Randomized controlled field trial to assess the efficacy of an intranasal Moraxella bovis cytotoxin vaccine against naturally occurring infectious bovine keratoconjunctivitis

Background

Infectious bovine keratoconjunctivitis (IBK; pinkeye) is generally considered to be caused by corneal infections with Moraxella bovis. Previous studies demonstrated that M. bovis cytotoxin-specific mucosal immune responses in the bovine eye can be stimulated by intranasal vaccination with a recombinant M. bovis cytotoxin subunit adjuvanted with polyacrylic acid.

Methods

A randomized controlled field trial (two-arm parallel design with blinding) was conducted in beef steers in Northern California to determine if this vaccine could prevent naturally occurring IBK and/or reduce morbidity rates associated with this disease. Beef steers were vaccinated intranasally on days 0 and 21 with either a recombinant M. bovis cytotoxin subunit adjuvanted with polyacrylic acid (Vaccine group) or adjuvant alone (Control group). Eye examinations were performed on all steers every 7 days for 16 weeks to document the occurrence of IBK and to determine sizes of corneal ulcers. Serum and tear samples were collected on days 0, 42, and 112 from a subset of animals to measure changes in systemic and ocular immune responses to M. bovis cytotoxin.

Results

The cumulative proportion of steers that developed IBK after 16 weeks did not differ between groups. Variables related to disease severity were numerically lower in steers that received the experimental vaccine. IBK-affected Vaccine group steers had a significantly lower number of observation weeks with severe ulcers versus Control group steers. Cytotoxin-specific tear IgA was significantly higher in Vaccine group compared to Control group steers on day 112. Conclusion: Although the proportion of animals that developed corneal ulcers associated with IBK did not differ between groups, the lowered metrics of disease severity in vaccinated steers suggests that intranasal vaccination with recombinant M. bovis cytotoxin can reduce the severity of IBK in cattle.

Investigation of the pathogens contributing to naturally occurring outbreaks of infectious bovine keratoconjunctivitis (pinkeye) using Next Generation Sequencing

Infectious bovine keratoconjunctivitis (IBK), commonly known as pinkeye, has a marked negative impact on the economy of the cattle industry. Moraxella species, including Mor. bovis and Mor. bovoculi, which have been associated with this disease, colonize clinically healthy eyes as well, suggesting that there are intrinsic changes that may occur to the ocular microbiota or the involvement of additional unrecognized organisms that contribute to IBK. To evaluate this, 104 ocular swabs collected from eyes with IBK or clinically healthy eyes from 16 different cattle herds were subjected to 16 S rRNA gene PCR and next generation sequencing (NGS) analysis. Organisms detected were similar across the herds and there was no difference in the total number of bacterial groups detected among IBK cases and controls. However, the percentages of the different organisms detected varied between the two groups, including Moraxella spp., with more Moraxella spp. in eyes with IBK than controls. Further, using culture and whole genome NGS, a new species of Moraxella (suggested name Mor. oculobovii) was detected from the eyes of cattle from two farms. This strain is non-hemolytic on blood agar, is missing the RTX operon, and is likely a non-pathogenic strain of the bovine ocular microbiome. Alteration of the ocular microbiota composition may have a predisposing role, enhancing bacterial infection and the occurrence of clinical IBK. Future studies are required to evaluate if these changes are permanent or if there is a shift in the microbiome following recovery from the infection and how antibiotics might affect the microbiome.

A randomised control trial to evaluate the effectiveness of a commercial vaccine for pinkeye in Australian beef cattle

Pinkeye (a generic term to describe infectious bovine keratoconjunctivitis) is a significant disease of cattle worldwide, impacting productivity and animal welfare. One commercial pinkeye vaccine, a systematically administered Moraxella bovis bacterin, has been available in Australia since 2007. This is the first field trial of the effectiveness of this vaccine for the prevention of naturally occurring disease in Australia. Extensively run beef herds in southwest Queensland that regularly experienced pinkeye were enrolled in the trial and animals were randomly allocated to vaccinated and control groups in different proportions in each herd. The subsequent incidence of clinical pinkeye between the two groups was compared for animals less than one-year-old. Data were analysed from 649 cattle from five herds over two pinkeye seasons: three herds of 390 calves from 1st November 2019 to 20th January 2020 and two herds of 259 calves from 23rd September 2020 to 21st April 2021. Pinkeye was common with 24% of all calves (156/649) contracting the disease during the trial. Univariable and multivariable binary logistic mixed-effect models were fitted to account for clustered data and potential residual confounding due to sex, weight, breed, coat colour, and periocular pigmentation. The incidence of pinkeye was not significantly different between vaccinated and control groups, both alone (p = 0.67) and after adjusting for sex and weight differences (p = 0.69). The vaccine was not protective against naturally occurring pinkeye under the field conditions.

