Staphylococcus aureus infections

International Common Names

• English: agalatiae mastitis-metritis syndrome; avian staphylococcal infections; bacterial chondronecrosis with osteomyelitis in broiler chickens; bacterial endocarditis in ruminants; bovine actinobaccillosis; comb necrosis in layer breeder chickens; enzootic calf pneumonia; folliculitis of sheep; gangrenous dermatitis of chickens and turkeys; goat mastitis due to staphylococcus aureus; impetigo of sheep; mammary abscess in pigs; mastitis; mastitis in ewes due to miscellaneous bacteria; ovine and bovine infectious keratoconjunctivitis; ovine interdigital pouch lameness; purulent osteomyelitis; pyoderma of sheep; scalded skin disease; septicemia of lambs or kids, tick pyemia; staphylococcal dermatitis; staphylococcal dermatitis in birds; staphylococcal dermatitis in cattle, impetigo, folliculitis; staphylococcal dermatitis, folliculitis, furunculosis, impetigo, of sheep; staphylococcal dermatitis, folliculitis, impetigo, of sheep; staphylococcal mastitis; staphylococcal, staphylococcus aureus, mastitis in cows; staphylococcus aureus mastitis in ewes; Staphylococcus aureus-induced septicaemia; staphylococcus infections in birds

Local Common Names

• Norway: hva; mastitt

Staphylococcus aureus

History

The early classical historical, but un-referenced, description of Micrococcus (Staphylococcus) was recorded by Merchant and Parker (1967). These authors stated that micrococci were first discovered, in pus, by Ogston in 1881, who then divided the organism (Micrococcus) into two other groups namely Staphylococci and Streptococci respectively in 1883.

Merchant and Parker (1967) further stated that in 1884, these organisms (Staphylococci and streptococci) were first cultivated in artificial medium. During this process two species of Staphylococci, Staphylococcus pyogenes aureus and Staphylococcus pyogenes albus emerged.

Scientists have demonstrated the relationship of these cocci organisms to human infections following experimental self-inoculations.

Taxonomy and Nomenclature

Migula, in 1895, officially assigned three organisms (two staphylococci and one streptococci) to the genus Micrococcus and they were then named Micrococcus aureus, albus and citreus respectively in 1900. Further nomenclature of staphylococci into chromogenic strains such as S. pyogenes albus (a non-pigmented variety), a golden form, and an intermediate yellowish variety of S. pyogenes citreus, was observed to be merely a physiological characteristic that was readily lost during primary and sub-cultures. Transformation in historical nomenclature occurred then, when Buchanan, in 1911, used the genus name Micrococcus for the group and later Staphylococcus in 1917.

The first well defined systematic classification of these genera was by Bergey in (1948), who maintained the genus name Micrococcus; and in the 1957 edition classified them in the genus Staphylococcus and family Micrococcaceae consisting of four genera, Planococcus, Stomatococcus, Micrococcus and Staphylococcus. Bergey has since classified Staphylococcus in the family Staphylococcaceae. More than nineteen species of the genus Staphylococcus are recognized and four groups are clearly distinct, based on deoxyribonucleic acid (DNA:DNA) hybridization analysis. These four groups are S. epidermidis, S. saprophyticus, S. simulans and S. sciuri. Of the large number of staphylococcal species, only three (S. aureus, S. epidermidis and S. saprophyticus) are commonly associated with some human and animal diseases. However, S. aureus, amongst the staphylococcal species is one of the most important causative pathogens in a variety of human and animal diseases.

Association of S. aureus with animal diseases

One of the most economically important diseases caused by S. aureus in dairy animals is mastitis. This is commonly defined as an inflammation of the mammary gland accompanied by physical, chemical, pathological and bacteriological changes in the milk and glandular tissues (Blood and Radostits, 1989; Ameh et al., 1993; Oliver et al., 1999; Apolo et al., 1999; Fitz-Patrick, 2000). S. aureus mastitis in cattle, sheep and goats has serious financial and milk production implications for intensive herds and flocks in the diary industry (Elecko, 1998; Smith et al., 1998; Saratis et al., 1998; Barkema et al., 1999; Sears and Smith, 1999).

