Prevalence and risk association for Trichinella infection in domestic pigs in the northeastern United States

Intrinsic and extrinsic factors related to pathogen infection in wild small mammals in intensive milk cattle and swine production systems

BACKGROUND

Understanding the ecological processes that are involved in the transmission of zoonotic pathogens by small mammals may aid adequate and effective management measures. Few attempts have been made to analyze the ecological aspects that influence pathogen infection in small mammals in livestock production systems. We describe the infection of small mammals with Leptospira spp., Brucella spp., Trichinella spp. and Cysticercus fasciolaris and assess the related intrinsic and extrinsic factors in livestock production systems in central Argentina at the small mammal community, population and individual levels.

METHODOLOGY/PRINCIPAL FINDINGS

Ten pig farms and eight dairy farms were studied by removal trapping of small mammals from 2008 to 2011. Each farm was sampled seasonally over the course of one year with cage and Sherman live traps. The 505 small mammals captured (14,359 trap-nights) included three introduced murine rodents, four native rodents and two opossums. Leptospira spp., anti-Brucella spp. antibodies and Trichinella spp. were found in the three murine rodents and both opossums. Rattus norvegicus was also infected with C. fasciolaris; Akodon azarae and Oligoryzomys flavescens with Leptospira spp.; anti-Brucella spp. antibodies were found in A. azarae. Two or more pathogens occurred simultaneously on 89% of the farms, and each pathogen was found on at least 50% of the farms. Pathogen infections increased with host abundance. Infection by Leptospira spp. also increased with precipitation and during warm seasons. The occurrence of anti-Brucella spp. antibodies was higher on dairy farms and during the winter and summer. The host abundances limit values, from which farms are expected to be free of the studied pathogens, are reported.

CONCLUSIONS/SIGNIFICANCE

Murine rodents maintain pathogens within farms, whereas other native species are likely dispersing pathogens among farms. Hence, we recommend preventing and controlling murines in farm dwellings and isolating farms from their surroundings to avoid contact with other wild mammals.

A study to demonstrate freedom from Trichinella spp. in domestic pigs in Switzerland

Trichinellosis is a food-borne zoonotic disease caused by the nematode Trichinella spp. Many omnivorous and carnivorous animal species can act as host for this parasite, including domestic pigs. To protect public health, it should be ensured that pork should not contain infective Trichinella larvae. Surveillance for Trichinella spp. can be done using direct (larval detection) and indirect (antibody detection) diagnostic techniques. The aim of this study was to demonstrate the absence of infection in Swiss domestic pigs. An ELISA was used as the initial screening test, and sera reacting in ELISA were further investigated using both a Western blot for serology and an artificial digestion test with 20 g of diaphragm tissue for larval detection. A total of 7412 adult pigs, 9973 finishing pigs and 2779 free-ranging pigs were tested. Samples from 17 (0.23%) adult pigs, 16 (0.16%) finishing pigs and nine (0.32%) free-ranging pigs were ELISA-positive, but all of these sera were subsequently negative by Western blot and by the artificial digestion method. Based on these findings, an absence of Trichinella infections in adult pigs (target prevalence 0.04%) and finishing pigs (target prevalence 0.03%) can be concluded. The results also demonstrated that the prevalence of Trichinella infections does not exceed 0.11% in free-ranging pigs, the group with the highest risk of exposure.

Quantitative risk assessment for the annual risk of exposure to Trichinella spiralis in imported chilled pork meat from New Zealand to Singapore

AIMS

To determine the annual likelihood of exposure to an infectious dose of Trichinella spiralis from consuming imported pork meat from New Zealand to Singapore.

METHODS

Input values specific for chilled pork meat imported into Singapore from New Zealand were used in a quantitative risk-assessment model. The model, designed to allow any combination of importing and exporting countries, was divided into two components, viz the release assessment, and the exposure assessment that assessed the annual risk of exposure to the consumer (ARC). The former estimated the likelihood that a contaminated fresh meat product from New Zealand would arrive at Singapore's border, and took into consideration the prevalence of disease on different types of farms. The latter determined the likelihood over a year that a person in Singapore would consume one or more servings of imported fresh meat from New Zealand that contained a burden of greater than or equal to one larva(e) of T. spiralis per gram after preparation for consumption.

RESULTS

The ARC for offal was 2.41 x 10(-7), which was below the pre-selected safety threshold of 1.00 x 10(-6). The ARC for lean meat was 2.39 x 10(-5), which was above the acceptable safety threshold.

CONCLUSIONS

The study demonstrated that continued routine testing at slaughter is unnecessary for pig offal produced commercially, and provided a model with which to further assess management of the risk of exposure to T. spiralis in lean meat.

