Sero-Conversion Status of Different Breeds of Chickens Vaccinated With Newcastle Disease I2 Vaccine Type around Hawassa, Ethiopia

Introduction

Majority (95 %) of Ethiopian chickens were kept in village scavenging systems [1]. Chickens in scavenging production systems in rural settings exist with little human input and are constrained by feed, management and disease problems [2]. Newcastle disease (ND) is a devastating disease of both commercial farms and village chickens [3]. Newcastle

disease (ND) is a highly contagious viral infection of avian species especially poultry caused by Newcastle disease virus (NDV), a Paramyxovirus called avian Paramyxovirus type 1 (APMV-1). It is a single-stranded non-segmented RNA virus with an approximately 15 kb genome of negative sense that codes for six proteins. Among these proteins, only hemagglutinin-neuraminidase (HN), fusion (F), and matrix (M) proteins interact with the viral envelope and contribute to the expression of the key antigenic and pathogenic properties of the virus [4]. Meanwhile, HN glycoprotein performs several functions during the infectious process of the virus, such as hemagglutination (HA), neuraminidase, as well as facilitating virus attachment, and is known as the main antigen of paramyxoviruses [4].

Although other host species are usually susceptible, the disease has a significant economic impact on poultry production [5]. There are about nine strains of NDV which are distinguished on the basis of pathogenicity test [6]. Based on the variation in strains of NDV, the rate of morbidity and mortality due to the disease in a flock varies from 90- 100% [7, 8]. The transmission of NDV occurs through newly introduced birds, selling of sick birds and exposure to fecal and other excretion from infected birds and contact with contaminated feed, water, equipment and clothing [9]. The disease is characterized by nervous, respiratory gastrointestinal and reproductive impairments [10, 11]. Vaccines are used for preventing the establishment of the disease. Currently many inactivated and live ND vaccines available around the world [12, 13].

The propagation of the virus in embryonated chicken eggs (ECEs) is the core element of developing licensed inactivated Newcastle vaccines [14]. Similar to other viruses, the life cycle of the NDV depends on the host cellular machinery [15]. In addition, it is of vital importance to investigate data on the growth of vaccine strains in host cells (HCs) [16].

Heat stable, non-pathogenic ND strains (I2 and V4) have been identified as an innovative alternative to traditional vaccines [17]. Heat stable vaccines, such as NDI2, are cheaper to produce, do not rely on a cold-chain and can easily be administered with feed grain or water without catching individual bird, and are thought to be suitable and fit for village chickens [9]. The reports from other countries indicated that ND-I2 vaccine retains potency in the absence of a cold chain, for eight weeks when stored in a cool, dark condition, or at 28 °C in a freeze-dried form [18]. Hence, the current study aims to observe the sero-conversion status of chickens with different breeds vaccinated with NDI2 vaccine and effect of various factors on immunological response of the vaccine.

Statistical Analysis

The data generated from the study was arranged, coded, and entered into an Excel spreadsheet. Stata MP statistical software version 13 was used for descriptive analysis. Pearson’s chi-squared test was also used to determine the association between the sero-conversion status and associated risk factors. P- Value less than 0.05 was considered statistically significant.

Discussion

Much of the samples 279/401 (69.6%) for these studies were collected from the scavenging chickens reared extensively constituting small number of birds per flock. Although the number of samples collected from each flock should be 18-22 to determine the level of sero-conversion due to vaccination, the flock size of chickens in the area were very few and subjected for the variation of sample sizes. Insufficient antibody titers can be caused by underlying immunosuppressive disorders such as Gumboro disease, Marek’s disease, or Chicken Anemia Virus [20]. The reason for insufficient sero-conversion in some flocks remains unclear and requires further investigation.

For the current study the individual bird level protection was 90.5% against the disease which was an acceptable level due to herd immunity status was expected to be above 80%. This result was in agreement with the findings of Kapczynski and King, [21] and Susta et al. [22] who reported ND vaccines offer substantial protection against clinical disease, although it fails to completely prevent infection. In this study, most of the chickens in the vaccinated flock generated the optimum amount of protective antibody titer (5945) compared to the minimum expected threshold value (993). Although, the highest antibody titer production does not mean the birds are contained from the disease where the study result is in agreement with the study done by Capua et al. [23]. Virulent ND virus was isolated from embryonated eggs from clinically normal breeder hens with high antibody titers to ND and clinically sick chickens produced from eggs. The sero- conversion status at flock level for this study was 86.2%, which was lower than the findings of Numan et al. [24] who obtained 98.07% of serum samples were positive for specific immunity against NDV in Pakistan. The difference might be attributed to vaccine strains used, route of administration, breed of chickens, type of production and management practices followed.

The uniformity of antibody titer was significantly varied from 21.2%-122.3% CV values where in most of the flocks (82.8%) of the CV% was beyond 50% which was considered as poor outcome.

This outcome might be emanated from various factors like age variability at the time of vaccination, difference among vaccinators, infection with other diseases, difference in management practice and breed variation.

Significant antibody titer variation was observed among Breeds (p=0.018) and Ages (0.001) of the birds in this study. Mixed type of breeds was more protective (97.8%) while local breed’s sero-conversion status was comparatively low (45.9%). Similarly, chickens with higher age categories were more protective than the rest which was in line with the reports of Okwor and Eze [25] that antibody titers against ND can vary in different breed’s and ages, because of differences of the speed of metabolism as well as the stress induced by the onset of lying. In addition, some chickens might be vulnerable to parasitism, inability in feeding competitively with others, vaccinator faults at the time of ocular droppings of the vaccine (dosage issues).

Based on number of samples collected, statistical significance (p=0.004) among the sample sizes of each flock was observed which might be due to the flock size for scavenging birds/ chickens reared extensively were by far fewer than commercially managed chickens, so that contributes for the collection of variable sample sizes. This also elevates the coverage of vaccination spots and vaccine delivery personnel numbers which might contribute to loosen antibody titer uniformity among the birds. Inadequate vaccination practices might result incidence of ND in vaccinated flocks as described by Dortmans et al. [26] in Algeria. De Wit and Cook [27] and Oberländer et al., [28], reported a vaccination’s success is determined by a variety of factors of which route of vaccination, vaccine storage, the hygiene of the administering vessel, and the number of vaccine doses per bird is all important aspects to be considered. All of these and other factors result in a reduction in vaccine dose per chicken or even the injection of entirely damaged virus, rendering immunization ineffective [29].

In general, the test result clearly showed that the type of vaccine used for the flocks was an excellent inducer of immunity and able to protect birds from the infection. However, factors that caused for the occurrence of a wide coefficient of variation should be well managed because the main cause of vaccination failure arises from the vaccine management itself.

Conclusion and Recommendations

Newcastle disease is a costly poultry disease that affects commercial farmers and poultry production sectors around the world. Although there is vaccination failure, the major strategy for controlling virulent NDV is to use vaccines which provide immunological protection against the disease. The vaccination campaign for the current study revealed ND I-2 vaccine type was effective in protecting the chickens from the disease in respective of the age of birds at vaccination although uniformity of antibody production is variably interrupted among the flocks. Therefore, the following recommendations were forwarded:

• The protective efficacy of the vaccine varies between flocks, so those flocks developing low antibody titers should need to be further assessed for poor sero- conversion and management conditions.
• The temperature, storage condition, way of administration, dose and transportation of the vaccine should be checked particularly in those flocks that induce low antibody titer.
• Finally, the vaccines should be administered by professionals, who have a good knowledge and skill about vaccination and poultry disease.

Acknowledgement

This research was supported by Animal Health Institute and Kyeema foundation and gratefully acknowledged.