In an extensive system of production, multiparous ewes have been shown to have higher milk somatic cell counts compared with primiparous ewes (Lafi et al., 1994). In another study, S. aureus was reported to be the dominant pathogen in clinical cases of mastitis from 145 different breeds of Nigerian goats, as well as from 173 quarter milk samples collected from 297 dairy cows, maintained extensively in Egypt (Shoukry and Shabana, 1997).

The eradication of S. aureus from extensive dairy herds or flocks is more difficult than in intensive systems due to a non-adherence to mastitis control guidelines (Zurzul, 1994). Persistence of this organism in affected udders results in poor milk production, low condition scores, culling and preponderance of resistant strains in affected animals (Elecko, 1998; Zecconi and Piccinini, 1999; Tyagunenko et al., 1999; Costa et al., 2000). It can also lead to zoonotic transmissions from milk to man (Blood and Radostits, 1989; Oboegbulem and Fidelis, 1996).

S. aureus has also been incriminated, or shown to be associated with a variety of human and animal diseases. It has been reported as causing a generalized scalded skin disease in lambs, resembling ‘scalded skin syndrome’ of man. The characteristic lesions of this disease include exfoliation with deep ulcerations in the affected areas (Scott and Murphy, 1997; Yeruham et al., 1999).

Other infections caused by S. aureus and reported in pigs, include purulent osteomyelitis with lesions located at the costo-chrondral junction of affected pigs (Jensen et al., 1999). This disease has been experimentally reproduced when eight calves were intramedullarly infused with a haemolytic strain of S. aureus. This induced traumatic osteomyelitis shown by arteriographic and radiographic analysis. The analysis clearly indicated areas marked by an increased sprouting of osteomyelitic arterial networks around the meta-tarsal bones (Das et al., 1998). Similar microscopic osteo-myelitic lesions have also been observed in experimentally infected goats (Hoque, 1997; Singh et al., 1998).

Staphylococcal dermatitis in some domestic animal species is associated with three bacterial species, S. aureus, S. hyicus and S. epidermidis. S. aureus induced septicaemia has also been reported in chickens and lambs (Pyrah et al., 1994; Rasmaswamy et al., 1997; Fisher et al., 1998). Other important diseases associated with S. aureus in domestic animals which have serious implications in intensive and extensive husbandry systems includes agalactiae mastitis-metritis syndrome in cows and pigs. This is characterized by a purulent uterine discharge (Aerts et al., 1995; Esmat and Badr, 1996). Other equally important diseases implicated or associated with S. aureus in domestic animals are: ovine interdigital pouch lameness (Egwu et al., 1994), bovine actinobaccillosis (Mondodori et al., 1994), abcessation (Sharma et al., 1994), porcine gastroenteritis (Ayoade et al., 1990) and ovine/bovine infectious keratoconjunctivitis (Egwu et al., 1989; Egwu, 1992; Egwu et al., 1993; Chhabra et al., 1996; Takele et al., 2000).

The ability of S. aureus to produce these diseases in domestic animals, is due to elaboration of cytolytic, sphingomyelinase, leukocidic, demonecrotic and entero-toxins (A-E). These include enzymes such as hyaluronidase, nucleases, penicillinase, sphingomylinase, coagulases and the production of ‘slime’, which is known to aid attachment, inversion and multiplication of the organism (Baselga, 1994).

Drastic reduction and elimination of S. aureus-induced diseases, particularly mastitis, should involve good management. This includes good housing, sanitation, maintenance of milking equipment, routine monitoring and surveillance, composite and bulk milk testing, milker's hygiene, accurate timing and strategic treatment or chemotherapy of all diseases induced or associated with S. aureus. These measures are important to ensure the production of good quality milk and profitability. More importantly they are ensuring the consumption of high quality milk as one of our essential dietary components, with reduced chances of contracting diseases such as tuberculosis, streptococcal sore throat, staphylococcal entero-toxin and brucellosis. These are all of zoonotic importance, and are commonly associated with inadequate food safety or hygiene.