CLINICAL RELEVANCE

The potential of Trichinella species to cause disease in humans is a public health concern, and has created adverse effects on the international trade of fresh lean meat without regard to the surveillance measures employed by particular pork-producing countries.

Serological detection of anti-Trichinella antibodies in wild foxes and experimentally infected farmed foxes in Norway

Trichinella surveillance in wildlife has relied on the detection of muscle larvae using digestion techniques. Serology has been proposed as more suitable for large-scale epidemiological studies in wildlife. In this study, 328 individual sera from wild red foxes and 16 sera from experimentally infected farmed foxes were serologically tested with both excretory/secretory antigen (E/S) and the synthetic beta-tyvelose glycan antigen, in indirect ELISA tests. The wild red foxes (Vulpes vulpes) had previously been examined for muscle larvae, using muscle digestion, whilst the experimentally infected farmed foxes were inoculated per os with either a low dose, 500 larvae, or a high dose, 10,000 of Trichinella nativa muscle larvae. Western blot (WB) was carried out on all seropositive samples using crude larval antigen. The present study found both beta-tyvelose and E/S antigen suited for the detection of antibodies to Trichinella spp., and T. nativa in particular, in foxes. Both ELISA antigens performed well, although, the E/S antigen was superior to the beta-tyvelose antigen, with sera that had been stored at -20 degrees C for more than 10 years. Neither antigen, however, detected all of the samples proven seropositive by WB: E/S detected 21 of the 27 wild red fox sera positive by WB; beta-tyvelose detected 22 positive sera; and in total 24 of the 27 positive WB sera were identified using both antigens. Serology alone, without WB or muscle digestion, led to a two- to threefold higher seroprevalence estimate, respectively. The use of E/S antigen in conjunction with the WB was the method of choice for the screening of wild red fox populations for Trichinella. Antibody persistence to T. nativa was short in the low dose group where antibody levels were not different from background by 32 wpi. In total, 7.3% (24/328) of the wild red fox population had antibodies to Trichinella on ELISA and WB. Antibodies were identified in foxes from a further two regions in Norway compared to the original muscle digestion results.

The effect of pest control on the incidence of trichinosis in Virovitica-Podravina County, Croatia

The aim of the study was to assess the role of pest control as a preventive measure in the control of trichinosis. The investigation was designed as a retrospective cohort study in the Virovitica-Podravina County. The incidence of trichinosis was determined according to the presence or absence of exposure to protective pest control measures. Differences in the rate of trichinosis infection were determined by calculating relative risk (RR), i.e., the ratio of absolute risk in the two groups. In this epidemiological study, data collected in the County area regularly covered by pest control (exposure area) were compared with other areas not covered by regular pest control measures (non-exposure area). The study included 132 subjects living in the County and locally infected with trichinosis. Patient data were obtained from their medical history, epidemiological survey and epidemiological questionnaire on the source (autochthony) of the disease. The study also included 133,028 domestic pigs, 875 wild animals (mostly wild boar) and 111 trapped rats. All samples were examined by the method of trichinoscopy at authorized veterinary institutions of the County. Trichinosis infection was recorded in 1012 of 79,534 domestic pigs in the area exposed to pest control measures and in 51 of 53,494 domestic pigs from the rest of the County. RR for the exposure area vs. non-exposure area was 13.35 (95% CI 10.08-17.68; p<0.05); according to particular areas, RR was 3.66 (95% CI 2.62-5.13) for Pitomaca, 22.47 (95% CI 13.27-38.08) for Slatina and 118.89 (95% CI 29.7-476.01) for Orahovica (p<0.05 all). Domestic pigs from the County areas exposed to regular pest control measures had a 13-fold likelihood of infection recorded in pigs from County areas where pest control measures were not or were only occasionally performed. Quite unexpectedly, these results revealed a failure of pest control to reduce the incidence of trichinosis. The spread of trichinosis obviously occurs by some routes obviating the impact of pest control.

Swine trichinella infection and geographic information system tools

Pastured pigs are vulnerable to Trichinella spiralis infection through exposure to wild reservoir hosts. To evaluate the potential impact of the expanding production of pork from pasture-raised pigs, we mapped locations of T. spiralis occurrence and pastured-pig farms in the United States. Twenty-eight farms were located within 50 km of previous infection.

