Author Names: Ellie Caskey (BSc (Hons) Animal Management) and Brian Evans
Infectious Bovine Rhinotracheitis (IBR) is a highly contagious respiratory disease in cattle caused by the bovine herpesvirus (BHV-1) belonging to the sub family Alphaherpesvirinae, which results in financial loss to cattle farmers worldwide. Vaccination is considered one of the most cost effective methods for prevention and control, and in some countries it is used for eradication purposes. However multiple vaccines are available to farmers for BHV-1 and IBR protection, resulting in varying immunity response, thus the efficacy of these vaccines has been investigated and tested. This literature review aimed to critically analyse research published from 1990-2015 on BHV-1 and IBR while evaluating the individual methodologies and results, in order to highlight any common limitations influencing the assessment of the vaccine efficacies. These limitations were considered and recommendations for more effective farming vaccine protocols were made. Strengths and limitations of the individual vaccines were discussed throughout the review. Live and inactivated marker vaccines had the most recent research and were identified to be the most commonly used for their serological capabilities. Whereas DNA immunisation, despite producing promising results for efficacy, has had limited recent research as it is still considered a modern technique. Therefore, before it can compete with the highly utilised marker vaccines, further research into immunity response needs to be conducted. Following in-depth analysis of the individual vaccines, common limitations were discussed. These identified that the current vaccination-challenge experiment design does not produce results which can be directly applied into farming protocol. Field studies needs to be utilised more to test results found in the vaccination-challenge research to allow for more application and understanding of the relationship between vaccination and the physical environment. Finally, different administration routes could also contribute to the varying efficacy of the vaccines which needs to be further investigated to produce a common protocol for farming vaccination.
Infectious Bovine Rhinotracheitis (IBR) is a highly contagious respiratory disease in cattle caused by bovine herpesvirus (BHV-1) (Mahajan et al. 2013; Kahrs, 1977) belonging to the sub family Alphaherpesvirinae (Bertolotti et al. 2015; Ackermann and Engels, 2005). The disease is characterised by inflammation of the upper respiratory tract with conjunctivitis and nasal discharge (Kamiyoshi et al. 2008; Yates, 1982). Secondary infection occurs due to immunosuppression resulting in further bacterial and viral infection; such as signs of fever, loss of appetite and reduction in milk yield (Kamiyoshi et al. 2008). The genital tract can also become infected with the most severe cases resulting in abortion (Nardelli et al. 2008). Once recovered from the primary infection, the cattle become latent carriers of IBR (Jones and Chowdhury, 2008; Muylkens et al. 2007). This phase is characterised by the absence of clinical symptoms and lifelong persistence of viral DNA in the trigeminal and other sensory ganglia (Bosch et la. 1998). After exposure to stressful situations the latent virus can reactivate thus allowing transmission of IBR to susceptible cattle (Raaperi et al, 2014, Solis-Calderon, et al, 2003; Bosch et al. 1998).
Direct and indirect transmission of the virus occurs through respiratory, ocular and genital discharges from the cattle and contaminated material (Solis-Calderon et al. 2003; Wentkin et al. 1993). Larger herds are more susceptible to the risk of IBR due to more transmission contact, and the potential integration of positively/latently infected cattle increasing likelihood of transmission (Woodbine et al. 2009; Nuotio et al. 2007). Within the UK 70% of herds have been exposed to IBR (Price, 2012) with increased abortions, reduced milk production, decrease in weight gain, and barriers to the international trade of livestock resulting in significant economic loss for farmers (Parreno et al. 2010; Nardelli et al. 2008).
Vaccination is considered to be one of the most successful cost-effective methods for the management of infectious diseases (Hurk, 2006), despite producing varying levels of protection in terms of such as reducing transmission, prevention and eradication (Tizzard, 2013; Normile, 2008; Hurk, 2006). For the disease to be effectively controlled, correct usage of the vaccine is required including the correct administration route and correct timing. Incorrect administration is expensive, wasteful and can lead to breakthrough disease (Salisbury et al. 2006). In a UK farmers based questionnaire 86% of respondents vaccinated their cattle with the majority being dairy famers (Cresswell et al. 2014). The results from the questionnaire also identified that farmers on larger farms vaccinated more regularly compared to small farms. This is beneficial as transmission is more likely in larger herds. However despite the high percentage, these results do not testify what vaccinations the cattle are being inoculated with. Therefore in relation to BHV-1 and IBR this data is difficult to apply, however, the highest uptake of vaccination was not for IBR but BVD (Bovine Viral Disease) (Cresswell et al. 2014) with some vaccines used to prevent both IBR and BVD.
