Figure 1. Percentage distribution of intramammary infections caused by Serratia spp. during a 10-year period in an experiment station herd.
J.S. Hogan and K.L. Smith
Department of Animal Sciences
Outbreaks of mastitis caused by Serratia spp. occurred in two herds in which mammary health was being monitored by routine bacteriological analyses of quarter milk samples. Approximately half of the Serratia spp. intramammary infections originated during the dry period. Average duration of intramammary infections exceeded 4 months in both herds and clinical signs were associated with 57% and 33% of cases in an experiment station and a commercial herd, respectively. Antibiotic therapy of clinical cases and dry cow therapy had little impact on the duration of infection. As with most environmental mastitis pathogens, the susceptibility to Serratia spp. intramammary infections appears to be greatest during the dry and periparturient periods. The persistency of these infections into lactation may shift attention of mammary health professionals toward milking hygiene and management when the etiological origin of a Serratia spp. outbreak is actually in nonlactating cows.
Serratia spp. are common bacterial inhabitants of water, soil, manure, feed, and bedding. Although these bacteria are generally considered harmless residents of organic materials, Serratia spp. were implicated as the cause of numerous diseases, including bovine mastitis (Yu, 1979). Barnum et al. (1958) reported the first thorough account of a Serratia spp. mastitis outbreak in a dairy herd nearly 40 years ago. Characteristics of Serratia spp. intramammary infections (IMI) reported by these investigators included: 1) IMI tend to be mild and chronic; 2) clinical signs were sporadic and did not involve systemic signs of disease; 3) bacteria were often shed in low numbers; and 4) isolates were resistant to most antibiotics commonly used to treat mastitis in lactating cows. These characteristics have been reiterated in a series of case study reports of Serratia spp. mastitis outbreaks (Bowman et al., 1986; Howell, 1972; Kamarudin et al., 1996; Ollis and Schoonderwoerd, 1989; Wilson et al., 1990). Outbreaks have been attributed to contaminated teat dips, contaminated water on affected farms, and bacterial colonization of teat skin following frost bite. Each of these factors is consistent with known etiological properties of Serratia spp. causing infectious diseases in humans. Serratia spp. infections in humans have arisen from contaminated surgical disinfectants and occur in immune compromised patients including those with dermal trauma (Ehrendranz et al., 1980; Yu, 1979).
A series of reports in the last decade have shifted the emphasis of etiological studies on Serratia spp. to dry and periparturient cows. Todhunter et al. (1990) reported the results of monitoring Serratia spp. IMI in an experiment station herd over 32 months. The majority of Serratia spp. IMI originated during the dry period and were not associated with dramatic events such as frost bite or the discovery of contaminated teat dip. Ruegg et al. (1992) and Kamarudin et al. (1996) isolated Serratia spp. in the bedding and litter in herds with outbreak of disease and implied teat contamination with these pathogens occurred primarily between milkings. Therefore, Serratia spp. were regarded as environmental mastitis pathogens that could be controlled by controlling pathogen loads in bedding and lots (Kamarudin et al., 1996).
The majority of reports on Serratia spp. mastitis have been retrospective analyses of outbreaks. A unique event occurred recently in which Serratia spp. mastitis outbreaks were recorded in two herds that mammary health was being monitored both preceding and during the outbreak. The purpose of this report is to present data regarding Serratia spp. mastitis outbreaks in an experiment station herd and a commercial dairy herd.
Experimental Herd. Intramammary infection status of cows in the Ohio Agricultural Research and Development Center (OARDC) herd was monitored from January 1, 1986, through October 31, 1995. During these 10 years, the OARDC herd averaged 120 lactating cows comprised of approximately 80% Holsteins and 20% Jerseys. Cows were milked twice daily in a parlor. Milking hygiene included premilking and postmilking disinfection of teats by immersion in a 4% sodium hypochlorite solution. Approximately 75% of the lactating herd was housed in free stalls bedded with recycled manure. The remaining 25% of the lactating herd were in tie-stalls bedded with pelleted corn cobs in 1986 and 1987. Tie stalls were bedded with wood shavings the remainder of the monitoring period.
