Abstract

Typhoid fever is a systemic prolonged febrile illness caused by certain Salmonella serotypes. Salmonella infections are the major cause of morbidity and mortality in developing countries thus constitute major public health problems in these countries. A number of tests are presently available for diagnosis, from molecular to immunological and biochemical to microbiological. Misdiagnosis is usually experienced since most hospitals use only the Widal test without confirmation of results with a second test method. This study aimed at comparing the diagnostic performance of the Widal test and stool culture in the laboratory diagnosis of Salmonella infections using blood culture as gold standard. Presenting patients aged between 7 months to 87 years with symptoms clinically suspected to be typhoid fevers visiting the District Hospital Limbe (Bota) for a period of 5 months were recruited for the study. Verbal consents were obtained from volunteers and guardians. Serum samples from a total of 108 patients were subjected to the Widal test. Blood and stool samples from the same patients were analyzed for typhoid fever infection using blood and stool cultures respectively. Serotyping was performed using agglutination with Salmonella O, H and Vi antisera. In the Widal agglutination test, titre values from 1:160 and above were regarded as significant and therefore positive for the Salmonella antigen. Isolation of Salmonella from stool and blood culture indicated an infection. Raw data were entered into Microsoft excel and analyzed using statistical package for social sciences (SPSS). A comparison of categorical data was made using Chi square or fisher’s exact test, as appropriate. The confidence intervals for sensitivity and specificity were computed using the Wilson’s score method. A P value ≤ 0.05 was considered statistically significant. The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for Widal test and stool culture were calculated. The total number of patients positive for typhoid fever based on the Widal test was 67 (62.0%), 61 (56.5%) and 21 (19.4%) for stool and blood culture respectively. The Widal test recorded 61.9% sensitivity, 37.9% specificity, 19.4% PPV and 80.4% NPV. Stool culture showed 80% sensitivity, 49.4% specificity, 27.9% PPV and 91.4% NPV. These results demonstrate that the Widal test is not very reliable for diagnosis of typhoid fever since false positive and false negative results are common. The low PPV means that the Widal test could only be useful for excluding the disease from the population hence health care personnel should not totally depend on this test alone for diagnosis of enteric fever but should use other diagnostic methods to differentiate Salmonella infection from other infections. There is therefore, an urgent need to develop a rapid, highly sensitive and cheap diagnostic tool for the diagnosis of typhoid fever.

CHAPTER ONE

INTRODUCTION AND LITERATURE REVIEW

1.1 Introduction

Typhoid fever is a systemic prolonged febrile illness (CDC, 2012) caused by certain Salmonella serotypes including Salmonella Typhi, S. Paratyphi A, S. Paratyphi B and S. Paratyphi C.

Human beings are the only reservoir host for typhoid fever, and the disease is transmitted by faecally contaminated water and food in endemic areas, especially by carriers handling food. Typhoid fever affects people of all walks of life in urban, semi-urban and rural areas. Infection occurs in all age groups with higher incidence and more variable clinical presentation in children (Gizachew, 2011). Salmonella infections are the major cause of morbidity and mortality in developing countries thus constitute major public health problems in these countries (Qamar et al., 2014).

In the U.S.A, about 400 cases occur each year and 70% of these are acquired while traveling internationally as such, visiting or living in areas where the disease occurs is a risk (Miriam, 2005). An estimated 17 million cases of typhoid fever and 600,000 deaths occur worldwide, annually (WHO, 2005). In Cameroon, the prevalence of typhoid fever is 7.9% (Ndip et al., 2015).

The illness begins with mounting fever, headache, abdominal pain and constipation. The patient reaches a state of prolonged apathy, toxemia, delirium, disorientation or coma, followed by diarrhea, which if left untreated, can lead to complications affecting various organs of the body (Fauci et al., 2008).

Options for the diagnosis of typhoid fever are clinical signs and symptoms, serological markers, bacterial culture, antigen detection and DNA amplification (Nsutebu et al., 2003). Blood, bone marrow and stool culture are the most reliable diagnostic methods, but they are expensive techniques and some bacterial culture facilities are often unavailable (Wain and Hosoglu, 2008). In many African countries including Cameroon, the Widal test is the routinely used test in typhoid fever diagnosis because it is relatively cheaper, easy to perform and requires minimal training and equipment (Ley et al., 2010). Although the Widal test has been in use for more than a century, the value of the test to diagnose typhoid fever has been debated for the past years (Oloponenia et al., 2000). It relies classically on the demonstration of a rising titer of antibodies in paired samples 10-14 days apart. Oloponenia et al. (2000) proved that in typhoid fever, such a rise is not always demonstrable, even in blood culture-confirmed cases. In addition, interpreting the test has been such a problem that different cut-offs have been reported from different regions (Ley et al., 2010). Furthermore, patient management cannot wait for results obtained with a convalescent-phase sample. For practical purpose, a treatment decision must be made on the basis of the results obtained with a single acute-phase sample (Khoharo et al., 2010).

