The world chickens population is estimated at 18 billion (Conan et al., 2012), and about 80% of chickens that are raised in Africa are indigenous local chickens (Conan et al., 2012). The majority (60%) of African households raise chickens   under   backyard  production   settings   where chickens are left to scavenge for their nutritional need (Mapiye et al., 2008; Mtileni et al., 2009). In Tanzania, 96% of livestock farmers keep local chickens, which supply 94% of poultry meat and eggs in rural areas (Ministry of Livestock and Fisheries Development- MLFD, 2015).

Local chickens are well adapted to harsh tropical environmental conditions and survive persistent exposure to endemic infectious diseases (Msoffe et al., 2001; Minga et al., 2004: Muchadeyi et al., 2017). The growth of chicken is rapid and its farming requires relatively small land size. More importantly, chicken has a high nutritive value from chicken eggs and meat, and serves as a chief source of high-quality meat protein among households in resource-poor countries in SSA (Ahlers et al., 2009; Mtileni et al., 2009; Martin et al., 2015). In Africa setting, chickens particularly local chickens play a crucial role in socio-cultural functions such as traditional ceremonies and rituals (Mtileni et al., 2009; Conan et al., 2012).

However, local chickens are characterized by low productivity traits like low growth performance and eggs production (Yakubu and Ari, 2018). Efforts are made to introduce improved chicken genetic resources adapted to harsh tropical environmental conditions, for example, with an introduction of kuroiler chicken, a dual-purpose scavenger hybrid chicken raised for meat and egg production, which was imported to Africa from India (Sharma et al., 2015; Fleming et al., 2016; Yakubu and Ari, 2018). Introduction of improved breeds is in an agreement with Tanzania Livestock Modernization Initiative (TLMI) (MLFD, 2015), which among other key priority actions in poultry modernization is the identification of dual-purpose breeds suitable for local free-range scavenging conditions (MLFD, 2015).

Newcastle disease (ND) caused by the Newcastle disease virus (NDV) is ranked as the number one killer of chickens under backyard production systems (Kitalyi, 1998; Gondwe and Wollny, 2007; Alders et al., 2014; Yune and Abdela, 2017; Absalón et al., 2019). Current disease prevention and control option largely depends on an appropriate vaccination of susceptible flocks (Alexander, 2001; Sharif et al., 2014). The ability of chickens to elicit protective antibody against the virus is crucial to prevent disease occurrence (Pevzner et al., 1981; Yune and Abdela, 2017). Chickens variation in antibody responses against NDV has been documented (Pevzner et al., 1981; Luo et al., 2013). Selection of chicken genotypes with high protective antibody titres may have significant contribution in preventing ND outbreak in chicken populations (Kapczynski et al., 2013; Luo et al., 2013). The present study was aimed to assess antibody titres and growth performance following ND immunization in local Tanzanian chicken, kuroilers, and broilers.

Chicken population and husbandry

Three chicken breeds (local Tanzanian chicken, kuroilers, and broilers), which were raised under the same environment and management condition were involved  in  the  present  study.  Local Tanzanian chicken and kuroiler eggs were obtained from Urio Cross and Pure Breeding LTD, a local farm in Tanzania (Tengeru, Arusha, Tanzania). Eggs were incubated to hatch at the Nelson Mandela African Institution of Science and Technology (NM-AIST) Laboratory egg incubators. The incubation conditions were 37.9°C temperature and 55% humidity. The eggs were candled at 10 and 18 days of incubation to detect infertile eggs and dead embryos. On the other hand, because of difficulty of obtaining broiler chicken eggs, instead, day-old broiler chicks were obtained from commercial poultry company in Arusha, Tanzania (Tanzania Poultry Farm, Usa River, Arusha Tanzania). Therefore, the experiment started with chicks of the same age, and was housed at Livestock Training Agency-Tengeru campus (LITA-Tengeru). The broiler starter feed (0 – 28 days) and finisher (29 – 50 days) were provided ad libitum. Light was provided throughout the experiment and was also used for room temperature adjustment during brooding. The feeds manufactured by commercial company (Harsho Milling Co.Ltd, Moshi Kilimanjaro, Tanzania) were used throughout the experiment.

Experimental design

One-day-old chicks were kept in brooding chicken facility at LITA-Tengeru, and kept for four weeks to allow maternal antibodies to wane (Jalil et al., 2010). Birds at three weeks of age before vaccination were wing tagged and randomly distributed between the control (A) and the vaccination (B) group. Birds in control and challenge groups were kept in separate houses. Group B birds were randomly allocated in three replicates for each chicken type, and birds in each replicate were randomly kept in separate pen of 1.5 m². A total of 358 chickens (Table 1) were raised for seven weeks.