blood and circulatory system diseases of large ruminants
blood and circulatory system diseases of pigs
blood and circulatory system diseases of poultry
blood and circulatory system diseases of small ruminants
bone, foot diseases and lameness in large ruminants
bone, foot diseases and lameness in pigs
bone, foot diseases and lameness in small ruminants
digestive diseases of large ruminants
digestive diseases of pigs
digestive diseases of poultry
digestive diseases of small ruminants
mammary gland diseases of large ruminants
mammary gland diseases of pigs
mammary gland diseases of small ruminants
multisystemic diseases of large ruminants
multisystemic diseases of pigs
multisystemic diseases of poultry
multisystemic diseases of small ruminants
muscular skeletal diseases of poultry
nervous system diseases of large ruminants
nervous system diseases of pigs
nervous system diseases of poultry
nervous system diseases of small ruminants
reproductive diseases of large ruminants
reproductive diseases of pigs
reproductive diseases of poultry
reproductive diseases of small ruminants
respiratory diseases of large ruminants
respiratory diseases of pigs
respiratory diseases of poultry
respiratory diseases of small ruminants
skin and ocular diseases of large ruminants
skin and ocular diseases of pigs
skin and ocular diseases of poultry
skin and ocular diseases of small ruminants
urinary tract and renal diseases of large ruminants
urinary tract and renal diseases of pigs
urinary tract and renal diseases of poultry
urinary tract and renal diseases of small ruminants

Gross pathology

Gross and histopathological changes are common features that are associated with mastitis infected glands. Gross lesions are usually obvious following palpation of the infected udder and can be either a result of pronounced or massive swellings. The udder is usually enlarged to approximately twice its normal size, with other associated signs which are discussed in the disease course text (Blood and Radostits, 1984; Egwu et al., 1994; Ameh et al., 1996; Fabre et al., 1997). Necrotizing gangrenous lesions are a common feature associated with mastitis caused by S. aureus, Clostridium perfringens and Escherichia coli (Blood and Radostits, 1989; Ameh et al., 1994). The affected gland shows marked necrosis of the parenchymal epithelium coupled with foci of epithelial erosions, and ulcerations of the ductal system and surrounding skin (Blood and Radostits, 1989; Ameh et al., 1994; Cifrain et al., 1995, Jones, 1998).

Though necropsy procedures are usually not common in animals that have died of gangrenous and other clinical forms of mastitis, further examination of histopathological changes in affected animals are important in order to ascertain the extent of tissue damage.

Histopathology

Lesions observed through histopathology, include accumulation of large numbers of PMNs and other inflammatory cell cytokines (Sanchez et al., 1994; Shoshani et al., 1995; Leitner et al., 2000). Histologically, the inter-lobular duct’s epithelium, in the infected mammary gland, has been displaced by bits of necrotic debris and bacterial clumps. These ducts (interlobular) are necrotic with fibrinous thrombi, and sometimes the vascular walls of the inter-lobular stroma exhibit thrombosis (Shibahara and Nkamura, 1999; Leitner et al., 2000).

Marked accumulation of PMNs, which is sometimes not reflected in the grossly affected gland, is the basis of sub-acute diagnosis of mastitis using the California, Wisconsin and other somatic cell counts (Lam et al., 1996; Hulsen, 1997). This histopathological change, though not diagnostic for S. aureus-induced mastitis, could reveal the causative agent of the mastitis pathogen if backed by a laboratory culture of the collected samples.

Pathogenesis

S. aureus induces disease by either toxin production (Cifrian et al., 1996; Quinones and Demo, 1997; Jones 1998), or by direct invasion and destruction of tissues (Kang et al., 1996; Quinones and Demo, 1997).

Other staphylococcal species can invoke disease by proliferation of the causative agent in conjunction with some virulent toxins and/or associated enzymes produced by these organisms (Blood and Radostits, 1989; Baselga, 1994; Bezek and Hull, 1995; Ichikawa 1996). Amongst the numerous diseases caused by S. aureus, mastitis is of a high priority in dairy animals. This disease (mastitis) can be classified as per-acute, sub-acute, acute and chronic (Blood and Radostits, 1989; Simpson et al., 1995; Topolko and Benice, 1997) forms or clinical stages.

