Understanding the epidemiology of Staphylococcus aureus (reservoirs, transmission pathways, and risk factors) has resulted in excellent control of this major mastitis pathogen in many herds. The major breakthrough in controlling S. aureus came with the realization that it was primarily transmitted from cow to cow during the milking process. Milking time hygiene measures that decreased cow to cow transfer were largely responsible for decreasing new S. aureus intramammary infections (IMI). However, milking time hygiene alone was insufficient in controlling the disease. The addition of dry-cow therapy, and especially, culling the chronically infected were needed to achieve low levels of S. aureus IMI. Some have claimed that S. aureus has been eradicated from the herd. However knowledge of the sources of S. aureus would suggest that total eradication is not currently possible.

There is little doubt that the primary and most important reservoir of S. aureus is the infected mammary gland. In 1961 Davidson reported that of all the sites where S. aureus has been isolated from cows, the infected mammary gland is considered the primary source for IMI. Many strains of S. aureus were eliminated from extramammary body sites once the udder was treated which is why Davidson theorized that the udder is the chief reservoir that seeded other areas. In a Washington state study milk of lactating cows, body sites of prepartum heifers, and environmental sites were examined from 7 dairies to identify potential sources of heifer S. aureus IMI (Roberson et al. 1998). The most common source of S. aureus on dairies was IMI in the lactating herd. The fact that S. aureus IMI can be reduced to very low levels with milking time hygiene, dry cow therapy, and culling also attests to the fact that the infected mammary gland is the most important reservoir. The producer should be aware that heifers, even heifers from herds with excellent mastitis control, can freshen with S. aureus IMI. Nearly one-third of new cases of S. aureus IMI in lactating herds were due to heifers that freshened with S. aureus IMI (Roberson et al., 1994). The new herd addition (whether cow or heifer) represents another source of contagious mastitis pathogens and milk culture prior to purchase would be prudent.

The most common transmission pathway consist of transfer from an infected mammary gland to an uninfected gland via fomites, such as milking equipment, common udder cloths, or the milker’s hands. In herds that do not practice backflushing, one can look inside one of the teat cups and see residual milk. If the last cow milked with that unit had S. aureus IMI, then the next cow milked, with the same unit, will be directly exposed to S. aureus-laden milk. If employed, common clothes or sponges can be a major means of spreading S. aureus as nearly every cow in the herd would be exposed on a daily basis. The importance of the milker’s hands in spreading S. aureus could be equally as important as a common udder cloth, especially in herds that practice forestripping. Latex or similar gloves have been advocated to decrease this means of spread; however if the gloves are not disinfected between cows, this preventive measure may be ineffective. Another potential fomite is the strip cup. If strip cups are used, they should be sanitized after each use. The milking parlor and the lactating period represent the time period and place where most new IMI occur, but S. aureus can result in new IMI during the dry period (Shultz and Mercer, 1976). Although the infected mammary gland could still be the source of these dry periods infections, the infected mammary gland is not the only reservoir.