1.1 Characteristics of BHV-1 and IBR Vaccines
There are varying vaccinations for BHV-1 and IBR prevention and control; modified live vaccines, live or inactivated marker vaccines (Oirschot et al. 1996) and less commonly DNA immunisation (Langellotti et al. 2011). Modified live vaccines (MLV), contain versions of the living microbe that has been weakened in the laboratory to prevent the disease from occurring (CDC, 2015). An advantage to this type of vaccination is that it elicits strong cellular and antibody responses associated with lifelong immunity after one or two doses (Alvarez et al. 2007). A limiting factor with this type of vaccination is the weakened microbe which could revert to a virulent form and cause the disease (World Health Organisation, 2016) within the cattle. In comparison some vaccines contain an inactivated virus which does not have the risk of reverting back to virulence as the disease-causing microbe is killed through chemical, heat or radiation thus is unable to revert. However these vaccines have the limitation of stimulating a weaker immune response compared to the live vaccines (NOAH, 2013).
Marker vaccines make it possible for serological differentiation between vaccinated and non-vaccinated cattle that have had the disease (Oirschot et al. 1996a). Deletion of a glycoprotein is a form of genetic modification making this vaccine virus serologically different to the wild type (WT) strain (Manoranjan, et al. 2014). Different glycoproteins have been deleted and tested for different immune responses (Traesel et al. 2015). This genetic modification can be applied to live or inactivated vaccine and both types are commercially used.
Less commonly used are DNA vaccines, which involve the construction of bacterial plasmids through in vivo administration to express and encode protein (Liu, 2011). DNA vaccinations produce multiple adaptive immunity responses from antibodies, T cells and cytolytic T-lymphocyte (CTLs) with the benefit of avoiding undesirable side effects and responses (Liu, 2011).
As a result of having different types of vaccines available for BHV-1 and IBR vaccination, varying levels of efficacy have been observed in preventing and controlling the disease. There is a strong argument for the need for this literature review to critically analyse the appropriate research available on BHV-1 and IBR vaccinations while evaluating the individual methodologies and results, and identifying any common limitations which could influence the assessment of different vaccine efficacies. Furthermore this allows recommendations to be made where appropriate whilst considering application into the wider context throughout the review.
A literature review surveys scholarly articles relevant to specific topics (BHV-1 and IBR vaccines) and critically evaluates each individual piece of research in order to offer an in depth analysis on the topic (Ridley, 2008). This literature review selectively analysed existing research encompassing factors which may influence the productivity of immune responses induced through different vaccinations and methods. Studies investigating similar vaccines were grouped together into sections and discussed thematically through analysis and evaluation. A conclusion was reached on the significant importance of the research and how valid the published results are to the scientific community (Cronin, Ryan and Coughlan, 2008).
2.1 Literature Search
A breadth of different literature on BHV-1 and IBR vaccination was analysed to develop a thorough background and a greater understanding of the prevalence including the impact on the cattle farming industry. A variety of suitable data-bases were used to search for the research literature including: Google Scholar, Pub Med, NCBI and Science Direct.
2.1.1 Inclusion Criteria
A combination of different key words and phrases were used for the inclusion criterion to determine what literature was necessary and relevant to the literature review’s aim. The search terms were broad to avoid inadvertent bias (Cronin, Ryan and Coughlan, 2008). All literature included was published from 1990 – 2015.
Key words: Infectious Bovine Rhinotracheitis, IBR, Bovine Herpesvirus, vaccine, vaccination (marker, live, attenuated, subunit, killed, inactive, active), immunisation, DNA, gene, deleted, gE, efficiency, efficacy, efficient, administration, route, intranasal, intramuscular, naïve calves, adult, young, dairy, beef, cattle, farms, cows, pregnant, lactation, milk yield, weight gain, transmission.
2.1.2 Critical analysis and evaluation
Literature reviews have been critiqued for not being sufficiently critical, lacking synthesis and analysis of the research and risking being biased (Pautasso, 2013). In order to overcome these limitations certain measures were followed such as critical analysis and evaluation of the selected literature, including assessment of methodologies and results, being undertaken. It was essential for validation of the argument and the review’s aim to understand the differences between facts and hypotheses and/or opinions, to recognise relevant evidence and to distinguish between relevant and irrelevant statements (Pautasso, 2013). Utilising the methodology and results from each piece of research through scrutinising the data, an unbiased conclusion was formed to achieve the overall aim of assessing the efficacy of BHV-1 and IBR vaccines with regards to common limitations identified from the research and allow suggestions to allow more effective farming vaccine protocols.
