Exploring the sero-prevalence of a dangerous zoonotic disease in El-Gadarif State
Imagine a shepherd in rural Sudan, tending his flock as generations before him have done. Suddenly, his sheep begin to mysteriously miscarry their lambs, and his family's livelihood dwindles. To make matters worse, members of his own household start experiencing recurring fevers and joint pain that persist for months. This isn't a fictional scenario—it's the reality for many in El-Gadarif State, where a dangerous bacterial disease called brucellosis silently undermines both animal and human health.
Brucellosis infects an estimated 2.1 million people annually worldwide 2
Capable of spreading from animals to humans, threatening communities
Recent research sheds light on disease prevalence in El-Gadarif's sheep
Brucellosis is caused by bacteria of the genus Brucella, with B. melitensis being the primary species affecting sheep and goats. These Gram-negative coccobacilli are facultative intracellular pathogens, meaning they can survive and multiply inside the host's own cells, particularly white blood cells called macrophages 8. This clever survival strategy helps them evade the immune system and explains why infections can become chronic and difficult to treat.
The disease causes significant economic losses through reduced milk production, weight loss, infertility, and abortion in animals 7.
In humans, it typically causes undulant fever (characterized by rising and falling temperature), severe fatigue, joint pain, and swelling of lymph nodes, liver, or spleen 1.
El-Gadarif State in eastern Sudan presents an ideal location for studying brucellosis. Characterized by a Sudano-Sahelian climate with distinct wet and dry seasons, the region supports various animal production systems, including mixed crop-livestock farming and nomadic pastoralism 8. These practices often bring animals and humans into close contact, creating opportunities for disease transmission.
The state shares an international border with Ethiopia, raising concerns about possible cross-border disease transmission through livestock movement. With approximately one million goats alone in the state, plus significant sheep populations, the potential for widespread transmission exists if control measures aren't implemented 8.
Goats in the state
With Ethiopia
Farming systems
Diagnosing brucellosis requires multiple complementary approaches, as no single test is perfect in all situations. The World Organisation for Animal Health (OIE) recommends a testing strategy that combines rapid screening tests with confirmatory methods 9.
The RBPT serves as a quick and inexpensive screening tool perfect for field use. The test involves mixing a small serum sample with a special rose Bengal-stained Brucella antigen 8.
If antibodies against Brucella are present in the serum, visible clumping or agglutination occurs within four minutes, indicating a positive result 5.
The SAT is a traditional confirmatory test that detects the presence of agglutinating antibodies against Brucella. The test involves creating serial dilutions of the serum sample and mixing them with a standardized Brucella antigen 8.
After overnight incubation, the highest dilution that shows 50% agglutination is considered the antibody titer, with higher titers indicating a higher likelihood of active infection 9.
The cELISA represents a more advanced serological test that can differentiate between antibodies produced in response to natural infection versus vaccination. This test is particularly valuable in regions where vaccination is part of control programs 8.
The cELISA works by measuring how effectively antibodies in a sample compete with known monoclonal antibodies for binding to Brucella antigens 10.
PCR is a molecular technique that detects Brucella DNA directly, offering several advantages over serological methods. It can identify the specific species and even biovars of Brucella, provides results more quickly than culture (which can take weeks), and is particularly useful when samples contain dead bacteria or when antibiotic treatment has been initiated 7.
A comprehensive survey was conducted in El-Gadarif State from March 2016 to March 2017 to determine the sero-prevalence of brucellosis in sheep 3. The study employed a cross-sectional design, collecting 558 serum samples from sheep across five different localities within the state.
The sampling strategy was designed to be representative of the broader sheep population in the region. Blood samples were collected aseptically from the jugular vein using vacutainer tubes and allowed to clot naturally. The sera were separated through centrifugation and stored at -20°C until testing to preserve antibody integrity 8.
