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 be mediated by the bacterial pili, although other ‘adhesins’ may also play a role.^{44, 45} More bacteria adhere to the so-called dark corneal epithelial cells (which contain fewer microplicae) than to the ‘light’ cells. Virulent M. bovis bacteria are cytotoxic to epithelial cells of the cornea, and invade these and the superficial stromal collagen even in the absence of prior mechanical injury.^43–45 After penetration, the bacteria are found inside the superficial epithelial cells of the cornea. This may be followed by oedema and the development of vesicles, erosions and ulcerations of the cornea and conjunctiva.^{1, 32, 44, 45} In vitro studies suggest that M. bovis possesses iron-acquisition systems such as siderophores and outer membrane receptors that bind bovine lactoferrin, which appears to play an important role in the ocular defence system.^{10, 21}

The earliest microscopic lesions develop in the bulbar conjunctiva and include oedema, hyperaemia and mononuclear cell infiltrates.^{17, 32, 44, 45} In experimental cases bacteria are visible within swollen epithelial cells of the palpebral conjunctiva within one or two hours of inoculation, and may result in erosion and ulceration.^{44, 45} The mechanism by which the erosions develop has not been elucidated; it has been suggested that M. bovis disrupts cell junctions without damaging the cells.^{44, 45} The exact cytotoxic factor responsible for the damage has not been identified yet, but haemolysins, a dermonecrotic toxin, and lipase and protease enzymes have been incriminated.^{7, 44, 45} No collagenase is produced.^{44, 45} Pustules and erosions of varying extent develop on the nictitating membrane, palpebral conjunctiva and margins of the eyelids 10 to 72 hours after inoculation,^{44, 45} and are accompanied by hyperaemia, oedema and neutrophil infiltration of the underlying lamina propria.^{44, 45}

Corneal ulcers are usually smaller than 5 mm in diameter³⁰ and are centrally situated.^{44, 45} The onset of corneal ulceration varies from one to four days after inoculation,^{32, 44, 45} and in untreated cases a purulent keratitis develops.³⁰ When this happens, there is a neutrophil infiltration of the tissue surrounding the ulcer, the adjacent swollen epithelium and the underlying stroma.^{1, 17, 44, 45}

After a week there is vascularization of the stroma extending from the limbus area.^{32, 43} In uncomplicated cases the necrotic epithelium of the cornea sloughs off and healing occurs with gradual epithelialization.^{32, 43}

Clinical signs and pathology

The incubation period of infectious bovine keratoconjunctivitis varies from two to three days to as long as three weeks in most natural and experimental cases.^{1, 7, 18, 32} One or both eyes may be affected.⁴³ The first signs are lachrymation, blepharospasm and photophobia,^{1, 5, 12, 17, 18, 40, 43} and conjunctivitis characterized by oedema, hyperaemia and swelling of the eyelids.^{5, 40, 43} The ocular discharge is initially serous but later becomes purulent, causing the eyelids to be glued together.^{5, 26, 43} The cornea is oedematous, and a whitish to yellowish-blue opaqueness, which spreads centrifugally, develops in its centre two to four days after the onset of clinical signs.^{1, 5, 12, 17, 43} Corneal vascularization heralds the recovery phase,^{12, 43} although in more severe cases the cornea may ulcerate.^{1, 40} Complications include keratoconus, hypopyon, iridocyclitis, or even lenticular enucleation.^{5, 17, 43} Various schemes to classify the severity of the disease, based on the severity of the lesions, have been described.¹¹

In most cases complete recovery of the eye (spontaneously or after treatment), with centrifugal disappearance of corneal opacity, occurs two to eight weeks after the initial clinical signs.^{5, 7, 11, 43} In severe cases permanent scarring of the cornea or even blindness may result.^{5, 7, 26, 40, 43} Systemic signs are rarely encountered, although fever or decreased milk yield may accompany the infection.⁷ Affected animals frequently fail to gain, or even lose, weight.^{7, 11}

Repeated occurrence of the disease is common, even though protective immunity may develop.³⁵

Diagnosis

The provisional diagnosis of infectious ophthalmia in cattle usually presents few problems because of the explosive nature and characteristic signs of the disease. For isolation of the causative organism, ocular swabs must be taken from the cornea and conjunctiva (preferably from early, untreated cases) and appropriate methods used to culture the organism involved.^{7, 17} Swabs should be submitted in a transport medium.