Cows were dried off approximately 60 days prior to anticipated calving by abrupt cessation of milking and all quarters were treated with a commercially available antibiotic preparation labeled for use in nonlactating cows. Dry cows were housed in free stalls bedded with recycled manure. Cows were moved to individual box stalls 3 to 7 days prior to anticipated calving and remained in the box stalls until 3 days after calving. Stalls were bedded with pelleted corncobs in 1986 and 1987. Wood shavings were used in box stalls 1988 through 1995.
Quarter foremilk samples were collected on days 0, 3, 7, 14, 21, and 30 after calving, at 30- day intervals during lactation, and days 14, 7, and 0 prior to drying off. Foremilk samples were collected from mammary quarters with clinical signs of mastitis during lactation on days 0, 1, 7 and 14 after initial diagnoses of clinical mastitis. Duplicate foremilk samples were collected from all mammary quarters at 30 days into the dry period. Milk and dry cow secretions were plated on blood-esculin agar (0.01 ml of sample) and MacConkey agar (0.1 ml of sample) as described in Todhunter et al. (1990). Gram-negative bacilli were delineated by motility, citrate utilization, lactose fermentation, triple-sugar-iron reactions, and API20E system (Analytab Products, Plainview, NY).
Diagnoses of Serratia spp. IMI, duration, and origin by stage of lactation were as described by Todhunter et al. (1990). Briefly, an IMI was diagnosed when Serratia spp. were either isolated from a quarter with clinical signs of mastitis, isolated from both of the duplicate samples taken from a quarter during the dry period, or from two of three consecutive quarter samples during lactation. Duration of Serratia spp. IMI was the number of days between the first and last isolation of the pathogen from a quarter. Intramammary infections first diagnosed at 30 days into the dry period were considered to originate during the first half of the dry period, those first diagnosed within 3 days after calving originated during the last half of the dry period, and all others were defined as originating during lactation.
Commercial Herd. Cows were in a herd of approximately 140 Holsteins involved in a postmilking teat dip trial from February 1994 through January 1995 (Hogan et al., 1995). The DHIA rolling herd averages for the month the trial began were 19,200 pounds of milk, 660 pounds of fat, and 598 X 103 SCC/ml. Lactating cows were in total confinement housing with fill sand used as bedding in clay based free-stalls. Dry cows were in loose housing bedded with a straw-manure pack. Cows were moved to individual box stalls bedded with long straw approximately 3 days prior to calving and remained in the stalls for 1 to 2 days after calving.
Cows were milked at 12-hour intervals in a double-nine herringbone parlor equipped with automatic take-offs. The milking system was serviced bimonthly as part of the experimental protocol. Premilking sanitation of teats included predipping with a 0.25% iodophor and drying teats with single service towels. Teats of half the cows were dipped postmilking in an experimental barrier product containing 0.55% chlorhexidine gluconate and teats of the remaining half of the herd were dipped in a 1% iodophor.
Duplicate samples of quarter foremilk were collected prior to trial. When bacteriological results did not agree, a third sample was obtained 14 days later. Single quarter foremilk samples were collected monthly during the trial. A second quarter milk sample was collected within 7 days from all quarters in which the bacteriological status of the gland changed. Duplicate milk samples were collected from all lactating quarters of cows within 7 days after calving, from quarters with clinical mastitis prior to antibiotic treatment, and within 7 days prior to drying off or leaving the herd.
Primary culture of all milk samples was on blood-esculin agar (0.01 ml of sample). Samples from quarters with clinical mastitis were also plated on MacConkey agar (0.1 ml) to aid detection of Gram-negative bacteria. Bacterial isolates were delineated as outlined in (Todhunter et al., 1991). Serratia spp. were identified by procedures described previously in this manuscript.
An IMI was diagnosed when Serratia spp. were isolated from 1) both of the duplicate samples taken from a quarter at calving, drying off, or with clinical mastitis, or 2) from two consecutive quarter samples during lactation. Duration of Serratia spp. IMI was the number of days between the first and last isolation of the pathogen from a quarter. Intramammary infections were defined either as originating at calving or as originating during lactation.
Experimental Herd. A total of 75 Serratia spp. IMI were diagnosed in 63 cows in the OARDC herd during the 10-year survey period. Annual rates of Serratia spp. IMI were similar the first 8 years of the study and increased dramatically in 1994 and 1995 (Figure 1). Incidence of new Serratia spp. IMI ranged from 1 to 4% of cows each year from 1986 through 1993. Serratia spp. IMI increased to approximately 10% of cows in 1994 and 17% of cows in 1995. Interestingly, season, stage of lactation, severity, and other factors did not differ among years. The only difference noted was that the rate of new IMI increased in 1994 and 1995.
