Several regions of Ethiopia are endemic for foot and mouth disease (FMD) [24]. Serotypes O, SAT2, and A have been linked to the majority of reported outbreaks. Quarantine, vaccination, and animal movement control were the main methods used to combat this disease [25]. It has been demonstrated that the routine immunization of cattle with vaccines and the emergency vaccination of all susceptible species can effectively control the disease and significantly reduce the spread of the virus to undetectable levels [26].
According to [27], the indicator used to evaluate preventive vaccination programs is post-vaccination sero-surveys for FMD. Effective vaccines and control measures have enabled the FMD unvaccinated, sero-negative herds in compliance with strict international trade policy [28]. The current study aimed to assess the post-vaccination state of immune response to homologues FMDV seed strain using VNT in order to determine the antibody titer of cattle and to detect the immune response to NSP from commercially available aqueous FMD polyvalent vaccine manufactured from NVI and its compatibility with DIVA test.
The A/ETH/21/2000, O/ETH/38/2005, and SAT2/ETH/64/2009 FMDV seed strains were used to test the sera. According to the study, after a single primary vaccination dose, the vaccine was able to induce antibody titers against the A, O, and SAT2 serotypes at a pre-specified level (≥ 1.5 log10) in cattle in the proportion of 69%, 73%, and 94%, respectively, on day 14 post-vaccination. [29] reviewed the stimulus variables of hosts, such as species, breed, individuality, age, health, state, and other stress factors (e.g., husbandry, climate), which affect FMD immune status and dose, route, volume, purity of virus, virus strain (e.g., physical and antigenic characteristics), and adjuvant(s) which influence the response to FMDV and vaccine, and concluded that different factors are often responsible for unprotected (< 1.5 log10) cattle. After the initial vaccination, peak antibody titers were observed on day 21 and 35 post-vaccination. These results were consistent with those of [30] and [29]. According to their findings, vaccinated cattle react fast to the initial dose and reach peak antibody titers 14–28 days after the single injection. Similarly, the immune response of sheep after an initial dose elicits production of antibody, which was observed as early as 7 dpv and the antibody titers peak in most animals within 28 days after vaccination [31]. In our study, cattle were reactive on day14 after initial vaccination and the proportion of cattle showing peak conversion were on day 35 post-vaccination.
In this study, the lowest antibody titer findings were 1.45 log10 for serotype SAT2 and 1.2 log10 for serotype A and O, which agrees with the findings of [32] who reported 1.28 log10 for serotype A and 1.14 for serotype O. Our results on day 14 post-vaccination showed values of 1.2 log10 for serotypes A and O, which were higher than those found by [31] in Egypt, where they discovered 1.05 log10 for serotype O and 0.95 log10 for serotype A in sheep. The difference could be due to top-type used in vaccine in previous study top-type O1 and A1 used in our situation top-type A21 and O38 used and species variation in previous study sheep’s were used. The serological response observed was ≤ 2 log10 titer in the majority of vaccinated cattle. But the study revealed that the SAT2 serotype had higher titers, followed by serotypes A and O. Peak antibody titers for the SAT2 serotype were recorded at up to 2.4 log10; this result was consistent with that of [29] in Zimbabwe and corroborated the findings of [33], who discovered that Nigerian strains of the SAT2 serotypes of FMDV used as vaccine antigen could elicit high antibody titers against FMD. The value of the other recorded peak antibody titer, which belonged to serotype A and was 2.1 log10, was found to be in disagreement with the findings of [32], who reported a value of 2.8 log10 for the age group of cattle older than 2 years. The difference might be due to mixed age group used in this study, where as in previous study the age group was analyzed separately and better responses were recorded in adult cattle.
According to our research, cattle that received a vaccination on 21 days post-vaccination had a mean antibody titer of 1.64 log10 for serotype A. This is in agreement with the findings of [34], a convincing study and report that had a mean antibody titer value of 1.53 log10. On day 21 following vaccination, the range value for serotype A was 1.35 to 1.8 log10, which was not comparable to the findings of [35], who reported 1.5 to 3.5 log10 from Eritrea. The type of vaccination that was administered in our case may have made a difference. [35] used monovalent vaccines; polyvalent vaccines are used because they have a different effect on capsid stability than monovalent vaccines.
The mean antibody titer for serotypes A and O on day 35 post-vaccination in our study was 1.82 log10, which was more in line with the findings of [36] who reported 1.95 log10 for these serotypes in Bangladeshi calves under 12 months of age, and [32], who reported 1.787 log10 in cattle under 2 years old from Pakistan. The results of [36] showed that serotype A causes 2.15 log10 and serotype O causes 2.19 log10 at ages greater than 12 months; however, our results (1.82 log10) did not match their results. [32] found that the mean values for serotypes A and O in the age group over two years were 2.25 log10 and 2.12 log10, respectively. The detection also showed discrepancies with our findings of 35 dpv (1.82 log10 value) for these serotypes. In our instance, mixed age groups were most likely utilized. However, in [36 and 32] findings, the cattle were divided into groups and had separate discussions. As a result, the age groups of > 12 months and > 2 years, or adults, respectively, showed better immune responses. Furthermore, antigen stability may play a role.