3.1 Efficacy of BHV-1 and IBR Vaccines
The common assessment for vaccine efficacy is by conducting a vaccination and challenge experiment, which involves inoculation of the chosen virus (BHV-1 and IBR) into a suitable host (cattle) followed by direct exposure to that virus (Hudgens and Gilbert, 2009). Subsequently the vaccine’s ability to reduce clinical signs and virus shedding can be recorded and antibody titres analysed from blood samples (Oischot et al. 1996). Despite the multiple vaccines available for protection against BHV-1 and IBR this method of testing efficacy is universally used as a standardisation approach (Weinberg and Szilgyi, 2010). It is important to establish the difference between testing for efficacy and effectiveness; vaccine efficacy refers to an individual level whereas effectiveness refers to the effect on the population level (Halloran et al, 1991). The majority of the research in this literature review tested vaccine efficacy.
MLV vaccines have been considered more effective for inducing strong immunity compared to inactivated vaccine (Montagnaro et al. 2014). This is due to the ‘live’ virus having higher virulence; therefore the body has a more aggressive reaction.
Fairbanks, Campbell and Chase (2004), investigated the onset of protection following a subcutaneous injection of MLV containing attenuated BHV-1 administered at different times. The time from inoculation to the onset of an immune response can affect the level of protection produced against IBR and BHV-1 (Fairbanks, Campbell and Chase 2004; Woolums et al. 2003; Sutton, 1980). Calves were divided into three groups (n=11); group 1 was vaccinated 96 hours before challenge, group 2: 72 hours before challenge and group 3: 48 hours before challenge. The final group (control) received no vaccination (n=10) prior to challenge. Post challenge the calves were monitored for clinical signs. Individual weight was recorded to assess differences in weight gain, blood samples were collected to determine IBR antibodies and nasal secretions were collected. The results from this study identified significant differences between the time of vaccination and clinical signs exhibited among the different groups (Table 1).
|Vaccinated 96hr pre challenge||Vaccinated 72hr pre challenge||Vaccinated 48hr pre challenge||Control Group|
Table 1 – Table showing the percentage differences of clinical signs monitored (Fairbanks, Campell and Chase, 2004).
Group 1 and 2 indicated a higher immune response compared to group 3 and the control group, emphasising the importance of time of delivery for the vaccine to be effective. Depression and nasal secretions were significantly reduced (P ≤.03) and (P ≤.03) respectively for group 1 and 2 and no differences were found between group 3 and the control. Ocular lesions were more severe and longer lasting in the control group compared to the other three groups. In addition, respiratory abnormalities were more extreme in group 3 and the control. Table 1 shows that group 1 had higher respiratory abnormality, this result may be an anomaly, however as the small sample sizes make it difficult to distinguish. Using a larger sample size in a future study would allow for more applicability (Button et al. 2013). The methodology does not disclose how the calves were monitored; therefore it is difficult to know the accuracy of monitoring and thus the reliability of the results presented. There is no regards to the safety of the vaccine. Many MLV are considered less safe than killed or inactivated virus vaccines despite the stronger immune response (Smith et al. 2015). Furthermore this vaccine may not be suitable for pregnant and lactating cows, as multiple studies have looked into the negative effects associated with MLV vaccination (Waltz et al. 2015, Elsworth et al. 2003). However this study also looked at body weight as BHV-1 and IBR has caused financial loss in the beef industry (Perry et al. 2013). Fairbanks, Campbell and chase (2004) also found Group 1 gained approximately 76% more weight compared with the control group, signifying the importance of vaccination to prevent financial losses in the industry.
Despite this study identifying the importance of time of vaccination for the most effective immune response, it is difficult to apply these results to daily farming practice. In the physical environment predicting the onset of BHV-1 and IBR outbreak within a herd is unlikely, thus the findings from this study could be considered futile. Nonetheless the results do emphasise the importance of placing new animals or recently vaccinated animals in quarantine for 92 hours before being returned into the herd allowing for the strongest immune response.