A diagnosis of infectious bovine keratoconjunctivitis can be confirmed by the fluorescent antibody technique, which is regarded as being more accurate and reliable than culture.^{5, 37, 43} ELISA has been used to determine serum antibody titres after infection or vaccination and to distinguish between virulent or potentially virulent strains of M. bovis.^{36, 38}

Differential diagnosis

Chemicals or foreign bodies (e.g. grass awns) sporadically cause keratoconjunctivitis in livestock. Other agents that have been incriminated as causes of infectious ophthalmia in cattle include C. pecorum, parainfluenza-3 and adenoviruses, ^{1, 5, 44, 45} Mycoplasma bovoculi and other Mycoplasma spp.,^{1, 5, 6, 43–45} Listeria monocytogenes⁵ and Thelazia spp. (‘eyeworms’).⁵

Keratoconjunctivitis also occurs in animals suffering from systemic infectious diseases such as infectious bovine rhinotracheitis, bovine virus diarrhoea/mucosal disease, bovine malignant catarrhal fever and rinderpest.

Control

When infectious ophthalmia appears in a herd it is important to separate affected animals from the rest of the herd and to keep them in a shady, dust-free area.^{5, 7, 43} Although affected eyes may recover spontaneously, treatment should be initiated as soon as possible for a better therapeutic response and to reduce the risk of complications and pain.^{43, 51, 49} Topical antibiotic therapy is the treatment of choice. Repeated treatment is usually necessary.^{7, 41}

Subconjunctival administration of antibiotics, although resulting in high concentrations of antibiotic in the tear film, is not recommended as it is very painful and irritating (especially the tetracyclines).^{11, 13, 22, 43} Ointment formulations are better than aerosol or powder formulations as the latter result in increased lachrymation and are therefore washed out more rapidly.^{11, 41, 43} Many ophthalmic preparations contain benzathine cloxacillin, as therapeutic levels of this antibiotic are maintained for a long time in tear secretions. ^{13, 22} However, resistance of M. bovis to cloxacillin has been described,^{3, 22, 43} as well as to streptomycin, tylosin, sulphonamides and lincomycin.^{22, 43} Chloramphenicol formulations are regarded as being the treatment of choice, but regulations regarding their use in food-producing animals restrict their usefulness.⁴³ Eye inserts and ring devices that result in the sustained release of antibiotics are under investigation. ^{5, 43} Bacterial culture from eye swabs and antibiograms may be necessary when selecting the most effective antibiotic.¹¹

Antibiotic therapy may, however, not eradicate the clinical state.¹¹ Intramuscular injection of long-acting tetracyclines at 20 mg/kg has been shown to be effective, as well as reducing the duration of the carrier state.¹¹ Preventive blanket treatment may be considered, although results may vary.^{11, 49} Management systems and cost effectiveness will largely determine the choice of antibiotic as well as the treatment regimen.¹¹

Although the use of corticosteroids in conjunction with antibiotics⁴³ has been propagated, their use is generally not recommended, especially in cases with corneal ulceration. Local anaesthetics are not advised as their prolonged use may result in corneal ulceration,⁴³ and the administration of vitamins A, D and E is also of doubtful value.^{5, 43} In complicated cases or where corneal lesions are advanced, the third eyelid may be sutured across the cornea.^{5, 7}

Fly control by means of dipping, spraying animals with insecticides or the use of insecticide-impregnated ear tags is important in the control of infectious ophthalmia.^{4, 5, 11, 23, 35}

Bacterial pili play an important role in the development of immunity to infectious bovine keratoconjunctivitis following both natural infection and vaccination with an inactivated, piliated bacterin.^{5, 36} Vaccination with non-piliated bacterins does not confer immunity to challenge with M. bovis^{.30, 36} Animals are usually immune to ocular challenge by homologous strains after natural infection or vaccination, but not to heterologous strains, although a degree of cross-immunity is reported between various strains.^{5, 36, 38, 43} Serum antibodies to M. bovis can be detected by means of ELISA or the modified gel diffusion precipitin test two to three weeks after infection or vaccination,^{7, 30, 36} but it is still uncertain whether these are due to a general immune response or to a local immune response.⁴³

An inactivated, piliated M. bovis bacterin vaccine is commercially available in some countries to control infectious bovine keratoconjunctivitis.^{30, 36, 38} Results obtained with vaccination have been very variable.¹¹ Multivalent and recombinant vaccines that afford greater protection are under investigation.¹¹

Moraxella ovis infection

In southern Africa, infectious ophthalmia, which may be caused in sheep and goats by a number of organisms, is widespread and responsible for economic losses.^{2, 26} Ophthalmia is very common in these species, but the aetiology is still largely unresolved.

