Epidemiology of brucellosis in humans and domestic ruminants in Bangladesh

Brucellosis is an ancient and one of the world's most widespread zoonotic diseases which is also endemic in Bangladesh. However, the epidemiological understanding of this disease in humans and animals in Bangladesh is not complete. The overall objective of this thesis was to investigate the epidemiology of brucellosis in humans and domestic ruminants in Bangladesh in terms of the evaluation of commonly used diagnostic tests, estimation of true prevalence, identifying risk factors and detection of Brucella species in order to provide information for appropriate future control strategy. This document consists of 13 chapters. The first three chapters are on summary, preamble and general introduction. Chapter 4 describes the objectives of this thesis. Chapter 5 to chapter 9 include 5 manuscripts published, submitted or prepared for submission. Chapter 5 describes the efficacy of three conditionally dependent serological tests like Rose Bengal test (RBT), Slow Agglutination test (SAT) and indirect enzyme linked immunosorbent assay (iLEISA) for the diagnosis of bovine brucellosis in naturally infected cattle in Bangladesh; the true prevalence of bovine brucellosis and stage of infection were also described. The performance of these three tests was also compared in low and high prevalence scenario. In chapter 6, the Bayesian latent class model based evaluation of three conditionally dependent serological tests like RBT, SAT and iELISA for the simultaneous diagnosis and estimation of true prevalence of brucellosis in small ruminants are described. The dominant species of Brucella prevalent in Bangladesh is described in chapter 7. In chapter 8, the prevalence and risk factors of brucellosis in high risk group of people in Bangladesh is reported. The chapter 9 describes the prevalence of brucellosis in pyretic people in Bangladesh and detection Brucella species from seropositive PUO patients. Randomly collected blood samples from pyrexia of unknown origin patients were used for this study. The same tests were also used in pyretic people. The chapter 10 describes the general discussion. Chronologically, chapter 11, chapter 12 and chapter 13 consist of conclusions, recommendations and bibliography. The main results are summarized below: Only 0.29% (95% CI: 0.06-0.86) cattle were found to be acutely infected whereas 0.49% (95% CI: 0.16- 8 1.1) were chronically infected with brucellosis in Mymensingh. On the other hand, in Central cattle breeding and dairy farm (CCBDF) 15.58% (95% CI: 11.89-19.89) cattle were acutely infected with brucellosis and only 3.2% (95% CI: 1.63-5.72) were chronically infected. The true prevalence of brucellosis among cattle in Mymensingh and CCBDF were 0.3% (95%CI: 0.03-0.7) and 20.5% (95% CI: 16.4-26.3) respectively. The performance of iELISA was best in both Mymensingh and CCBDF with the sensitivity of 90.5% and 91.3% and specificity of 99.3% and 99.2% respectively. The performance of RBT was better in Mymensingh than CCBDF with 81.0% and 76.1% sensitivity and 99.0% and 95.6% specificity respectively. Similar to RBT, the performance of SAT was also better in Mymensingh than CCBDF with 63.5% and 79.7% sensitivity and 98.6% and 95.3% specificity respectively. Through this test validation study, a new cut-off of 5 IU/ml for iELISA was recommended both in low (as at Mymensingh) and high prevalence scenario in cattle population (as at CCBDF) for routine screening. It was recommended to do nothing for the control of bovine brucellosis under small-scale dairy and subsistence management system in Bangladesh. However, vaccination should be applied in herds where the prevalence is very high as like CCBDF. The true prevalence of brucellosis in goats and sheep were estimated as 1% (95% CI: 0.7-1.8) and 1.2% (95% CI: 0.6-2.2) respectively. The sensitivity of iELISA was 92.9% in goats and 92.0% in sheep with corresponding specificities of 96.5% and 99.5% respectively. The sensitivity and specificity estimates of RBT were 80.2% and 99.6% in goats and 82.8% and 98.3% in sheep. The sensitivity and specificity of SAT were 57.1% and 99.3% in goats and 72.0% and 98.6% in sheep. Only B. abortus DNA was amplified from 19 seropositive human (both HROG and PUO) and six animal samples (3 cows milk, one goat milk, one gayal milk and one bull semen). No Brucella like organisms was observed under microscope in stained smears. Similarly, no Brucella organism was isolated from any of the clinical samples. The prevalence of brucellosis in occupationally exposed people (HROG) using three tests was observed to be 4.4% based on a parallel interpretation. The results of the multiple random effects logistic regression analysis with random intercept for district revealed that the odds of brucellosis seropositivity among individuals who had been in contact with livestock for more than 26 years was about 14 times higher as compared to those who had less than 5 years of contact with livestock. In addition, when the contact was with goats, the odds of brucellosis seropositivity were about 60 times higher as compared to when contact was with cattle only. The seroprevalence of brucellosis among patients with pyrexia of unknown origin (PUO) was estimated to be 2.7% (95% CI: 1.2-5.2). The age, residence, type of patient, contact with animals, type of animal handled, arthralgia and backache were found to be significantly associated with a positive serological result in bivariable Firth's logistic regression. Brucella abortus was also detected from seropositive pyretic patients.