Preventive therapy for latent tuberculosis infection-the promise and the challenges

Around one third of the world's population may harbour latent tuberculosis infection (LTBI), an asymptomatic immunological state that confers a heightened risk of subsequently developing tuberculosis (TB). Effectively treating LTBI will be essential if the End TB Strategy is to be realized. This review evaluates the evidence in relation to the effectiveness of preventive antibiotic therapy to treat LTBI due to both drug-susceptible and drug-resistant bacteria. Current national and international preventive therapy guidelines are summarized, as well as ongoing randomized trials evaluating regimens to prevent drug-resistant TB. Populations that may benefit most from screening and treatment for LTBI include close contacts of patients with TB (particularly children under 5 years of age) and individuals with substantial immunological impairment. The risks and benefits of treatment must be carefully balanced for each individual. Electronic decision support tools offer one way in which clinicians can help patients to make informed decisions. Modelling studies indicate that the expanded use of preventive therapy will be essential to achieving substantial reductions in the global TB burden. However, the widespread scale-up of screening and treatment will require careful consideration of cost-effectiveness, while ensuring the drivers of ongoing disease transmission are also addressed.

Passive case finding for tuberculosis is not enough

Current World Health Organisation targets calling for an end to the global tuberculosis (TB) epidemic by 2035 require a dramatic improvement in current case-detection strategies. A reliance on passive case finding (PCF) has resulted consistently, in over three million infectious TB cases per year, being missed by the health system, leading to ongoing transmission of infection within families and communities. Active case finding (ACF) for TB has been recognized as an important complementary strategy to PCF, in order to diagnose and treat patients earlier, reducing the period of infectiousness and therefore transmission. ACF may also achieve substantial population-level TB control. Local TB epidemiology and the resources available in each setting will influence which populations should be screened, and the types of ACF interventions to use for maximal impact. TB control programs should begin with the highest risk groups and broaden their activities as resources allow. Mathematical models can help to predict the population-level effects and the cost-effectiveness of a variety of ACF strategies on different risk populations.

Preventing the spread of multidrug-resistant tuberculosis and protecting contacts of infectious cases

Prevention of multidrug-resistant and extensively drug-resistant tuberculosis (MDR/XDR-TB) is a top priority for global TB control, given the need to limit epidemic spread and considering the high cost, toxicity and poor treatment outcomes with available therapies. We performed a systematic literature review to evaluate the evidence for strategies to reduce MDR/XDR-TB transmission and disease progression. Rapid detection and timely initiation of effective treatment is critical to rendering MDR/XDR-TB cases non-infectious. The scale-up of rapid molecular testing has transformed the capacity of high-incidence settings to identify and treat patients with MDR/XDR-TB. Optimized infection control measures in hospitals and clinics are critical to protect other patients and healthcare workers, whereas creative measures to reduce transmission within community hotspots require consideration. Targeted screening of high-risk communities may enhance early case-detection and limit the spread of MDR/XDR-TB. Among infected contacts, preventive therapy promises to reduce the risk of disease progression. This is supported by observational cohort studies, but randomized trials are urgently needed to confirm these observations and guide policy formulation. Substantial investment in MDR/XDR-TB prevention and care will be critical if the ambitious global goal of TB elimination is to be realized.

Housing and tuberculosis in an Inuit village in northern Quebec: a case-control study

BACKGROUND

Between November 2011 and November 2012, an Inuit village in Nunavik, Quebec experienced a surge in the occurrence of active TB; contact investigations showed that TB infection was highly prevalent (62.6%), particularly among those over age 14 years (78.8%). A nested case-control study showed that nutritional inadequacy was associated with acquisition of infection but not progression to disease. We performed a study to determine whether characteristics of one's dwelling were associated with 1) acquisition of newly diagnosed TB infection and 2) progression to confirmed or probable disease among those with TB infection.

METHODS

In this nested case-control study, we enrolled 200 people who were household or social contacts of at least 1 person with active TB or had received a diagnosis of active TB and assessed whether characteristics of their dwellings were associated with their odds of having newly diagnosed TB infection and/or odds of progression to disease between November 2011 and November 2012. For our first objective, we compared participants with newly diagnosed TB infection (regardless of their disease status) to a control group of contacts who were uninfected. For the second objective, we compared participants with confirmed or probable disease to a control group consisting of those with infection but no disease. We used information collected during investigation of the contacts and from study questionnaires to determine whether participants may have been exposed to TB in their own home (if they had shared a dwelling with someone who had smear-positive TB during the outbreak) or in other dwellings that they visited at least weekly.

RESULTS

The participants lived in 79 dwellings. The mean number of people per room was 1.1 (standard deviation [SD] 0.5). The mean room size and ventilation level of the common living space (kitchen and living/dining rooms) were 67.9 (SD 9.4) m3 and 1.69 (SD 0.26) air changes per hour, respectively. After adjustment for potential confounders, the number of people per room was positively associated with the odds of newly diagnosed infection and odds of disease, but only among participants who lived with someone with smear-positive TB (the minority of participants). Other dwelling characteristics were not associated with either outcome.

INTERPRETATION

Reducing household crowding may contribute to TB prevention. Overall, our investigations have not identified associations that explain the elevated disease risk in this village. In light of our results and considering the high prevalence of TB infection, treatment of latent infection is an essential intervention for long-term reduction of TB incidence in this village.