Although Moraxella ovis has been isolated from cases of infectious ophthalmia in sheep and goats,^{12, 40} it is regarded by some authors as being of doubtful aetiological significance. ⁷ Recent isolates from various cases of infectious ophthalmia in sheep in South Africa yielded only M. ovis and Mycoplasma spp.³ Attempts to reproduce the disease in lambs by using cultures of M. ovis obtained from these clinical cases were, however, unsuccessful.³

A number of other organisms have been incriminated in infectious ophthalmia in sheep and goats. Rickettsia (Colesiota) conjunctivae was first demonstrated in South Africa in conjunctival scrapings from sheep with infectious ophthalmia by Coles,²⁶ but the opinion has been expressed by some workers that it could have been Chlamydia psittaci (renamed C. pecorum).^{8, 12} Chlamydophila pecorum causes ophthalmia in various species, including sheep^{7, 8, 32} (see Chlamydiosis), and the organism can be demonstrated in conjunctival or corneal scrapings stained with Gimenez.²⁶ Mycoplasma conjunctivae,M. arginini, M. agalactiae, M. mycoides mycoides, and other Mycoplasma spp.have been incriminated as causes of infectious ophthalmia of sheep and goats. The condition has been experimentally reproduced in the United Kingdom, Europe and South Africa using cultures of M. conjunctivae.^{7, 20, 28, 31, 32, 52, 53, 54} The general principles for the treatment and control of infectious ophthalmia in cattle appear also to apply to sheep and goats (see Moraxella bovis infection, above). The intramuscular injection of long-acting tetracyclines has proved to be very effective in cases of infectious ophthalmia; a single injection is usually effective.^{24, 34} No vaccine is available for use in sheep and goats.

