer nuclear layer and increased inner retinal thickness in a dose-dependent manner (both were maximal at CNTF doses of 1-6 ng day-1). Significant cone loss or reduction of inner nuclear layer width in rcd-1 did not occur in this model, therefore we were unable to assess the protective effect of CNTF upon these parameters. CNTF did not afford significant ganglion cell protection. CNTF induced morphologic changes in rods and ganglion cells, as well as reducing expression of hCAR and rhodopsin, but not recoverin. The dose of CNTF which provided optimal outer nuclear layer protection also resulted in several other effects, including altered ganglion cell morphology, increased thickness of the entire retina, and reduced expression of some phototransduction proteins. These changes were more marked in rcd-1 retinas than in wild-type retinas. This implies that the consequences of CNTF treatment may be substantially influenced by the cellular context into which it is introduced.

The Dynamic Nature of Mobility Disability in Older Persons

OBJECTIVES

To determine the rates of clinically meaningful transitions in mobility disability; evaluate how these transitions differ according to age, sex, and physical frailty; and depict the duration of the resulting episodes of mobility disability.

DESIGN

Prospective cohort study.

SETTING

General community in greater New Haven, Connecticut, from March 1998 to October 2004.

PARTICIPANTS

Seven hundred fifty-four community-living older persons, aged 70 and older, who were nondisabled (i.e., required no personal assistance) in four activities of daily living.

MEASUREMENTS

Mobility disability, defined as the inability to walk one quarter of a mile and to climb a flight of stairs, respectively without personal assistance, was assessed every month for up to 5 years.

RESULTS

For both mobility tasks, rates per 1,000 person-months were higher for transitions from no disability to intermittent disability (34.7 for walking one quarter of a mile and 17.4 for climbing a flight of stairs), intermittent to continuous disability (52.0 and 42.5), continuous to intermittent disability (35.4 and 31.5), and intermittent to no disability (68.6 and 85.4) than for other transitions. Older age, female sex, and physical frailty were associated with greater likelihood of transition to states of greater disability and lower likelihood of regaining independent mobility.

CONCLUSION

Mobility disability in older persons is a highly dynamic process, characterized by frequent transitions between states of independence and disability. Programs designed to enhance independent mobility should focus not only on the prevention of mobility disability but also on the restoration and maintenance of independent mobility in older persons who become disabled.

Hospitalization, restricted activity, and the development of disability among older persons

CONTEXT

Preventing the development of disability in activities of daily living is an important goal in older adults, yet relatively little is known about the disabling process.

OBJECTIVES

To evaluate the relationship between 2 types of intervening events (hospitalization and restricted activity) and the development of disability and to determine whether this relationship is modified by the presence of physical frailty.

DESIGN, SETTING, AND PARTICIPANTS

Prospective cohort study, conducted in the general community in greater New Haven, Conn, from March 1998 to March 2003, of 754 persons aged 70 years or older, who were not disabled (ie, required no personal assistance) in 4 essential activities of daily living: bathing, dressing, walking inside the house, and transferring from a chair. Participants were categorized into 2 groups according to the presence of physical frailty (defined on the basis of slow gait speed) and were followed up with monthly telephone interviews for up to 5 years to ascertain exposure to intervening events and determine the occurrence of disability.

MAIN OUTCOME MEASURE

Disability, defined as the need for personal assistance in bathing, dressing, walking inside the house, or transferring from a chair.

RESULTS

During the 5-year follow-up period, disability developed among 417 (55.3%) participants, 372 (49.3%) were hospitalized and 600 (79.6%) had at least 1 episode of restricted activity. The multivariable hazard ratios for the development of disability were 61.8 (95% confidence interval [CI], 49.0-78.0) within a month of hospitalization and 5.54 (95% CI, 4.27-7.19) within a month of restricted activity. Strong associations were observed for participants who were physically frail and those who were not physically frail. Hospital admissions for falls were most likely to lead to disability. Intervening events occurring more than a month prior to disability onset were not associated with the development of disability. The population-attributable fractions associated with new exposure to hospitalization and restricted activity, respectively, were 0.48 and 0.19; 0.40 and 0.20, respectively, for frail participants and 0.61 and 0.16, respectively, for nonfrail participants.

