Active environmental surveillance of SARS-CoV-2 in Midwestern United States meatpacking plants.

This pilot project investigated environmental SARS-CoV-2 presence in seven Midwestern meatpacking plants from May 2020 to January 2021. This study investigated social distancing and infection control practices and incorporated environmental sampling of surfaces and air in employee common areas. All plants increased their social distancing efforts, increased the frequency of cleaning and disinfecting worker areas, and screened for symptomatic people to prevent entry into the workplace. 575 samples from common areas were collected and evaluated with RT-qPCR for the presence of SARS-CoV-2. 42/367 surface samples were positive, while no virus was detected in air samples. Case positive data from the counties surrounding each plant showed peak positive SARS-CoV-2 cases from 12-55 days before the virus was detected in the plant, indicating that environmental sampling is likely a lagging indicator of community and plant infection.

Invisible No More: The Impact of COVID-19 on Essential Food Production Workers.

From the farms to the packing plants, essential workers in critical food production industries keep food on our tables while risking their and their families' health and well-being to bring home a paycheck. They work in essential industries but are often invisible. The disparities illuminated by COVID-19 are not new. Instead, they are the result of years of inequities built into practices, policies, and systems that reinforce societal power structures. As a society, we are now at an antagonizing moment where we can change our collective trajectory to focus forward and promote equity and justice for workers in agriculture and food-related industries. To that end, we describe our experience and approach in addressing COVID-19 outbreaks in meat processing facilities, which included three pillars of action based on public health ethics and international human rights: (1) worksite prevention and control, (2) community-based prevention and control, and (3) treatment. Our approach can be translated to promote the health, safety, and well-being of the broader agricultural workforce.

Prevalence, genotypic diversity and detection of virulence genes in thermotolerant Campylobacter at different stages of the poultry meat supply chain.

Thermotolerant Campylobacter is the leading bacterial cause of foodborne illness in humans worldwide. The objectives of this study were to estimate prevalence and to identify and characterize potential sources of thermotolerant Campylobacter contamination in broilers on farms and at the slaughterhouse; to evaluate the clonal relationship among thermotolerant Campylobacter isolates from different stages of the broiler meat supply chain, and to analyze the presence of virulence genes in different sources of thermotolerant Campylobacter. A total of 1210 samples were collected from three broiler meat supply chains in Santa Fe, Argentina. At the farms, the sampling collection included broilers one week prior to slaughter, wild-living birds, domestic dogs, wild rodents, farm workers' boots, litter, feed, drinking water, flies, and darkling beetles (Alphitobius diaperinus). At the slaughtering line, the samples taken were from the evisceration zone (broiler cecum, working surfaces, evisceration knives and workers' hands), from the chiller zone (surfaces and direct supply water) and from the packing zone (work surfaces, workers' hands and broiler carcasses). The samples taken along each supply chain were in the same batch. The isolates obtained were identified to the species level (C. jejuni and C. coli) by multiplex PCR and were analyzed using pulsed-field gel electrophoresis to compare different profiles according to the source. Finally, the presence of 11 virulence genes was examined (cadF, cdtA, cdtB, cdtC, ciaB, flaA, flhA, iam, wlaN, virB11, racR). From 254 isolates, 128 (50.4%) were Campylobacter jejuni and 126 (49.6%) Campylobacter coli. C. jejuni was the species most prevalent in farm and C. coli the species most prevalent at the slaughterhouse. We detected thermotolerant Campylobacter in samples of wild birds, darkling beetles, farm workers' boots, flies and litter. At the slaughterhouse, the prevalence varied along the process line. By analyzing PFGE results, C. jejuni showed 21 profiles with three predominant genotypes, while C. coli showed 14 profiles with four predominant genotypes. A high genotype diversity was found; however, relationships between isolates from different stages of the broiler meat chain, between broiler and potential sources of thermotolerant Campylobacter contamination and between strains in the farm and in the slaughterhouse were detected. Furthermore, there was evidence of cross-contamination at the slaughterhouse. FlaA, flhA genes were detected in all strains, and the third most prevalent virulence gene was cadF. Only those strains obtained from flies, wild-living birds and broiler carcass samples harbored 10 of 11 pathogenic genes. The prevalence of some pathogenic genes between C. jejuni and C. coli was different. This evidence should contribute the scientific basis to implement risk management measures in public health.