Figure 1. Percentage distribution of intramammary
infections caused by Serratia spp. during a 10-year period
in an experiment station herd.
The origin of Serratia IMI differed among stages of lactation with a greater proportion of IMI originating during the dry period than during lactation (Figure 2). The proportion of Serratia spp. IMI detected during the dry period and at calving was 62% compared with 38% during lactation. Proportion of new Serratia IMI originating during lactation increased as days in milk advanced.
Figure 2. Percentage distribution of intramammary
infections caused by Serratia spp. by origin during the dry
period and lactation in an experiment station herd.
Figure 3. Percentage distribution of intramammary
infections caused by Serratia spp. by parity in an experiment
station herd.
Season of the year affected the incidence of IMI. Incidence of new Serratia spp. IMI was greatest in Summer and lowest in Spring. Percentage of Serratia spp. IMI originating during Summer, Fall, Winter, and Spring were 32, 27, 26, and 15, respectively. July was the month with the greatest percentage of new IMI (20%) and December was the month with the lowest percentage of new IMI (1%). Incidence of new Serratia spp. IMI differed among parity groups. A greater percentage of Serratia spp. IMI occurred in parity >3 cows than in parity 1, 2, and 3 cows (Figure 3). A total of 43% of IMI occurred in parity >3 cows compared with 25%, 15%, and 17%, in parity 1, 2, and 3 cows, respectively.
Fifty-seven percent of Serratia IMI were diagnosed as clinical during lactation. The proportion of Serratia IMI that became clinical was dependant upon the stage of lactation that IMI originated (Figure 4). Only 32% of IMI originating during the first half of the dry period became clinical during lactation compared with 68% of those originating the last half of the dry period and 75% of IMI originating during lactation. Clinical cases tended to be mild with only 3% of clinical cases involving systemic clinical signs. The mean duration of IMI was 131 days, ranging from 1 to 1113 days. Duration of IMI was not affected by parity or stage of lactation when the IMI originated. Eighty percent of IMI first detected at 30 d into the dry period were still present at calving.
Figure 4. Distribution of intramammary
infections caused by Serratia spp. that were clinical during
lactation by origin of infection in an experiment station herd.
Spontaneous cure of IMI was the most common means by which Serratia spp. IMI were eliminated. Thirty-seven percent of Serratia spp. IMI were eliminated as spontaneous cures. Only 11% of IMI were eliminated following intramammary infusion of antibiotic during lactation and 7% were eliminated after dry cow therapy. Culling infected cows eliminated 17% of cows with IMI and 28% remained in the herd at the end of the study period.
Commercial Herd. Forty-three Serratia spp. IMI were diagnosed in 36 cows during the 12-month survey period (Figure 5). Eight (19%) Serratia IMI were present at the initial herd survey in January 1994. Sixteen (43%) of the new Serratia spp. IMI during the trial were diagnosed in March 1994. At least one new Serratia IMI was detected each month; however, no more than three new IMI were detected in a single month other than March 1994.
Figure 5. Percentage distribution of new intramammary
infections caused by Serratia spp. by month in a
commerical herd.
A total of 33% of Serratia IMI were clinical during the trial. Systemic clinical signs were not associated with any of the Serratia IMI. Fifty percent of IMI first detected at calving became clinical compared with 24% of IMI originating during lactation. Rate of new Serratia IMI differed among months of lactation. Sixteen (37%) of the total IMI were detected at calving and 11 (26%) were first detected the second month of lactation (Figure 6). No other month of lactation accounted for more than 12% of new IMI. Mean duration of IMI was 139 days ranging from 4 to 315 days.
Figure 6. Percentage distribution of intramammary
infections caused by Serratia spp. by origin during lactation
in a commerical herd.
Incidence of IMI differed among cows in the two experimental teat dip groups. Sixty-five percent of Serratia spp. IMI originating during lactation occurred in quarters dipped with the experimental chlorhexidine barrier product and 35% were diagnosed in quarters with teats dipped in an 1% iodophor.