CONCLUSIONS

Illnesses and injuries leading to either hospitalization or restricted activity represent important sources of disability for older persons living in the community, regardless of the presence of physical frailty. These intervening events may be suitable targets for the prevention of disability.

Optimizing replacement of dairy cows: modeling the effects of diseases

We modified an existing dairy management decision model by including economically important dairy cattle diseases, and illustrated how their inclusion changed culling recommendations. Nine common diseases having treatment and veterinary costs, and affecting milk yield, fertility and survival, were considered important in the culling decision process. A sequence of stages was established during which diseases were considered significant: mastitis and lameness, any time during lactation; dystocia, milk fever and retained placenta, 0-4 days of lactation; displaced abomasum, 5-30 days; ketosis and metritis, 5-60 days; and cystic ovaries, 61-120 days. Some diseases were risk factors for others. Baseline incidences and disease effects were obtained from the literature. The effects of various disease combinations on milk yield, fertility, survival and economics were estimated. Adding diseases into the model did not increase voluntary or total culling rate. However, diseased animals were recommended for culling much more than healthy cows, regardless of parity or production level. Cows in the highest production level were not recommended for culling even if they contracted a disease. The annuity per cow decreased and herdlife increased when diseases were in the model. Higher replacement cost also increased herdlife and decreased when diseases were in the model. Higher replacement cost also increased herdlife and decreased the annuity and voluntary culling rate.

A mathematical model of Staphylococcus aureus control in dairy herds

An ordinary differential equation model was developed to simulate dynamics of Staphylococcus aureus mastitis. Data to estimate model parameters were obtained from an 18-month observational study in three commercial dairy herds. A deterministic simulation model was constructed to estimate values of the basic (R0) and effective (Rt) reproductive number in each herd, and to examine the effect of management on mastitis control. In all herds R0 was below the threshold value 1, indicating control of contagious transmission. Rt was higher than R0 because recovered individuals were more susceptible to infection than individuals without prior infection history. Disease dynamics in two herds were well described by the model. Treatment of subclinical mastitis and prevention of influx of infected individuals contributed to decrease of S. aureus prevalence. For one herd, the model failed to mimic field observations. Explanations for the discrepancy are given in a discussion of current knowledge and model assumptions.

Cow- and quarter-level risk factors for Streptococcus uberis and Staphylococcus aureus mastitis

This study was designed to identify risk factors for intramammary infections with Streptococcus uberis and Staphylococcus aureus under field conditions. An 18-mo survey with sampling of all quarters of all lactating cows at 3-wk intervals was carried out in three Dutch dairy herds with medium bulk milk somatic cell count (200,000 to 300,000 cells/ml). Quarter milk samples were used for bacteriology and somatic cell counting. Data on parity, lactation stage, and bovine herpesvirus 4-serology were recorded for each animal. During the last year of the study, body condition score, and teat-end callosity scores were recorded at 3-wk intervals. A total of 93 new infections with Strep. uberis were detected in 22,665 observations on quarters at risk for Strep. uberis infection, and 100 new infections with Staph. aureus were detected in 22,593 observations on quarters at risk for Staph. aureus infection. Multivariable Poisson regression analysis with clustering at herd and cow level was used to identify risk factors for infection. Rate of infection with Strep. uberis was lower in first- and second-parity cows than in older cows, and depended on stage of lactation in one herd. Quarters that were infected with Arcanobacterium pyogenes or enterococci, quarters that had recovered from Strep. uberis- or Staph. aureus-infection in the past, and quarters that were exposed to another Strep. uberis infected quarter in the same cow had a higher rate of Strep. uberis infection. Teat-end callosity and infection with coagulase-negative staphylococci or corynebacteria were not significant as risk factors. Rate of Staph. aureus infection was higher in bovine herpesvirus 4-seropositive cows, in right quarters, in quarters that had recovered from Staph. aureus or Strep. uberis infection, in quarters exposed to other Staph. aureus infected quarters in the same cow, and in quarters with extremely callused teat ends. Infection with coagulase-negative staphylococci was not significant as a risk factor. The effect of infection with corynebacteria on rate of infection with Staph. aureus depended on herd, stage of lactation, and teat-end roughness. Herd level prevalence of Strep. uberis or Staph. aureus, and low quarter milk somatic cell count were not associated with an increased rate of infection for Strep. uberis or Staph. aureus.

