Experimental trichinellosis in horses: biological and parasitological evaluation

Experimental infection of tigerfish (Hydrocynus vittatus) and African sharp tooth catfish (Clarias gariepinus) with Trichinella zimbabwensis

Trichinella zimbabwensis naturally infects a variety of reptilian and wild mammalian hosts in South Africa. Attempts have been made to experimentally infect piranha fish with T. zimbabwensis and T. papuae without success. Tigerfish (Hydrocynus vittatus) and African sharp tooth catfish (Clarias gariepinus) are accomplished predators cohabiting with Nile crocodiles (Crocodylus niloticus) and Nile monitor lizards (Varanus niloticus) in southern Africa and are natural hosts of T. zimbabwensis. To assess the infectivity of T. zimbabwensis to these two hosts, 24 African sharp tooth catfish (mean live weight 581.75 ± 249.71 g) randomly divided into 5 groups were experimentally infected with 1.0 ± 0.34 T. zimbabwensis larvae per gram (lpg) of fish. Forty-one tigerfish (mean live weight 298.6 ± 99.3 g) were randomly divided for three separate trials. An additional 7 tigerfish were assessed for the presence of natural infection as controls. Results showed no adult worms or larvae of T. zimbabwensis in the gastrointestinal tract and body cavities of catfish sacrificed at day 1, 2 and 7 post-infection (p.i.). Two tigerfish from one experimental group yielded 0.1 lpg and 0.02 lpg of muscle tissue at day 26 p.i. and 28 p.i., respectively. No adult worms or larvae were detected in the fish from the remaining groups sacrificed at day 7, 21, 28, 33 and 35 p.i. and from the control group. Results from this study suggest that tigerfish could sustain T. zimbabwensis under specific yet unknown circumstances.

Epidemiology and hypothetical transmission cycles of Trichinella infections in the Greater Kruger National Park of South Africa: an example of host-parasite interactions in an environment with minimal human interactions

Knowledge on the epidemiology, host range and transmission of Trichinella spp. infections in different ecological zones in southern Africa including areas of wildlife-human interface is limited. The majority of reports on Trichinella infections in sub-Saharan Africa were from wildlife resident in protected areas. Elucidation of the epidemiology of the infections and the prediction of hosts involved in the sylvatic cycles within specific ecological niches is critical. Of recent, there have been reports of Trichinella infections in several wildlife species within the Greater Kruger National Park (GKNP) of South Africa, which has prompted the revision and update of published hypothetical transmission cycles including the hypothetical options based previously on the biology and feeding behaviour of wildlife hosts confined to the GKNP. Using data gathered from surveillance studies and reports spanning the period 1964-2019, confirmed transmission cycles and revised hypothesized transmission cycles of three known Trichinella species (T. zimbabwensis, Trichinella T8 and T. nelsoni) are presented. These were formulated based on the epidemiological factors, feeding habits of hosts and prevalence data gathered from the GKNP. We presume that the formulated sylvatic cycles may be extrapolated to similar national parks and wildlife protected areas in sub-Saharan Africa where the same host and parasite species are known to occur. The anecdotal nature of some of the presented data confirms the need for more intense epidemiological surveillance in national parks and wildlife protected areas in the rest of sub-Saharan Africa to unravel the epidemiology of Trichinella infections in these unique and diverse protected landscapes.

Detection of natural Trichinella murrelli and Trichinella spiralis infections in horses by routine post-slaughter food safety testing

Trichinella spiralis typically infects domestic swine, wild boar and occasionally horses, has a cosmopolitan distribution, and consequently is most frequently associated with food-borne outbreaks of trichinellosis in humans. Trichinella murrelli is typically found in wild carnivores in temperate areas of North America, where it has been responsible for outbreaks of human trichinellosis due to consumption of infected wild game. There has previously been only indirect evidence of natural infection with T. murrelli in a horse originating from Connecticut and implicated in an outbreak of trichinellosis in France in 1985. We describe a T. murrelli infection detected during routine testing of a horse from the USA imported to Canada for slaughter and export to the European Union (EU). Approximately 5 or more larvae per gram were recovered from digested tongue and diaphragm samples and identified as T. murrelli by PCR. This case provides the first direct evidence of naturally acquired T. murrelli infection in a horse, and further supports the potential food safety risk posed by this parasite species. It is the first instance in Canada of the detection of a Trichinella-infected horse via routine post-mortem testing. Trichinella spiralis-infected horses have been similarly detected by regulatory testing in France, and further details of two such previously reported cases are also provided here. The cases described herein underscore the importance of continued vigilance in quality assured food safety testing of horse meat to mitigate the risk of human trichinellosis.