Whole genome sequencing of Moraxella bovis strains from North America reveals two genotypes with different genetic determinants

BACKGROUND

Moraxella bovis and Moraxella bovoculi both associate with infectious bovine keratoconjunctivitis (IBK), an economically significant and painful ocular disease that affects cattle worldwide. There are two genotypes of M. bovoculi (genotypes 1 and 2) that differ in their gene content and potential virulence factors, although neither have been experimentally shown to cause IBK. M. bovis is a causative IBK agent, however, not all strains carry a complete assortment of known virulence factors. The goals of this study were to determine the population structure and depth of M. bovis genomic diversity, and to compare core and accessory genes and predicted outer membrane protein profiles both within and between M. bovis and M. bovoculi.

RESULTS

Phylogenetic trees and bioinformatic analyses of 36 M. bovis chromosomes sequenced in this study and additional available chromosomes of M. bovis and both genotype 1 and 2 M. bovoculi, showed there are two genotypes (1 and 2) of M. bovis. The two M. bovis genotypes share a core of 2015 genes, with 121 and 186 genes specific to genotype 1 and 2, respectively. The two genotypes differ by their chromosome size and prophage content, encoded protein variants of the virulence factor hemolysin, and by their affiliation with different plasmids. Eight plasmid types were identified in this study, with types 1 and 6 observed in 88 and 56% of genotype 2 strains, respectively, and absent from genotype 1 strains. Only type 1 plasmids contained one or two gene copies encoding filamentous haemagglutinin-like proteins potentially involved with adhesion. A core of 1403 genes was shared between the genotype 1 and 2 strains of both M. bovis and M. bovoculi, which encoded a total of nine predicted outer membrane proteins.

CONCLUSIONS

There are two genotypes of M. bovis that differ in both chromosome content and plasmid profiles and thus may not equally associate with IBK. Immunological reagents specifically targeting select genotypes of M. bovis, or all genotypes of M. bovis and M. bovoculi together could be designed from the outer membrane proteins identified in this study.

Effects of eye patches on corneal ulcer healing and weight gain in stocker steers on pasture: a randomized controlled trial

Infectious bovine keratoconjunctivitis (IBK) is a painful ocular disease in cattle that is characterized by the presence of a corneal ulcer and production losses. A common industry practice is to cover an affected eye with a piece of cloth to reduce exposure to face flies and ultraviolet light with the goal of alleviating pain, accelerating healing, and reducing spread. To study the efficacy of eye patches in the treatment of IBK, a group of 216 clinically normal Angus crossbred steers were followed between April and August 2019 and evaluated weekly for the development of IBK. Eyes of cattle that developed IBK were enrolled with a blocked randomization scheme based on ulcer severity score to receive either an eye patch (treatment group) or no eye patch (control group). All treatment and control group animals received parenteral antimicrobial and nonsteroidal anti-inflammatory treatments and were housed in a pasture separated from the rest of the cohort for a maximum of 28 d or until clinical cure. Corneal ulcer areas were measured, and body weights were recorded twice weekly for steers in the treatment and control groups. Weights of all steers in the cohort were recorded three times during the trial period. The primary outcome, rate of corneal ulcer healing, was higher (P = 0.001) for lesions in eyes receiving an eye patch as determined by a linear mixed model that controlled for ulcer severity score at enrollment and previous IBK in the opposite eye. Median corneal ulcer healing time was 10 (IQR [Interquartile range] 7–17) d for patched eyes vs. 14 (IQR 7–21) d for unpatched eyes. In a Cox proportional hazards model adjusted for severity score at diagnosis, the hazard ratio for ulcer healing was 1.62 (95% CI: 1.02–2.56, P = 0.042) for eyes that received a patch compared to eyes that did not. Among all 216 steers in the cohort, those that were diagnosed with IBK had a numerically higher average daily gain (ADG) (0.45 [±SE 0.01] kg) vs. those that were not (0.42 [±SE 0.12] kg; P = 0.06). In enrolled steers that received a patch, the secondary outcome ADG was 0.47 (±SE 0.02) kg compared to 0.43 (±SE 0.02) kg in controls (P = 0.22). Weight gain may have been confounded by pasture during the treatment period. Results of this trial support the use of this low-cost intervention; further investigation into possible reasons for observed differences in weight gain may be warranted.

