Mycobacterium avium subspecies paratuberculosis is the causative agent of Johne's disease, a chronic enteritis in ruminants including cattle, sheep, goats, and farmed deer. Recently, this bacterium has received an increasingly wide interest because of a rapidly growing body of scientific evidence which suggests that human infection with this microorganism may be causing some, and possibly all, cases of Crohn's disease. Recent studies have shown that a high percentage of people with Crohn's disease are infected with M. avium subsp. paratuberculosis; whether the association of this bacterium and Crohn's disease is causal or coincidental is not known. Crohn's disease is a gastrointestinal disease in humans with similar histopathological findings to those observed in the paucibacillary form of Johne's disease in cattle. The search for risk factors in Crohn's disease has been frustrating. However, epidemiologists have gathered enough information that points to an association between M. avium subsp. paratuberculosis and Crohn's disease. This paper reviews epidemiological models of disease causation, the major philosophical doctrines about causation, the established epidemiological criteria for causation, and the currently known epidemiological evidence of M. avium subsp. paratuberculosis as a possible cause of Crohn's disease.

We examined different patient outcomes following diagnosis of tuberculosis (TB). Incident cases were reported to the enhanced surveillance system in the East of England, between 2000 and 2003. For the 575 cases reported in 2001 and 2002, outcomes were assessed 1 year after initiating treatment. The crude clinical incidence rate of TB was 6·0 cases/100 000 person-years (pyr) [95% confidence interval (CI) 5·7–6·4], highest in the 25–29 years age group (14·9, 95% CI 12·9–17·1 cases/100 000 pyr) and among Black Africans (328·6, 95% CI 286·9–374·6 cases/100 000 pyr). Patients born abroad were 2·35 (95% CI 1·03–5·32) times more likely to be lost to follow-up than those born in the United Kingdom. Age at diagnosis (OR 1·05, 95% CI 1·04–1·07) and pulmonary disease (OR 2·73, 95% CI 1·21–6·15) were independently associated with mortality. Elderly patients and those with pulmonary TB appear to have worse outcomes despite treatment. Foreign-born patients may need closer follow-up to ensure favourable outcomes.

Our aim was to obtain knowledge of how meteorological conditions affect community epidemics of respiratory syncytial virus (RSV) infection. To this end we recorded year-round RSV activity in nine cities that differ markedly in geographic location and climate. We correlated local weather conditions with weekly or monthly RSV cases. We reviewed similar reports from other areas varying in climate. Weekly RSV activity was related to temperature in a bimodal fashion, with peaks of activity at temperatures above 24–30°C and at 2–6°C. RSV activity was also greatest at 45–65% relative humidity. RSV activity was inversely related to UVB radiance at three sites where this could be tested. At sites with persistently warm temperatures and high humidity, RSV activity was continuous throughout the year, peaking in summer and early autumn. In temperate climates, RSV activity was maximal during winter, correlating with lower temperatures. In areas where temperatures remained colder throughout the year, RSV activity again became nearly continuous. Community activity of RSV is substantial when both ambient temperatures and absolute humidity are very high, perhaps reflecting greater stability of RSV in aerosols. Transmission of RSV in cooler climates is inversely related to temperature possibly as a result of increased stability of the virus in secretions in the colder environment. UVB radiation may inactivate virus in the environment, or influence susceptibility to RSV by altering host resistance.

We compared the burden of illness due to a spectrum of respiratory diagnostic categories among persons presenting in a sentinel general practice network in England and Wales during periods of influenza and of respiratory syncytial virus (RSV) activity. During all periods of viral activity, incidence rates of influenza-like illness, bronchitis and common cold were elevated compared to those in baseline periods. Excess rates per 100 000 of acute bronchitis were greater in children aged <1 year (median difference 2702, 95% CI 929–4867) and in children aged 1–4 years (994, 95% CI 338–1747) during RSV active periods rather than influenza; estimates for the two viruses were similar in other age groups. Excess rates of influenza-like illness in all age groups were clearly associated with influenza virus activity. For common cold the estimates of median excess rates were significantly higher in RSV active periods for the age groups <1 year (3728, 95% CI 632–5867) and 5–14 years (339, 95% CI 59–768); estimates were similar in other age groups for the two viruses. The clinical burden of disease associated with RSV is as great if not greater than influenza in patients of all ages presenting to general practitioners.