Japanese encephalitis: the vectors, ecology and potential for expansion.

BACKGROUND: Japanese encephalitis (JE) is a viral disease predominantly located in South East Asia and commonly associated with transmission between amplifying hosts, such as pigs, and the mosquito Culex tritaeniorhynchus, where human infection represents a dead end in the life cycle of the virus. The expansion of JE beyond an Asiatic confine is dependent on a multitude of complex factors that stem back to genetic subtype variation. A complex interplay of the genetic variation and vector competencies combine with variables such as geography, climate change and urbanization. METHODS: Our understanding of JE is still at an early stage with long-term longitudinal vector surveillance necessary to better understand the dynamics of JE transmission and to characterize the role of potential secondary vectors such as Cx. pipiens and Cx. bitaeniorhynchus. The authors review the vectors indicated in transmission and the ecological, genetic and anthropological factors that affect the disease's range and epidemiology. CONCLUSION: Monitoring for the presence of JE virus in mosquitoes in general can be used to estimate levels of potential JE exposure, intensity of viral activity and genetic variation of JEV throughout surveyed areas. Increased surveillance and diagnosis of viral encephalitis caused by genotype 5 JE virus is required in particular, with the expansion in epidemiology and disease prevalence in new geographic areas an issue of great concern. Additional studies that measure the impact of vectors (e.g. bionomics and vector competence) in the transmission of JEV and that incorporate environmental factors (e.g. weekly rainfall) are needed to define the roles of Culex species in the viral pathogenesis during outbreak and non-outbreak years.

Association between butchers and cancer mortality and incidence: A systematic review and meta-analysis.

BACKGROUND: In this study, we evaluated whether increased risks of mortality and cancer incidence exist among butchers worldwide. To achieve this goal, we conducted a systematic review and meta-analysis to investigate the correlations of the risks of cancer death and incidence with male and female butchers. METHODS: We obtained data by performing a comprehensive literature search in several databases for eligible studies published before March 2017. Multivariable-adjusted standardized mortality ratios (SMRs) and odds ratio (OR), as well as associated 95% confidence intervals (CIs) and those by subgroups, were extracted and pooled. RESULTS: A total of 17 observational studies comprising 397,726 participants were included in the meta-analysis. The butcher occupation was not associated with all-cancer mortality risk, with pooled overall SMRs of 1.07 (95% CI 0.96-1.20). However, the pooled ORs revealed that butchers hold an elevated risk of total cancer incidence (OR, 1.51; 95% CI, 1.33-1.73). No proof of publication bias was obtained, and the findings were consistent in the subgroup analyses. CONCLUSION: Our results suggest that working as butchers did not significantly influence all-cancer mortality risk but significantly contributed to elevated all-cancer incidence risk. Nevertheless, well-designed observational studies on this topic are necessary to confirm and update our findings.

Association of Poultry Processing Industry Exposures With Reports of Occupational Finger Amputations: Results of an Analysis of OSHA Severe Injury Report (SIR) Data.

OBJECTIVE: This analysis was conducted to identify industry exposures strongly associated with reports of finger amputation. METHODS: Reports of severe occupational injuries in the Occupational Safety and Health Administration (OSHA) Severe Injury Report (SIR) database were analyzed in relation to U.S. Census Bureau industry employment data. Industries with significantly elevated reporting odds ratios (RORs) and relative reporting risks (RRRs) were identified. Multiple population association measures including population attributable fraction (PAF) were calculated by industry. RESULTS: Among industries with statistically significant RRR and ROR, the poultry processing industry (RRR = 12.60, ROR = 3.37) accounted for the highest PAF by RRR (2.34%) and ROR (1.65%) CONCLUSION:: The results of this analysis identify the poultry processing industry as a leading source of reports of occupational finger amputations and substantiate the need for further collaboration with this industry.

Seroreactivity to Q Fever Among Slaughterhouse Workers in South Korea.