Risk factors associated with pinkeye in Australian cattle

Pinkeye is the most important bovine ocular disease worldwide and a major welfare and economic concern to the Australian cattle industry. Pinkeye can occur in epidemic proportions, but severity and susceptibility vary within and between herds, indicating that the disease is multifactorial. This study was conducted to identify the on-farm risk factors associated with pinkeye disease in Australian cattle. Data were gathered from cattle farmers using a custom designed online questionnaire. Farmer responses suitable for assessment (n = 999) were analysed with descriptive, univariable and multivariable logistic regression to evaluate the association of 26 explanatory variables with pinkeye within-herd prevalence. Results revealed that farm location, farm grazing area, farmer-reported dust levels, fly levels, rain levels, animal zebu content and cattle age were significantly associated with pinkeye prevalence. More specifically, having a farm located in southern Australia, of smaller grazing area with cattle ≤ 2 years of age, was associated with a higher pinkeye prevalence. Pinkeye prevalence was also greater if respondents ranked their farms as having high fly levels compared to moderate and low fly levels, respectively and on farms ranked low for rainfall compared to moderate and high rainfall, respectively. Those that ranked their farms as having high dust levels had more pinkeye compared to moderate and low dust levels, but moderate dust levels were protective compared to low dust levels. The results confirm that pinkeye disease is multifactorial and is associated with a range of host and environmental factors. These findings should be used to assist in the control of the disease and improve pinkeye outcomes in Australian cattle.

Defining and Diagnosing Infectious Bovine Keratoconjunctivitis

Pinkeye and infectious bovine keratoconjunctivitis (IBK) are imprecise terms that describe diverse ocular diseases. Moraxella bovis is the major causative agent of IBK; however, disease epidemiology is not fully known. Not all cases referred to as pinkeye are of infectious origin, and not all IBK involve M bovis. This article suggests the term pinkeye should no longer be used, offers a case definition for IBK (a herd disease), and suggests describing ocular signs of IBK using existing clinical descriptors rather than resorting to novel scores. A new term "ocular moraxellosis" is defined as IBK from which Moraxella spp are demonstrated.

The Evidence Base for Prevention of Infectious Bovine Keratoconjunctivitis Through Vaccination

Pili and cytotoxins are important virulence factors and antigens for Moraxella spp. Local and systemic immunity may play a role in the body's response to infectious bovine keratoconjunctivitis (IBK). No evidence exists that eliminating the carrier state for IBK is possible or beneficial. Evidence for efficacious transfer of passive immunity from dams to calves is conflicting. Autogenous vaccines and commercial vaccines for putative pathogens for IBK have not yet shown efficacy in blinded randomized field trials. Study design features, such as randomization, blinding, diagnostic criteria, and use of a placebo, reduce the risk of bias in vaccine studies for IBK.

Bovine Immune Responses to Moraxella bovis and Moraxella bovoculi Following Vaccination and Natural or Experimental Infections

Studies have sought to develop effective vaccines against infectious bovine keratoconjunctivitis (IBK). Most research has focused on parenterally administered vaccines against Moraxella bovis antigens; however, researchers have also included Moraxella bovoculi antigens in vaccines to prevent IBK. Critical knowledge gaps remain as to which Moraxella spp antigens might be completely protective, and whether systemic, mucosal, or both types of immune responses are required for protection against IBK associated with Moraxella spp. Immune responses to commensal Moraxella spp residing in the upper respiratory tract and eye have not been analyzed to determine if these responses control colonization or contribute to IBK.

Applying Concepts of Causal Inference to Infectious Bovine Keratoconjunctivitis

Establishing causation, otherwise known as causal assessment, is a difficult task, made more difficult by the variety of causal assessment frameworks available to consider. In this article, Bradford Hill viewpoints are used to discuss the evidence base for Moraxella bovis and Moraxella bovoculi being component causes of infectious bovine keratoconjunctivitis. Each of the nine Bradford Hill viewpoints are introduced and explained: strength, consistency, specificity, temporality, biologic gradient, plausibility, coherence, experiment, and analogy. Examples of how the viewpoints have been applied for other causal relations are provided, and then the evidence base for M bovis and M bovoculi is discussed.