Antibody titers measurement

Antibody titers were quantified using NDV ELISA antibody test kit (BioCheck (UK) Ltd., Hounslow, London). Samples were tested in duplicate in accordance with the manufacturer’s instructions. Sample optical density determined spectrophotometrically using 96-wells microliter plate reader (SYNERGY|^HTX multi-mode reader, BioTek Instruments Inc, Winooski, VT, USA) at 405 nm wavelengths. The calculation and interpretation of ELISA results was in accordance with the manufacturer guidelines. Briefly, the S/P ratio was calculated by using the formula: (mean of test sample−mean of negative control)/(mean of positive control−mean of negative control), and then at 1:500 dilution, the Log₁₀ titer =1.0×Log (S/P) +3.52. Serum sample with S/P value ≥ 0.35 or titer value ≥ 1159 was considered positive for antibodies against NDV.

Data analysis

Descriptive and inferential statistical analyses were performed using R software (Team, 2017). One-way analysis of variance (ANOVA) and Turkey’s HSD were conducted to assess difference in BWG and average antibody titers between chicken types. Also, the Student t-test was used to test difference between BWG between control and challenge birds, and to test difference between average antibody titers between two time points (days 10 and 21 post challenge). The relationship between BWG and antibody titers was evaluated by performing correlation test. Significance differences were taken at p <0.05 levels.

Ethical statement

The experiment was conducted in compliance with the Guidelines on the Humane Treatment of Laboratory Animals as stipulated in the Tanzania Animal Welfare Act, 2008.

Local chickens are well adapted to harsh tropical environmental conditions and survive persistent exposure to endemic infectious diseases (Msoffe et al., 2001; Minga et al., 2004; Mpenda et al., 2019).  However,  local chickens are characterized by low productivity traits like low growth performance and eggs production (Yakubu and Ari, 2018). Efforts are made to introduce improved chicken genetic resources adapted to harsh tropical environmental conditions, for example, with an introduction of kuroiler chicken (Yakubu and Ari, 2018). Introduction of improved breeds is in agreement with TLMI (MLFD, 2015), which among other key priority actions in poultry modernization is the identification of dual-purpose breeds suitable for local free-range scavenging conditions. The present study was aimed to assess antibody responses following ND vaccination and growth performance of kuroilers, broilers, and local Tanzanian chickens.

Findings demonstrated higher (P < 0.05) antibody titres in kuroilers (3.81 ± 0.06) as compared to local chicken (3.73 ± 0.07) and broilers (3.53±0.06) at day 10 post-vaccination. The antibody titres remained higher in kuroilers and local Tanzanian chickens as compared to antibody titres in broilers at day 21 post-vaccination (Table 3). As expected, the highest growth performance observed in broiler chicken as compared to kuroilers and local Tanzania chickens (Table 2). However, the growth performance was not significant different (P > 0.05) between kuroilers and local Tanzanian chickens.

Body weight and body weight gain

The weekly means ± standard errors of body weight and BWG post-vaccination are presented in Table 2. The broilers had the highest growth performance throughout the experiment followed by kuroilers. The kuroiler chickens had higher (P>0.05) body weight than local chickens except at the first three weeks (W1, W2, and W3), whereas, the body weights of kuroilers and local Tanzanian chickens were not different (P>0.05) from week four (W4) to the rest of the experimental duration.  Furthermore, the BWG was higher (P>0.05) in the control group than in the challenge group (Figure 1).

The broilers average body weights of 1407.39 ± 25.12 g and 2528.06 ± 48.95 g at 4 and 7 weeks of age, respectively reported in the present study are higher as compared to the previous reports (Munisi et al., 2015; Adeleke et al., 2011). In the previous report in Tanzania, the mean broilers body weights were 396 and 1255 g at 4 and 8 weeks of age, respectively (Munisi et al., 2015). In another study that was conducted in Nigeria, the broiler mean body weight was 2360 g at 20 weeks of age (Adeleke et al., 2011). The higher average broilers body weight observed in the present study may be explained by breed difference of the broilers involved in the studies. Also, the differences may be  explained  by  feed  content variations between the studies. For example, 4 weeks old broilers that were fed with feed supplemented with baobab seed oil cake had body weight of 1266 g, which is comparable to the broilers mean body weight of 1407.39 ± 25.12 g reported in the present study (Chisoro et al., 2018).