OBJECTIVES: Q fever is a zoonotic disease that occurs worldwide; however, little is known about its prevalence in South Korea. We attempted to determine the prevalence of Q fever seroreactivity among Korean slaughterhouse workers and the risk factors for seroreactivity according to the type of work. METHODS: The study was conducted among 1503 workers at a total of 73 slaughterhouses and 62 residual-product disposal plants. During the study period, sites were visited and surveys were administered to employees involved in slaughterhouse work, and serological tests were performed on blood samples by indirect immunofluorescence assays. Serological samples were grouped by job classification into those of slaughter workers, residual-product handlers, inspectors and inspection assistants, and grading testers and testing assistants. Employee risk factors were analyzed according to the type of work. RESULTS: Out of 1481 study subjects who provided a blood sample, 151 (10.2%) showed reactive antibodies. When these results were analyzed in accordance with the type of work, the result of slaughter workers (11.3%) was similar to the result of residual-product handlers (11.4%), and the result of inspectors and assistants (5.3%) was similar to the result of grading testers and assistants (5.4%). Among those who answered in the affirmative to the survey question, "Has there been frequent contact between cattle blood and your mouth while working?" the proportions were 13.4 and 4.6%, respectively, and this was identified as a risk factor that significantly varied between job categories among slaughterhouse workers. CONCLUSIONS: This study found a Q fever seroreactivity rate of 10.2% for slaughterhouse workers, who are known to be a high-risk population. Contact with cattle blood around the mouth while working was the differential risk factor between job categories among slaughterhouse workers.

Non-cancer mortality in workers in the meat and delicatessen departments of supermarkets (1950-2006).

We studied non-cancer mortality in 10,701 workers in the meat and delicatessen departments of supermarkets because they have increased exposure to a variety of microorganisms that infect and cause disease in food animals such as cattle, pigs, sheep, and poultry, to which subjects in the general population are also exposed, albeit to a lesser degree. These workers were also exposed to fumes from the wrapping machine. Standardized mortality ratios were estimated in the cohort as a whole and in race/sex subgroups, using the US population for comparison. Study subjects were followed up from January 1950 to December 2006. Significantly increased deaths from diabetes, ischemic heart disease, pulmonary embolism, chronic bronchitis, peritonitis, intracranial and intraspinal abscess, other bacterial diseases, and significantly decreased deaths from diffuse diseases of connective tissue, functional diseases of the heart, intracerebral hemorrhage, occlusion/stenosis of the precerebral and cerebral arteries, and various types of accidents were observed in certain race/sex subgroups or in the cohort as a whole. The observed increased risks of several infectious conditions suggest that the increased occupational exposure to microorganisms may be responsible for at least some of the observed excess deaths, while exposure to fumes may also contribute to the excess of chronic bronchitis. The findings are important not only for supermarket workers and other workers in the meat and poultry industries, but also because the general population is exposed to these microorganisms found in food animals and their products. Nested case-control studies within cohorts that include both workers in supermarkets and other sectors of the meat and poultry industries, are now needed to examine specific risks from occupational exposures while adequately controlling for confounding factors, so that the role of these infectious agents in the occurrence of these diseases in workers and in general population subjects can be adequately assessed.

Workers' Health Surveillance in the Meat Processing Industry: Work and Health Indicators Associated with Work Ability.

BACKGROUND: Workers' health surveillance (WHS) programs commonly measure a large number of indicators addressing health habits and health risks. Recently, work ability and functional capacity have been included as important risk measures in WHS. In order to address work ability appropriately, knowledge of associations with work and health measures is necessary. The objective of this study was to evaluate which of the factors measured in a WHS are independently associated with work ability in a group of meat processing workers. METHODS: A cross-sectional study was performed in a large meat processing company in The Netherlands. Data were collected during a WHS between February 2012 and March 2014. Personal characteristics, health habits and health-risk indicators, functional capacity, and work-related factors were measured. Work ability was measured with the Work Ability Index and was used as dependent variable. Univariable and multivariable logistic regression analyses were conducted, a receiver operating characteristic curve was constructed and the area under the curve (AUC) was calculated. RESULTS: Data sets from 230 employees were used for analyses. The average age was 53 years and the average work ability index score was 39.3. In the final multivariable model age (OR 0.94), systolic blood pressure (OR 1.03), need for recovery (OR 0.56), and overhead work capacity (OR 3.95) contributed significantly. The AUC for this model was 0.81 (95% CI 0.75-0.86). CONCLUSION: Findings from the current study indicate that multifactorial outcomes (age, systolic blood pressure, need for recovery, and overhead work capacity) from a WHS were independently associated with work ability. These factors can be used to assess employees at risk for low work ability and might provide directions for interventions.