Trichinella spp. imported with live animals and meat

Nematodes of the genus Trichinella are widely distributed throughout the world in omnivorous and carnivorous animals (mammals, birds, and reptiles) and in incidental hosts. To prevent the transmission of these zoonotic parasites to humans, meat samples from Trichinella spp. susceptible animals are tested at the slaughterhouse or in game processing plants. The aim of the present review was to collect documented cases on Trichinella infected animals, meat, or meat derived products which reached the international trade or were illegally introduced from one to another country in personal baggage. In the course of the last 60 years in the international literature, there have been 43 reports of importation of Trichinella spp. infected animals or meat, most of which (60%, 26/43) related to live horses or their meat. Meat or meat derived products from pigs, wild boar and bears, account only for 18.6% (8/43), 4.7% (3/43), and 14.3% (6/43), respectively. However, only live horses or their meat intended for human consumption, meat from a single wild boar, and live polar bears caught in the wild for zoos, were imported through the international market; whereas, meat from pigs, wild boars and bears were illegally introduced in a country in personal baggage. Trichinella infected animals or meat which were officially or illegally introduced in a country were the source of 3443 Trichinella infections in humans in a 40-year period (1975-2014). Most of these infections (96.8%) have been linked to horsemeat consumption, whereas meat from pigs, wild boars and bears accounted only for 2.2%, 0.7% and 0.3% of cases, respectively. This review shows the Trichinella spp. risk in the international animal and meat trade has been linked mainly to horses and only one time to wild boar, if they carcasses are not adequately tested, whereas pigs and other wild animals or their derived products infected with Trichinella spp. are unlikely to reach the international market by the official animal and meat trade.

Assessment of selected biochemical parameters and humoral immune response of Nile crocodiles (Crocodylus niloticus) experimentally infected with Trichinella zimbabwensis

Fifteen crocodiles were randomly divided into three groups of five animals. They represented high-infection, medium-infection and low-infection groups of 642 larvae/kg, 414 larvae/kg and 134 larvae/kg bodyweight, respectively. The parameters assessed were blood glucose, creatine phosphokinase (CPK), lactate dehydrogenase (LDH), aspartate transaminase (AST) and alanine transaminase (ALT). The humoral immune response to Trichinella zimbabwensis infection was evaluated in all three groups by an indirect ELISA method. The results showed deviations from normal parameters of blood glucose, CPK, LDH, AST and ALT when compared with reported levels in uninfected reptiles. Contrary to studies involving mammals, hypoglycaemia was not observed in the infected groups in this study. Peak values of blood glucose were reached on post-infection (PI) Day 49, Day 42 and Day 35 in the high-infection, medium-infection and low-infection groups, respectively. Peak values of LDH and AST were observed on PI Day 56, Day 49 and Day 42 in the high-infection, medium-infection and low-infection groups, respectively. Peak values of CPK were observed on Day 35 PI in all three groups. Peak ALT values were reached on Day 56 in the high-infection group and on Day 28 PI in both the medium-infection and low-infection groups. No correlations between the biochemical parameters and infection intensity were observed. Peak antibody titres were reached on Day 49 PI in the medium-infection group, and on Day 42 PI in both the high-infection and low-infection groups. Infection intensity could not be correlated with the magnitude of the humoral immune response or time to sero-conversion. Results from this study were in agreement with results reported in mammals infected with other Trichinella species and showed that antibody titres could not be detected indefinitely.

