Vi serology in screening of typhoid carriers: improved specificity by detection of Vi antibodies by counterimmunoelectrophoresis

New advances in the diagnosis of typhoid and detection of typhoid carriers

For effective management of typhoid, diagnosis of the disease must be done with speed and accuracy. Development of such a test would require antigens that are specific for typhoid diagnosis. Attempts at finding the specific antigen have been carried out throughout the years. The finding of such an antigen can lead to carrier detection as well. Candidate antigens have been used in the development of antigen or antibody detection tests with variation in sensitivity and specificity. Further characterization and understanding of the candidate antigens combined with use of innovative technologies will allow for the ideal test for typhoid and typhoid carriers to be within reach.

Epidemiology of typhoid fever in Singapore

A total of 1452 cases of typhoid fever was notified in Singapore from 1980-9. The morbidity rates of indigenous cases showed a steady decline from 5.9 per 100,000 population in 1980 to 1.2 per 100,000 population in 1989. The mean case fatality was 0.8%. Children, adolescents and young adults were most susceptible to typhoid fever. There was no significant difference in morbidity rates between the major ethnic groups. The vast majority of indigenous cases were sporadic while outbreaks accounted for almost one third of them. Food was the main vehicle of transmission. The commonest indigenous phage types were B1, D1 and A. Antimicrobial resistance was infrequently seen. The proportion of imported cases rose from 32% in 1980 to 72% in 1989. Almost half (48.5%) of all imported cases were local residents who contracted typhoid fever while travelling in endemic countries. As imported cases assume greater importance in the epidemiology of typhoid fever in Singapore, further drop in typhoid fever incidence would require reduction of travel-related cases through greater awareness of food hygiene and effective vaccination.

Epidemiology and control of an outbreak of typhoid in a psychiatric institution

An outbreak of typhoid caused by Salmonella typhi of the same Vi-phage type (D1) and of the same antibiogram was reported in a large psychiatric institution in Singapore. A total of 95 (4.8%) of the 1965 inmates were infected, 47 with symptoms and 48 asymptomatic. Transmission was through close person-to-person contact and not through contaminated food or water. The source of infection could not be established. The outbreak was brought under control by maintaining a high standard of environmental sanitation, active search for fever and diarrhoeal cases, identification of asymptomatic cases by rectal swabbing, and isolation of those found to be infected. Mass immunization with two doses of heat-phenol inactivated typhoid vaccine was also carried out concurrently. The vaccine was found to have an efficacy of 65.8% in preventing clinical illness.

Restaurant-associated outbreak of typhoid fever in Maryland: identification of carrier facilitated by measurement of serum Vi antibodies

Ten cases of typhoid fever occurred between 24 August and 1 September 1986 in the vicinity of Silver Spring, Md. Shrimp salad served in a fast-food restaurant was implicated as the source of infection. Stool cultures were obtained from 104 employees, and serum Vi antibodies were assayed in 97 of the employees. Salmonella typhi was isolated from stool cultures of an 18-year-old asymptomatic female employee, who was a food handler. A high level of Vi antibodies (79.0 micrograms/ml), measured by radioimmunoassay, was found in her serum. She had emigrated from an endemic area at the age of 14 years and had visited that endemic area 2 years previously. The causal relation between the carrier and the 10 cases of typhoid fever was confirmed by a common bacteriophage type, denoted "degraded Vi resembling O," in the S. typhi isolates. This phage type is rare in the western hemisphere but common in the endemic area from which the carrier had emigrated. The high level of Vi antibody in the asymptomatic carrier, in contrast to the lower levels in the convalescent- and postimmunization-phase sera, facilitated the identification of the source infection in this outbreak. This radioimmunoassay offers a rapid and standardized method for identifying carriers of S. typhi.

Prevention of typhoid fever in Nepal with the Vi capsular polysaccharide of Salmonella typhi. A preliminary report

We conducted a pilot study followed by a large clinical trial in Nepal of the use of the capsular polysaccharide of Salmonella typhi (Vi) as a vaccine to prevent typhoid fever. In the pilot study, involving 274 Nepalese, there were no significant side effects of the Vi vaccine; about 75 percent responded with a rise in serum antibodies of fourfold or more. In the clinical trial, residents of five villages were given intramuscular injections of either Vi or, as a control, pneumococcus vaccine dispensed in coded, randomly arranged, single-dose syringes. There were 6907 participants, of whom 6438 were members of the target population (5 to 44 years of age); each was visited every two days. Those with temperatures of 37.8 degrees C or higher for three consecutive days were examined and asked to give blood for culture. Typhoid was diagnosed as either blood culture-positive or clinically suspected on the basis of bradycardia, splenomegaly, and fever, with a negative blood culture. Seventeen months after vaccination, the codes were broken for the 71 patients meeting the criteria for either culture-positive or clinically suspected typhoid. The attack rate of typhoid was 16.2 per 1000 among the controls and 4.1 per 1000 among those immunized with Vi (P less than 0.00001). The efficacy of Vi was 72 percent in the culture-positive cases, 80 percent in the clinically suspected cases, and 75 percent in the two groups combined. These data provide evidence that Vi antibodies confer protection against typhoid. Surveillance continues to determine the duration of Vi-induced immunity.

Production of Vi monoclonal antibodies and their application as diagnostic reagents

Serum antibodies to Vi antigen were detected in mice immunized with the purified antigen but not with Vi-bearing Salmonella typhi whole cells. Fusion of the spleen cells from one of the Vi antibody-producing mice with NSI myeloma cells produced four stable hybridomas that secreted antibodies to Vi. Monoclonal antibodies from these four clones were all of the immunoglobulin G class and, as determined by competition, appeared to have the same epitope specificity. Despite their immunoglobulin G nature, mouse ascitic fluids induced by one of the hybridomas strongly agglutinated the Vi-positive strains of S. typhi, S. dublin, and Citrobacter strain 5396/38. Thus, 10 clinical isolates of S. typhi but not 98 strains of other bacteria were reactive in slide agglutination tests with the monoclonal antibodies.

Vi serology in detection of chronic Salmonella typhi carriers in an endemic area

A passive haemagglutination assay measuring antibody to highly purified Vi antigen, known to be sensitive and specific for the detection of chronic Salmonella typhi carriers in a non-endemic area, was assessed in an endemic area. A reciprocal serum Vi antibody titre of 160 was found to have a sensitivity of 75%, specificity of 92%, and a high predictive value in screening for chronic S typhi carriers in high-risk population group (eg, women over 40 years). This simple assay can screen for chronic S typhi carriers even in areas where typhoid fever is highly endemic.