Brucella ovis infection

GENERAL INTRODUCTION: GRAM-NEGATIVE AEROBIC OR CAPNOPHILIC RODS AND COCCI Brucella spp. infections

Introduction

Brucella ovis infects primarily sheep and is one of the most common causes of epididymitis in rams and a rare cause of abortion in ewes and neonatal mortality in lambs. Low reproductive rates may occur in affected flocks.⁴ Similar clinical manifestations have been occasionally reported in male red deer (Cervus elaphus) in New Zealand.⁸⁹ It is a non-zoonotic disease.

Brucella-like organisms, which were first isolated from the lesions of rams with epididymitis and from ewes in Australia and New Zealand in 1953,^{18, 98} were assigned to a new species, Brucella ovis.¹⁷ In South Africa, rams with clinical or subclinical epididymitis and low fertility as a result of B. ovis infection were diagnosed for the first time in 1956.¹⁰⁴ Since then the disease has also been diagnosed in Namibia and Zimbabwe.¹⁰⁵

With few exceptions B. ovis infections occur practically in all sheep-raising countries. In Australia it has been reported that up to 80 per cent of young rams in certain studs or flocks are infected.^{63, 86} In Europe, B. ovis infections have been described mainly in Southern European countries and more recently also in Eastern Europe.^{83, 84} The infection is rare in Northern America, whereas B. ovis infection is one of the major causes of low fertility or infertility in rams in Argentina³ and Brazil.³⁶ The infection is widespread in South Africa.^{34, 104, 105, 106} Results of a survey in 1984 revealed that 4,3 per cent of rams in the Free State Province of South Africa were infected.³⁴

Aetiology

Brucella ovis is similar to the other Brucella spp. in its morphology, staining properties and cultural characteristics,^{17, 62, 77} but it can be differentiated on the basis of its colonial morphology, as it is always isolated in the rough phase, negative reactions in the oxidase and urease tests, lack of agglutination with specific A and M antisera, and susceptibility to the R/C phage.^{2, 29}

Culture of the organism requires an atmosphere containing five to 10 per cent carbon dioxide,^{18, 62, 98}  and enrichment of the culture media with five to 10 per cent blood or serum.^{18, 62, 65, 98}  Optimum growth occurs at 37°C while no growth occurs at temperatures below 20°C or above 40°C.¹⁸  On primary culture after three to four days of incubation on media such as blood agar, the colonies are greyish-white, up to 0,5 to 2,5 mm in diameter, non-haemolytic, circular, convex, rough phase, glistening, translucent and entire-edged.^{17, 18, 62, 98}

Epidemiology

Sheep are naturally infected by B. ovis. Besides experimental infection,^{23, 43} there is serological evidence of natural exposure in billy goats.³⁰   Red deer (Cervus elaphus) are also susceptible and transmission occurs through direct contact with infected rams.⁹⁰  White-tailed deer (Odocoileus virginianus)⁷ are susceptible to experimental infection.
The disease is spread primarily by infected rams. Brucella ovis may be excreted in the semen of infected rams even before the development of lesions.^{22, 49, 64}  Clinically or subclinically infected rams may excrete B. ovis in their semen for years.^{17, 49}

Infection may spread from ram to ram as a result of homosexual activity or venereally during coitus when uninfected rams mate with ewes that passively harbour the bacteria.^{6, 14, 23, 25, 49, 63, 79, 95, 99, 100, 110, 111}  Ewes generally only transfer organisms mechanically from infected to uninfected rams when they are mated in succession by different rams during the same heat period. Vasectomized and cryptorchid rams are equally vulnerable as a result of direct transmission through contact, homosexual activity and venereal spread through ewes.

Direct ram-to-ram and venereal transmission result in a higher prevalence of infection in flock rams than in stud rams.^{6, 37, 38, 63, 82, 110, 114} This is probably due to the differences in mating and management practices between these two categories of rams, such as the regular examination of breeding rams and new introductions, individual versus mass mating, and the more isolated rearing and housing of stud rams. High prevalences and even epidemics have been reported in non-breeding rams housed together. Most ram-to-ram infections occur via the oral route. Housed rams establish hierarchies (head-to-head combats), and it is frequent that ‘dominated’ rams, after being ‘mated’ by the dominant rams, lick the prepuce of these dominant rams as an act of submission. If these dominant rams are infected, the probability of harbouring B. ovis in...