The present study on ND from non-vaccinated chicken using RT-PCR for F gene assay revealed an overall result of 7.8% [95%CI=(4.69–12.63)%] in the study area from Seka Chokorsa town and JUCAVM veterinary clinics. This disclosed that ND is one of the major diseases of chicken occurring in the study area. Other scholars like Chaka et al.,(2012) reported 4.9% in Eastern Shoa Zone, Terefe, et al., 2015 in Rift Valley areas,3.2% in Chintu et al.,2022 in Kerala, India,4.2% of the 2079 samples tested for the F gene were positive,4.4% of the F gene of NDV also recorded in India of Assam region from chicken (Sangeeta et al.,2021) and another research done in Barda region Azerbaijan indicates that from 341 studied chickens, all become negative and they reasoned out the absence of actively circulating Newcastle disease virus in the region (Zeynalova et al.,2015). This variation in different findings probably shows a phase of infection at the time of sampling, a difference in the study design, and agroecology.
Female chickens were twice more likely affected by NDV as compared to male chickens[AOR = 2.1,95% CI(1.03–4.3)] which indicates a statistically significant difference(p = 0.04). This might be due to the presence of stress because of egg production and the care of small chicks which slow down immunity and make the chicken more exposable to the disease(Wigley et al.,2005, Johnston et al.,2012).Chickens that had flock to flock contacts were highly affected by the Newcastle disease virus compared to fenced yard chickens that showed a statistically significant difference [AOR = 7,95% CI(1.9–62)]with (P = 0.034). This is probably due to these roaming birds were easily get the virus by ingestion of contaminated forage with feces of infected birds through the fecal oral route and inhalation (Martin, 1992).
In the current serological study, samples of chickens from the Seka Chokorsa town and JUCAVM veterinary clinics were found to have ND circulating antibodies. When samples were taken, it was confirmed by individual owners and clinicians that none of them had given their chickens the Newcastle Disease virus vaccine. Therefore the current result revealed that the 28.3%(95% CI=(22.26–35.32)%) presence of antibodies to ND in the chicken was considered to be evidence of exposure to natural infection. Different reports are available on this disease by Tadesse et al.,2005 (28.6%) from Debre Birhan, Jarso,2015 (28.6%) East Showa Zone (Lume and Modjo districts), Geresu et al.,2016 (27.86%) in Agarfa and Sinana districts of Bale, Mamo and Yimer,2021 (30%) at Buno Bedelle Zone, Tadesse et al.,2005 (32.22%) central Ethiopia, Vui et al.,2002 (28.4%) Viet Nam and Sule et al.,2012 (34.5%) Yobe, Nigeria, Gutierrez et al.,2000, Mexico (2.2%), Getachew et al., 2014 (5.6%), Kersna Kondaltity, Chaka, et al., 2012 (5.9%) Eastern Shewa zone, Sori et al., 2016 (11.34%), Sebata Hawas district, Terefe et al., 2015 (11.61%) Rift-Valley Areas (Debre Zeit, Ziwaye, and Tikur wuha), Haile, 2017 (12.7%) Rift valley areas( Bishoftu, Batu, and Hawassa), Zeleke et al., 2005 (19.78%), Rift valley and southern(Hawassa, Butajira, Alage, and Hossana) and Abraham et al.,2014 (17%) from Abuja, Nigeria of local chickens.
These discrepancies in results may be from the methodological difference, year of study, and sample size. A constant cycle of infection caused by the presence of lentogenic or possibly mesogenic NCDV in backyard poultry flocks in a region can periodically improve the immunity of all exposed chickens, resulting in a higher percentage of birds with antibodies (Chaka et al.,2012). The ecological aspects of a location, including climate, human settlement patterns, sanitary and socioeconomic practices, high interaction between free ranging chickens, and management approaches can play a great role in having different findings among researchers (Awan et al.,2007).
Female chickens were two times more likely affected by NDV as compared to male chickens[AOR = 2.3,95% CI(1.1–4.8)]. It was observed that this difference is statistically significant (𝑃 < 0.05). This result is in line with the findings of Geresu et al.,2016 in (females 30.53% and males 14.29%), Appiah et al.,2020 reported that (68.8% of males and 69.1% of females), and Betela et al.,2022 in female (67% and in males 33%)were significant. Those scholars reported the presence heavy burden in females than males. The presence of more previous infections in females might be the result of hens being kept for long term production purposes more often than males. Males have short life spans which are typically used for nonproductive uses like food, earning a living, and performing social, cultural, and religious rites(Mamo and Yimer,2021).