Non-antimicrobial approaches for the prevention or treatment of infectious bovine keratoconjunctivitis in cattle applicable to cow-calf operations: A scoping review

Infectious bovine keratoconjunctivitis (IBK) is a common ocular disease in cattle that causes economic losses to producers and negatively impacts animal welfare. In a 2016 survey of cow-calf producers in California, IBK was identified as the disease for which antimicrobials are most frequently used. The presented scoping review examined the available literature for methods to prevent IBK and for alternatives to antimicrobials to treat the disease that can be applied in cow-calf operations. Online databases were searched for publications about IBK in cattle populations that were reported from 1950 to 2020. Citations were systematically evaluated in a multi-stage approach using commercial software and summarized in a scoping review format. For the studies included in the review, most research (n = 50) has focused on the development of vaccines for the prevention of IBK. Although the quality of publications has improved over time, there is a lack of consistent evidence for vaccine efficacy against IBK in post-2000 experimental and conventional vaccine trials. A systematic analysis of vaccine studies is warranted. A limited number (n = 6) of studies evaluated the prevention of IBK through fly control, where most have found efficacy of this control measure. Several treatment options (n = 5) that do not include the use of antimicrobials have been investigated but remain at the preliminary stage of testing. Differences in breed susceptibility has been demonstrated with breeds belonging to the Bos indicus subspecies less frequently affected compared to those belonging to the Bos taurus subspecies. Hereford cattle and those lacking pigmentation around the eyelid margin are more frequently affected than other breeds. At present, there are few evidence-based measures that producers can utilize to reduce the burden of IBK in their herds and more research into the efficacy of fly control measures, non-antimicrobial treatment options, the continued search for a viable vaccine, as well as identifying genetic markers associated with traits that confer resistance to the disease are needed.

Whole genome sequencing of Moraxella bovoculi reveals high genetic diversity and evidence for interspecies recombination at multiple loci

Moraxella bovoculi is frequently cultured from the ocular secretions and conjunctiva of cattle with Infectious Bovine Keratoconjunctivitis (IBK). Previous work has shown that single nucleotide polymorphism (SNP) diversity in this species is quite high with 81,284 SNPs identified in eight genomes representing two distinct genotypes isolated from IBK affected eyes (genotype 1) and the nasopharynx of cattle without clinical IBK signs (genotype 2), respectively. The goals of this study were to identify SNPs from a collection of geographically diverse and epidemiologically unlinked M. bovoculi strains from the eyes of IBK positive cattle (n = 183) and another from the eyes of cattle (most from a single population at a single time-point) without signs of IBK (n = 63) and to characterize the genetic diversity. Strains of both genotypes were identified from the eyes of cattle without IBK signs. Only genotype 1 strains were identified from IBK affected eyes, however, these strains were isolated before the discovery of genotype 2, and the protocol for their isolation would have preferentially selected genotype 1 M. bovoculi. The core genome comprised ~74% of the whole and contained >127,000 filtered SNPs. More than 80% of these characterize diversity within genotype 1 while 23,611 SNPs (~18%) delimit the two major genotypes. Genotype 2 strains lacked a repeats-in-toxin (RTX) putative pathogenesis factor and any of ten putative antibiotic resistance genes carried within a genomic island. Within genotype 1, prevalence of these elements was 0.85 and 0.12 respectively in strains from eyes that were IBK positive. Recombination appears to be an important source of genetic diversity for genotype 1 and undermines the utility of ribosomal-locus-based species identification. The extremely high genetic diversity in genotype 1 presents a challenge to the development of an efficacious vaccine directed against them, however, several low-diversity pilin-like genes were identified. Finally, the genotype-defining SNPs described in this study are a resource that can facilitate the development of more accurate M. bovoculi diagnostic tests.