Comorbidity of tinea pedis and onychomycosis and evaluation of risk factors in Latino immigrant poultry processing and other manual laborers.

OBJECTIVES: Latino immigrant workers experience elevated rates of skin disease that result from their working and living conditions. Working in manual occupations exposes workers to a variety of challenges, including occlusive shoes, vigorous physical activity, and wet conditions. These challenges predispose workers to fungal infection. The objectives of this article are to examine the comorbidity of tinea pedis and onychomycosis and to identify possible risk factors among Latino immigrant poultry and nonpoultry workers in western North Carolina. METHODS: Data were obtained from a cross-sectional study conducted between June 2009 and November 2010 in rural western North Carolina among 518 manual Latino immigrant workers to assess their occupational injuries. Participants completed a face-to-face interview and a dermatologic examination. RESULTS: Nearly one-third of the participants (32%) were diagnosed as having onychomycosis and more than one-third (37.8%) were diagnosed as having tinea pedis. There was a greater prevalence of tinea pedis in men than women (71.3% vs 28.7%, respectively). Of the 518 participants, 121 (23.5%) had both conditions. Participants who reported the use of occlusive shoes as "always" or "most of the time" had a higher prevalence of comorbid onychomycosis and tinea pedis than the rest of the group. CONCLUSIONS: Comorbidity of tinea pedis and onychomycosis is common among immigrant Latino men and women who perform manual labor. Further studies confirming the presence and type of dermatophyte should be conducted.

Methicillin-resistant Staphylococcus aureus in cattle food chains - prevalence, diversity, and antimicrobial resistance in Germany.

Livestock-associated methicillin-resistant Staphylococcus aureus (MRSA) have been found in various farm animal species throughout the world. It was the objective of this study to estimate the prevalence of MRSA in different cattle food chains (milk, beef, and veal) in Germany, to analyze the MRSA diversity along each food chain and to compare the characteristics of the different subtypes. Samples were collected between 2009 and 2012 from dairy herds (bulk tank milk), veal herds (dust from the stables), veal calves, and beef cattle at slaughter (nasal swabs) and carcasses of veal calves (surface cuts) and beef as well as veal at retail. Sampling was proportionally distributed over the country according to the cattle population (on-farm sampling), slaughterhouse capacity (abattoir samples), and the human population (meat at retail). Methicillin-resistant S. aureus were isolated using harmonized methods from all sample types and populations investigated. The highest proportion of positive samples was found in nasal swabs from veal calves at slaughter in 2012 (144/320; 45.0%) and the lowest rate in bulk tank milk in 2009 (14/388; 4.1%). Most isolates, irrespective of the origin, were from spa types t011 and t034. Both have been assigned to the clonal complex (CC) 398. Few isolates (15/632; 2.4%) were from spa types not associated with the CC398. Spa-type patterns were similar along individual food chains but differed between food chains. Antimicrobial resistance patterns differed between isolates from the different food chains and spa types. Isolates from the veal chain displayed the highest resistance rates. We conclude that there is substantial diversity in the MRSA prevalence across different cattle production sectors.

Cancer mortality in workers employed in cattle, pigs, and sheep slaughtering and processing plants.

OBJECTIVES: We studied mortality in two separate cohorts of workers in abattoirs (N=4996) and meat processing plants (N=3642) belonging to a meatcutters' union, because they were exposed to viruses that cause cancer in food animals, and also to chemical carcinogens at work. METHODS: Standardized mortality ratios (SMRs) and proportional mortality ratios (PMRs) were estimated for each cohort as a whole and in subgroups defined by race and sex, using the US general population mortality rates for comparison. Study subjects were followed up from January 1950 to December 2006, during which time over 60% of them died. RESULTS: An excess of deaths from cancers of the base of the tongue, esophagus, lung, skin, bone and bladder, lymphoid leukemia, and benign tumors of the thyroid and other endocrine glands, and possibly Hodgkin's disease, was observed in abattoir and meat processing workers. Significantly lower SMRs were recorded for cancer of the thymus, mediastinum, pleura, etc., breast cancer, and non-Hodgkin's lymphoma. CONCLUSION: This study confirms the excess occurrence of cancer in workers in abattoirs and meat processing plants, butchers, and meatcutters, previously reported in this cohort and other similar cohorts worldwide. Large nested case-control studies are now needed to examine which specific occupational and non-occupational exposures are responsible for the excess. There is now sufficient evidence for steps to be taken to protect workers from carcinogenic exposures at the workplace. There are also serious implications for the general population which may also be exposed to some of these viruses.