References

1. AIKMAN, J.G., ALLAN, E.M. & SELMAN, I.E., 1985. Experimental production of infectious bovine keratoconjunctivitis. The Veterinary Record, 117, 234–239.
2. ANON., 1979–1986. Annual Reports of the Directorate of Veterinary Services, South Africa.
3. ANON., 1985. Regional Veterinary Laboratory, Stellenbosch, South Africa. Unpublished data.
4. ARENDS, J.J., WRIGHT, R.E., BARTO, P.B. & LUSBY, K.S., 1984. Transmission of Moraxella bovis from blood agar cultures to Hereford cattle by face flies (Diptera: Muscidae). Journal of Economic Entomology, 77, 394–398.
5. BAPTISTA, P.J.H.P., 1979. Infectious bovine keratoconjunctivitis. A review. British Veterinary Journal, 135, 225–242.
6. BARBER, D.M.L., JONES, G.E. & WOOD, A., 1986. Microbial flora of the eyes of cattle. The Veterinary Record, 118, 204–206.
7. BLOOD, D.C., RADOSTITS, O.M. & HENDERSON, J.A., 1983. Veterinary Medicine/Small Animal Clinician. 6th edn. London: Baillière Tindall.
8. BOGAARD, A.E.J.M. (JR), 1984. Inclusion keratoconjunctivitis (‘pink eye’) in sheep. A proposal for a new name for chlamydial keratoconjunctivitis in sheep and comment on recent clinical trials. The Veterinary Quarterly, 6, 229–235.
9. BOVRE, K., 1984. Moraxella. In: krieg, n.r. & holt, j.g., (eds). Bergey’s Manual of Systematic Bacteriology. Vol. I. Baltimore: Williams and Wilkins
10. BROWN, M.H., 1995. Moraxella bovis outer membrane proteins bind bovine lactoferrin. Manhattan, KS: Kansas State University, Master’s Thesis.
11. BROWN, M.H., BRIGHTMAN, A.H., FENWICK, B.W. & RIDER, M.A., 1998. Infectious bovine keratoconjunctivitis: A review. Journal of Veterinary Internal Medicine, 12, 259–266.
12. BULGIN, M.S. & DUBOSE, D.A., 1982. Pinkeye associated with Branhamella ovis infection in dairy goats. Veterinary Medicine/Small Animal Clinician, 77, 1791–1793.
13. BUSWELL, J.F. & HEWETT, G.R., 1983. Single topical treatment for bovine keratoconjunctivitis using benzathine cloxacillin. The Veterinary Record, 113, 621–622.
14. BÜTTIKER, W., 1973. Further records of eye-frequenting Noctuidae (Lepidoptera) from South Africa. South African Journal of Science, 69, 337–341.
15. BÜTTIKER, W. & BEZUIDENHOUT, J.D., 1974. First records of eye-frequenting Lepidoptera from South West Africa. Journal of the Entomological Society of Southern Africa, 37, 73–78.
16. BÜTTIKER, W. & WHELLAN, J.A., 1966. Records of eye-frequenting moths from Rhodesia. Bulletin 2306. Reprinted from March/April, 1966 issue of the Rhodesia Agricultural Journal, Ministry of Agriculture, Government Printer, Salisbury, 1–4.
17. CHANDLER, R.L., TURFREY, B.A. & SMITH, K., 1983. Laboratory model for infectious bovine keratoconjunctivitis: The pathogenicity of different strains of Moraxella bovis, pathology and ultrastructure observations. Research in Veterinary Science, 35, 277–284.
18. CHANDLER, R.L., TURFREY, B., SMITH, K. & GOURLAY, R.N., 1980. Virulence of Moraxella bovis in gnotobiotic calves. The Veterinary Record, 106, 364–365.
19. COX, P.J., LIDDELL, J.S. & MATTINSON, A.D., 1984. Infectious bovine keratoconjunctivitis: Isolation of Moraxella bovis from two groups of young beef cattle in fly control field trials during 1981. The Veterinary Record, 115, 29–32.
20. EGWU, G.O., FAULL, W.B., BRADBURY, J.M. & CLARKSON, M.J., 1989. Ovine infectious keratoconjunctivitis: A microbiological study of clinically unaffected and affected sheep’s eyes with special reference to Mycoplasma conjunctivae. The Veterinary Record, 125, 253–256.
21. FENWICK, B. & RIDER, M., 1996. Iron repressible outer membrane proteins of Moraxella bovis and demonstration of siderophore-like activity. eterinary Microbiology, 48, 315–324.
22. GEORGE, L.W., WILSON, W.D., BAGGOT, J.D. & MIHALYI, J.E., 1984. Antibiotic treatment of Moraxella bovis infection in cattle. Journal of the American Veterinary Medical Association, 185, 1206–1209.
23. GLASS, H.W. (JR) & GERHARDT, R.R., 1984. Transmission of Moraxella bovis by regurgitation from the crop of the face fly (Diptera: Muscidae). Journal of Economic Entomology, 77, 399–401.
24. GREIG, A., 1989. Ovine keratoconjunctivitis. In Practice, 11, 110–113.
25. HALL, R.D., 1984. Relationship of the face fly (Diptera: Muscidae) to pinkeye in cattle: A review and synthesis of the relevant literature. Journal of Medical Entomology, 21, 361–365.
26. HENNING, M.W., 1956. Animal Diseases in South Africa. 3rd edn. Pretoria: Central News Agency.
27. HENTON, M.M., 1988. Veterinary Research Institute, Onderstepoort. Personal communication.
28. HOSIE, B.D., 1988. Keratoconjunctivitis in a hill sheep flock. The Veterinary Record, 122, 40–43.
29. HUGHES, D.E. & PUGH, G.W. (JR), 1970. Isolation and description of a Moraxella from horses with conjunctivitis. American Journal of Veterinary Research, 31, 451–462.
30. JAYAPPA, H.G. & LEHR, C., 1986. Pathogenicity and immunogenicity of piliated and nonpiliated phases of Moraxella bovis in calves. American Journal of Veterinary Research, 47, 2217–2221.