It has been known for several years that S. aureus may colonize the body sites of mammals, including human. Davidson (1961) isolated S. aureus from the teat skin, udder skin, nose, lips, eyes, vagina, rectum, poll, chest, sacral region, caudal folds and belly of cows. Hajek and Marsalek (1969) isolated S. aureus from the external nares of healthy cattle (12.5%), but found a marked difference between strains from bovine mastitis cases and strains isolated from the nares of cattle. They also note that S. aureus was often found on the teat skin without being shed in the milk. Spencer and Lasmanis (1952) concluded that the principal extramammary reservoir of pathogenic staphylococci were the skin of the teats, finding that 4.8% (12/248 teat skin samples) were colonized with S. aureus. Thorne and Wallmark (1960) isolated S. aureus from the udder skin of 3 cows and found that the phage type from the udder skin S. aureus was identical to the phage type of milk samples from cows in the herd. Evidence by Fox et al. (1991) suggest that teat skin is not a significant reservoir for S. aureus IMI because less than 4% of cows had the same type of S. aureus on the teat skin as that causing IMI. However, the 4% would be consistent with an expected prevalence of S. aureus IMI in heifers at freshening and perhaps the percent of new IMI occurring during the dry period. Matos (1988) found S. aureus in the lesions of 40% of 70 lesions sampled on teats and udders of lactating animals, demonstrating that this organism readily colonizes wounds. The second most common source of S. aureus recovered from heifer colostrum at freshening in the Washington study were strains previously recovered from heifer body sites (Roberson et al., 1998). Overall, 35% of 700 heifers were colonized with S. aureus on a body site at least once (Roberson et al., 1994). Although most body site colonizations appeared to be transient, a few heifers were colonized on the same site for one year. This persistent colonization was also documented in a Louisiana study (Boddie et al., 1987). It has been documented in humans that S. aureus is not capable of sustaining colonization in certain individuals whereas other individuals become colonized for prolonged periods of time (Ehrenkranz, 1965). The same may be true for cattle. Although many modern dairies use individual housing for preweaned calves, few provide individual housing after calves are weaned. Contact with the adult herd is usually limited to the first few days after birth, whereas there is usually extensive contact with other heifer calves after weaning. Thus, an important reservoir of S. aureus for the heifer and dry cow may be the persistently colonized animal. Phage typing data by Devriese et al. (1981) suggest that rabbits derive S. aureus directly or indirectly from their parents, littermates, stablemates rather than from other sources. Although the prevalence of S. aureus isolated from heifer calves is usually low, most researchers have been successful in isolating S. aureus from heifers. Matos (1988) studied 103 heifers and identified S. aureus from 5 sites; the nares, hair coat, streak canal, vagina, and perineum. He found a higher recovery rate in the 3-12 month age group and suggest an environmental influence rather than an age effect in that this group of heifers were kept on pasture where the heifers were exposed to a very heavy fly infestation. In a study by Roberson et al. (1994), S. aureus was most prevalent in the preweaned and pregnant groups and was least prevalent in the grower stage (3-12 months). The presence of S. aureus during the preweaning stage could be possibly explained by the feeding of waste milk containing S. aureus whereas this source of S. aureus is removed at weaning. The relatively high prevalence seen in the breeding age and pregnant heifers may relate to hormonal changes at puberty and or increased contact as heifers are brought in for breeding. Whether it be social contact or suckling, it is likely that the heifer calf carrying S. aureus could play a role in the epidemiology of S. aureus IMI in heifers because she is a source of S. aureus that would be constantly present to herdmates. Likewise adult body site carriers of S. aureus may represent the reservoir both for themselves and other cows during the dry period. It is generally accepted that S. aureus gains entrance to intramammary tissue via the teat duct (Anderson, 1983). This premise was based on conclusions of researchers who found that contamination of teats with bacteria leads to IMI (Little, 1937; Miller and Heishman, 1943). Staphylococcus. aureus placed on the teat orifice readily colonizes the tip of the teat and this colonization frequently precedes IMI (Bramley et al., 1979). Forbes (1968) demonstrated that the invasion of S. aureus into the teat sinus occurred between milkings which suggests growth of the organism through the teat canal. Thus, it has been accepted that S. aureus can readily colonize the teat skin and teat orifice and from this location invade the teat cistern via the teat canal. Staphylococcus aureus colonization of the teat skin of dairy cows has been recognized as an important risk factor to S. aureus IMI, because teat dipping or spraying with a germicidal solution immediately after every milking is regarded as the single most effective practice for the prevention (Pankey, 1984) and control (Philpot and Pankey, 1978) of new S. aureus IMI in lactating dairy cows. Prepartum heifers with lacteal secretions with S. aureus were at significantly greater risk (P < .05) of S. aureus IMI at parturition (Roberson et al., 1994). Staphylococcus aureus teat skin colonized heifers were 3.34 times more likely to have S. aureus IMI at parturition (P = .06) than noncolonized heifers. These studies strongly suggest that teat site colonization by Staphylococcus aureus are important risk factors both for cows and heifers.