The serological test indicated that the difference in the NDV infection in adults was 33.7%([AOR = 2.4,95%CI=(1.1-5.0),] compared to young (23.1%) was statistically significant (p < 0.05), which agrees with the finding of vui et al.,2002 which stated that the young had a significantly lower NDV antibody than the adult. It is also consistent with the finding of Getachew et al., 2014, Derbew et al.,2016 and Betela et al.,2022. The occurrence of ND was strongly associated with the age of chickens. The disease was higher in adult chickens than the young ones. This variation can be suspected due to the frequent exposure of adult chickens at an earlier age to the field virus (Biswas et al., 2005).
The present antibody detection showed that chickens that had contact with other flocks were 2.3 times more affected compared with noncontact chickens by the virus [AOR = 2.3,95% CI(1.2–5.8),p < 0.05]. Chaka et al.,2012 indicated that the chance of mingling flocks from various households increased the transmission of illness with increasing proximity. This could be due to the chickens that were mixed with the flock from different areas increasing the chances of disease transmission among birds as they might be the main source of infection even clinically healthy chickens might be the cause of the illness in the flock of birds (Jibril et al.,2014).
Competative ELISA also confirmed that chickens that contact with wild birds were 8.7 times more likely infected relative to the noncontact [AOR = 8.7,95% CI(1.1–81),p < 0.05]. Finding Wodajo et al .,2023 also supports this point explaining that backyard chickens that mixed with wild birds have increased odds of ND seropositivity by 63.7 times over chickens that weren’t allowed to mingle with the wild birds scavenging in the environment. In this way, the possibility of traditionally managed free-range backyard chickens contracting NDV from wild birds and vice versa may significantly contribute to the bilateral transmission of the virus between wild birds and domestic chickens. A virulent NDV was identified in both wild and domestic birds, according to Schelling et al.,1999 and Koch and Elbers (2006).
The current finding shows that the disease was higher in the chicken that comes from the area of dead chickens improper disposal system was practiced than proper disposal system was common [AOR = 9,95% CI(1.1–73),p < 0.05]. This result coincides with the finding of Wodajo et al.,2023 who reported that the odds of ND positivity in the household flocks practicing dead chicken disposal by throwing carcasses of dead chickens far away may chicken houses decrease by 93% less likely in comparison with the households throwing nearby. Similarly, Belgrad et al.,2018 found that dead chicken disposal practices with ND positivity were significantly associated. There is also another report that shows a significant association between the mode of disposal of dead birds with ND outbreaks in village chickens in Kenya (Njagi et al.,2010; cited Chaka et al.,2013)This might be improper disposal system the carcass is easily accessible to healthy scavenging chickens, dogs, and cats that can easily contaminate the environment and exaggerate the transmission of the virus (Awan et al.,2007).
This study also found a higher risk of ND in the sick chicken that was not isolated from the healthy birds [AOR = 6.5,95% CI(1.4–32),p = 0.02]. This is in agreement with the study of Roky et al.,2020 confirmed that the disease was 2.8 times higher in households isolation was not practiced comparing isolated chicken. Sick infectious chickens shed viruses and might act as the source of infection for the rest of the birds (Zhou et al., 2020). Hence, keeping normal and abnormal chicken in the same place amplify the risk of disease in the backyard.
When compared to daily cleaning frequency, the chances of ND-positive were substantially greater in chicken houses that were not cleaned every day (AOR = 1.2; 95% CI: 1.3–3.4; P = 0.043). This study found that daily poultry house cleaning lowered the likelihood of transmission and decrease ND positivity. This study supports the findings of Chaka et al.,2013 and Wodajo et al.,2023 who identified chicken house cleaning frequency as a risk factor. This is probably because daily removal of feces can minimize viral load in the house since NDV has been shown to survive in chicken excrement for several days to weeks, depending on temperature (Kinde et al., 2004). This discussion point contradicts the work of Belgrad et al.,2018 daily cleaning may increase the spread of NDV due to disturbing the litter and source of excessive stress on chickens which predisposes them to infection. Another study conducted by Anderson et al.,1966 demonstrated that higher dust, ammonia, and carbon dioxide levels caused by high cleaning frequency increase the risk of virus transmission. However, the studies did not consider NDV harbored within the feces.
Using binary logistic regression analysis, no association between risk factors (clinical sign, site, and breed) and the detection of NDV in the chicken was found at the set statistical significance level. In this study, some chicken's results indicated positive on the PCR test but negative when detected by ELISA. This is probably due to complications of Newcastle disease with infectious bronchitis virus (IBV). When chickens are infected by IBV and NDV at the same time the level of antibody response to NDV can decrease because the infectious bronchitis virus can reduce the Harderian Gland's capacity to produce antibodies that can protect the upper respiratory tract and eye region by affecting plasma cells (Dohms et al.,1988). The presence of the two viruses in the same tropism can be also another reason for the interference of each other that replicate in the cytoplasm of epithelial cells respiratory tract(Gelb et al.,2004).