Clinicopathological and Molecular Diagnosis of Newcastle Disease in Ethiopia

Technical Summary: Clinicopathological and Molecular Diagnosis of Newcastle Disease in Ethiopia

Problem Statement
Newcastle Disease (ND) remains a highly contagious viral disease causing significant economic losses to the poultry industry globally and specifically in Ethiopia. Despite Ethiopia possessing the largest livestock population in Africa, with a chicken population estimated at approximately 57 million, poultry production is severely constrained by high mortality rates (31.78%), largely attributed to infectious diseases. ND is identified as a primary cause of recurrent outbreaks, particularly in central and peri-urban regions. A critical gap exists in the integrated documentation of ND outbreaks in Ethiopia, specifically lacking comprehensive reports that combine clinical observation, postmortem pathology, and molecular confirmation. This study addresses the need for accurate diagnosis and characterization of ND outbreaks in the Modjo area to support effective control strategies.

Methodology
The study employed a cross-sectional design conducted from November 2024 to July 2025 in four purposively selected layer chicken farms around Modjo town, central Ethiopia. The study population consisted of 2,500 chickens across the four farms.

• Sampling: From a total of 112 birds exhibiting clinical signs suggestive of ND, 20 representative cases were selected using proportional systematic random sampling for detailed investigation.
• Clinical and Postmortem Examination: Affected birds were examined for clinical signs including neurological and systemic manifestations. Postmortem examinations were conducted following OIE guidelines to identify gross pathological lesions in the respiratory, gastrointestinal, lymphoid, and musculoskeletal systems.
• Sample Collection: Tracheal, cecal tonsil, and cloacal samples were aseptically collected from recently dead and clinically sick birds. Samples were transported in Viral Transport Medium (VTM) under cold chain conditions to the Animal Health Institute (AHI) in Sebeta and stored at -80°C.
• Molecular Diagnosis: RNA was extracted from the samples and subjected to one-step real-time reverse transcription polymerase chain reaction (rRT-PCR) targeting the fusion (F) gene of the Newcastle Disease Virus (NDV). Amplification was performed using specific primers and a fluorescent hydrolysis probe. Samples with Cycle Threshold (Ct/Cq) values ≤ 35 were considered positive.
• Differential Diagnosis: The study systematically excluded other major poultry diseases, including Infectious Bursal Disease, Marek's Disease, Infectious Laryngotracheitis, and Avian Influenza, based on the absence of their characteristic clinical and pathological features.

Key Results

• Clinical Findings: Affected chickens presented with recumbency, facial swelling, torticollis (neck twisting), leg paralysis, greenish diarrhea, and an arched body posture.
• Pathological Findings: Postmortem examinations revealed marked tracheal congestion and hemorrhages, inflamed air sacs with caseous exudates (airsacculitis), pinpoint hemorrhages at the tips of the proventricular glands, severe hemorrhagic intestinal lesions, and enlarged hemorrhagic cecal tonsils. These findings indicated a severe multisystemic viral infection.
• Molecular Confirmation: rRT-PCR analysis confirmed the presence of NDV in 4 out of the 20 tested samples. The positive samples exhibited clear exponential amplification curves with Ct values between 18 and 28.
• Exclusion of Differentials: The specific combination of proventricular gland hemorrhages, enlarged cecal tonsils, and positive molecular detection for NDV, alongside the absence of lesions typical of other diseases (e.g., bursal enlargement for IBD or visceral tumors for Marek's), confirmed ND as the etiological agent.

Significance and Claims
The study concludes that Newcastle Disease remains a major constraint to poultry production in the Modjo area and central Ethiopia. The research demonstrates that an integrated diagnostic approach—combining clinical evaluation, postmortem lesion analysis, and molecular confirmation—is essential for the accurate and timely diagnosis of ND outbreaks.

The authors claim that while advanced molecular facilities are not always available in remote settings, the combination of field-based epidemiological investigation and gross pathology remains critical for timely outbreak response. The study highlights that effective management requires strict biosecurity measures, proper vaccination schedules, and the isolation of infected flocks. Furthermore, the authors note that while vaccine-associated reactions can occur, the current outbreaks are likely driven by circulating virulent strains (potentially genotype VII) that may differ antigenically from commonly used genotype II vaccines, leading to incomplete protection. The paper advocates for continued molecular surveillance and comprehensive epidemiological studies to better understand circulating strains and improve disease control strategies in the region.

Limitations
The authors acknowledge that the study did not include serological assays (such as Hemagglutination or Hemagglutination Inhibition tests) or genomic sequencing due to resource constraints. Consequently, the study provides evidence based on clinical, pathological, and targeted molecular data but does not offer a full antigenic or genetic characterization of the circulating strains.