Reassessment of the positive predictive value and specificity of Xpert MTB/RIF: a diagnostic accuracy study in the context of community-wide screening for tuberculosis

BACKGROUND

Community-wide screening for tuberculosis with Xpert MTB/RIF as a primary screening tool overcomes some of the limitations of conventional screening. However, concerns exist about the low positive predictive value of this test in screening settings. We did a cross-sectional assessment of this diagnostic test to directly estimate the actual positive predictive value of Xpert MTB/RIF when used in the setting of community-wide screening for tuberculosis, and to draw an inference about the specificity of the test for tuberculosis detection.

METHODS

Field staff visited households in 60 randomly selected villages in Ca Mau province, Vietnam. We included people aged 15 years or older who provided written informed consent and were able to produce 0·5 mL or more of sputum, irrespective of reported symptoms. Participants were tested with Xpert MTB/RIF, then those with positive results had two further sputum samples tested for smear microscopy and culture, and underwent chest radiography at the provincial TB Health Center. The positive predictive value of Xpert MTB/RIF was compared against two reference standards for tuberculosis diagnosis-a positive sputum culture for Mycobacterium tuberculosis, and a positive sputum culture or a chest radiograph consistent with active pulmonary tuberculosis. We then calculated the specificity of Xpert MTB/RIF for tuberculosis detection on the basis of these positive predictive values and disease prevalence in this setting.

FINDINGS

43 435 adults consented to screening with Xpert MTB/RIF. Sputum samples of 0·5 mL or greater were collected from 23 202 participants, producing 22 673 valid results. 169 participants had positive Xpert MTB/RIF results (0·39% of those screened and 0·75% of those with valid sputum results). The positive predictive value of Xpert MTB/RIF was 61·0% (95% CI 52·8-68·7) when compared against a positive sputum culture and 83·9% (76·8-89·2) when compared against a positive sputum culture or chest radiograph consistent with active tuberculosis. On the basis of these positive predictive values, the specificity of Xpert MTB/RIF was determined to be between 99·78% (95% CI 99·71-99·84) and 99·93% (99·88-99·96).

INTERPRETATION

The positive predictive value and specificity of Xpert MTB/RIF in the context of community-wide screening for tuberculosis is substantially higher than that predicted in previous studies. Our findings support the potential role of Xpert MTB/RIF as a primary screening tool to detect prevalent cases of tuberculosis in the community.

FUNDING

Australian National Health and Medical Research Council.

Tuberculosis contact investigation in low- and middle-income countries: standardized definitions and indicators

There is a need for better utilization of program data for global tuberculosis (TB) control. Significant information could be gained from data collected by TB programs that could supplement traditional sources of evidence and contribute to policy development. For this operational information to be useful, it must be collected in a uniform manner, using standardized definitions and approaches to evaluation. As an example of an approach to uniformity in generating useful program data, we present recommendations for the standardization of definitions and indicators for the investigation of contacts of persons with infectious TB in low- and middle-income countries.

Epidemiology of tuberculosis immunology

Immunological impairment plays a major role in the epidemiology of TB. Globally, the most common causes of immunological impairment are malnutrition, diabetes, HIV/AIDS, aging, and smoking. With the notable exception of HIV, each factor leads to relatively mild immunological impairment in individuals. However, as these conditions affect a significant proportion of the population, they contribute substantially to the incidence of TB at a global scale. Understanding immunological impairment is central to understanding the global TB pandemic, and vital to the development of effective disease control strategies.

Effect of temperature and lymphokines on mixed lymphocyte and mitogen responses of chicken lymphoid cells in vitro

The effect of temperature on T cell mitogen and mixed lymphocyte responses (MLR) of chicken lymphoid cells in vitro was examined. Responses at 40 degrees C were much higher than at 37 degrees C. This difference did not appear to be due only to faster kinetics of the responses at 40 degrees C. At the lower temperature the MLR could be enhanced by polyethylene glycol (PEG) for spleen cells and by Con A induced lymphokines for peripheral blood cells (PBL). The positive effect of PEG on the chicken spleen cell MLR appeared to be determined at the stimulator cell level. Responses to mitogens at 37 degrees C of both spleen cells and PBL were enhanced by lymphokines.

Potentials for pheromones in chimpanzee vaginal fatty acids

52 vaginal lavages from 33 chimpanzees (Pan troglodytes) of various age classes and reproductive states were examined for volatile fatty acid content by means of liquid gas chromatography. Features of the acid complex in chimpanzee vaginal secretions distinguish these apes from other anthropoids investigated and may constitute and odor signal that identifies chimpanzees. It was not possible to discriminate between the most and least fertile stages of the estrous cycle or among individuals on the basis of acid characteristics. If volatile fatty acids in chimpanzee vaginal secretions are components of a sexual pheromone or pheromones, it is more likely that they are immediate releasers of attraction than of sexual behavior.

Some comparisons between siamang and gibbon behaviour

A family of captive siamangs at the Milwaukee County Zoo was observed for 100 h during an 8-month period. The adults were originally feral animals which have been mated in captivity. All of the offspring, a subadult female, a juvenile male and an infant are theirs. They exhibit a family composition known to exist in the wild. Behavioural similarity to wild gibbons was noted, particularly in the peripheralization of the subadult female, whose isolation is both spatial and social. She was almost never the object of grooming activity and she was allowed to eat only when the family had finished eating. Aggression at feeding time, particularly by the adult male, was both covert and overt. This isolation of the maturing young seems to be but one of the indicators of the similarity between gibbon and siamang behaviour in the wild and in captivity.