The transmission pathways responsible for new S. aureus IMI occurring during the dry period may be similar to transmission pathways for prepartum heifers in that both groups of animals have similar environmental circumstances. The source of S. aureus colonization of the heifer's teat skin and IMI has largely been attributed to the feeding of mastitic milk and then allowing the heifers to suckle each other (Nickerson, 1987; McDonald, 1982). This theory has never been documented for S. aureus although Schalm (1942) concluded that this was the case for the other major contagious pathogen Streptococcus agalactiae. Barto and coworkers (1982) concluded that exposure of heifers to S. aureus of known pathogenicity by feeding milk containing the organism did not increase the incidence of udder infection at first calving when heifers are maintained in individual pens or otherwise prevented from nursing one another following feeding. A more important point of Barto's study is that 14% of heifer calves fed non-S. aureus pasteurized milk freshened with S. aureus IMI. This suggests that a major reservoir or mode of transmission, besides feeding of mastitic milk and suckling, was available to this control group of heifers. Another interesting finding by Barto was the failure to isolate S. aureus from the following external and internal sites: nostrils, lips, medial canthus of the eyes, poll, side of chest, area over sacrum, tailhead, caudal folds, external area of anus, area of the teats, tonsils, sublingual lymph node, lung, mediastinal lymph node, mesenteric lymph node, liver, spleen and mucosal surface of the rumen, duodenum, and colon of 10 bull calves which had been fed milk containing S. aureus. More recent findings by Bushnell (1989) show that 6.3% of heifers fed pasteurized milk and 6.4% of heifers fed raw milk containing S. aureus freshen with S. aureus IMI. The findings by Barto and later Bushnell suggest a less important role for mastitic milk in the epidemiology of S. aureus IMI in heifers. Research from Washington suggests no correlation between feeding mastitic milk and the prevalence of S. aureus IMI in heifers at freshening (Roberson et al., 1990). The evidence presented was that of herds with a low prevalence of S. aureus IMI, 8% of heifers freshened with S. aureus IMI compared to 9% of heifers in herds with a high prevalence of S. aureus IMI. The difference between the prevalence of the two groups of heifers was not statistically significant. Herds with a higher prevalence would have been expected to have more S. aureus milk fed calves and therefore more heifers freshening with S. aureus IMI. Nonetheless, feeding calves milk containing S. aureus is one method of getting the organism to the heifer calf. Additionally, newborn calves allowed to nurse their dam, may be directly inoculating themselves with S. aureus as discussed by Keys (1980). Because waste milk (colostrum, clinically mastitic milk, and residue milk) cannot be sold for human consumption, it is often fed to preweaned calves. Thus, waste milk represents one common source of S. aureus for calves. Further, fresh salable milk fed to calves is likely to contain at least small numbers of S. aureus organisms since roughly 80% of dairies have at least one cow with S. aureus IMI. An unpublished Washington study found that calves within 8 hours of birth already had body sites contaminated with S. aureus (Roberson). The calves received their dams milk only and the dams did not have S. aureus IMI. The S. aureus positive sites on the dams were the vagina, the muzzle, and the teat skin. The earliest possible age at which a heifer calf could be exposed to S. aureus is in utero. In a study by Eduvie et al. (1984), S. aureus was isolated from the uterus 1 day postpartum. Thus, either S. aureus was present in the gravid uterus or was a contaminate possibly from the vaginal flora. In either case, the calf would have been exposed to S. aureus either before or during parturition. Staphylococcus aureus has been recovered from heifer calves less than one day old (Harmon, 1989; Roberson (unpublished); Woodward et al., 1988). Davidson (1961) found the vagina of dairy cows to be the third most common site to recover S. aureus. These data suggests that colonization of the heifer by S. aureus can occur quite early, possibly in utero and implicates the reproductive tract of the dam as a possible source of S. aureus for the newborn calf. Thus, preventing S. aureus from coming in contact with heifer calves appears to be impossible.