Study initiation and sample collection begins
Blood collection, serum separation, and storage at -20°C
RBPT screening followed by SAT confirmation
Study completion and data analysis
The testing protocol followed a sequential approach to maximize both efficiency and accuracy:
All samples using the Rose Bengal Plate Test (RBPT)
RBPT-positive samples using the Serum Agglutination Test (SAT)
Random testing of RBPT-negative samples to check for false negatives
The laboratory findings told a compelling story about brucellosis presence in the region's sheep. The initial RBPT screening identified a significant number of positive animals, but the more specific SAT test provided confirmation of active infection in a subset of these.
| Test Method | Percentage Positive | Primary Use |
|---|---|---|
| RBPT (Screening) | Not fully reported | Initial field screening |
| SAT (Confirmation) | 1.3% of total samples (estimated) | Confirmatory diagnosis |
| Risk Factor | Association with Infection | Possible Explanation |
|---|---|---|
| Location | Significant variation (χ²=31.62, p=0.001) | Differences in animal density, management practices, or environmental conditions |
| History of Abortion | Increased odds of RBPT positivity | Aborting animals likely shed more bacteria, increasing transmission risk |
| Age and Sex | Not specifically reported in this study | Typically, adults and postpartum females show higher susceptibility |
When compared with studies from other regions, the prevalence in El-Gadarif appears moderate but still concerning. For instance, a large study in the Arabian Gulf region that examined 8,500 sheep and goats found an overall brucellosis rate of just 0.48% 7. The higher prevalence in El-Gadarif suggests possible differences in animal management practices, environmental factors, or control program implementation.
Brucellosis research relies on specialized reagents and diagnostic tools that enable scientists to accurately detect and characterize the disease. These tools have been refined over decades to improve both accuracy and practicality for field use.
| Reagent/Tool | Function | Application in Brucellosis Research |
|---|---|---|
| Rose Bengal Antigen | Stained Brucella cells for agglutination tests | Initial screening of serum samples in RBPT |
| SAT Antigen | Killed Brucella abortus strain 119-3 | Detection of agglutinating antibodies in SAT |
| cELISA Kits | Coated plates with Brucella antigens | Specific antibody detection; differentiation from vaccine strains |
| PCR Master Mixes | Enzymes and reagents for DNA amplification | Molecular detection and characterization of Brucella species |
| Bacterial Culture Media | Nutrient substrates for bacterial growth | Gold standard confirmation and biovar identification |
Commercial test kits like the SVANOVIR® Brucella-Ab C-ELISA have become invaluable tools, offering high sensitivity (99.5-100%) and specificity (99.5-100%) for detecting antibodies against multiple Brucella species including B. abortus, B. melitensis, and B. suis 10.
Such standardized commercial tests are particularly important for international trade, where consistent and reliable results are essential.
The findings from El-Gadarif State carry significant implications for both public health and livestock economics in the region. The demonstrated presence of brucellosis in sheep represents not just an animal health concern but a very real human health threat, particularly for farmers, herders, slaughterhouse workers, and those who consume raw dairy products.
In high-prevalence localities identified through the research
For livestock owners about disease transmission risks
Measures to reduce transmission between flocks
Of positive animals in commercial operations
The silent spread of brucellosis through sheep flocks in El-Gadarif State represents more than just a veterinary concern—it's a complex challenge that intersects with public health, economic development, and food security. The research we've explored reveals both the magnitude of the problem and the sophisticated tools scientists now have to combat it.
As we've seen, understanding and controlling diseases like brucellosis requires multiple scientific disciplines, from field veterinarians collecting samples to laboratory specialists running advanced molecular tests. Each piece of data brings us closer to effective control strategies that can protect both animal and human populations.
The story of brucellosis research in El-Gadarif is ultimately a reminder that in our interconnected world, animal health and human health are inseparable. By investing in understanding and controlling diseases at their source in animal populations, we protect not just livestock but human communities and livelihoods, creating a healthier future for all.