Towards a risk-based surveillance for Trichinella spp. in Danish pig production

Increasing demands for cost-effectiveness in surveillance for human health hazards from animal origins can be met by introducing risk-based principles. This e.g. implies targeting subpopulations with higher risk of infection compared to the whole population. Furthermore, historical data from surveillance can be collated and used to assess future risk of infection. To demonstrate the effectiveness of combining these two approaches, we used a model called "Discounting historical evidence". It depends mainly on the annual risk of introduction (PIntro) and the surveillance system sensitivity (SSe) (ability to detect infection if present). The model implies simulations that reiterate for a number of years. For each year the output is updated with the confidence on absence in infection. Trichinella spiralis infection in pigs is used as an example. In Denmark, more than 20 million pigs are tested annually. Despite more than 70 years of testing no pigs have been found positive for Trichinella. Hence, PIntro is low. SSe can be estimated from the maximum number of infected carcasses expected under the specified design prevalence, and the sensitivity of the test applied. According to our assessment, the current prevalence of Trichinella in Danish pigs is less than one case per million, which we interpret as negligible risk. Based on this, a risk-based surveillance programme for Trichinella is designed that targets all out-door reared pigs as well as all sows and boars (current total 610,000 slaughtered annually). These subpopulations are judged to have higher risk of getting Trichinella. Again, SSe and PIntro are estimated and the model results show that risk-based surveillance can be applied without jeopardizing human health. Finally, we incorporate wildlife surveys and test quality assurance in the programme. The results of the simulation model were included in an application to the European Commission concerning Denmark's status as a region with negligible risk of Trichinella. In July 2007, the European Commission granted status as "negligible risk" to Danish pigs and pork.

Orientation of a monoclonal antibody adsorbed at the solid/solution interface: a combined study using atomic force microscopy and neutron reflectivity

Conformational orientations of a mouse monoclonal antibody to the beta unit of human chorionic gonadotrophin (anti-beta-hCG) at the hydrophilic silicon oxide/water interface were investigated using atomic force microscopy (AFM) and neutron reflectivity (NR). The surface structural characterization was conducted with the antibody concentration in solution ranging from 2 to 50 mg.L(-1) with the ionic strength kept at 20 mM and pH = 7.0. It was found that the antibody adopted a predominantly "flat-on" orientation, with the Fc and two Fab fragments lying flat on the surface. The AFM measurement revealed a thickness of 30-33 A of the layer formed in contact with 2 mg.L(-1) antibody in water, but, interestingly, the flat-on antibody molecules formed small nonuniform clusters equivalent to 2-15 antibody molecules. Parallel AFM scanning in air revealed even larger surface clusters, suggesting that surface drying induced further aggregation. The AFM study thus demonstrated that the interaction between protein and the hydrophilic surface is weak and indicated that surface aggregation can be driven by the attraction between neighboring protein molecules. NR measurements at the solid/water interface confirmed the flat-on layer orientation of adsorbed molecules over the entire concentration range studied. Thus, at 2 mg.L(-1), the adsorbed antibody layer was well represented by a uniform layer with a thickness of 40 A. This value is thicker than the 30-33 A observed from AFM, suggesting possible layer compression caused by the tip tapping. An increase in the antibody concentration to 10 mg.L(-1) led to increasing surface adsorption. The corresponding layer structure was well represented by a three-layer model consisting of an inner sublayer of 10 A, a middle sublayer of 30 A, and an outer sublayer of 25 A, with the protein volume fractions in each sublayer being 0.22, 0.42, and 0.10, respectively. The structural transition can be interpreted as a twisting and tilting of segments of the adsorbed molecules, driven by an electrostatic repulsion between them that increases with the surface packing density. Hindrance of antigen access to antibody binding sites, resulting from the change in surface packing, can account for the decrease in antigen binding capacity (AgBC) with increasing surface density of the antibody that is observed.

Absence of Trichinella infection in adult pigs slaughtered in Palmas, State of Parana (Brazil), detected by modified artificial digestion assay

Samples of 2,490 adult pigs, slaughtered under federal inspection between May 2004 and February 2005 in the county of Palmas, State of Paraná, Brazil, were examined by pooled sample artificial digestion with magnetic stirrer assay for a survey of Trichinella spp. larvae. Animals originated from 53 counties in three states of southern Brazil. Test sensitivity was increased with modifications of the European standard for artificial digestion. In this survey, a 5-g sample of tongue and 5-g sample of diaphragm pillar were collected from each pig into a pool of 100 g (up to 10 animals for each assay). A 355-microm mesh sieve was used, but no larvae were detected in the pigs, indicating that trichinellosis does not occur in the examined stocks.

