Estimating and interpreting secondary attack risk: Binomial considered biased

The household secondary attack risk (SAR), often called the secondary attack rate or secondary infection risk, is the probability of infectious contact from an infectious household member A to a given household member B, where we define infectious contact to be a contact sufficient to infect B if he or she is susceptible. Estimation of the SAR is an important part of understanding and controlling the transmission of infectious diseases. In practice, it is most often estimated using binomial models such as logistic regression, which implicitly attribute all secondary infections in a household to the primary case. In the simplest case, the number of secondary infections in a household with m susceptibles and a single primary case is modeled as a binomial(m, p) random variable where p is the SAR. Although it has long been understood that transmission within households is not binomial, it is thought that multiple generations of transmission can be neglected safely when p is small. We use probability generating functions and simulations to show that this is a mistake. The proportion of susceptible household members infected can be substantially larger than the SAR even when p is small. As a result, binomial estimates of the SAR are biased upward and their confidence intervals have poor coverage probabilities even if adjusted for clustering. Accurate point and interval estimates of the SAR can be obtained using longitudinal chain binomial models or pairwise survival analysis, which account for multiple generations of transmission within households, the ongoing risk of infection from outside the household, and incomplete follow-up. We illustrate the practical implications of these results in an analysis of household surveillance data collected by the Los Angeles County Department of Public Health during the 2009 influenza A (H1N1) pandemic.     

Poultry Slaughtering Practices in Rural Communities of Bangladesh and Risk of Avian Influenza Transmission: A Qualitative Study

Slaughtering sick poultry is a risk factor for human infection with highly pathogenic avian influenza and is a common practice in Bangladesh. This paper describes human exposures to poultry during slaughtering process and the customs and rituals influencing these practices in two Bangladeshi rural communities. In 2009, we conducted 30 observations to observe slaughtering practices and 110 in-depth and short interviews and 36 group discussions to explore reasons behind those practices. The villagers reported slaughtering 103 poultry, including 20 sick poultry during 2 months. During different stages of slaughtering, humans, the environment, healthy poultry, and other animals were exposed to poultry blood and body parts. Women performed most of the slaughtering tasks, including evisceration. Defeathering required the most time and involved several persons. During festivals, ceremonies, and rituals, many people gathered and participated in the slaughtering of poultry. Exposure to poultry slaughtering created numerous opportunities for potential avian influenza transmission. Strategies that can be further tested to determine if they reduce the risk of transmission include skinning the carcasses of sick poultry, using hot water for defeathering and cleaning, using a bucket to contain slaughtering blood and carcass, burying the offal and encouraging handwashing.     

A randomized controlled trial of interventions to impede date palm sap contamination by bats to prevent nipah virus transmission in bangladesh

Background

Drinking raw date palm sap is a risk factor for human Nipah virus (NiV) infection. Fruit bats, the natural reservoir of NiV, commonly contaminate raw sap with saliva by licking date palm’s sap producing surface. We evaluated four types of physical barriers that may prevent bats from contacting sap.

Methods

During 2009, we used a crossover design and randomly selected 20 date palm sap producing trees and observed each tree for 2 nights: one night with a bamboo skirt intervention applied and one night without the intervention. During 2010, we selected 120 trees and randomly assigned four types of interventions to 15 trees each: bamboo, dhoincha (local plant), jute stick and polythene skirts covering the shaved part, sap stream, tap and collection pot. We enrolled the remaining 60 trees as controls. We used motion sensor activated infrared cameras to examine bat contact with sap.

Results

During 2009 bats contacted date palm sap in 85% of observation nights when no intervention was used compared with 35% of nights when the intervention was used [p<0.001]. Bats were able to contact the sap when the skirt did not entirely cover the sap producing surface. Therefore, in 2010 we requested the sap harvesters to use larger skirts. During 2010 bats contacted date palm sap [2% vs. 83%, p<0.001] less frequently in trees protected with skirts compared to control trees. No bats contacted sap in trees with bamboo (p<0.001 compared to control), dhoincha skirt (p<0.001) or polythene covering (p<0.001), but bats did contact sap during one night (7%) with the jute stick skirt (p<0.001).

Conclusion

Bamboo, dhoincha, jute stick and polythene skirts covering the sap producing areas of a tree effectively prevented bat-sap contact. Community interventions should promote applying these skirts to prevent occasional Nipah spillovers to human.     

Influenza in Outpatient ILI Case-Patients in National Hospital-Based Surveillance,Bangladesh, 2007–2008

Background

Recent population-based estimates in a Dhaka low-income community suggest that influenza was prevalent among children. To explore the epidemiology and seasonality of influenza throughout the country and among all age groups, we established nationally representative hospital-based surveillance necessary to guide influenza prevention and control efforts.

Methodolgy/Principal Findings

We conducted influenza-like illness and severe acute respiratory illness sentinel surveillance in 12 hospitals across Bangladesh during May 2007–December 2008. We collected specimens from 3,699 patients, 385 (10%) which were influenza positive by real time RT-PCR. Among the sample-positive patients, 192 (51%) were type A and 188 (49%) were type B. Hemagglutinin subtyping of type A viruses detected 137 (71%) A/H1 and 55 (29%) A/H3, but no A/H5 or other novel influenza strains. The frequency of influenza cases was highest among children aged under 5 years (44%), while the proportions of laboratory confirmed cases was highest among participants aged 11–15 (18%). We applied kriging, a geo-statistical technique, to explore the spatial and temporal spread of influenza and found that, during 2008, influenza was first identified in large port cities and then gradually spread to other parts of the country. We identified a distinct influenza peak during the rainy season (May–September).

Conclusions/Significance

Our surveillance data confirms that influenza is prevalent throughout Bangladesh, affecting a wide range of ages and causing considerable morbidity and hospital care. A unimodal influenza seasonality may allow Bangladesh to time annual influenza prevention messages and vaccination campaigns to reduce the national influenza burden. To scale-up such national interventions, we need to quantify the national rates of influenza and the economic burden associated with this disease through further studies.     

Isolation of Salmonella Virchow from a Fruit Bat (Pteropus giganteus)

Detection of zoonotic pathogens carried by bats is important both for understanding disease ecology and for developing preventive measures. Pteropus fruit bats have been identified as potential carriers of Salmonella enterica serotype Typhi. A cross-sectional study was conducted to determine the prevalence of Salmonella Typhi and other Salmonella serotypes in Pteropus giganteus fruit bats in Bangladesh. Rectal swabs were collected from 302 bats and cultured for Salmonella species. The bats were trapped in three districts (Faridpur, Rajbari, and Cox’s Bazar). Salmonella Typhi was not found but one juvenile female bat from Faridpur district was positive for Salmonella Virchow. Close associations between frugivorous bats, humans, and livestock in rural Bangladesh make it likely that the bat was infected by consuming contaminated water.