Kuroiler  is  dual-purpose  chicken  raised  for  egg  and meat production (Dessie and Getachew, 2016). Like local chickens, kuroilers can thrive under harsh tropical environmental conditions, and they can scavenge for nutrition needs just like local chickens (Ahuja et al., 2008; Dessie and Getachew, 2016).  The breed outperforms local chickens in terms of meat and egg production (Ahuja et al., 2008; Yakubu and Ari, 2018). However, in the present study, kuroilers (886.80 ±25.63) and local Tanzania chicken (880.07 ±29.67) body weights were comparable (P > 0.05). The discrepancy may be explained by differences in chicken breeds involved in the studies. Local Tanzania chickens are not well characterised and due to lack of genetic resources conservation programs, possibly local chicken involved in the present study have acquired high growth performance genetic materials from improved breed through interbreeding. Also, feed regime used in the present study probably had significant contribution on growth performance observed in kuroilers and local Tanzanian chickens. This is corroborated by higher body weights observed in the present study as compared to the previous reports. For example, Egyptian chicken (Mandarah) mean body weights at 4 and 7 were 299 and 747 g, respectively (Taha et al., 2012). In another study conducted in Nigeria to evaluate growth characteristic of kuroiler and Nigerian local chicken (Fulani), at 6 weeks of age the mean body weight of kuroiler and Fulani was 450 and 228 g, respectively (Yakubu and Ari, 2018). In another  study   conducted   in   Tanzania  involving  local chickens, the mean body weights at 4 and 7 weeks of age were 151 and 419 g, respectively (Munisi et al., 2015).

Antibody titres following ND vaccination

The means ± standard errors of antibody response against NDV at two time points (days 10 and 21 post-vaccination) are presented in Table 3. The antibody titers were highest in kuroiler chickens compared to antibody titers in local and broiler chickens. The antibody titers of kuroiler and local chickens at 10 days post-vaccination were different (P < 0.05) from that of broilers. The kuroiler chickens had higher (P > 0.05) antibody titres than local chickens at two time points. Overall, the antibody titers were different (P < 0.0001) between days 10 and 21 post challenge. Mean (3.91 ± 0.04) antibody titer at 21-days post challenge was higher than the mean (3.69 ± 0.04) antibody titer at 10-days post-vaccination (Figure 2). Furthermore, there was positive correlation of antibody titers between days 10 and 21 post challenge. The correlation of antibody titer between day 10 and 21 was intermediate in broiler (r = 0.52; P = 4.1e-5) and local (r =0.4; p=0.006) chickens as compared to weak correlation in kuroiler (r = 0.1; p = 0.4) chickens.

Although not supported by empirical studies, kuroilers are said to be resistant to infectious diseases (Sharma et al., 2015; Fleming et al., 2016). The higher antibody titres observed in kuroilers likely corroborated the previous reports, which suggest increased disease resistance in kuroilers.

The antibody titers were different (P < 0.0001) between control and challenged group in all chicken types at the two time points (Figure 2). The antibody titers were not different (P = 0.37) in the three chicken types at days 10 and 21 in the control group (Figure 2).

Generally, the observed antibody titers in the present study are relatively higher compared to the previous findings in other parts of the world. For example, Luo et al. (2013), reported mean antibody titer of 3.3 at 41 days after second immunization among Chinese-yellow broiler chickens. Another report found the mean antibody titer of 3.2 at 15 days post challenge among commercial meat chicken type (Tabidi et al., 2004). Generally, the comparison of the antibody responses in the present study with the previous reports is difficult because of the difference in environmental conditions, NDV strains used in vaccination, and detection tests involved.

Kuroiler chickens demonstrated higher antibody titers following ND vaccination as compared to local Tanzania chickens and broilers raised under the same environmental and management condition. However, it remains to be established whether the high antibody titres observed in kuroiler chickens can be translated to an increased protective effect against virulent NDV infection in the field. Also, the growth performance of kuroilers and local Tanzanian chicken were not different, which suggests that breeds and feeding regiment employed in the present study had an influence on the observation. Overall, the finding contributes to on-going work on understanding chicken immune responses against NDV and informs breeding programs designed for developing chickens that have increased resistance to NDV. Also, findings suggest genetic characterisation of local chicken genetic resources and to establish the levels of gene flows between local chicken and improved commercial breeds existing in Tanzania.

We thank the Bill & Melinda Gates Foundation for funding this research (grant #OPP1083453) through the Program for Enhancing Health and Productivity of Livestock (PEHPL) and the Nelson Mandela African Institution of Science and Technology in Arusha (NM-AIST), Tanzania for hosting the program.

The authors have not declared any conflict of interests.