Intravenous fluids and electrolytes may be given in the management of typhoid, and appropriate antibiotics must be given to inhibit growth or kill the bacteria. The prognosis of typhoid usually resolves in 2 to 4 weeks with treatment. The outcome is good with early treatment, but becomes poor if complications develop. Relapse may occur if treatment did not fully eradicate the infection (Tuise, et al., 2005).

Since the late 1940s, typhoid fever was successfully treated with one of several antibiotics (Chloramphenicol, Ampicillin and Cotrimoxazole). However, since 1990, multi-drug resistant strains (MDRS) to previously useful antibiotics have emerged and the treatment for such strains requires more potent Quinolone antibiotics, such as oral Ciprofloxacin, or third generation Cephalosprins such as Ceftriaxone (WHO, 2006). Although the global burden of typhoid fever has reduced, emergence of multi drug resistant S. Typhi (MDRST) is still a major threat to public health.

Like many other developing countries, accurate diagnosis of infectious diseases remains a challenge due to lack of skilled man-power and equipment in making clinical diagnosis a common practice. Hence, the specific aim of this study is to compare stool culture and the Widal test as diagnostic methods for Salmonella infection in febrile patients visiting the District Hospital Limbe (Bota-Limbe) at the time of the study.

1.2 Literature Review (See full project)

1.3 Statement Of The Problem

Laboratory diagnosis of Salmonella infection requires the isolation and identification of Salmonella Typhi and Salmonella Paratyphi. In many areas where the disease is endemic, laboratory capacity is limited (Shrivastava et al., 2011).

There have been cases of misdiagnosis because most of our health facilities adopt the Widal test as the routine diagnostic method for the diagnosis of Salmonella infection, as it is quicker, simpler and less costly. The role of the Widal test has been to increase the index of suspicion for the presence of typhoid fever by demonstrating a positive agglutination; unfortunately, in most developing countries, including Cameroon, the situation is quite different as the test continues to suffer from serious cross-reactivity with other infectious agents that produce false-positive results.

Clinical diagnosis is not reliable because it is based on symptoms, and some diseases could present the same in terms of signs and symptoms.

Self-diagnosis could lead to misdiagnosis and improper treatment. This could bring about drug resistance in the future.

In endemic regions, where there are difficulties in establishing steady states or baseline titre of Widal agglutination, questions the usefulness of the test. Therefore, diagnosis on clinical ground alone is difficult as many other infections have the same clinical presentations (Nsutebu et al., 2002).

Thus, there is need to evaluate the performance of the Widal test and stool culture diagnostic methods to guide on the need to review the testing algorithms which will benefit patients and the society at large.

1.4 Rationale Of The Study

Proper diagnosis of Salmonella will go a long way to reduce the incidence and prevalence of Salmonella infections. There is need to determine the best and less costly method of diagnosis of the infection since outbreaks of the infections could be due to misdiagnosis. Use of insensitive diagnostic methods may lead to wrong medication, prolonged pathogenesis and bacterial resistance to antibiotics, which can be fatal; hence the need for re-evaluation of diagnostic methods in order to advise medical practitioners on the most sensitive method of diagnosing Salmonella infections. The Widal test is relatively cheap and do not require complicated technical expertise hence is applicable in all settings unlike the stool and blood culture. It is hoped that results from this study would be useful in ascertaining the diagnostic potential of the widal test or stool culture in the diagnosis of Salmonella infections and recommendations made to the relevant departments in the health sector and other agencies appropriately.

1.5 Hypothesis

The sensitivity, specificity, positive predictive values and negative predictive values of the Widal test and stool culture in detecting Salmonella infections are not the same.

1.6 Objectives Of The Study

1.6.1 Main Objective

To compare the diagnostic performance of the Widal test and stool culture in the laboratory diagnosis of Salmonella infections in febrile patients in the District Hospital Limbe.

1.6.2 Specific Objectives

1. To isolate and identify Salmonella species from stool samples of febrile patients.
2. To screen the presence of antibodies against Salmonella species in patient’s serum using the Widal test.
3. To determine the sensitivity and specificity of the Widal test and stool culture in detecting Salmonella infections in febrile patients.
4. To determine the positive and negative predictive values of the Widal test and stool culture in detecting Salmonella infections in febrile patients.