The evolution of risk perceptions related to bovine spongiform encephalopathy--Canadian consumer and producer behavior.

In this study the dynamics of risk perceptions related to bovine spongiform encephalopathy (BSE) held by Canadian consumers and cow-calf producers were evaluated. Since the first domestic case of BSE in 2003, Canadian consumers and cow-calf producers have needed to make decisions on whether or not their purchasing/production behavior should change. Such changes in their behavior may relate to their levels of risk perceptions about BSE, risk perceptions that may be evolving over time and be affected by BSE media information available. An econometric analysis of the behavior of consumers and cow-calf producers might identify the impacts of evolving BSE risk perceptions. Risk perceptions related to BSE are evaluated through observed market behavior, an approach that differs from traditional stated preference approaches to eliciting risk perceptions at a particular point in time. BSE risk perceptions may be specified following a Social Amplification of Risk Framework (SARF) derived from sociology, psychology, and economics. Based on the SARF, various quality and quantity indices related to BSE media information are used as explanatory variables in risk perception equations. Risk perceptions are approximated using a predictive difference approach as defined by Liu et al. (1998). Results showed that Canadian consumer and cow-calf producer risk perceptions related to BSE have been amplified or attenuated by both quantity and quality of BSE media information. Government policies on risk communications need to address the different roles of BSE information in Canadian consumers' and cow-calf producers' behavior.

Retrospective survey of human hydatidosis in Bahir Dar, north-western Ethiopia.

To determine the prevalence of human hydatidosis in Bahir Dar and the sex and age distribution and awareness, we used a questionnaire and retrospective analysis covering 5 years of the case reports at public health institutions in Bahir Dartown and the surrounding areas. We interviewed 244 individuals: 150 households, 32 abattoir workers, 30 butchers and 32 health professionals. None of the householders or butchers knew about cystic echinococcosis, nor did 25% of the health professionals; however, 100% of the abattoir workers and 75% of the health professionals recognized it. The householders owned on average 3 livestock and 1 dog and during holidays, ceremonies and other feasts, the community mostly practised backyard slaughter. Case book analysis between January 2002 and December 2006 showed that out of the total of 36,402 patients admitted for ultrasound examination, 24 hydatidosis cases were registered, giving a mean annual incidence rate of approximately 2.3 cases per 100,000 per year. Ultrasound was the most frequently used diagnostic method.

Risk assessment to estimate the probability of a chicken flock infected with H5N1 highly pathogenic avian influenza virus reaching slaughter undetected.

Highly pathogenic avian influenza (HPAI) H5N1 is an infectious disease of fowl that can cause rapid and pervasive mortality resulting in complete flock loss. It has also been shown to cause death in humans. Although H5N1 HPAI virus (HPAIV) has not been identified in the United States, there are concerns about whether an infected flock could remain undetected long enough to pose a risk to consumers. This paper considers exposure from an Asian lineage H5N1 HPAIV-infected chicken flock given that no other flocks have been identified as H5N1 HPAIV positive (the index flock). A state-transition model is used to evaluate the probability of an infected flock remaining undetected until slaughter. This model describes three possible states within the flock: susceptible, infected, and dead, and the transition probabilities that predict movements between the possible states. Assuming a 20,000-bird house with 1 bird initially infected, the probability that an H5N1 HPAIV-infected flock would be detected before slaughter is approximately 94%. This is because H5N1 HPAIV spreads rapidly through a flock, and bird mortality quickly reaches high levels. It is assumed that approximately 2% or greater bird mortality due to H5N1 HPAIV would result in on-farm identification of the flock as infected. The only infected flock likely to reach slaughter undetected is one that was infected within approximately 3.5 days of shipment. In this situation, there is not enough time for high mortality to present. These results suggest that the probability of an infected undetected flock going to slaughter is low, yet such an event could occur if a flock is infected at the most opportune time.

Non-cancer mortality in supermarket meat workers.