The number of deaths attributable to influenza is believed to be considerably higher than the number certified by vital statistics registration as due to influenza. Weekly mortality data for Canada from the 1989/1990 to the 1998/1999 influenza seasons were analysed by cause of death, age group, and place of death to estimate the impact of influenza on mortality. A Poisson regression model was found to accurately predict all-cause, as well as cause-specific mortality, as a function of influenza-certified deaths, after controlling for seasonality, and trend. Influenza-attributable deaths were calculated as predicted less baseline-predicted deaths. In summary, throughout the 1990s there were on average just under 4000 deaths attributable to influenza annually (for an influenza-attributable mortality rate of 13/100 000 persons), varying from no detectable excess mortality for the 1990/1991 influenza season, to 6000–8000 influenza-attributable deaths for the more severe influenza seasons of 1997/1998 and 1998/1999. On average, 8% (95% CI 7–10) of influenza-attributable deaths were certified as influenza, although this percentage varied from 4% to 12% from year to year. Only 15% of the influenza-attributable deaths were certified as pneumonia, and for all respiratory causes, 40%. Deaths were distributed over most causes. The weekly pattern of influenza-certified deaths was a good predictor of excess all-cause mortality.

Surveillance is an important component of influenza control. This report describes the establishment and first results of the Korean Influenza Surveillance Scheme (KISS), an integrated clinical and laboratory surveillance network involving 622 public health centres (PHCs) and private clinics. Sentinel physicians reported cases of influenza-like illness (ILI) weekly and forwarded specimens for virus isolation and characterization. Influenza activity during the opening 2000–2001 season was milder and delayed compared with previous years. The ILI consultation rate corresponded well with the number of influenza virus isolates, both peaking in week 10 of 2001. Influenza A(H3N2) was the dominant isolate. The peak ILI consultation rate was higher in private clinics than in PHCs (5·04 vs 1·79 cases/1000 visits). An evaluation questionnaire generated potential enhancements to the scheme. KISS appears to represent the pattern of influenza activity accurately and will have a valuable role in monitoring and preventing epidemics in Korea.

Planning adequate public health responses against emerging infectious diseases requires predictive tools to evaluate the impact of candidate intervention strategies. With current interest in pandemic influenza very high, modelling approaches have suggested antiviral treatment combined with targeted prophylaxis as an effective first-line intervention against an emerging influenza pandemic. To investigate how the effectiveness of such interventions depends on contact structure, we simulate the effects in networks with variable degree distributions. The infection attack rate can increase if the number of contacts per person is heterogeneous, implying the existence of high-degree individuals who are potential super-spreaders. The effectiveness of a socially targeted intervention suffers from heterogeneous contact patterns and depends on whether infection is predominantly transmitted to close or casual contacts. Our findings imply that the various contact networks' degree distributions as well as the allocation of contagiousness between close and casual contacts should be examined to identify appropriate strategies of disease control measures.

To determine the potential benefits of regionally targeted mass vaccination as an adjunct to other smallpox control strategies we employed a spatial metapopulation patch model based on the administrative districts of Great Britain. We counted deaths due to smallpox and to vaccination to identify strategies that minimized total deaths. Results confirm that case isolation, and the tracing, vaccination and observation of case contacts can be optimal for control but only for optimistic assumptions concerning, for example, the basic reproduction number for smallpox (R0=3) and smaller numbers of index cases (∼10). For a wider range of scenarios, including larger numbers of index cases and higher reproduction numbers, the addition of mass vaccination targeted only to infected districts provided an appreciable benefit (5–80% fewer deaths depending on where the outbreak started with a trigger value of 1–10 isolated symptomatic individuals within a district).