Trichinae certification in the United States pork industry

Control of Trichinella infection in U.S. pork has traditionally been accomplished by inspection of individual carcasses at slaughter or by post-slaughter processing to inactivate parasites. We propose that an alternative to individual carcass testing or processing can be used when pigs are raised in production systems where risk of exposure to Trichinella spiralis has been mitigated. Declines in prevalence of this parasite in U.S. domestic swine during the last 30 years, coupled with improvements in pork production systems, now allow Trichinella control to be shifted to the farm through implementation of specific pork production practices. Knowledge of risk factors for exposure of swine to T. spiralis was used to develop an objective audit of risk that can be applied to pork production sites. In a pilot study, 461 production site audits were performed by trained veterinary practitioners. The on-farm audit included aspects of farm management, bio-security, feed and feed storage, rodent control programs and general hygiene. Of the 461 production site audits, 450 audits (97.6%) indicated compliance with the required good production practices. These sites are eligible for certification under the U.S. Trichinae Certification Program and will be audited regularly to maintain that status. The described trichinae certification mechanism will establish a process for ensuring the Trichinella safety of swine, and ultimately food products derived from swine, at the production level.

Trichinella pseudospiralis from a wild pig in Texas

In December 2001, the routine inspection of a wild boar intended for human consumption revealed the presence of Trichinella ssp. larvae. Biological, morphological and genetic analyses demonstrated the parasite to be Trichinella pseudospiralis. This is the second report of T. pseudospiralis in the United States and the first report of the parasite in a food animal species in the U.S.

Epidemiology of Trichinella infection in the horse: the risk from animal product feeding practices

A discovery in 2002 of a Trichinella spiralis-infected horse in Serbia offered an opportunity to conduct needed epidemiological studies on how horses, considered herbivores, acquire a meat-borne parasite. This enigma has persisted since the first human outbreaks from infected horse meat occurred in then 1970s. The trace back of the infected horse to a farm owner was carried out. Interviews and investigations on the farm led to the conclusion that the owner had fed the horse food waste in order to condition the horse prior to sale. Further investigations were then carried out to determine the frequency of such practices among horse owners. Based on interviews of horse producers at local horse markets, it was revealed that the feeding of animal products to horses was a common practice. Further, it was alleged that many horses, particularly those in poor nutritional condition would readily consume meat. A subsequent series of trials involving the experimental feeding of 219 horses demonstrated that 32% would consume meat patties. To confirm that horses would eat infected meat under normal farm conditions, three horses were offered infected ground pork balls containing 1100 larvae. All three became infected, and at necropsy at 32 weeks later, were still positive by indirect IFA testing, but not by ELISA using an excretory-secretory (ES) antigen. This result indicates that further study is needed on the nature of the antigen(s) used for potential serological monitoring and surveillance of horse trichinellosis, especially the importance of antigenic diversity. The experimentally-infected horses also had very low infection levels (larvae per gram of muscle) at 32 weeks of infection, and although the public health consequences are unknown, the question of whether current recommended inspection procedures based on pepsin digestion of selected muscle samples require sufficient quantities of muscle should be addressed. It is concluded that horses are more willing to consume meat than realized and that the intentional feeding of animal products and kitchen waste is a common occurrence among horse owners in Serbia (and elsewhere?). This is a high risk practice which demands closer scrutiny by veterinary and food safety authorities, including the implementation of rules and procedures to ensure that such feeds are rendered safe for horses, as is now required for feeding to swine.

Persistence of Trichinella spiralis in rat carcasses experimentally mixed in different feed

Trichinella spiralis infected rat carcasses were incubated for 6 weeks in several animal feeds to assess how long Trichinella can present a risk for an outbreak in contaminated feeds. In groups of 6, 24 infected target rats were placed in silage, grained barley, propionic acid-preserved feed, and also into simulated pasture conditions. Test environments were sampled after one-, 2-, 4-, and 6-week-incubations. Trichinella larvae were recovered by digestion, and their infectivity was evaluated in rats. A two-week incubation reduced the number of recovered larvae, but still after 6 weeks low numbers were isolated from all feeds except from the experimental group simulating pasture conditions. After 2 weeks storage, the larvae were infective in all storage environments. However, up to 4 weeks, they survived only in the propionic acid-fermented feed and there in small numbers with reduced reproductive capability. This indicates the possibility of farm animals to get infection from rats or other infected material being hazardously mixed with hay or other feed. If silage is stored for at least one month before use, however, the risk from this forage appears to be minimized.

Hand rash in trichinosis

Trichinosis patients may develop peri-orbital oedema, conjunctival haemorrhages, splinter haemorrhages of the fingernails, and nonspecific skin rashes. Here, we describe an unusual hand rash noted in several patients enrolled in a treatment study for trichinosis.