Censoring in survival analysis: a simulation study of the effect of milk yield on conception

Survival-analysis methods often are used to analyze data from dairy herds where the outcome of interest is the interval from calving to conception. The purpose of this study was to determine whether an association between milk yield and culling biases the estimation of the effect of milk yield on conception. This was done by simulating four different scenarios modeling dairy-cattle milk yield and reproductive performance with known relationships among study factors. Cox's proportional-hazards model was used to analyze the effect of milk yield on days open under the following four scenarios: (1) no association between milk yield and culling or between milk yield and conception; (2) association between milk yield and culling only; (3) association between milk yield and conception only; (4) associations between milk yield and both culling and conception. The analyses also were repeated for data sets with an association between milk yield and culling, but with probabilities of culling ranging from 0.01 to 0.4. An effect of milk production on culling appeared to cause a small increase in the parameter estimates for the association of milk yield and days open - particularly when the probability of culling was high. The effect of high milk production on median days open (as estimated by survival functions) changed by 2 to 4 days when an association between milk yield and culling was programmed in the simulated data sets.

Analysis of an outbreak of Streptococcus uberis mastitis

An outbreak of Streptococcus uberis mastitis was described to gain insight into the dynamics of Strep. uberis infections at a herd level. Data were obtained from a longitudinal observational study on a commercial Dutch dairy farm with good udder health management. Quarter milk samples for bacteriological culture were routinely collected at 3-wk intervals from all lactating animals (n = 95 +/- 5). Additional samples were collected at calving, clinical mastitis, dry-off, and culling. During the 78-wk observation period, 54 Strep. uberis infections were observed. The majority of infections occurred during a 21-wk period that constituted the disease outbreak. The incidence rate was higher in quarters that had recovered from prior Strep. uberis infection than in quarters that had not experienced Strep. uberis infection before. The incidence rate of Strep. uberis infection did not differ between quarters that were infected with other pathogens compared with quarters that were not infected with other pathogens. The expected number of new Strep. uberis infections per 3-wk interval was described by means of a Poisson logistic regression model. Significant predictor variables in the model were the number of existing Strep. uberis infections in the preceding time interval (shedders), phase of the study (early phase vs. postoutbreak phase), and prior infection status of quarters with respect to Strep. uberis, but not infection status with respect to other pathogens. Results suggest that contagious transmission may have played a role in this outbreak of Strep. uberis mastitis.

Teaching dairy herd health dynamics using a web-based program

INTRODUCTION

This course teaches veterinary students basic principles of epidemiology. Dynamic relationships of dairy herd performance parameters are demonstrated.

METHODOLOGY

Courseware combines lectures (both in-class and Web-based) and problem-based exercises using two computer simulation models. The format of this eight-week, one-credit course is a lecture followed by exercises in a computer laboratory working with simulation models. Currently, Cornell University and nine test sites use the courseware.

CURRENT STATUS

The course has been taught for two years, with students and experts providing evaluations and critical feedback for courseware modification.