The disease process is dependent on the ability of the organism (S. aureus) to survive in the vicinity of the dairy animal, and its ability to confer resistance on a variety of disinfectants and antibiotics used in the farm (Boddie and Nickelson, 1997).

The strain variation of S. aureus accounts for the degree of pathogenicity (Bansal et al., 1995; Hermans et al., 2000). However, the size of the inoculum, stage of lactation, injuries on the teat and elaboration of toxins and enzymes will determine the type of mastitis and pathology induced in the hosts (Sultra and Poutrel, 1994; Bansal et al., 1995; Ameh et al., 1993; Quinones and Demo, 1997; Fitzgerald et al., 2000). In spite of these aforementioned factors, the adherence of S. aureus to the epithelial cells of the mammary gland, is the first process required for the initiation of pathogenesis (Katic et al., 1996).

Clinical Forms of Mastitis

Per-acute

This occurs most frequently during infections of early lactation (Blood and Radostits, 1989). It is sometimes accompanied by gangrene of the udder, due to the necrotizing action of the alpha toxins (Cifrain et al. 1995; Jones 1998). Per-acute mastitis usually occurs during the first few days of lactation and is accompanied by a severe temperature elevation to 41 - 42^°C, tachycardia, pronounced depression, absence of rumen motility and muscular weakness (Blood and Radostits, 1989). The affected quarter or half of the animal is swollen and hard. There is subcutaneous emphysema, blister formation and gangrene in some affected cases. These inflammatory processes can subside within a few days, and the infected gland is then replaced by proliferation of connective tissue, marked by blockage and atrophy of the ductal area.

Sub-acute

The sub-acute form of S. aureus mastitis is transient and mimics the clinical signs of either the acute or chronic forms, or both. The milk may show some degree of infection as evidenced by blood clots (Blood and Radostits, 1989; Petrovic et al., 1997) with an elevation of the leukocyte counts (Lam et al., 1996). The California mastitis test (CMT) is routinely used on individual, composite and bulk milk samples (Simpson et al., 1995; Leitner et al., 2000) to detect levels of polymorphonuclear cells (PMNs). The temperature of the affected animal may or may not become elevated (Blood and Radostits, 1989; Egwu et al., 1994; Rapoport et al., 1994).

Acute

The acute form of mastitis is characterised by a visible enlargement of the udder. There are marked changes in the milk observed, such as evidence of blot clots, fibrin, watery consistency, lobules and coloration. The temperature of the animal is elevated and the enlarged udder is painful on palpation. The affected animal is weak, comatose and sometimes becomes recumbent in unfavourable situations (Ameh et al., 1996; Fabre et al., 1997). Other associated signs include dullness, anorexia, inappetence and general malaise. Milk quality and production are reduced drastically (Wilson et al., 1997). This form is clinically recognizable by the farmer and provides an opportunity for reporting such cases urgently to the veterinarian or other appropriate authorities.

Chronic

If an animal is suffering from the chronic form of mastitis, the functional capacity of the udder is compromized. There is atrophy or shrinkage of the udder and milk production is also drastically reduced. The glandular tissue becomes fibrosed, and sometimes the ducts are blocked with difficulty in milk letdown through the teat sphincter (Shoshani et al., 1995). Most causal agents persist in the udder during this stage and with appropriate treatment, recovery from this clinical form of mastitis is high (Deutz et al., 1995). Farmers usually try to maintain patency of the teat canal by making use of teat cannulas. Total milking out by farmers or veterinarians can assist in the successful application of chemotherapy at this stage.

The staphylococcal species, with particular reference to S. aureus and associated diseases, are ubiquitous worldwide and affect a whole range of domestic and wild animals (Blood and Radostits, 1989; Oliver et al., 1999; Smith 1999).

S. aureus is always a dominant causal agent of mastitis (Smith et al., 1998; Elecko, 1998; Oliver et al., 1999) and is also highly predisposed to many organs of animal and human hosts, thereby causing different diseases (Egwu et al., 1989; Mondodori et al., 1994; Esmat and Badr, 1996; Ramaswamy et al., 1997; Takele and Zerihun, 2000).