This study investigated the infectiousness of smallpox relative to the onset of fever using a likelihood-based estimation procedure based on the observed transmission network (n=223) and on the distribution of the incubation period (n=379). Who-infected-whom information enabled us to back-calculate the infectiousness by disease age, employing a step function model for infectiousness. Frequency of secondary transmissions was highest between 3 and 6 days after onset of fever, yielding an expected daily frequency of 20·6% (95% CI 15·1–26·4) of the total number of secondary transmissions, which is consistent with previous observations. The estimated cumulative frequency suggests that 91·1% of secondary transmissions occurred up to 9 days after onset of fever. The proposed method appeared to be useful for diseases with an acute course of illness, where transmission was not hampered by depletion of susceptible contacts.

An outbreak of Campylobacter jejuni infection occurred in a South Wales Valleys housing estate. Illness in estate residents was associated with tap water consumption [population attributable risk (PAR) 50%, relative risk (RR) 2·53, 95% confidence interval (CI) 1·9–3·37] and residence in the upper estate (PAR 49%, RR 2·44, 95% CI 1·83–3·24). Amongst upper estate residents, rates of diarrhoeal illness increased with rates of water consumption (OR 18, 95% CI 3·5–92·4 for heaviest consumers, χ2 trend P<0·0001). The upper estate received mains water via a covered holding reservoir. A crack in the wall of the holding reservoir was identified. Contamination with surface water from nearby pasture land was the likely cause of this outbreak. Service reservoirs are common in rural communities and need regular maintenance and inspection. The role of water in sporadic cases of campylobacter enteritis may be underestimated.

Water treatment plants in the United Kingdom at significant risk of cryptosporidiosis and using conventional filtration methods have been required to install 24-h monitoring systems since April 2000. No major waterborne outbreaks have been described since 2001. Small outbreaks have been associated with water. This paper describes such an outbreak. Data from a local multi-agency surveillance system was used to describe the outbreak, including mapping cases against water supply zones. A case-control study investigated hypotheses raised. All cases were genotype 1. Early cases were in the supply zone of a surface water-treatment plant that had met treatment standards. Later cases included residents in a different supply zone that temporarily received water from the implicated plant. Cases reported more consumption of domestic mains water than controls. Descriptive and analytical epidemiology thus supported drinking water as a source of cryptosporidiosis from a plant meeting regulatory requirements. The evidence for setting drinking-water standards needs review.

Salmonella Braenderup is an uncommon serotype in the United States. In July 2004, a multistate outbreak of Salmonella Braenderup diarrhoeal infections occurred, with 125 clinical isolates identified. To investigate, we conducted a case-control study, enrolling 32 cases and 63 matched controls. Cheese, lettuce and tomato eaten at restaurants all appeared to be associated with illness. To further define specific exposures, we conducted a second study and asked managers of restaurants patronized by patients and controls about cheese, lettuce and tomato varieties used in dishes their patrons reported consuming. This information was obtained for 27 cases and 29 controls. Roma tomatoes were the only exposure significantly associated with illness (odds ratio 4·3, 95% confidence interval 1·2–15·9). Roma tomatoes from two restaurants were traced back to a single tomato packing house. The methods used in this field investigation to define specific exposures may be useful for other foodborne outbreaks.

In three successive years, we visited petting farms (n=132), care farms (n=91), and farmyard campsites (n=84), respectively, and completed a standard questionnaire with the objective of determining the hygienic status of these farms and describing hygiene measures implemented to reduce the risk of transmission of zoonotic agents from the animals to humans. For at least 85% of the farms, the overall impression of hygiene was recorded as good. However, more attention must be paid to: informing visitors on hygiene and handwashing, provision of handwashing facilities, and a footwear cleaning facility. Examination of samples of freshly voided faeces resulted in the detection of Shiga toxin-producing Escherichia coli O157 and/or Salmonella spp. and/or Campylobacter spp. at almost two-thirds (64·9%) of the petting farms, and around half of the care farms (56·0%) and farmyard campsites (45·2%). These data reinforce the need for control measures for both public and private farms to reduce human exposure to livestock faeces and thus the risk of transmission of zoonotic diseases. Public awareness of the risk associated with handling animals or faecal material should be increased.