Disease management research using event graphs

Event Graphs, conditional representations of stochastic relationships between discrete events, simulate disease dynamics. In this paper, we demonstrate how Event Graphs, at an appropriate abstraction level, also extend and organize scientific knowledge about diseases. They can identify promising treatment strategies and directions for further research and provide enough detail for testing combinations of new medicines and interventions. Event Graphs can be enriched to incorporate and validate data and test new theories to reflect an expanding dynamic scientific knowledge base and establish performance criteria for the economic viability of new treatments. To illustrate, an Event Graph is developed for mastitis, a costly dairy cattle disease, for which extensive scientific literature exists. With only a modest amount of imagination, the methodology presented here can be seen to apply modeling to any disease, human, plant, or animal. The Event Graph simulation presented here is currently being used in research and in a new veterinary epidemiology course.

Simulated effects on dairy cattle health of extending the voluntary waiting period with recombinant bovine somatotropin

We simulated the effect of extending the voluntary wait period by 100 days on disorder-frequency measures that were based on cow-years (from lactations completed during the 4-year simulation horizon), metric tons of milk yield, and lactational incidence risks. A dynamic stochastic discrete-event simulation model that focuses on clinical and subclinical intramammary infections (IMI), plus clinical metabolic (left-displaced abomasum, ketosis, milk fever) and reproductive (cystic ovarian disease, dystocia, retained placenta, twinning, uterine infection) disorders in dairy herds was used. Although the voluntary wait period was increased by 100 days (50 vs. 150), the predicted difference in simulated days to conception was only 89 days for the extended voluntary wait-period group (which we attributed to higher fertility later in lactation). Herds that had a voluntary wait period of 150 days (compared to the control herds' voluntary wait period of 50 days) were predicted to have significantly lower rates of metabolic and reproductive disorders and clinical mastitis on both cow-year and milk-yield bases. Simulated control herds, on average, produced 8539 kg of milk in an average lactation of 325 days and simulated herds with a 150-day voluntary wait period 10893 kg of milk in an average lactation of 409 days. There was a significantly lower predicted rate and risk of culling for reproductive failure in the extended voluntary wait period group. The predicted lactational incidence risks for subclinical IMI were 18% higher for the extended voluntary wait period group - but extending the voluntary wait period by 100 days was predicted not to increase the risk of any of the other 10 disorders.

Optimizing replacement decisions for Finnish dairy herds

The purposes of the study were to determine how "an optimal herd" would be structured with respect to its calving pattern, average herdlife and calving interval, and to evaluate how sensitive the optimal solution was to changes in input prices, which reflected the situation in Finland in 1998. The study used Finnish input values in an optimization model developed for dairy cow insemination and replacement decisions. The objective of the optimization model was to maximize the expected net present value from present and replacement cows over a given decision horizon. In the optimal solution, the average net revenues per cow were highest in December and lowest in July, due to seasonal milk pricing. Based on the expected net present value of a replacement heifer over the decision horizon, calving in September was optimal. In the optimal solution, an average calving interval was 363 days and average herdlife after first calving was 48.2 months (i.e., approximately 4 complete lactations). However, there was a marked seasonal variation in the length of a calving interval (it being longest in spring and early summer) that can be explained by the goal of having more cows calving in the fall. This, in turn, was due to seasonal milk pricing and higher production in the fall. In the optimal solution, total replacement percentage was 26, with the highest frequency of voluntary culling occurring at the end of the year. Seasonal patterns in calving and replacement frequencies by calendar month and variation in calving interval length or herdlife did not change meaningfully (< 1%-2% change in the output variables) with changes in calf, carcass or feed prices. When the price of a replacement heifer decreased, average herdlife was shorter and replacement percentage increased. When the price increased, the effect was the opposite.