31. JONES, G.E., 1983. Mycoplasmas of sheep and goats: A synopsis. The Veterinary Record, 113, 619–620.
32. JUBB, K.V.F., KENNEDY, P.C. & PALMER, N., 1985. Pathology of Domestic Animals. 3rd edn. Orlando: Academic Press.
33. KODJO, A., TONJUM, T., RICHARD, Y. & BOVRE, K., 1995. Moraxella caprae sp. nov., a new member of the classical Moraxellae with very close affinity to Moraxella bovis. International Journal of Systematic Bacteriology, 45, 467–471.
34. KÖNIG, C.D.W., 1983. ‘Pink eye’ or ‘zere oogjes’ or keratoconjunctivitis infectiosa ovis (KIO). Clinical efficacy of a number of antimicrobial therapies. The Veterinary Quarterly, 5, 122–127.
35. KOPECKY, K.E., PUGH, G.W. (JR) & MCDONALD, T.J., 1986. Infectious bovine keratoconjunctivitis—contact transmission. American Journal of Veterinary Research, , 622–624.
36. LEHR, C., JAYAPPA, H.G., GOODNOW, R.A., 1985. Controlling bovine keratoconjunctivitis with a piliated Moraxella bovis bacterin. Veterinary Medicine/Small Animal Clinician, 80, 96–100.
37. LEPPER, A.W.D. & BARTON, I.J., 1987. Infectious bovine keratoconjunctivitis —seasonal variation in cultural, biochemical and immunoreactive properties of Moraxella bovis isolated from the eyes of cattle. Australian Veterinary Journal, 64, 33–39.
38. LEPPER, A.W.D. & HERMANS, L.R., 1986. Characterisation and quantitation of pilus antigens of Moraxella bovis by ELISA. Australian Veterinary Journal, 63, 401–405.
39. MOORE, L. & LEPPER, A., 1991. A unified serotyping scheme for Moraxella bovis. Veterinary Microbiology, 29, 75–83.
40. OSUAGWUH, A.I.A. & AKPOKODJE, J.U., 1979. Infectious keratoconjunctivitis in sheep and goats in Nigeria. The Veterinary Record, 105, 125–126.
41. PIERCY, B.W.T., 1985. Factors influencing the penetration of therapeutic substances in the eye following topical application. The Veterinary Record, 116, 99–104.
42. PUGH, G.W. (JR) & MCDONALD, T.J., 1986. Identification of bovine carriers of Moraxella bovis by comparative cultural examinations of ocular and nasal secretions. American Journal of Veterinary Research, 47, 2343– 2345.
43. NCH, P.I. & SLATTER, D.H., 1984. A review of infectious bovine keratoconjunctivitis. Veterinary Bulletin, 54, 193–207.
44. ROGERS, D.G., CHEVILLE, N.F. & PUGH, G.W. (JR), 1987. Pathogenesis of corneal lesions caused by Moraxella bovis in gnotobiotic calves. Veterinary Pathology, 24, 287–295.
45. ROGERS, D.G., CHEVILLE, N.F. & PUGH, G.W. (JR), 1987. Conjunctival lesions caused by Moraxella bovis in gnotobiotic calves. Veterinary Pathology, 24, 554–559.
46. ROSENBUSCH, R.F., 1983. Influence of Mycoplasma preinfection on the expression of Moraxella bovis pathogenicity. American Journal of Veterinary Research, 44, 1621–1624.
47. ROSENBUSCH, R.F. & OSTLE, A.G., 1986. Mycoplasma bovoculi infection increases ocular colonisation by Moraxella bovis in calves. American Journal of Veterinary Research, 47, 1214–1216.
48. RUEHL, W. & MARRS, C., 1993. Infection rates, disease frequency, pilin gene rearrangement, and pilin expression in calves inoculated with Moraxella bovis pilin-specific isogenic variants. American Journal of Veterinary Research, 54, 248–253.
49. SARGISON, N.D., HUNTER, J.E.B., WEST, D.M. & GWOZDZ, M.J., 1996. Observations on the efficacy of mass treatment by subconjunctival penicillin injection for the control of an outbreak of infectious bovine keratoconjunctivitis.ew Zealand Veterinary Journal, 44, 142–144.
50. SCHURIG, G.G., LIGHTFOOT, D.R., TROUTT, H.F. & FINKLER, B.I., 1984. Genotypic, phenotypic and biological characteristics of Moraxella bovis. American Journal of Veterinary Research, 45, 35–39.
51. SINCLAIR, J.A., COOPER, B.S. & STEFFERT, I.J., 1986. A survey of infectious bovine keratoconjunctivitis in the Gisborne and Hunterville regions of New Zealand. New Zealand Veterinary Journal, 34, 121–125.
52. TER LAAK, E.A., SCHREUDER, B.E.C., KIMMAN, T.G. & HOUWERS, D.J., 1988. Ovine keratoconjunctivitis experimentally induced by instillation of Mycoplasma conjunctivae. The Veterinary Quarterly, 10, 217–224.
53. TER LAAK, E.A., SCHREUDER, B.E.C. & SMITH-BUYS, C.M.C., 1988. The occurrence of Mycoplasma conjunctivae in the Netherlands and its associations with infectious keratoconjunctivitis in sheep and goats. The Veterinary Quarterly, 10, 73–83.
54. VAN HALDEREN, A., VAN RENSBURG, W.J.J., GEYER, A. & VORSTER, J.H., 1994. The identification of Mycoplasma conjunctivae as an aetiological agent of infectious keratoconjunctivitis of sheep in South Africa. Onderstepoort Journal of Veterinary Research, 61, 231–237.
55. VEDROS, N.S., 1984. Neisseria. In: krieg, n.r. & holt, j.g., (eds). Bergey’s Manual of Systematic Bacteriology. Vol. I. Baltimore: Williams and Wilkins.
56. WEBBER, J.J. & SELBY, L.A., 1981. Risk factors related to the prevalence of infectious bovine keratoconjunctivitis. Journal of the American Veterinary Medical Association, 179, 823–826.
57. WILCOX, G.E., 1970. Bacterial flora of the bovine eye with special reference to the Moraxella and Neisseria. Australian Veterinary Journal, 46, 253–257.