Direct replication (cell division) of S. aureus is by binary fission and intermediate hosts or vectors are not necessary in the development of this organism. However, it is interesting to note that tick-pyaemia of lambs, characterized by elaboration of neuro-toxins, as a result of tick bites on the definitive host (lambs) is enhanced by S. aureus (Clarkson and Faull, 1985; Blood and Radostits, 1989). The obvious lesions resulting from these septicaemic staphylococcal infections are lameness, abcessation of vital organs, unthriftiness and general weakness of affected lambs. Experimentally, ixodes ricinus ticks acting as intermediate hosts infected with S. aureus have also reproduced typical lesions of tick pyaemia (Webster and Mitchell, 1986). The disease (tick pyaemia) has been reported as occurring naturally and concurrently with other diseases of domestic animals such as, enterotoxaemia, tick-borne fever, louping ill and lamb dysentry (Clarkson and Faull, 1985; Blood and Radostits, 1989).

Incubation Period and Transmission

The primary barrier protecting against the entry of S. aureus into the mammary gland is the teat sphincter. When the pathogen (S. aureus) overcomes this barrier, if not washed out by the physical act of milking, the bacteria multiplies, invades and proliferates in the udder tissue (Das and Khanna, 1994; Sultra and Poutrel, 1994).

Considerable variation occurs amongst susceptible animals, between infection and appearance of clinical signs. The incubation period may be for 3 - 4 days following infection of the teat (Blood and Radostits, 1989; Fox et al., 1995), with inflammatory processes coming into action 3 - 5 days later. These processes however, depend on the virulence, host immune mechanisms and weight of infecting pathogen. These factors also determine the degree of pathology and type of mastitis produced (Blood and Radostits, 1989; Das and Khanna, 1994; Sutra and Poutrel, 1994; Bansel et al., 1995).

Direct and indirect transmission of S. aureus, are usually the main avenues of disseminating an infection. Usually, mammary gland infection occurs via the teat canal and/or environment (Blood and Radostits, 1989; Bansal et al., 1995; Bouchard et al., 1996; Kosev et al., 1996). It can also occur through other unhygienic measures such as, housing, defects in milking machines and management factors (Demchonko, 1994; Vasil, 1994; Refsdal, 1996; Fox et al., 1995; Benda 1998; Barkema et al., 1999). Other inter-alia factors include: the use of clean milking towels for pre and post cleaning of mammary teats, clean calving pens, unhygienic housing condition, improper adjustments of milking machine, dirty teat cups or the use of unclean hands during hand milking. The importance of all factors mentioned cannot be overemphasized in their respective roles in the transmission of mastitis pathogens.

S. aureus is known to co-exist with many other bacterial pathogens occurring in the environment and other animal sites (Poorima and Updhye, 1995; Poorima and Upadhye, 1995; Huan and Huan, 1996; Yanni, 1999). However, the ability of S. aureus to persist in the tissue of the udder makes it somewhat more ameliorable to treatment, compared to other organism such as Escherichia coli, Streptococcus dysagalactiae, S. uberis and Pseudomonas aeruginosa that are all found in the environment (Sultra and Poutrel, 1994).

Since S. aureus is ubiquitous in distribution and is also involved in many other animal diseases, contact transmission, aerosol transmission, wounds and abrasions on the body surface play an important role in the transmission of S. aureus (El-Sukhon 1995; Poorima and Upadhye 1995; Yanni, 1999; Moller et al., 2000). They also play a role in other induced diseases such as mastitis-metritis syndrome, abcessation, gastroenteritis, septicaemia, osteomyelitis and urinary infections, including respiratory and environmentally acquired infections. Dissemination of the organism from one organ to the other is also possible, depending on the invasiveness or multiplication of the organism in the host tissues.

Economic impact of Mastitis

Mastitis caused by Staphylococcus aureus, is a major disease for the dairy industry to contend with, if productivity and health of the animals are to be ensured.