Serratia spp. mastitis outbreaks in the OARDC and the commercial dairy herd were caused by new IMI occurring in both the dry period and lactation. Earlier reports of Serratia spp. mastitis outbreaks had concentrated on lactating cows and contaminating sources unique to lactating cows, such as contaminated teat dips and wash water (Howell, 1972; Van Damme, 1982). Results of the current survey and the study reported by Todhunter et al. (1991) revealed Serratia spp. IMI were highly associated with the dry period. Sixty-two percent of Serratia spp. IMI in the OARDC herd originated during the dry period. In addition, 80% of IMI detected at 30 days into the dry period persisted until the next lactation. Serratia spp. were highly adapted to multiplying in cell free dry cow secretion compared with Escherichia coli that often were unable to survive in that environment (Todhunter et al., 1991). The limiting element for sustaining growth of many bacterial species in dry cow secretion is iron. Serratia spp. have a highly efficient iron sequestering mechanism via outer membrane proteins that may allow for the survival and multiplication in the involuted gland.
A characteristic of Serratia spp. mastitis consistent in most reports is that IMI tend to be chronic and cause only mild clinical signs. Fifty-seven percent of IMI in the OARDC herd had a duration of > 60 d and 40% had a duration of > 120 d. Duration and severity of IMI by Gram-negative bacteria are often functions of the speed and efficacy that neutrophils migrate into the gland and eliminate the bacteria. Some strains of Serratia spp. produce cytotoxins that are specific for leukocytes (Goluszko and Nowacki, 1989). This virulence factor and others may afford the bacteria protection against neutrophil phagocytosis and killing. The most common reason for elimination of Serratia spp. IMI in the OARDC herd was spontaneous elimination by the cow. The use of antibiotics to shorten duration of IMI appears futile. Only 11% of IMI were eliminated after antibiotic therapy during lactation. Case studies reporting cures of Serratia spp. IMI by use of antibiotics have involved drugs not currently labeled for use in lactating cows (Barnum et al., 1958; Van Damme, 1982).
One of the more intriguing aspects of the outbreak in the OARDC herd was that the seasonal effects, parity effects, percentage of IMI originating during the dry period, duration, and percentage of IMI becoming clinical did not differ among IMI occurring prior to the outbreak and those originating in 1994 and 1995. These data imply a shift in risk either by an increase in exposure to pathogens and/or a decrease in resistance to Serratia spp. that were independent of these factors. In other words, the underlying cause of the outbreak appeared to be an unknown factor(s) that increased incidence of new IMI, but did not alter the characteristics of IMI compared with those prior to the outbreak. These observations are consistent with the theory that this outbreak was not caused by a sudden and dramatic event that heightened the risk to a subpopulation of cows, such as contaminated teat dip affecting lactating animals. Rather, the increase in causative factors appeared more consistent among the environments of both dry and lactating cows.
The outbreak of Serratia spp. IMI in the commercial herd was noticed within weeks of a period in which the ambient temperature was -20o F three consecutive mornings during March 1994. Frost bite was evident only on a few teats, but visible chapping was common on teat skin following this extreme weather. A direct relationship between chapping and incidence of IMI, however, was not evident. Bowman et al. (1986) reported an outbreak of Serratia spp. mastitis following frost bite in a herd and concluded that the dermal lesions may have contributed to the high prevalence of mastitis caused by these bacteria. Serratia spp. are considered opportunist pathogens to humans and can create skin infections when the integrity of the dermal surface is compromised (Yu, 1979).
Investigators of previous Serratia spp. mastitis outbreaks have assigned responsibility to contaminated teat dips. Although a larger proportion of Serratia spp. IMI in the commercial herd occurred in quarters dipped with the experimental chlorhexidine gluconate dip, neither the experimental product nor the 1% iodophor were contaminated (Hogan et al., 1995). In addition, a substantial number of IMI occurred in quarters dipped with both dips and only approximately one-half of the IMI originated during lactation. The greater proportion of IMI in the chlorhexidine gluconate dipped quarters compared with iodophor dipped quarters may relate to bactericidal activity of the solutions. Numerous reports indicate chlorhexidine gluconate antiseptics are ineffective germicides against Serratia spp. Therefore, these data imply the use of the iodophor as a postdip had an advantage in controlling new Serratia IMI during lactation compared with a barrier product containing a germicide regarded as ineffective against Serratia spp.
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