Optimizing breeding decisions for Finnish dairy herds

The purpose of this study was to determine the effect of reproductive performance on profitability and optimal breeding decisions for Finnish dairy herds. We used a dynamic programming model to optimize dairy cow insemination and replacement decisions. This optimization model maximizes the expected net revenues from a given cow and her replacements over a decision horizon. Input values and prices reflecting the situation in 1998 in Finland were used in the study. Reproductive performance was reflected in the model by overall pregnancy rate, which was a function of heat detection and conception rate. Seasonality was included in conception rate. The base run had a pregnancy rate of 0.49 (both heat detection and conception rate of 0.7). Different scenarios were modeled by changing levels of conception rate, heat detection, and seasonality in fertility. Reproductive performance had a considerable impact on profitability of a herd; good heat detection and conception rates provided an opportunity for management control. When heat detection rate decreased from 0.7 to 0.5, and everything else was held constant, net revenues decreased approximately 2.6%. If the conception rate also decreased to 0.5 (resulting in a pregnancy rate of 0.25), net revenues were approximately 5% lower than with a pregnancy rate of 0.49. With lower fertility, replacement percentage was higher and the financial losses were mainly from higher replacement costs. Under Finnish conditions, it is not optimal to start breeding cows calving in spring and early summer immediately after the voluntary waiting period. Instead, it is preferable to allow the calving interval to lengthen for these cows so that their next calving is in the fall. However, cows calving in the fall should be bred immediately after the voluntary waiting period. Across all scenarios, optimal solutions predicted most calvings should occur in fall and the most profitable time to bring a replacement heifer into a herd was in the fall. It was economically justifiable to keep breeding high producing cows longer than low producing cows.

Approaches to modeling intramammary infections in dairy cattle

In this paper, three approaches (Markov processes, discrete-event simulation, and differential equations) to modeling intramammary infections (IMI; focusing on the dynamic changes between uninfected, subclinical, and clinical udder health states) are described. The objectives were to describe the various approaches to modeling intramammary infections, determine if simulations of the examples of the three approaches yield stable prevalences, and discuss the approaches' limitations. The literature review showed that there is no agreement on the proportion of animals that change health states. The approach of discrete-event simulation modeling included the most cow-level risk factors and udder-health states (hence, was judged to replicated best the dynamics of the infection process) and yielded stable prevalences for all udder-health states. However, there remain parts of the dynamics that need further research. These include the pathogen-specific probabilities and times of occurrence for: regression of clinical IMI to subclinical IMI, flare-up of subclinical IMI to clinical IMI, and incidence of subclinical IMI. Also, the assumption in all current approaches of homogeneous mixing is violated because the primary contact structure for contagious pathogens during milking is either between cows through residual infectious milk in the milking machine or within a cow by vacuum fluctuations or teat-cup liner slips. Better contact structures should be incorporated so that the effects of control strategies can be better-estimated. Moreover, the three modeling approaches discussed assumed that all non-infected quarters are susceptible to infection--which might be denied by work in genetic resistance.

Partial budget of the discounted annual benefit of mastitis control strategies

The objective of this study was to rank the benefits associated with various mastitis control strategies in simulated herds with intramammary infections caused by Streptococcus agalactiae, Streptococcus spp. other than Strep. agalactiae, Staphylococcus aureus, coagulase-negative staphylococci, and Escherichia coli. The control strategies tested were prevention, vaccination for E. coli, lactation therapy, and dry cow antibiotic therapy. Partial budgets were based on changes caused by mastitis control strategies from the mean values for milk, fat, and protein yields of the control herd and the number of cows that were culled under a fixed mastitis culling criterion. Each annual benefit (dollars per cow per year) of a mastitis control strategy was compared with the revenue for the control herd and was calculated under two different milk pricing plans (3.5% milk fat and multiple-component pricing), three net replacement costs, and three prevalences of pathogen-specific intramammary infection. Twenty replicates of each control strategy were run with SIMMAST (a dynamic discrete event stochastic simulation model) for 5 simulated yr. Rankings of discounted annual benefits differed only slightly according to milk pricing plans within a pathogen group but differed among the pathogen groups. Differences in net replacement costs for cows culled because of mastitis did not change the ranking of control strategies within a pathogen group. Both prevention and dry cow therapy were important mastitis control strategies. For herds primarily infected with environmental pathogens, strategies that included vaccination for mastitis caused by E. coli dominated strategies that did not include vaccination against this microorganism.