Generally in most affected animals, losses occurring from fatalities amongst dairy herds are very minimal compared to those resulting from milk production (Blood and Radostits, 1989; Deutz et al., 1995). Affected cows with sub-clinical mastitis have been shown to produce low birth weight calves (Dominigues et al., 1996). This may pose very serious economic costs and returns in rearing the dairy calves.

Contaminated milk has transmitted some zoonotic bacteria to man, when the milk has not been properly pasteurized before human consumption. Such diseases caused by this transmission include tuberculosis, streptococcal sore throat, brucellosis and staphylococcal food poisoning (Blood and Radostits, 1989; Mousing et al., 1997; Hernandez and Baca, 1998). Most of these diseases are becoming devastating in most developing countries and some are emerging as diseases in parts of the developed world.

Economic effects resulting from quarter or udder half infections of the mammary gland are drastically reduced milk yield, monetary returns and profit to the dairy farmer. This is dependant on the stage of lactation and time of infection (Shoshani et al., 1995; Deutz et al., 1995; Ameh et al., 1996; Fabre et al., 1997). Infection of cows during the dry period could reduce milk yield by up to 11%, whereas those occurring at late lactation could reduce percentage yield by up to 48% (Blood and Radostit, 1989; Deutz et al., 1995). This constitutes tremendous economic waste and financial returns to the dairy farmer.

The prevalence of specific mastitis pathogens such as S. aureus varies from herd to herd. In a high prevalence herd, conservative economic milk losses resulting from S. aureus mastitis on a ‘cow basis’, could be in excess of US $90-250, particularly in large dairy herds (Blood and Radostits, 1989).

Quarters or halves of udders from animals infected with S. aureus, alone or in combination with other mastitis pathogens, can result in low or poor quality milk and market sales (Shoshani et al., 1995; Lam et al., 1996; Fabre et al., 1997). Losses emanating from premature culling as a result of unthriftiness or poor condition of acutely or chronically affected animals, could seriously undermine the productivity of the dairy farm as well as monetary returns (Deutz et al., 1995; Dominigues et al., 1996; Moclerson et al., 2000).

The greatest economic losses for dairy farms with mastitis are usually associated with the costs of veterinary medicine and care, particularly in those herds with acute or chronic forms of the disease. The use of pre-dipping disinfectants, intra-mammary antibiotics and other systemic antibiotics can impact large economic losses in monetary terms to the dairy farmers. This is especially the case, if these are carried out routinely in the process of trying to eliminate persistent and recalcitrant strains of S. aureus on a dairy farm.

Apart from those practices aimed at reducing intra-mammary infections, there are other diagnostic costs incurred by the farmer. These include routine somatic milk counts on individual, composite and bulk milk samples and the increased labour costs for this. There are obvious advantages in monitoring levels of milk infection, but it does create an extra cost.

Other Economic Impact of S. aureus Associated or Induced Diseases

Other losses occurring from S. aureus induced or associated diseases, include blindness in small and large ruminants. This could be temporary or permanent blindness as observed in sheep, goats and cattle suffering from ovine, caprine and bovine kerato-conjunctivitis (Egwu et al., 1989; Al-Gaabary et al., 1998; Takele and Zerihun, 2000). Affected animals with this acute or chronic ocular disease are unthrifty, off-feed, emaciated and generally lose sight of their environment. It is not clearly known what role S. aureus plays in initiating the disease or exacerbating the disease condition, but its frequent association with the disease process is likely to indicate a possible role of toxins and enzymes, and other ocular inflammatory mediators in the disease process. Corneal ulceration (see picture) which is usually purulent, or scarification on the cornea following healing, has been reported to be associated with S. aureus following natural and experimental infections (Egwu et al., 1989; Egwu and Faull, 1991, Egwu and Faull, 1993).

Tick pyaemia causes the early death of lambs, particularly those with good traits that make them potential replacements for breeding ewes. Losses from this disease could be as a result of septicaemia, lameness, arthritis and disseminated abscessation of vital organs (Clarkson and Faull, 1985; Webster and Mitchell, 1986).