Microbial community sequencing analysis of the calf eye microbiota and relationship to infectious bovine keratoconjunctivitis

Infectious bovine keratoconjunctivitis (IBK) is an important production limiting disease in cattle. Moraxella bovis has historically been considered the primary causal agent; however, vaccines have not been consistently shown as effective in controlling disease incidence. The purpose of this study was to examine the bacterial community of calf eyes prior to disease onset using high-throughput sequencing of 16S ribosomal RNA and determine if it was associated with IBK occurrence. The study was designed as a case-control nested within a randomized controlled trial (RCT). Eye swabs were collected from all spring-born calves without clinical signs of IBK (t₀ swabs) on a research farm with a previous history of IBK disease outbreaks. At follow-up or weaning, calves were diagnosed as IBK positive or negative. The lag time between enrollment swabs (t₀) and IBK diagnosis ranged from approximately one to three months. Cases were randomly selected from IBK positive calves and controls were selected from IBK negative calves (i.e. calves that did not exhibit clinical signs of IBK throughout the course of the RCT). Analysis of the fold-change differences between cases and controls did not reveal large-scale distinctions in bacterial composition. However, principal component analysis suggested bacterial composition differences between calf management groups, which were based on dam parity. Moraxella was found to be among the top ten most abundant genera in our population; however, the difference in abundance was not significant between the cases and controls. No large-scale differences in the bacterial communities of calves that did or did not develop IBK were observed in our population. Nevertheless, it remains unclear whether the "natural" bacterial population of the calf might ultimately impact disease status. Further study is warranted to examine bacterial taxa that were observed to be significantly more abundant in the cases or controls as potential vaccines/therapeutic targets.

Ocular immune responses in steers following intranasal vaccination with recombinant Moraxella bovis cytotoxin adjuvanted with polyacrylic acid

Infectious bovine keratoconjunctivitis (IBK) caused by Moraxella bovis is the most common eye disease of cattle. The pathogenesis of M. bovis requires the expression of pili that enable the organism to attach to the ocular surface and an RTX (repeats in the structural toxin) toxin (cytotoxin or hemolysin), which is cytotoxic to corneal epithelial cells. In this pilot study, ocular mucosal immune responses of steers were measured following intranasal (i.n.) vaccination with a recombinant M. bovis cytotoxin adjuvanted with polyacrylic acid. Beef steers were vaccinated with either 500 μg (n = 3) or 200 μg (n = 3) of recombinant M. bovis cytotoxin plus adjuvant. Control group steers (n = 2) were vaccinated with adjuvant alone, and all steers were given a booster on day 21. Antigen-specific tear IgA and tear IgG, tear cytotoxin-neutralizing antibody responses, and serum cytotoxin-neutralizing antibody responses were determined in samples collected prevaccination and on days 14, 28, 42, and 55. Changes in tear antigen-specific IgA levels from day 0 to days 28, 42, and 55 were significantly different between groups; however, in post hoc comparisons between individual group pairs at the tested time points, the differences were not significant. Our results suggest that i.n. vaccination of cattle with recombinant M. bovis cytotoxin adjuvanted with polyacrylic acid effects changes in ocular antigen-specific IgA concentrations. The use of intranasally administered recombinant M. bovis cytotoxin adjuvanted with polyacrylic acid could provide an alternative to parenteral vaccination of cattle for immunoprophylaxis against IBK.

Randomized blinded challenge study to assess association between Moraxella bovoculi and Infectious Bovine Keratoconjunctivitis in dairy calves

The objective of this study was to evaluate if Moraxella bovoculi was associated with Infectious Bovine Keratoconjunctivitis (IBK) using a corneal scarification model in calves. A 3-arm single-eye block-randomized and blinded challenge study was designed as follows: corneal scarification only, corneal scarification and inoculation with M. bovoculi (ATCC strain: BAA-1259; origin: CA) and corneal scarification and inoculation with Moraxella bovis (strain Epp63-300; origin: NADC). The study was conducted in 3 replicates of 10-12 animals housed in individual pens with no nose-to-nose contact. Calves were enrolled after an ophthalmologist confirmed the absence of corneal, conjunctival, and eyelid abnormalities. Calves were scarified and inoculated in one randomly selected eye, then observed for the primary outcome of interest (corneal ulcers) until euthanized 10 days following scarification. Research group members assessing the outcome were blind to allocation status. The study was approved by the institutional animal care and use committee. Of 36 animals purchased for the study, 5 were excluded prior to enrollment due to ophthalmic abnormalities. Of the 31 enrolled calves, 9/10 (90%) of M. bovis calves, 0/10 (0%) of M. bovoculi calves and 1/11 (9%) of control calves developed corneal ulcerations consistent with IBK in the scarified eyes. The absence of corneal ulcerations in M. bovoculi BAA-1259 inoculated calves suggests it is not a causal organism for IBK in this model and the pathogenicity of this ATCC strain has not been established. Consistent corneal ulceration development in the M. bovis inoculated group demonstrates the ability of the model to induce IBK ulcers.