A simulation of strategies to lower bulk tank somatic cell count below 500,000 per milliliter

In the future, the Pasteurized Milk Ordinance may make milk quality standards more stringent by lowering the somatic cell count (SCC) limit on Grade A raw milk to 500,000/ml. Therefore, using a discrete event simulation model, we investigated the effects of the prevention of intramammary infection (as recommended by the National Mastitis Council), lactation therapy, and dry cow therapy (all seven possible combinations) on bulk tank SCC; milk, fat, and protein yields; prevalence of intramammary infection; and culling for mastitis. Untreated controls were also tested. Ten replicates of each intervention and each control were run for 2 simulated yr, including the daily sampling of 100 cows. The goal was to lower bulk tank SCC < 500,000/ml in the 2nd yr for herds that previously had stable bulk tank SCC between 500,000 and 750,000/ml. Although all strategies occasionally met this goal, on no occasion did all replicates perform without a violation in the 2nd yr of the study (median last month of violation ranged from mo 12 to 23). The combination of the prevention of intramammary infection, lactation therapy, and dry cow therapy resulted in the lowest bulk tank linear score, most replicated without a violation in the 2nd yr, fewest months with a bulk tank linear score > or = 5.3, and fewest mastitis culls. The combination of the prevention of intramammary infection and dry cow therapy also was favorably ranked (highest milk yield, fewest clinical intramammary infections during lactation, and highest percentage of uninfected cows).

Design and validation of a dynamic discrete event stochastic simulation model of mastitis control in dairy herds

A dynamic stochastic simulation model for discrete events, SIMMAST, was developed to simulate the effect of mastitis on the composition of the bulk tank milk of dairy herds. Intramammary infections caused by Streptococcus agalactiae, Streptococcus spp. other than Strep. agalactiae, Staphylococcus aureus, and coagulase-negative staphylococci were modeled as were the milk, fat, and protein test day solutions for individual cows, which accounted for the fixed effects of days in milk, age at calving, season of calving, somatic cell count (SCC), and random effects of test day, cow yield differences from herdmates, and autocorrelated errors. Probabilities for the transitions among various states of udder health (uninfected or subclinically or clinically infected) were calculated to account for exposure, heifer infection, spontaneous recovery, lactation cure, infection or cure during the dry period, month of lactation, parity, within-herd yields, and the number of quarters with clinical intramammary infection in the previous and current lactations. The stochastic simulation model was constructed using estimates from the literature and also using data from 164 herds enrolled with Quality Milk Promotion Services that each had bulk tank SCC between 500,000 and 750,000/ml. Model parameters and outputs were validated against a separate data file of 69 herds from the Northeast Dairy Herd Improvement Association, each with a bulk tank SCC that was > or = 500,000/ml. Sensitivity analysis was performed on all input parameters for control herds. Using the validated stochastic simulation model, the control herds had a stable time average bulk tank SCC between 500,000 and 750,000/ml.