There is an important public health aspect of S. aureus infections. Staphylococcal food poisoning is becoming important in the milk and meat industry (Blood and Radostits, 1989; Mousing et al., 1997; Hernandez and Baca, 1998). Nosocomial (hospital acquired) infections are common sources of acquiring S. aureus. The need for pasteurization of milk before consumption, and the sterilization of prosthetic surgical equipment before open surgery must also be ensured. Many other pathogens of zoonotic importance apart from S. aureus are involved in ‘mastitides’ (Blood and Radostits, 1989). These include Mycobacterium spp. (El-Haenaeey et al., 1994; Klinger and Rosenthal, 1997), and commonly incriminated species of this genera (mycobacterium) include: M. bovis, M. fortuitum, M. smegmatis, all of which are commonly acquired from contaminated milk and its products. Infections can also be obtained from infections of the hosts or from contaminated unhygienic preparations of milk and meat products (El-Haenaeey et al., 1994; Klinger and Rosenthal, 1997). Clostridium perfringens type A, Bacillus spp., Campylobacter jejuni, and Brucella abortus and milletensis are also commonly acquired pathogens from improperly pasteurized milk and its products (Egwu et al., 1994; Huan and Huan 1996; Waage et al., 1999).

The isolation of S. aureus from local milk products (nono) or local yoghurts, even at low pH has been reported by Egwu et al. (1995b). Consumers should therefore ensure that milk and meat products are wholesome before taking them to market, to ensure freedom from some of these zoonotic diseases. This is particularly important in developing countries where consumption of local milk products is popular.

Consideration for the treatment of S. aureus induced diseases should involve the target sites involved in the disease process. These sites include the udder, respiratory tract, eyes, bones and skin where the diseases are manifested. Priority is however ascribed to S. aureus mastitis, as it stands out prominently because of the economics involved in controlling the disease. Treatment should aim at combating or controlling the severity of mastitis through chemotherapy, with the use of the right antibiotics following adequate sensitivity tests or anti-biogrammes (Sandgren and Ekman, 1996).

Antibiotics

A number of first generation antibiotics including penicillins, streptomycin, cloxacillin, penicillin/streptomycin combination and nitrofurazone, have been used at various doses and regimens for the treatment of S. aureus mastitis (Blood and Radostits, 1989; Owens et al., 1997; Egwu et al., 1994; Dhabele et al., 1997; Schlegelova and Sediva, 1999).

The intramammary and/or systemic routes are usually employed during the chemotherapy of mastitis (Egwu et al., 1994; Pyorala and Pyorala, 1994; Dhabele et al., 1997; Owens et al., 1997).

Most drugs used in the treatment of S. aureus mastitis tend to be more sensitive in vitro than in vivo (Egwu et al., 1994; Owens et al., 1994; Dhabele et al., 1997), following experience from laboratory and field investigations.

The increasing incidence of S. aureus resistance following mastitis treatment has resulted in inefficacy treatment in the use of antibiotics (Allen et al., 1994; Simko and Bartes, 1996; Schlegelova and Sediva, 1999).

There is a need for antibiotic sensitivity testing prior to treatment of mastitis to determine the best drug of choice (Cruz et al., 1997; Kumajock et al., 1999). This should involve serum blood concentrations of the drug of choice, minimum inhibitory concentration studies and disk sensitivity antibiotic testing (Owens et al., 1994; Egwu et al., 1994; Allen et al., 1994; Simko and Bartes, 1996), so as to allow the veterinarian to select the best antibiotics.

Ethnoveterinary Practice

Ethnoveterinary practice has not assumed any significance in recent times for the treatment of mastitis and other related diseases. However, the efficacy of this method of treatment has not been properly assessed in line with current antibiotic usage. This practice in nomadic pastoralism could offer some benefits to the nomadic farmer, who may find costs of modern antibiotic treatment very exorbitant. Some of these practices may be handed from generation to generation in the family line.

Chemotherapy of Other S. aureus Diseases

Infections like osteomyelitis, keratoconjunctivitis, and staphylococcal dermatitis can be treated with oxytetracycline, erythromycin, demofloxacin and other suitable antibiotics (Egwu, 1992; Egwu and Faull, 1993; Al-Gaabary et al., 1998).

The commonly used drugs, dose, adverse reactions and possible emergence of resistant strains in the disease treatment table.