Our research revealed a recently observed elevated immune response to serotype O. However, this finding contradicted the findings of [37], who reported that serotype O was the early responder in sheep in India. The dose and species used in the prior study, however, may be the reason for the difference from this investigation. Sheep was the species, and the dosage was 5 milliliters. However, in our study, cattle were utilized, and the dosage was 4 milliliters. In our study, serotype O antibodies begin to decline more quickly than those of the other three serotypes. When compared to other studies that supported our findings, the immune response for serotype O led decreased quickly in the same study by [37].
FMDV has high rate of genetic diversity because of frequent mutations [31]. This variation is reflected in the immunogenicity of different strains and in the immune response induced by FMDV. [37] Report, some strains are more immunogenic than other. Regarding to the above reasons, different strains tend to induce production of variable levels of antibody titers. However, the level of immunity induced by each serotype differed in each vaccination protocol.
When it comes to serotype O, 93% of cattle and 100% of cattle for serotype A and SAT2 maintain antibody titers to the pre-specified level (> 1.5 log10) on day 80. This shows that most of the cattle that received vaccinations were able to continue for up to 80 days after the shot. Cattle that received the aqueous FMD polyvalent vaccine for serotypes A, O, and SAT2 maintained antibody titers on 125 dpv in 86%, 82%, and 96% of cases, respectively. This observation is consistent with the findings of [38], who observed a rapid decline in antibody titers induced by aqueous FMD vaccines within 2 and 4 months of vaccination, and [39] in Mongolia, who observed a titer drop in the induced immune response to cattle 3 months after vaccination. From other research’s the developed antibody titer to lower potency vaccine without booster dose exist in between to 4 to 6 month [31]. As of our study finding, Rodriguez and [6] reports was found to be disagreed with our outcome, inactivated virus vaccines can elicit high levels of neutralizing antibodies and offer efficacious protection against homologous serotypes. The immune status of the immunized animals and the vaccine's capsid stability could be the cause of this discrepancy.
Based on the findings in naïve cattle, low humoral immune response to inactivated FMD vaccine was recorded, these was agreed with report of [39; 40], from this result, both in experimental animals and pigs, low antibody titer has been seen in naïve population. The low proportion of animals with protective levels of antibodies matches with the results obtained by [29] according to [22], in general an initial vaccination with aqueous vaccines does not induce a potent and durable immunity. Because of this, boosters are required to induce and maintain a high level of protection.
All naive cattle in this study were negative on day zero of the study. Three samples from ID numbers #4698, #4713, and #4718) were positive by ID vet 3ABC ELISA kit for NSP only once (point positive) on days (21, 35, and 80 dpv, respectively, after the first vaccination. Nevertheless, samples are being identified with a different second test kit (IDEXX FMD 3ABC Bo-Ov antibody test kit), and the results of this test were NSP-negative. NSP serology offers significant advantages over other tests for the DIVA strategy, even in the absence of the second confirmation test [40]. These advantages include its high throughput and the persistence of antibody response against NSPs [41]. Again, observing the illness's clinical symptoms is necessary for diagnosing FMD. A second NSP antibody assay with at least equivalent sensitivity and specificity is typically used to confirm positive results from tests with high diagnostic sensitivity, such as 3ABC ELISA, which are typically used to screen the sera [42] in Italy pointed out test gaps in kits. According to [43] study, the 3ABC ELISA misclassified 58 out of 1595 negative cattle in Australia as positive. The study also revealed that the kit's sensitivity exceeds 96%. A review by [44] also noted that the kit's repeated limitations were observed in the diagnosis of various scholars. When clinical signs or epidemiologic correlations do not exist, it is important to secure the reliability of test results through the evaluation of the profiling of Electro-Immuno Transfer Blotting (EITB) and multiple NSP antibodies for the confirmation of the test results [19].
In this longitudinal study, throughout study period vaccinated or unvaccinated cattle, no detection of NSPs were assumed. This indicates that, vaccination with inactivated FMD vaccine produced in NVI does not induce antibodies to 3ABC protein and there was no disease exposure was noted during the study period as seen in unvaccinated group. This finding agreed with study of [45] in Poland and [46] in Argentina and they found no vaccine induced NSP in the experimental and control animal. Our findings are disagreed with [41], they were stated that, 1.809% (76 from 4200) positive cattle are found after vaccination. The difference could be due to the level of NSP content in the vaccine used in the studies.
The purpose of this study was to provide guarantees regarding the potency and purity of inactivated vaccines sold in Ethiopia with regard to the amount of non-soy protein present. It demonstrates that serological surveys to assess viral activity are unaffected by the conventional vaccine with adequate potency used in Ethiopia. This means that after an outbreak, this indicator can be used to identify infected animals. It is imperative to ensure that FMD vaccination of cattle with commercially available FMD polyvalent vaccine produced in NVI does not induce NSP antibodies. The effectiveness of vaccination campaigns is largely dependent on the caliber of vaccines administered.As my knowledge this is the first study to demonstrate that a vaccine produced by conventional methods achieve a degree of purity, in terms of NSP content. Therefore, this allows for an accurate interpretation of the results of sero-epidemiological surveillance; this accuracy is important, as the detection of NSP antibodies during surveillance provides supporting evidence to confirm the animal disease status of FMD free zones.
Finally, as clearly stated by [47] it is difficult to extrapolate the findings directly with different results with literature there is a large variation in the types and quality of vaccine available from global market, so comparisons between reports in the literatures on FMD vaccines should be made with caution.