MLV vaccines are highly regarded for providing lasting immunity and stronger protection against BHV-1 (Rodning et al. 2010). However concerns regarding the safety of these vaccines has been highlighted as a result of abortions and some infertility in cows and still needs resolving (O’Toole et al. 2012; Givens, 2006). Despite efficiently providing immune protection, without being safe for use, these vaccines cannot be used as the financial cost is too high (Fulton et al. 2015). The main aim of Walz et al’s. (2015) study was to analyse pregnant dairy cows vaccinated with either a MLV or a killed virus (KV) vaccine. The study was carried out on a large scale dairy farm using 692 cattle randomly assigned into group A (MLV) or Group B (KV). All subjects had been previously vaccinated. The data presented in this study found no significant differences in pregnancy rate or abortion between group A and B, suggesting the MLV and KV vaccine used for this study are safe to use. However a significant difference was identified with group B having significantly higher (p<0.0001) antibody response compared to group A. These are surprising results as MLV vaccine research associates higher immunity with live virus (Kalthoff el al. 2010), though this unexpected result may be due to pregnancy which could explain the difference in antibody response (Walz et al. 2015; Dubovi et al. 2000).
Correspondingly, Lee et al. (2015) collected blood samples from 28 heifers and calves (mixed range of ages and vaccination status) with the aim to evaluate antibody titres. The subjects used were obtained from a dairy farm whose vaccination policy involved two administrations of an MLV vaccine at 2.5-3 months and repeated again at 3.5-4 months plus one single MLV administration 30-40 days prior to breeding. The data produced from this study found no antigenic variation between vaccine virus and disease isolate, suggesting despite subjects producing the same number of antibodies, abortion still occurred. Whilst this may suggest a causative agent of abortion, it is not clear if the abortion was caused by infection due to lower immunity in pregnant cows or from the agents present in the vaccine. Another option may be an unknown factor causing abortion. Furthermore this data produces an unclear conclusion on the effectiveness and safety of MLV vaccines and further testing into possible causative agents should be undertaken. Methodological limitations also contributed to the unreliability of the results such as number of subjects used and differences in vaccination status.
3.3 Live and Inactivated Marker Vaccines (Gene Deleted Vaccines)
Modified-live vaccines and inactivated vaccines have been associated with complications post inoculation resulting in abortion, infertility, and incompetent protection (HIPRA, 2016; Ostertag-Hill et al. 2015). As a result research has looked into genetic modification, specifically the deletion of glycoproteins. Glycoproteins are responsible for cell-cell interactions within viral biology, however not all are essential for virus replication, thus can be deleted from attenuated marked vaccine strains (Weiss et al. 2015). Many vaccinations against BHV-1 and IBR contain the deletion of glycoprotein E (gE) as it is responsible for virulence (Belknap et al. 1999). A specific advantage to gE deletion is serological differentiation, limiting the chances of reversion to virulence and providing a safer vaccine whilst maintaining a strong immune response (Kaashoek et al. 1996). Vast amounts of research into gE deleted vaccines for BHV-1 and IBR has been undertaken into the efficacy of protection, however, differences in research suggest varying advantages and limitations with this method of vaccination. Applying gene deletion to inactivated vaccines and MLV is widely used for prevention and control of BHV-1 and IBR therefore prevalence of the disease is wide spread (Mineo et al. 2006). Currently the deletion of gE is highly utilised in control programmes (Foley and Hill, 2005).
3.3.1 Controlled Studies
Romera et al. (2014) hypothesised using both an inactivated and MLV combined with gE deletion would produce an efficient vaccine candidate. The results reflect the hypothesis, finding no differences in virus shedding post challenge in subjects inoculated with either vaccine and can be used efficiently combined with gE deletion. However these results fail to consider the administration route of the vaccine. The methodology explains groups were either inoculated intranasally, intramuscularly or intravenously. Despite no difference in virus shedding post challenge, differences in antibody titres and cellular response were recorded, highlighting differences between the administration routes. Intranasal administration had a 95% cellular response compared to 60% response for intramuscular and 20% response for intravenous. In comparison the intranasal administered vaccines had significantly lower antibody titres (P < 0.001) than the other administration routes. With these conflicting results, it is difficult to understand how the researchers arrived at the conclusion of both vaccines being equally effective against BHV-1 and IBR. Another limitation is the reliance on a Likert scale for monitoring clinical symptoms in the calves. Though this allows for severity of symptoms to be recorded it is still largely based on assumption producing subjective results. Overall this research, despite its findings, does not present a clear understanding on the efficacy of either live or inactivated gE-deleted vaccines.