Selecting linear-score distributions for modelling milk-culture results

The data for this cross-sectional retrospective study are from surveys of 65 dairy-cattle herds in central New York, USA sampled between February, 1993 and March, 1995. The objective was to identify probability distributions of logarithmically transformed somatic-cell counts (linear score) for use in a simulation model of mastitis and milk quality. Probability density functions were estimated using maximum-likelihood estimators for the linear score of individual-cow composite-milk samples culture negative and culture positive for the pathogens Streptococcus agalactiae, Streptococcus non-agalactiae, Staphylococcus aureus, and coagulase-negative staphylococci for the complete dataset and by bulk-tank somatic-cell count group (< 500,000, > or = 500,000 SCC/ml). Based on the rankings of three goodness-of-fit tests (Anderson-Darling, Kolmogorov-Smirnov and chi 2), the Weibull distribution (among the three top-ranking distributions for 14 out of 15 cases) may be used to model the individual-cow linear-score response by culture-result-specific bulk-tank somatic-cell count group. A beta distribution was among the three top-ranking distributions for nine out of 15 culture-result-specific bulk-tank somatic-cell count groups and has a logical relationship to linear score because it is defined on a fixed interval. On the other hand, the normal distribution had a poorer fit than the Weibull and at least two other distributions for all culture negative and coagulase-negative staphylococci samples. We do not assume that the underlying biological processes are fully explained by either Weibull or beta distribution--but modelling the linear score for the above culture results with these distributions provided an adequate fit to the survey data, reduced the need for two-sided truncation that open intervals needed, and had errors that did not appear to be systematically positive or negative.

Effects of season, herd size, and geographic region on the composition and quality of milk in the northeast

Our objectives were to describe the milkshed comprising herds in New York, western New Jersey, and central and eastern Pennsylvania in regard to milk yield, composition, and quality and also to estimate the effects of season, herd size, and geographic area on those same variables. Data were collected from July 1993 through June 1994 from 3450 herds. The effect of a somatic cell count (SCC) limit of 500,000/ml on milk yield and the composition of monthly bulk tank milk for all marketed milk was estimated as was the frequency of deliveries of milk that contained SCC that were greater than this limit. All general linear models for mean monthly yield of milk and milk components (fat and protein) and SCC were significant for fixed effects of month and herd size within quartiles for herd size (defined by the number of lactating cows) and significant absorbed effects of herds within quartiles for herd size within subregion. Milk yield, milk components (kilograms), true protein percentage, and SCC were significantly higher in spring than in fall for both data files (complete data file and data file containing only herds with SCC < 500,000/ml). Thirty-five percent of herds with < 27 lactating cows but only 15.3% of herds with > 62 lactating cows had > or = 1 mo with an SCC > 500,000/ml. For herds in the subregions, percentages of shipments with an SCC > or = 500,000/ml ranged from 10.5 to 20.2%. Herds with < 27 lactating cows contributed to the milkshed a disproportionate percentage of SCC (11%) compared with their percentage of contribution of milk (5%).

A decision support system for evaluating mastitis information

A data-driven decision support system, MAST, was developed to summarize systematically the DHI data related to mastitis. MAST determines weaknesses and problems of a mastitis control strategy by pinpointing problem areas, highlighting the scope of the mastitis problem, providing reference values for comparison, offering potential solutions to problem areas, and monitoring changes in the control strategy. Advantages of a DHI data-driven system include accuracy of input data, fast execution speeds, access to untapped information available in DHI data, and a consistent analysis of data. An on-farm decision support system allows the evaluation of the mastitis control strategy at any time, which in turn increases the value of DHI information. The sequence of the graphical results allows users to understand intuitively the overall mastitis problems, find the potential origins, and assess their impact on the herd. Color-coded graphs with supplementary text allow for a rapid analysis of level and trends over time, as well as comparisons among parities, lactation stages, and active and culled cows.

An approach to summarize somatic cell score trends for a data-driven, decision support system

A set of analytical routines were developed to determine significant trends in values for herd DHI somatic cell scores. These trends were used as input to a data-driven, decision support system to aid mastitis management of dairy cattle. The trends of interest were those experienced over the last six DHI sample periods for the entire herd, for three parity groups, and for three stages of lactation. First, cows within the herd were split randomly into two equal groups to account for within-herd variation. For each group, linear regression was calculated for somatic cell score over time within each parity by stage of lactation group. The 18 slope estimates were then analyzed using a two-way ANOVA to test for main fixed effects of parity, stage of lactation, and their interaction. The results of the trend analyses were converted to facts that were asserted to an embedded expert system for further evaluation.