Distribution patterns and predilection muscles of Trichinella zimbabwensis larvae in experimentally infected Nile crocodiles (Crocodylus niloticus Laurenti)

No controlled studies have been conducted to determine the predilection muscles of Trichinella zimbabwensis larvae in Nile crocodiles (Crocodylus niloticus) or the influence of infection intensity on the distribution of the larvae in crocodiles. The distribution of larvae in muscles of naturally infected Nile crocodiles and experimentally infected caimans (Caiman crocodilus) and varans (Varanus exanthematicus) have been reported in literature. To determine the distribution patterns of T. zimbabwensis larvae and predilection muscles, 15 crocodiles were randomly divided into three cohorts of five animals each, representing high infection (642 larvae/kg of bodyweight average), medium infection (414 larvae/kg of bodyweight average) and low infection (134 larvae/kg of bodyweight average) cohorts. In the high infection cohort, high percentages of larvae were observed in the triceps muscles (26%) and hind limb muscles (13%). In the medium infection cohort, high percentages of larvae were found in the triceps muscles (50%), sternomastoid (18%) and hind limb muscles (13%). In the low infection cohort, larvae were mainly found in the intercostal muscles (36%), longissimus complex (27%), forelimb muscles (20%) and hind limb muscles (10%). Predilection muscles in the high and medium infection cohorts were similar to those reported in naturally infected crocodiles despite changes in infection intensity. The high infection cohort had significantly higher numbers of larvae in the sternomastoid, triceps, intercostal, longissimus complex, external tibial flexor, longissimus caudalis and caudal femoral muscles (p < 0.05) compared with the medium infection cohort. In comparison with the low infection cohort, the high infection cohort harboured significantly higher numbers of larvae in all muscles (p < 0.05) except for the tongue. The high infection cohort harboured significantly higher numbers of larvae (p < 0.05) in the sternomastoid, triceps, intercostal, longissimus complex, external tibial flexor, longissimus caudalis and caudal femoral muscles compared with naturally infected crocodiles. Results from this study show that, in Nile crocodiles, larvae of T. zimbabwensis appear first to invade predilection muscles closest to their release site in the small intestine before occupying those muscles situated further away. The recommendation for the use of masseter, pterygoid and intercostal muscles as sampling sites for the detection of T. zimbabwensis in crocodiles is in contrast to the results from this study, where the fore- and hind limb muscles had the highest number of larvae. This study also supports the use of biopsy sampling from the dorso-lateral regions of the tail for surveillance purposes in both wild and commercial crocodile populations.

Clinical, haematological, biochemical and economic impacts of Trichinella spiralis infection in pigs

The purpose of this work was to assess the clinical, haematological and biochemical responses of pigs experimentally inoculated with Trichinella spiralis. Groups of three pigs were inoculated per os with 100, 500 and 5000 T. spiralis muscle larvae, two pigs were used as control. Clinical evaluation of disease in pigs included daily examination, rectal temperature measurements and cardiac and respiration rates. Haematological studies included: hematocrit (%), hemoglobin (g/dl), and white cell, neutrophil, lymphocyte and eosinophil counts. Blood biochemistry included: bun (mg/dl), creatinine (mg/dl), AST (UI/l), ALT (UI/l), CPK (UI/l) and ALP (UI/l). No significant differences were observed in rectal temperature and in cardiac and respiration rates between inoculated animals and the control group (p> or =0.05). Significant differences were detected (p< or =0.05) in the values of % hemoglobin, and eosinophils, as well as in the values of CK, ALP, AST and ALT. The variations observed in some cases were related to the number of T. spiralis larvae inoculated and varied with the number of days post-infection. Inoculated pigs showed significant differences (p< or =0.05) in weight gain when compared with uninoculated controls. This study has clinical, haematological, and enzyme alterations in Trichinella infected pigs provides a better understanding of acute and chronic trichinellosis in pigs.

Larval viability and serological response in horses with long-term Trichinella spiralis infection

The horse is considered an aberrant host for the nematode parasite Trichinella spiralis, and many aspects of the biology and epidemiology of Trichinella infection in the horse are poorly understood. It has been reported that experimentally-infected horses produce a transient serological response to infection and that muscle larvae are cleared more rapidly than in parasite-adapted hosts such as the pig and humans. However, limited numbers of animals have been studied, and both the longevity of larvae in horse musculature and the immune response to Trichinella larvae remain unclear. In this study, we infected 35 horses with 1000, 5000, or 10,000 T. spiralis muscle larvae and followed the course of infection for 1 year, assessing larval burdens in selected muscles, the condition and infectivity of recovered larvae, and the serological response of infected horses. The results demonstrated that T. spiralis establishes infection in horses in a dose dependent manner. Anti-Trichinella IgG antibodies peaked between weeks 6-10 post-inoculation. Viable, infective larvae persisted in horse musculature for the duration of the study (12 months), and exhibited no apparent reduction in muscle burdens over this period. Encapsulated larvae showed no obvious signs of degeneration in histological sections. Larval capsules were surrounded by infiltrates consisting of mature plasma cells and eosinophils. Macrophages were notably absent. Given the lack of a detectable serological response by 26 weeks p.i. and the persistence of infective muscle larvae for at least 1 year, parasite recovery methods are currently the only suitable detection assays for both meat inspection and epidemiological studies of Trichinella infection in the horse.