Cases of ornithosis amongst workers on a rural duck abattoir and farm were notified from late 2003 to health authorities in Victoria, Australia. In May 2004 we conducted a serological survey to identify the extent of exposure to Chlamydophila psittaci amongst workers and a case control study to identify high-risk work areas for ornithosis-related pneumonia. Some workers in all occupational groups showed serological evidence of exposure, while those with pneumonia were more likely to have worked in the slaughtering area of the abattoir (adjusted odds ratio 16·7, 95% confidence interval 1·3–207). High exposure to blood and feathers from recently killed birds is likely to represent an occupational hazard to workers, but pneumonia did occur in workers without these exposures. We recommended respiratory protection for all abattoir workers and improvements to airflow and reduction of environmental contamination in high-risk work areas to prevent further cases.

Norway rats (Rattus norvegicus) carry several zoonotic pathogens and because rats and humans live in close proximity in urban environments, there exists potential for transmission. To identify zoonotic agents carried by rats in Baltimore, Maryland, USA, we live-trapped 201 rats during 2005–2006 and screened them for a panel of viruses, bacteria, and parasites. Antibodies against Seoul virus (57·7%), hepatitis E virus (HEV, 73·5%), Leptospira interrogans (65·3%), Bartonella elizabethae (34·1%), and Rickettsia typhi (7·0%) were detected in Norway rats. Endoparasites, including Calodium hepatica (87·9%) and Hymenolepis sp. (34·4%), and ectoparasites (13·9%, primarily Laelaps echidninus) also were present. The risk of human exposure to these pathogens is a significant public health concern. Because these pathogens cause non-specific and often self-limiting symptoms in humans, infection in human populations is probably underdiagnosed.

A series of transmission studies was conducted to investigate the aetiology, or aetiologies, of emerging fatal epidemic disease syndromes affecting the common frog (Rana temporaria) in Britain. The syndromes, characterized by skin ulceration or systemic haemorrhages, were induced upon exposure to lesion homogenates or cultured ranavirus. The re-isolation of ranavirus from experimentally affected frogs fulfilled Koch's postulates. Aeromonas hydrophila, previously associated with similar lesions, was not significant to disease development. Unexpectedly, disease outcomes were influenced by both the source of agent and the route of exposure, indicating that different ranaviruses with different tissue tropisms and pathogeneses (possibly similar to quasi-species in RNA virus populations) are circulating in the British common frog population. Our findings confirm that ranavirus disease has emerged as an important cause of amphibian mortality in Britain.

During investigations of epidemic frog mortality in Britain, a novel fatal systemic haemorrhagic disease of common toads was discovered. This disease resembles a systemic haemorrhagic disease of common frogs in Britain, which is one of a range of fatal disease syndromes, characterized by systemic haemorrhages, skin ulceration or a combination of these lesions, caused by ranavirus infection. Ranavirus previously isolated from diseased toads was inoculated into common frogs to evaluate if this virus could infect and cause disease in common frogs. All virus-inoculated frogs died with systemic haemorrhages between 6 and 8 days post-inoculation, giving similar results to those produced by the inoculation of frogs with ranavirus cultured from naturally diseased frogs. These results indicate that the same, or similar, viruses are affecting both frogs and toads in the field and confirm that ranavirus has emerged as an important cause of amphibian mortality in Britain.

To enhance the detection of bacterial meningitis in an East Asian surveillance study, we employed cerebrospinal fluid (CSF) bacterial culture, latex agglutination (LA) and polymerase chain reaction–enzyme immunoassay (PCR–EIA) testing for Haemophilus influenzae type b (Hib) and Streptococcus pneumoniae (Sp). The sensitivity and specificity of CSF PCR–EIA testing was compared to LA and culture. A meningitis case was defined by one positive result for any of the three tests. The sensitivity of H. influenzae CSF PCR–EIA, LA, and culture was 100%, 40% and 57·5% respectively; and for Sp CSF PCR–EIA, LA and culture, the sensitivity was 100%, 58·3% and 66·7%, respectively. Hib and Sp specificity was 100% by each method. CSF PCR–EIA was more sensitive than culture or LA for the detection of Hib and Sp meningitis cases increasing their incidence by 74% and 50% compared to culture respectively. CSF PCR–EIA should be included for the detection of bacterial meningitis in surveillance studies.