An outbreak of trichinellosis in farmed wild boar in Finland

Nine farmed wild boar out of 25 slaughtered from a single farm were condemned at meat inspection because of trichinellosis. With RAPD-PCR, Trichinella spiralis was identified in all positive wild boar. Out of the available serum samples (n=7), all wild boar which had failed the meat inspection showed seroconversion in ELISA and Western blotting, as did one additional animal which had passed the inspection. The animals became infected during an invasion of rats from an improperly closed dump near the farm. Unfortunately, by the time trichinellosis was discovered in the wild boar, the invasion had already been brought under control; thus, no samples from rats were available. However, having lived through the rat invasion was shown to be a risk factor for trichinellosis in wild boar (relative risk, RR=6.3). In wildlife samples from surrounding areas, sylvatic trichinellosis was found to be very common (74%; n= 19 red foxes). Intriguingly, the prevalent species in trichinella-positive foxes differed from that in wild boar, Trichinella nativa and T. spiralis being found in 12 foxes and in one fox, respectively.

Adsorption of Immunoglobulin G on Core-Shell Latex Particles Precoated with Chaps

The aim of this work is to investigate the adsorption behavior of a monoclonal antibody (immunoglobulin G, IgG) on latex particles, possessing reactive chloromethyl groups, precoated with 3-([3-cholamidopropyl]dimethylammonio-1-propanesulfonate (Chaps). The amount and reactivity of the surface chloromethyl groups were monitored by the nucleophilic attack of glycinate to the functional groups as a function of time at 22 and 36 degrees C. The extent of displacement of Chaps by IgG and the enthalpy of the process were determined under two different conditions of precoating the latex particles with Chaps, at 22 and 36 degrees C. The adsorption of IgG takes place in two steps; the first one involves physical interaction between IgG and the surface. This step is relatively fast (in the range of minutes) and independent of temperature. In the second step covalent bonding between the protein and the active surface groups occurs. This reaction is improved by raising the temperature because Chaps desorption, which exposes the reactive chloromethyl groups on the latex particles, is kinetically and thermodynamically favored at 36 degrees C and the covalent bonding of IgG is faster at 36 degrees C. Copyright 2000 Academic Press.

Trichinellosis: the zoonosis that won't go quietly

Trichinellosis, is normally not included among those regarded as emerging zoonoses because it has been a public health threat for more than 150 years. However, its dramatic re-emergence in many areas around the world over the past 10-20 years, inspite of a century of veterinary public health efforts to control and eradicate it, justifies it being included in this group. The reasons for this re-emergence are diverse, and include human pertubation and manipulation of ecosystems, war and political turmoil, rapidly changing food distribution and marketing systems, and even, surprisingly, rising affluence in developing countries. These influences, and their impact on the epidemiology of both domestic and sylvatic trichinellosis, are discussed, along with recommendations for confronting this altered status as a public health threat.

Prevalence of Toxoplasma gondii infection in domestic pigs in the New England states

To determine regional prevalence of infection with Toxoplasma gondii, 1897 pigs from 85 farms in five New England states were tested using a modified direct agglutination test. Sera were diluted 1:25 and a titer at this dilution was indicative of T. gondii infection. Farm management questionnaires were completed at the time of blood collection and were used to develop descriptive statistics on farms tested and to determine measures of association for risk factors for the presence of T. gondii-seropositive pigs. A total of 900 seropositive pigs were identified for a prevalence rate of 47.4%. Of 85 herds tested, 77 had at least one positive pig for a herd prevalence rate of 90.6%. Within herd prevalence ranged from 4% to 100% (mean = 48.4%). All farms studied had one or more risk factors for exposure to T. gondii. However, statistical associations with individual risks could not be made, most likely due to the extremely high prevalence. The results obtained here suggest that education on farm management practices to reduce exposure to T. gondii should be targeted to include small producers.

Seroprevalence of Trichinella infection in domestic swine based on the National Animal Health Monitoring System's 1990 and 1995 swine surveys

Swine sera collected by the US Department of Agriculture's Center for Animal Health Monitoring during 1990 and 1995 was tested for antibodies to Trichinella spiralis using an enzyme immunoassay. From a total of 3048 sera collected from lactating sows in 1990, five sera tested positive for a prevalence of 0.16%. From a total of 7987 sera collected from both finishing pigs and gestating sows in 1995, one serum was positive for a prevalence of 0.013%. Responses to questionnaires administered at the time of serum collection showed that seropositive farms had management variables consistent with known risk factors for exposure to trichinae.