Moraxella spp. infections

A VAN HALDEREN AND M M HENTON

GENERAL INTRODUCTION: GRAM-NEGATIVE AEROBIC OR CAPNOPHILIC RODS AND COCCI

Introduction

Of the known Moraxella spp., only Moraxella bovis, the cause of infectious bovine keratoconjunctivitis in cattle, is recognized as being an important pathogen of livestock.⁹ Moraxella bovis is also isolated from sheep suffering from keratoconjunctivitis on rare occasions, while Moraxella equi has been isolated from horses with keratoconjunctivitis.²⁹ The role of Moraxella ovis in infectious ophthalmia in sheep and goats is uncertain, as it has also been isolated from the conjunctiva of healthy animals.^{9, 55} It has occasionally been associated with pneumonia and pleuritis in sheep and goats, and with pituitary abscesses in goats in South Africa. ²⁷ Moraxella osloensis has been isolated from a lamb suffering from pneumonia, aborted calves, and from cattle with mastitis, while Moraxella phenylpyruvica has been associated with septicaemia in a calf and a sheep.^{9, 55} Moraxella caprae has been isolated from healthy goats and, although it may be biochemically similar to M. bovis, it can be distinguished by an inability to liquefy gelatin.³³

Moraxella bovis infection

Infectious bovine keratoconjunctivitis caused by M. bovis is characterized by lachrymation, conjunctivitis and keratitis.^{5, 43} It has been described under a variety of names including New Forest disease, pink eye, blight, and infectious keratitis. ⁴³ This condition was first described as keratitis contagiosa by Billings in the USA in 1889, and has subsequently been reported throughout the world and in many countries is regarded as an important economic disease.^{5, 11, 26, 43}

Moraxella bovis is generally accepted as being the primary cause of infectious ophthalmia of cattle.^{3, 6, 27, 36, 38, 43–45}

Aetiology

Moraxella bovis is a non-motile, non-sugar fermenting, Gram-negative, short and plump diplobacillus.⁵⁰ Different strains of M. bovis, which vary in virulence, occur,^{5, 30} but only haemolytic,^{6, 37, 43–45} piliated or fimbriated^{30, 43–45} strains are pathogenic. Various types of pili have been described, and each is associated with the production of specific antibodies. ⁴⁸ Pili have been arranged in seven groups (A–G) in a unified pili serotyping scheme.³⁹ The production of certain enzymes, such as lipases and proteases,^{30, 44, 45} and the presence of a dermonecrotic toxin^{7, 44, 45} also play an important role in the virulence of M. bovis. The ability of M. bovis to change spontaneously from virulent to avirulent strains in vivo and in vitro^{43, 50} may be the reason why many transmission experiments have been unsuccessful. Phase variation, which is a reversible genetic mechanism, controls pilus expression. ^{30, 36} Much confusion surrounds the description of colony morphology with regard to piliated or non-piliated strains — both kinds having been variously described as rough or smooth.^{5, 30, 37, 38, 43, 50}

Epidemiology

Infectious bovine keratoconjunctivitis is highly contagious and usually occurs in epidemics; individual cases are rare.^{5, 43} The prevalence in a herd may vary from 10 to 76 per cent,²⁶ but although the morbidity rate is high, the mortality rate is negligible.⁷ The disease is more common and usually more severe in young animals,^{7, 32, 53, 56} although previously unexposed adult cattle may be affected.⁵⁶ There are definite breed differences in susceptibility — the Hereford and its crosses appear to be most susceptible although Channel Island breeds, Aberdeen Angus and Charolais also appear to be highly susceptible,¹¹ while the Bos indicus breeds are least susceptible.^{5, 43, 51, 56, 57} Animals having pigmented skin around the eye are more resistant to infection.

Clinically healthy carrier animals harbour the organism in the oculonasal tract,^{36, 37, 42} and play an important role in maintaining the infection.^{7, 32, 38, 42, 43} They may be responsible for introducing the disease into previously clean herds.⁵¹ Transmission may be either direct, by means of droplet infection from oculonasal secretion,^{35, 42} or indirect by insect carriers such as the house fly (Musca domestica), face fly (Musca autumnalis) or stable fly (Stomoxys calcitrans)^{5, 11, 25, 35, 43} (see Vectors: Muscidae.) Moraxella bovis has been transmitted experimentally by Musca autumnalis^{4, 23} and by Arcyophora longivalvis, a moth that feeds on the ocular secretion of cattle. Other eyefrequenting Lepidoptera have also been implicated in southern Africa as possible transmitters of M. bovis and other micro-organisms of the eyes.^{14–16, 26}

Infectious ophthalmia is most prevalent during the summer months,^{6, 42, 51, 56} as dust and other mechanical irritants, increased ultraviolet radiation, increased fly populations, and crowding of animals predispose to the infection, ^{5, 6, 12, 19, 35–37, 42, 51} while vitamin A deficiency has also been suggested as a possible predisposing factor.^{5, 43} Stress such as transport may aggravate the disease,⁴² as may concurrent or prior infection with other infectious agents such as Chlamydophila pecorum, infectious bovine rhinotracheitis, parainfluenza-3 and adenoviruses as well as certain Mycoplasma spp.^{1, 6, 11, 43, 46, 47} The infection does not appear to be transmitted from cattle to sheep.²⁶

Pathogenesis

Pathogenic strains of M. bovis have the ability to adhere to and penetrate the superficial epithelial cells of the cornea and conjunctiva. Adhesion appears to...