Staphylococcus aureus infections

M M HENTON

GENERAL INTRODUCTION: GRAM-POSITIVE COCCI Staphylococcus spp. infections

Introduction

Staphylococcus aureus is a commensal bacterium of animals and humans that most commonly occurs on the skin and in the nasopharynx, but may also be present in the alimentary and genital tracts.²⁰ It is a potential pathogen and may cause a range of pyogenic conditions, the major one in livestock being mastitis in cattle, sheep and goats. Other conditions for which it is periodically responsible include tick pyaemia of lambs;¹³ chronic pyogranulomatous in- flammation known as ‘botryomycosis’ which may occur in horses, cattle and pigs; folliculitis and furunculosis in horses, goats and sheep;¹⁴ pyoderma in goats, piglets and cattle;¹¹ staphylococcal dermatitis in sheep (also known as facial or periorbital eczema¹²); polyarthritis in young animals; impetigo or subcorneal pustular dermatitis of piglets; and dermatitis of the udder in goats.¹⁹ Wounds may become infected with S. aureus, and it may play a primary or secondary role in a great variety of infections. These include purulent bronchopneumonia and pyothorax, osteomyelitis, salpingitis and abscess formation. A variety of virulence determinants have been described,¹⁰ but the effects of these have only been studied in detail in human infections and bovine mastitis. In addition, many strains of S. aureus are an important cause of food poisoning in humans, but animals are resistant.^{10, 20}

Staphylococcus aureus of cattle, pigs, horses, poultry and humans can be distinguished from each other, and may be regarded as representing different ecovars (or biovars).²⁰ There is an association between biovars and their hosts; biovar A consists of human strains, B of pig and poultry strains, C of cattle and sheep strains, D of hare strains, and E, which is now called Staphylococcus intermedius, is commonly isolated from dogs, horses and pigeons.⁶ The association between biovars and their hosts is not absolute, e.g. human strains (A) can cause mastitis in animals.

Some of the conditions mentioned above, and of which S. aureus is the cause, are discussed below.

Tick pyaemia of lambs

Synonym: Cripples

Tick pyaemia of lambs is a pyogenic staphylococcal infection of two- to ten-week-old animals reported in the United Kingdom and Ireland, resulting from infestation with the Ixodes ricinus tick. Affected animals often suffer concurrent tick-borne fever caused by Ehrlichia (formerly Cytoecetes) phagocytophila (see Lesser-known rickettsias infecting livestock).^{2, 25} The syndrome has not been reported in southern Africa.

Staphylococcus aureus appears to be transmitted by I. ricinus, although the process would seem to be purely mechanical: the bacteria enter the skin of lambs via the wounds produced by the mouth parts of infected ticks. From these primary sites of infection the bacteria spread haematogenously to localize in many organs and tissues of the body, particularly the joints, where they produce a suppurative in- flammatory reaction.

As only young lambs between two and ten weeks of age are affected, it seems probable that an acquired immunity to S. aureus develops in older animals.¹³ It appears that tick-borne fever, though it is usually a mild disease, may predispose to the development of tick pyaemia by the suppression of the humoral and cell-mediated immune responses which occur during the course of the disease.¹ Additionally, the parasites also infect neutrophils, thus further impairing the defence against infection with S. aureus. ¹³

The disease occurs most often on hill farms in Scotland, Wales, north-east England and Ireland, and can result in serious economic losses. Prevalence rates as high as 29 per cent have been reported on some farms, although they are generally less than this; a figure of about 5 per cent is usual on farms on which the disease occurs.²³

The most commonly recognized forms of tick pyaemia are characterized by the development of suppurative polyarthritis that results in lameness (crippling) and multiple abscesses in other organs and tissues, or of vertebral abscessation with pressure on the spinal cord and consequent paresis or paralysis. Abscesses may, however, develop in virtually any organ or tissue, resulting in lambs which are thin, dull and unthrifty, or even in sudden death if one of the vital organs is involved.¹³

Conditions such as joint ill (both suppurative and nonsuppurative polyarthritides), lameness caused by foot rot, foot abscess or toe abscess (see Fusobacterium necrophorum, Dichelobacter (Bacteroides) nodosus and Bacteroides spp. infections), nutritional muscular dystrophy (white muscle disease), ill-thrift, and tick paralysis should be considered in the differential diagnosis of tick pyaemia.

The treatment of lambs suffering from tick pyaemia is often disappointing, as by the time the disease is noticed, lesions are well advanced and irreparable damage has occurred.¹³ The control of the disease is primarily dependent upon the control of the vector tick. Alternatively, holding ewes and their lambs in a fenced, tick-free or relatively tickfree-pasture until the lambs are about six weeks old is effective. In addition, suppression of either the S. aureus or E. phagocytophila infections by the administration of long-acting antibiotics to which the organisms are susceptible may reduce the prevalence of tick pyaemia.^{2, 13} Long-acting benzathine...