Earlier research evaluated the immunogenicity, efficacy and safety of a gene deleted vaccine compared to a commercially available MLV (Belknap et al. 1999). Subjects were 5-7 month old calves which were all seronegative to BHV-1 and IBR prior to the study. The subjects were split into five groups of five; group 1 was the control group and received no vaccination, group 2 received the commercial vaccine and groups 3, 4 and 5 all received a gene deleted vaccine at different doses all administered intramuscularly. Prior to challenge the calves were monitored for nasal and ocular discharge, respiratory rate was measured and rectal temperature was taken. Depending on what clinical signs were expressed, a point was allocated for each symptom to each calf. These points were added together and the sums averaged, indicating how many calves experienced symptoms. However this method of monitoring is limited as it does not allow for the differentiation in symptom severity to be identified, thus a calf with severe ocular discharge will be considered the same as a calf with mild ocular discharge. Therefore it is difficult to interpret which vaccination resulted in more severe clinical signs, producing conflicting results on the safety of the vaccines leading to part of the aim not being met. Post challenge serum was collected on days 0, 14, 26 and 40 from the jugular vein which was tested for virus neutralisation to determine antibody titres (Table 2).
|Day 14 p.v.||Day 26 p.v.||Day 7 p.c.||Day 14 p.c.|
Table 2 – data representing antibody titres on day 14 and 26 post vaccination (p.v.) and antibody titres on day 7 and 14 post challenge (p.c.)
The results presented no clinical signs of infection in the calves vaccinated with the gene deleted vaccine, including those vaccinated with the lowest dose which had significantly lower clinical scores. These findings are supported by similar results from other research on gene deletion vaccination (Kaashoek et al. 1995; Kaashoek et al. 1994). However the results also highlight that no significant differences were found between all vaccinated groups and the amount of viral shedding produced, suggesting the gene deleted vaccines are no more efficient at immune protection than the commercial vaccines. These data demonstrate gene deleted vaccines have the same efficacy as the commercial vaccines. Despite this, gene deleted vaccines maintain the safety advantage when using MLV compared to conventional MLV (Frey, 2006). Interpretation of these results is difficult as the method does not identify if a live or inactivated gE deleted vaccine was used for the study limiting the replicability. Regardless of what the data suggests, applicability of these results would be restricted, thus further research using a live and inactivated gE deleted vaccine compared to a MLV would be recommended. This study was carried out in confined conditions to allow for precise control over the extraneous and independent variables and to determine the cause and effect relationship (Clarke and Dawson 2012) between the different vaccines and immune response. However these controlled conditions do not reflect the natural environment where this virus would occur thus reducing the ecological validity and applicability. Another limitation with this study design is the possibility of demand characteristics, which could result in biased results affecting reliability (Patton, 2015). Nonetheless this research does identify that gene deleted vaccines can be used to produce immune protection with similar efficacy to conventional vaccines.
Traditional MLV vaccines are considered efficient in reducing viral shedding and clinical signs (Wei et al. 2012; Jones and Chowdhury, 2010; Ackermann and Engles, 2006). Latency reactivation and shedding has also been associated with MLV vaccine due to the down regulation of cellular response (Jones and Chowdhury, 2010). A consequence of this effect is that many EU countries have mandated the use of gE deleted marker vaccines. Despite gE deleted marker vaccines having the ability of serological differentiation and being safer to use, the vaccine has been considered less efficacious when compared with the MLV, gC-, gG- and TK deleted viruses as demonstrated by Kaashoek et al (1998) which found that gG was significantly more immunogenic, however not as safe (Chowdhury et al. 2014; Kaashoek et al. 1998; Kaashoek et al. 1996; Kaashoek et al. 1996a).
Chowdhury et al (2014) tested the efficacy of a constructed novel vaccine BHV-1 triple mutant virus (BHV-1 tmv) incorporating individual deletions and mutations compared to a standard BHV-1 gE deleted vaccine. To determine protective efficiency, 18 BHV-1 and BVD negative, four month old calves were used and randomly assigned into three rooms (6 per room). Recording of clinical signs, rectal temperatures, nasal discharge and lesions were carried out and assigned a parameter (0-4). As expected following challenge, the control group displayed clinical signs compared to both vaccinated groups which did not. However the BHV-1 tmv immunised calves only produced viral shedding for seven days compared to the BHV-1 gE deleted immunised calves and the control group. Also the BHV-1 tmv displayed a significant reduction in the quantity of virus shed compared to the other groups. Furthermore these data indicate that the BHV-1 tmz vaccination produced a significantly more rapid and efficient immune response compared to the other two groups. ANOVA was selected for statistical analysis due to being a repeated measures experimental design. Using ANOVA is the most efficient and appropriate method for statistical analysis with this methodology as it identifies if any groups are significantly different. A limitation with the methodology is the utilisation of a Likert scale for recording clinical signs exhibited by the calves. This scale is subjective and can differ between researchers using the scale for measurements; therefore one opinion on the severity of a symptom could be different to another. Ultimately this scale could limit the accuracy of the results and the validity and reliability of the results could be questioned. Nonetheless this scale is universally used and a common method for measurement among vaccination research. This study contradicts previous work suggesting the BHV-1 tmv is safer to use and significantly better for inducing a protective immune response compared to gE deletion vaccines. However it does support genetic modification in vaccination protocol as a effective tool for prevention and control of BHV-1 and IBR.