OBJECTIVE: To report a non-cancer mortality update in supermarket meat workers. METHODS: Mortality of 10,383 supermarket meat workers was compared with that of a control group of non-meat workers and the US population from 1949 to 1989. RESULTS: Compared with both controls, male supermarket meat workers had an elevated relative risk for diabetes. There is also a suggestion of an increase in deaths from ischemic heart disease, other diseases of the kidney and ureter, alcoholism, and subarachnoid hemorrhage. Women had elevated relative risks for chronic bronchitis and ischemic heart disease, and possibly alcoholism. Numbers were too small to interpret the apparent increase in deaths from intracranial and intraspinal abcesses and acute nephritis in men and peritonitis in women. CONCLUSIONS: The role of occupational exposure to transmissible agents and exposure to fumes from the wrapping machine warrants further investigation.

Laceration injuries among workers at meat packing plants.

BACKGROUND: Employees in meat packing experience one of the highest occupational laceration injury rates in the US. METHOD: A retrospective study was conducted using OSHA 200 injury and illness logs and First Reports of Injury from two large US meat packing plants from 1998 to 2000. The total workers observed during the study period ranged between 2,449 and 2,682 per year. RESULTS: Laceration injury incidence rates in Plant 1 were 14.0 injuries per 200,000 person hours (per 100 workers per year) in 1998, 11.5 in 1999, and 8.3 in 2000, whereas in Plant 2 the overall incidence rate was 3.7 in 1998, 4.8 in 1999, and 3.0 in 2000. Laceration injury rates in Plant 2 were close to the expected OSHA recordable laceration injury rate in 1999 (3.0 per 100 workers per year), but Plant 1 was considerably higher. Plant 1 had a kill support department, and removed animal hides whereas Plant 2 did not. Handheld non-powered tools were the most common contact objects whereas the slaughter department had the highest number of injuries. Finger injuries from a handheld non-powered tool were the most frequent. CONCLUSIONS: Findings confirm the high rate of injury from laceration in this industry and indicate hazard varies across time into shift, task being performed, and type of tool being used.

Health hazards in the meatpacking industry.

Workers in the meatpacking industry face many health risks. Musculoskeletal disorders, including both acute injuries and repetitive strain injuries, are the most commonly reported problem in these workers. Other health hazards include infectious diseases, skin and respiratory disorders, and problems caused by environmental stressors such as cold, heat, noise, chemical exposures, explosions, fires, and work stress. Several studies are reviewed to show strengths and weaknesses of the purported association between work in the meatpacking industry and the development of cancer. Workplace programs designed to decrease health risks in this industry and governmental initiatives and legislation are discussed.

Protracted outbreak of Salmonella typhimurium definitive phage type 170 food poisoning related to tripe, 'pig bag', and chitterlings.

An outbreak of food poisoning around South Yorkshire due to Salmonella typhimurium definitive phage type 170 related to eating tripe, 'pig bag', and chitterlings was associated with a common supplier. Possible links with other suppliers were considered. Twenty-two cases occurred between 11 April 1995 and 7 May 1995. The situation was complicated by the complex distribution network of suppliers and by possible cross contamination in the retail outlets. This complexity created difficulties in the collection, recording and subsequent analysis of the data. The investigation of the outbreak and the subsequent control measures are discussed. Some of the difficulties of monitoring foodborne intestinal disease are highlighted.

[An evaluation of the scope of the circulation of Salmonella among the workers of commercial poultry- and meat-packing enterprises based on serological study data in the PHA test]. / Otsenka shiroty tsirkuliatsii sal'monell sredi rabotnikov predpriiatii promyshlennogo ptitsevodstva i miasokombinata po dannym serologicheskikh issledovanii v RPGA.

Serum samples from 641 workers of large poultry and meat-packing plants were studied in the passive hemagglutination test with the use of Salmonella complex and serogroup diagnostica. A specific increase in the level of anti-Salmonella antibodies in 60.7% of poultry plant workers and in 9.8% of meat-packing plant workers was established. Among the workers of the poultry plants the most pronounced immune shifts were detected in persons having contacts with sick poultry and pathological material and among the employees of the meat-packing plant, in those who ate raw sausage meat. A high level of antibodies in the professional groups under study was observed as early as in the first year of work at the plant and persisted over the whole period of this work. Under the conditions of constant contamination of the workers of poultry and meat-packing plants with small doses of salmonellae specific immunity to this infection was seemingly induced, which inhibited the development of the manifest forms of infection, but did not prevent the formation of chronic carrier state.