Pseudomonas spp. infections

The genus Pseudomonas is a complex group of Gram-negative bacteria, which are aerobic, straight or slightly curved rods 0,5−1,0 × 1,5−5,0 μm in size, and motile. The cells contain one or several polar flagella. Some species produce lateral flagella and possess pili, which are important in the attachment of the bacteria to cell surfaces and affect phagocytosis.¹ Many of the Pseudomonas spp. produce pigments, the best-known being pyocyanin and the fluorescent pyoverdin, which are produced in profusion in media of low iron content. Members of this genus are characterized by their ability to grow in simple media of neutral reaction.

Glanders and melioidosis in animals are caused by Burkholderia (previously Pseudomonas) mallei and Burkholderia (previously Pseudomonas) pseudomallei ⁸ respectively. They are also important zoonotic diseases. Glanders is a disease particularly of solipeds. It was a common and important disease at the time when equids were used extensively for draught and transport purposes, but it has been eradicated from many countries. Currently, glanders still occurs in eastern Europe and Asia. Melioidosis, which is primarily a disease of rodents, is not an important disease of livestock. It has, however, been sporadically diagnosed in various livestock species in tropical countries, particularly in south-east Asia. Melioidosis in a goat has been reported from southern Africa.⁷

Pseudomonas spp. are frequently isolated from a variety of biological tissue and organ specimens obtained from diseased animals and humans.¹ They are often the cause of nosocomial infections, particularly in immunocompromised humans and animals. Infections by them (and particularly P. aeruginosa) are opportunistic and of relatively little importance.² Pseudomonas aeruginosa is sporadically isolated from specimens collected from the lesions of bovine mastitis, equine endometritis, equine and bovine pyelonephritis and pyometra, and bovine suppurative bronchopneumonia and valvular endocarditis.²

Pseudomonas aeruginosa is, in addition, considered by some authors to be the sole causative agent of the disease known as fleece rot in woolled sheep, and it may be responsible for the production of pigmented wool. One study³ showed that the presence of P. aeruginosa was much lower (14 per cent) in cases of fleece rot than previously reported. However, this report also showed that the presence of P. aeruginosa was associated with more severe cases of fleece rot, and predisposed the sheep to subsequent fly strike.³

Fleece rot which is also known as wool rot, yolk rot or water rot, has been reviewed by Salisbury and Barrowman. ⁵ It is an acute superficial exudative dermatitis caused by prolonged wetting of the fleece and skin, and marked proliferation of bacteria on the skin surface. The wet conditions provide a suitable environment for bacterial proliferation, which may occur within 24 hours of wetting. The pathogenicity of P. aeruginosa has been attributed mainly to its extracellular products, such as the pigments, which may suppress the action of other microorganisms, proteases of localized activity, haemolytic toxins, enterotoxin, and the lethal toxin (exotoxin A), an acidic protein capable of inhibiting protein synthesis.⁴ Sheep with well-grown fleeces are more prone to develop the disease, and in Merino sheep the lesion tends to develop particularly in the skin and fleece of the withers and backline. The skin in the affected area assumes a deep purple colour and the seropurulent exudate causes a band of matted fleece to develop. The wool appears saturated and has a leached and dingy appearance.¹ In severe cases the wool can be easily plucked from the lesion and the skin may tear when the fleece is parted.

Fleece rot is manifested by seropurulent exudation with resultant matting of the wool fibres and the formation of a band of matted fleece adjacent to the skin. Pigmentation of the fleece commonly accompanies the disease, in which case the affected fleece may assume shades of green, brown, orange, pink or blue. Bacterial coloration of the fleece without skin exudation is not considered to be fleece rot.

The odours produced by bacterial decomposition in the lesion encourage blowfly strike, principally by Lucillia cuprina, which gives rise to the most economically significant feature of the disease. Fleece rot, together with the associated body blowfly strike, represents one of the major causes of production loss in the Australian sheep industry.

The two most important differential diagnoses of fleece rot are sheep scab due to Psoroptes communis var. ovis infestation and lumpy wool caused by Dermatophilus congolensis (see Dermatophilosis).

Pigmentation or discoloration of sheep’s wool in the absence of fleece rot has been reviewed by Van Tonder.⁶ The condition is caused by the growth of a number of chromogenic bacteria in the fleece, including P. aeruginosa, Chromobacterium violaceum, Serratia marcescens, Pseudomonas indigofera, Arthrobacter sp. and others. The ensuing coloration may be green, brown, yellow, violet, blue, purple, pink or red, depending on the species of bacterium present. Bacterial wool pigmentation has been described in several countries, including Australia, New  Zealand, the USA and South Africa, and is associated with very damp environmental conditions. In the case of P. aeruginosa infection the pigment pyocyanin that is elaborated by the organism is the...