3.3.2 Field Studies
In comparison to vaccination-challenge studies, there were a limited number of studies investigating the efficacy of BHV-1 and IBR in the field. The advantage of field studies are that they allow for real-world results to be produced and more easily applied into farming practice.
A large scale field study testing the effectiveness of a live gE deleted BHV-1 vaccine was conducted by Mars et al. (2001). In this study they investigated 84 dairy herds situated across the Netherlands and concluded that the live gE deleted BHV-1 vaccine could be used in eradication programmes with the vaccinated group being significantly (P=0.038) more efficient compared to the placebo group. Although the results support the efficacy of live gE deleted vaccines, the study has limitations which may affect the reliability of the results. Being a field experiment produced high ecological validity thus application was easier, however, control over variables was restricted. A major disadvantage for testing the efficacy of this vaccine is difficulty recording the severity and number of infections. Age variation and breed of cattle could also limit the specificity of the results, though the large number of herds used for this study was beneficial as detection of subtler and more complex effects can be identified (Lin, Lucas and Shmueli, 2013). A methodological limitation in this research was the reliance on the farmer’s ability to report adverse events following vaccination such as abortion or clinical signs. This could lead to false results being recorded through measurement error and inexperience (Cheisa and Hobbs, 2008). However, this does allow for reduction in demand characteristics as the participating farmers were not aware of the study aim (McCambridge, Witton and Elbourne, 2014). Therefore the data produced identifies the advantage of using a live gE deleted vaccine within the field, demonstrating high efficacy with low abortion rates (1.4%) and high immune response (Mars et al. 2001; Bosch et al. 1998). Despite the limitations of this study, the results are more representative of the challenges faced by farmers and ultimately establish that the live gE deleted vaccine is efficient against BHV-1 and IBR.
Three European countries participated in field studies to determine the efficacy of a live IBR marker vaccine (Makoschey et al. 2007). Following vaccination seroconversions decreased from 16.4% to 0.1% in Germany, 25.7% to 11% in Italy and 71% to 17% in Hungary. These percentages indicate that the live marker vaccine effectively reduced IBR in field conditions. Each country has different vaccination protocols which need to be considered when considering these results in terms of efficacy i.e. in Germany it is mandatory to vaccinate all cattle against BHV-1 thus the different numbers in vaccinated cattle will affect the percentage of virus control.
Raaperi et al. (2015) observed the effect of BHV-1 vaccine on infected dairy cattle herds in Estonia. In comparison to Mars et al. (2001) only seven vaccinated (V+) herds were used and vaccinated with a gE inactivated marker vaccine twice a year. Seven unvaccinated (V-) herds (against IBR and BHV-1) were selected for comparison measures. The methodology used a qualitative approach distributing questionnaires to participating farmers, asking specific questions on changes in clinical signs, disease incidences in calves including mortality rate, abortion incidences in heifers and milk production plus treatment costs. The results indicated a reduction in respiratory disease for calves and heifers and mortality rate following the vaccination period compared to the V- herds (95% higher culling due to respiratory abnormality). The questionnaire also identified an improvement in reproduction status and no increase in abortion from the V+ herds. A limitation with these results is the reliance on data collected based on farmer opinion thus producing subjective results. However similar to Mars et al. (2001) this study design is more realistic and applicable to cattle farmers, thus the result should not be underestimated. A larger sample size would be more appropriate allowing for closer evaluation of the control effects of the vaccine. Despite these limitations the study has produced results highlighting the benefit of utilising a gE inactivated marker vaccine for efficient protection against IBR and BHV-1.
Applying gene deletion to either live or inactivated vaccines is beneficial to allow developments in IBR and BHV-1 prevention and control. The research discussed has limitations, however what can be identified is the improvement in safety concerning MLV and the stronger immune response induced from inactivated vaccines.
