Histophilus somni disease complex in cattle

Histophilus somni (Haemophilus somnus) disease complex in cattle

P N THOMPSON

Introduction

Histophilus somni is the cause of a variety of clinical syndromes in cattle, including those associated with septicaemia, thrombotic meningoencephalitis, pneumonia, myocarditis, arthritis, mastitis, and disease of the reproductive tract in both cows and bulls. Disease caused by H. somni occurs in many countries, but it is seldom recognized in southern Africa and has only been reported from South Africa50, 67, 77, 102 and Zimbabwe.95

Disease due to H. somni was first described in 1956 in feedlots in the USA.8, 39, 65 This disease manifested in the form of an infectious meningoencephalitis, and has been variously termed the ‘sleeper syndrome’, thromboembolic meningoencephalitis (TEME), infectious thromboembolic meningoencephalitis (ITEME) and, more recently, thrombotic meningoencephalitis (TME). The last name for this syndrome is preferred because it has been established that embolism does not play a role in the pathogenesis of the lesions.45 Although TME was thought to be the major disease problem, fewer cases of it are being encountered, while some of the other syndromes are becoming more common and new manifestations of infection are being identified.45

When disease due to H. somni first breaks out in a region, it is predominantly seen as TME, but as the infection becomes endemic there is a gradual change in its manifestation, and the respiratory form attains a high profile.80

Histophilus somni has been isolated from cattle in many parts of South Africa, but clinical disease as a consequence of the infection has been diagnosed relatively infrequently.50, 67, 68, 77, 102

There are several excellent reviews on H. somni and the disease syndromes that it produces.10, 45, 57, 71, 119

Aetiology

Bacteria of the genus Haemophilus (meaning ‘bloodloving’) belong to the family Pasteurellaceae and occur as obligatory on the mucous membranes of humans and a variety of animal species.66 Histophilus somni is a facultatively anaerobic, microaerophilic, small, pleomorphic, non-motile, Gram-negative, non-acid-fast coccobacillus.57, 66

The taxonomic status of H. somni is still unresolved.66 It has been suggested that H. somni, Haemophilus agni and Histophilus ovis may belong to a single taxon.117 Owing to the similarity of these organisms, those isolated from sheep in North America are commonly identified as Histophilus somni, although cattle and sheep isolates have distinctly different biochemical and other characteristics130 and may be differentiated by polymerase chain reaction (PCR)-based DNA fingerprinting and plasmid profile analysis.5 Histophilus ovis causes a meningoencephalitis in sheep remarkably similar to TME in cattle.97 Further investigation into the relationship between them and Actinobacillus seminis is required.

Although there is a possibility that all isolates of H. somni are potentially pathogenic, they differ in virulence,41, 57, 72, 128 and H. somni is both a pathogen and a commensal organism.21

An atmosphere containing 5 to 20 per cent carbon dioxide is required for primary isolation of H. somni. It grows well at 37 °C on media supplemented with 10 per cent blood of cattle or sheep, although brain heart infusion agar supplemented with 0,5 per cent yeast extract may also be used.30, 37 Histophilus somni is easily overgrown by most contaminants and a selective medium, incorporating vancomycin, neomycin, sodium azide, nystatin and cyclohexamide in 5 per cent horse blood agar, is more successful for primary isolation.113

Histophilus somni is both X-factor (protoporphyrin, or protohaeme) and V-factor (nicotinamide adenosine dinucleotide, or NAD) independent, whereas most other Haemophilus spp. require the heat-stable X-factor and/or the heat-labile V-factor for in vitro growth.109 The colonies are about 1 to 2 mm in diameter after two to three days’ growth and are non-haemolytic, convex, circular and pale yellow in colour. The colour of the colonies is more conspicious when they are raised on a bacteriological loop.142 Identification of H. somni is based on morphological and colony characteristics, the fact that it is not dependent on either X- or V-factors, and biochemical testing,57, 70 as well as gel-diffusion tests with reference antisera,117 and disc electrophoresis of acid-phenol-soluble proteins.13 Commercially available techniques include the API ZYM system (API System S.A., La Balme les Grottes, France) which measures bacterial enzyme activity23 and the RapID NH system (Innovative Diagnostics, Atlanta, Ga., U.S.A.).104 Respiratory and genital isolates of H. somni can be differentiated by biotyping (sugar fermentation) and a variety of PCR methods.6, 29

Epidemiology

Thrombotic meningoencephalitis has been recorded in North and South America, Europe, Russia, Japan, New Zealand, South Africa and Zimbabwe.39, 46, 57, 67, 80, 95, 129 The organism has been isolated from the respiratory and reproductive tracts of cattle in North America and Europe,57 and in South Africa,11, 50, 98 where it has recently been associated with endometritis and infertility in KwaZulu-Natal.77 Abortion in cattle as a result of H. somni infection has been reported in several countries, including South Africa.21, 45, 50, 76, 121

Cattle between one and three years of age...

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