It has been suggested that fly control may help reduce transmission of S. aureus to prepartum heifers. Palmer et al. (1941) observed flies on the teats of heifers at pasture and suggested that these vectors may have been responsible for the spread of S. aureus. In an experimental setting, Sanders (1940) succeeded in transmitting mastitis pathogens (not specified) from infected to noninfected cows by the common housefly, Musca domestica, and concluded that it is a natural vector of bovine mastitis. Ewing (1942) identified pathogenic staphylococci from 5 of 2629 flies captured in a dairy barn which housed 61 milking cows. A 1985 study by Bramley et al, recovered S. aureus, usually in low numbers, from 10 out of 2347 flies which presumably acquired the organism from the skin surface. Of 30 flies sampled in the Matos (1987), no S. aureus was recovered. Roberson and coworkers (1994) isolated S. aureus from flies captured from high S. aureus prevalence (> 10%) dairy herds but not from flies captured from low prevalence herds. Overall, S. aureus was recovered from 14 of 1069 flies, 9 of which were from the preweaned calf area and 10 of which were collected during the summer sampling period. If calves are receiving milk containing S. aureus, this milk is the likely source for the flies in the preweaned area. Over half of the flies with S. aureus were collected from the preweaned area of one herd. This herd had the highest prevalence of S. aureus IMI in the lactating cows, fed waste milk to heifer calves, and differed from the other six dairies in that calves up to 6 months of age were in close proximity to, and under the same roof as, the lactating cows. In the other six herds, the preweaned area was either outside or in a barn separate from the lactating cows. The role of flies in the aforementioned study was determined by comparing "fingerprints" of the fly strains to heifer IMI strains (Roberson et al., 1998). Over 50% of the flies with S. aureus were captured on a single dairy. The fly isolates were the same as 8 of the heifer S. aureus IMI isolates. However, the flies were captured during the summer of 1990 on a separate farm, than where these 8 breeding age and pregnant heifers were kept. Thus, it is highly unlikely that these flies were responsible for the transmission of S. aureus to these heifers. However, the fly isolates were also the same as isolates from milk of cows and body sites of preweaned heifer calves which were maintained on the same farm. This data suggests that the flies may be important vehicles of transmission. The study by Owens and others (1998) leaves little doubt as to the importance of flies. They reported that horn flies are capable of transmitting S. aureus induced IMI to heifers; they also noted that scabs on teats are a potential source of S. aureus. Owens also notes that S. aureus was present on at least some of the flies for up to 96 hours. Thus, there appears to be ample evidence suggesting that flies spread S. aureus and that fly control may be an important control procedure in dairy herds, especially those with a moderate to high prevalence of S. aureus.

The role of environmental S. aureus isolates appears less important in the epidemiology of this disease, and although S. aureus does not appear to compete well in the environment, it can be recovered from practically any site.

Air can be a means of spread of S. aureus. Lidwell and Brock (1973) report that airborne dispersal of S. aureus in human hospitals does occur. Shimizu (1979) has isolated S. aureus from air of poultry houses. Staphylococcus aureus was recovered from air inside a milking parlor both before and during milking, but was not recovered from the holding area (Matos et al., 1991). Staphylococcus. aureus was isolated from 5 air samples from a barn housing breeding age heifers, a pasture with pregnant heifers, a preweaning heifer shed, a calf hutch, and one unknown area (Roberson et al., 1994). Of these 5 samples only one was considered the same strain as that of heifer IMI (Roberson et al, 1998). These results provide some evidence for a possible role of air as a vehicle for spread of this organism. The data also suggest that S. aureus recovery from air is more likely in enclosed or crowded areas.