3.4 DNA Immunisation
An alternative and modern vaccination approach is the utilisation of antigen-encoding DNA, inducing protective immune responses against BHV-1 (Dominguez et al. 2015; Cox et al. 1993). This method is considered to be a beneficial alternative to conventional vaccines due to low cost, long lasting immunity and the ability to be cohesive with maternal antibodies when inoculated into a suitable host (Hurk et al. 1999; Hurk et al. 1998). Intramuscular inoculation was the traditional route of DNA administration (Wolff et al. 1990), however, research now suggests inoculation at a mucosa or epidermal site may be a more efficient route (Zheng et al. 2005). Research into the efficacy of DNA vaccination is largely tested on mice providing useful data for understanding the effect of the antigen form and changes in immune response. However DNA vaccination studies on larger animals has allowed for the understanding of the correct dosage required, route and method of administration thus application to cattle is more reliable.
Hurk et al. (1998) used intradermal delivery for inoculation of BHV-1 free 6-7 month old calves. This method was selected because the epidermis has higher immune surveillance with the presence of Langerhans cells resulting in greater DNA diffusion compared to muscle tissue (Toussaint et al. 2005; Hurk et al. 1998). The study separated calves into two trials, trial one which immunised groups of ten calves intramuscularly and trial two which immunised groups of six calves intradermally through the ear. A control group was present during both trials to allow for comparison. The results identified that the intramuscular immunised calves did seroconvert but with low antibody titres in comparison to the intradermally immunised calves which all seroconverted following one inoculation of the DNA vaccine with statistically significant (p<0.05) higher antibodies. Therefore the intradermal inoculation route can be seen to be more efficient, demonstrating the importance of administration site for efficacy. A limiting factor of this study is that, despite the significant results, the trials contained unequal subject numbers potentially affecting the application of the results into the wider context. Also the methodology does not specify the conditions that the calves were inoculated in thus impacting on the external validity of the study.
Braun et al. (1999) tested the efficacy of DNA immunisation using mice and cow subjects to identify the differentiating effects dependent on the species. In comparison to Hurk et al’s. (1998) methodology Braun et al. (1999) used a gene gun for delivery method of the DNA vaccine, which involved the propulsion of plasmid coated balls into the epidermal cells of the host. Results from this study support Hurk et al. (1998) findings and the ability for an induced immune response in neonates without maternal antibody interfering with the efficacy of the vaccine (Braun et al. 1999). The mice used by Hurk et al. (1998) only required one immunisation of the vaccine before antibodies developed. In comparison despite the calves having proliferative responses following immunisation, which was maintained for up to 12 weeks; it was necessary for a second immunisation of the DNA vaccine before increases in antigen specific proliferation and serum antibody production could occur. Following the second immunisation, a significant (p<0.05) increase in antibodies in the vaccinated group was reported. These findings were in conjunction with a report by Macklin et al (1998) suggesting reliability of the results; however, Macklin et al’s. (1998) study looked at pigs affecting application of the results to cattle. The requirement for two immunisations in the calves and not in the mice is a result of physiology. The Langerhans cells are important targets for the plasmid; in mice these cells are present throughout the epidermis whereas in cattle they are resident at the lower region of the epidermis (Bancherau et al. 1998). However other considerations such as the size difference between species and physiological differences could also influence the inoculation effectiveness. The immune response generated in the calves suggests that DNA immunisations using the gene gun method can be effective for vaccination.
Utilisation of a gene gun also contributes to the benefits of DNA immunisation through reducing unwanted side effects due to the lower dose requirements of DNA (Toussaint et al. 2005; Braun et al. 1999). Another benefit associated with the gene gun method is the strong life-long responses expected (Loehr et al. 2000); however this may be due to the DNA vaccine alone and not the gene gun method. Despite the results of Braun et al’s. (1999) study identifying clear benefits to DNA immunisation and the gene gun method, other factors need to be evaluated to understand how effective this vaccination is for cattle inoculation against BHV-1 and IBR.
In contrast Loehr et al. (2000) carried out a similar study testing the efficacy of administration route with a gene gun either at a mucosal or epidermal site. The results identified that DNA immunisation of the vulval mucosa induced a stronger immune response (p<0.001) than the intradermal vaccination (Loehr et al. 2000). An explanation for this may be due to more effective antigen presentation by antigen presenting cells (APCs) and Langerhans cells in the mucosa being present throughout the epithelium (Loehr et al. 2000). In comparison the Langerhans cells are only found in the basal region of the epidermis (Braun et al. 1999) suggesting the site of inoculation plays an important role in efficacy of the DNA vaccine. Despite these data identifying the advantages to DNA immunisation it is important that the temporal validity is considered as technological developments could alter these findings. A repetition of these studies would be beneficial to uphold the findings of DNA immunisation efficacy for IBR.
In more recent studies Yao et al. (2015) supports the role of Langerhans cells producing stronger immunity through intradermic or mucosa administration compared to more traditional routes, supporting the earlier research (Loehr et al. 2000; Braun et al. 1999). DNA immunisation has been further improved by exploiting multiple gene delivery methods such as DNA vaccine priming and recombinant protein boosting (Jiang et al. 2007). It also has continued advantages such as manufacturing simplicity, biological stability, low cost and the ability to function when maternal antibodies are present (Chase et al. 2008; Zamorano et al.2002). Successive priming with plasmid DNA and boost with inactivated vaccination can induce a higher immune response than vaccinations administered separately (Toussaint et al. 2005). The results found from this study are consistent with other research into the efficacy of DNA immunisation (McShane, 2002) suggesting reliability. Toussaint et al. (2007) demonstrated that the administration of an inactivated vaccine resulted in a substantial rise of immune response in calves which was statistically significant (p<0.05). This suggested that the combination of DNA vaccination with an inactivated vaccine is more efficient than DNA vaccination alone. However compared to commercially used vaccines, there is little evidence suggesting DNA vaccinations are a more efficient barrier against the disease; nonetheless the ability for DNA vaccinations to be efficient with the presence of maternal antibodies is a key advantage over other vaccines. Further study into this method of vaccination needs to be undertaken before the strengths can be fully utilised in farming vaccination protocol.
3.5 Common Limitations
Reliance on highly controlled, vaccination-challenge experimental designs has been identified as a common methodological limitation throughout this review. Although this approach has the advantages of high variable control, establishing cause and effect relationships and precise measurement, the ecological validity is low. Until these results can be applied to practical contexts these studies will have little relevance for prevention or eradication of BHV-1 and IBR. The characteristics of a vaccination-challenge study design requires the cattle to be exposed to the virus (challenge) allowing for any immunity response to be recorded. The difficulty with challenging the cattle is that each study uses varying strains and doses of the virus and vaccine, in turn producing specific results individual to that study. Utilising these results for appropriate, efficient vaccination protocols is problematic as no one method is entirely applicable to farming. Farmers are unable to predict the onset of the virus or its severity, emphasising the impractical experiment design. Conducting further field studies would allow understanding of the vaccine and disease relationship in the physical environment thus being more applicable to farmers. Using the vaccination and challenge method is important though for identifying key characteristics of the vaccine being tested; including unwanted side effects such as abortion and weight loss which can be economically detrimental to farmers (Zhao and Xi 2011). The results produced from vaccination-challenge research should be applied in developing a large scale field study allowing for ecological validity to be increased and a suitable vaccine protocol to be established.
Varying administration routes have also been identified in the analysis of the individual research methodologies throughout this review, producing different immune responses. This suggests that it plays a key role in the efficacy of the vaccine. Currently the common protocol for vaccine administration is intramuscular; however some research proposes intranasal administration as a more suitable inoculation method. Despite this there is contradictory research that has found little efficacy significance between the different routes, therefore further research needs to be carried out to allow the best method to be established. The varying routes for administration emphasise the difficulty in comparing efficacy of the vaccines, as all have marginally different variables even though the aims are similar. Further research reducing the number of confounding variables will allow for greater understanding of the most suitable inoculation route and identify the most efficient vaccine; producing a gold standard for vaccination protocol which can be applied to practical farming methods.
Ultimately the studies evaluated in this literature review identify multiple variables that need to be considered when analysing the efficacy of the results for BHV-1 and IBR vaccines. Individually the vaccines all have strengths and limitations. Inactivated vaccines are considered safer to use on pregnant cattle although produce a weaker immune response compared to MLV. Safety of the vaccine needs to be considered as otherwise despite the efficacy strength it will not be suitable for use. Marker vaccines have had more research into their efficacy, including field studies, and are commonly used for their serological capabilities whilst producing a substantial immune response. Promising research from DNA immunisation suggests low cost production and induced immunity in the presence of maternal antibodies, however, it is still considered a modern technique thus more substantial evidence for strong viral protection is required before it can compete with the more utilised marker vaccines. Deciding if one vaccine is superior in efficacy is challenging due to confounding variables in the individual studies, therefore this literature review concludes that marker vaccines and DNA immunisation have encouraging results, however, the application of these results via field studies needs to be carried out before full confidence of BHV-1 IBR vaccine efficacy can be established and turned into effective farming protocol.
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