Bedding is considered by Rendos et al. (1975) to have little bearing on the epidemiology of S. aureus IMI. Other studies support this conclusion. Spencer and Lasmanis (1952) did not isolate S. aureus on the floor of the barn beneath the cows' udders. Matos (1988) found S. aureus in only 1 of 10 bedding samples and agreed with the concept that bedding is not a significant source of this organism. However, Thorne and Wallmark (1960) isolated S. aureus from the floor of the stalls of dairy cows and found the phage pattern was identical to that found in milk samples from these cows. Spencer and Lasmanis (1952) were able to maintain S. aureus in vitro on sterile straw for at least 49 days which suggests that S. aureus could survive in bedding for an unspecified period of time. Four of 208 bedding samples from high S. aureus prevalence herds were positive for S. aureus whereas 0 of 183 in low prevalence herds were positive; again suggesting that herd prevalence may be directly related to environmental contamination (Roberson et al., 1994). Although S. aureus of the same strain as found in heifer IMI at parturition was recovered from bedding (Roberson et al., 1998), an epidemiologic role is possible but appears limited.

Reports indicate that from 10 to 40% of people not associated with hospitals are carriers of S. aureus with the predominate sites being the throat and anterior nares (Noble et al., 1967; Tuazon and Sheagren, 1974; Kloos and Musselwhite, 1975). In dairy workers, the hands may be considered the most important site of S. aureus because the hands are in intimate contact with the udder. Based on biotyping or phage typing patterns, there have been several reports of human strains of S. aureus isolated from dairy cow IMI. Devriese (1984) notes that of 94 strains of S. aureus isolated from bovine mastitis 4% were of the human ecovar. Using the Devriese (1984) biotyping system, Matos (1988) found that 11% and 23% of subclinical and clinical cases of bovine mastitis were of the human ecovars. Swartz et al. (1985) found that 71.3% of bovine S. aureus IMI strains were typeable with the human phage set suggesting humans as the source of many of these infections. Davidson (1961a) identified one strain of S. aureus from a milker's hands that was isolated from a case of mastitis and was typical of human staphylococci. Hence, human carriers may be regarded as important factors in the spread of this disease. Humans may also be carriers of bovine strains of S. aureus. Peel et al., 1975, referenced a Simmons and Frost (unpublished ,1972) study stating that nasal swabs from abattoir workers showed some strains of S. aureus typical of human as well as bovine phage patterns. Matos (1988) found that 6.5% of S. aureus isolated from his research team were of the bovine ecovar. These latter studies indicate that humans can carry bovine strains of S. aureus. Even if a human is not a carrier of S. aureus, humans may still act as mechanical vectors in transmission of S. aureus. Bramley and Dodd (1984) state that even under the best milking routines, handling of the teats before milking will disseminate bacteria. Dodd et al. (1966) found that 50% of the milker’s hands were infected with S. aureus before milking and during the milking period 100% were infected. Edwards and Rippon (1957) found staphylococci on the hands of the milkers; the staphylococci from these sites had the same phage pattern as those excreted in the milk of cows in the same herd. Saperstein et al. (1988) identified S. aureus on the hands of milkers and noted that it could survive between milkings. Dairy workers from 4 of 7 dairies were colonized by S. aureus (Roberson et al., 1994) and 2 heifers with S. aureus IMI at parturition had the same strain as that isolated from the nose of a dairy worker (Roberson et al., 1998). These studies attest to the role of dairyworkers in epidemiology of S. aureus IMI.

Staphylococcus aureus has been isolated from equipment, instruments, housing, water, feedstuffs, and non-bovine species (Roberson et al., 1994). Water and feedstuffs are common sources shared by cattle and logically could play a role in the transmission of S. aureus. However, the importance of water, non-bovine animals, and equipment sources of S. aureus remains unclear because these sources yielded different S. aureus strains than those from heifer IMI (Roberson et al., 1998).

1. The infected mammary gland is the primary and most important reservoir of S. aureus.

2. The infected mammary gland is not the only source/reservoir of S. aureus.

3. Heifers may freshen with S. aureus IMI (~ 4%) and cows may develop S. aureus IMI during the dry period.

4. Contagious mastitis control measures should continue even in herds with low to non-existent levels of S. aureus IMI.

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