Viability and infectivity of Trichinella spiralis muscle larvae in frozen horse tissue

Many aspects of the biology and epidemiology of Trichinella infection in the horse are poorly understood, including survival of Trichinella spp in horse muscle. In this study, we have assessed the freeze tolerance of T. spiralis in horse meat stored at 5, -5, and -18 degrees C for 1 day to 24 weeks. Results demonstrate a steady reduction in the number of live ML recovered from the cold stored meat samples. On Day 1, recovery of live larvae had been reduced by 18.6%, 50.1%, and 37.2%, and by 4 weeks, recovery of larvae had been reduced by 65.4%, 66.5%, and 96.2% in samples stored at 5, -5, and -18 degrees C, respectively. Infectivity results (measured as reproductive capacity index (RCI)) from mice inoculated with larvae recovered from non-frozen meat samples at day 0 was 23.5. Following storage at -18 degrees C for one and two days, the RCIs were 2.09 and 0.99, respectively. Small numbers of infective larvae were still present in meat samples stored at -18 degrees C for 4 weeks. The RCI of ML recovered from meat samples stored at -5 degrees C was 14.99 and 6.36 at 2 weeks and 4 weeks respectively; the RCI of samples stored at 5 degrees C was 23.1 at 8 weeks, and fell rapidly thereafter (12 week RCI 1.33; 0 at 24 weeks). These data demonstrate that infective T. spiralis, a non-freeze tolerant species, can survive for at least 4 weeks in horse tissue frozen at -5 or -18 degrees C, and that the numbers of infective larvae decrease substantially by day 2 at -18 degrees C and by week 4 at -5 degrees C.

Anti-Trichinella antibodies detected in chronically infected horses by IFA and Western blot, but not by ELISA

In the Balkan countries, where trichinellosis is a re-emerging zoonosis, it is of great importance to determine Trichinella infection prevalence among the major hosts, including horses. One method for monitoring prevalence is serological surveillance; however, the validity of serological methods in horses is not well understood. The dynamics of anti-Trichinella IgG production and circulating excretory/secretory (ES) antigens were investigated in three horses experimentally-infected with Trichinella spiralis. Horses were slaughtered at 32 week post infection (p.i.). Low worm burdens were found in all three animals. Anti-Trichinella IgG was detected up to 32 weeks p.i. by an indirect immunofluorescence assay (IFA) and by Western blot (Wb), but not by ELISA. The ELISA test detected antibodies for only a short period of time (up to 18 weeks p.i. using ES antigen or up to 20 weeks p.i. using tyvelose-BSA antigen). The presence of circulating muscle larvae ES antigen in sera of infected horses was observed by dot blot from the 4th week p.i. up to the 32nd week p.i.

The broad spectrum of Trichinella hosts: from cold- to warm-blooded animals

In recent years, studies on Trichinella have shown that the host range is wider than previously believed and new Trichinella species and genotypes have been described. Three classes of vertebrates are known to act as hosts, mammals, birds and reptiles, and infected vertebrates have been detected on all continents but Antarctica. Mammals represent the most important hosts and all Trichinella species are able to develop in this vertebrate class. Natural infections with Trichinella have been described in more than 150 mammalian species belonging to 12 orders (i.e., Marsupialia, Insectivora, Edentata, Chiroptera, Lagomorpha, Rodentia, Cetacea, Carnivora, Perissodactyla, Artiodactyla, Tylopoda and Primates). The epidemiology of the infection greatly varies by species relative to characteristics, such as diet, life span, distribution, behaviour, and relationships with humans. The non-encapsulated species Trichinella pseudospiralis, detected in both mammals (14 species) and birds (13 species), shows a cosmopolitan distribution with three distinguishable populations in the Palearctic, Nearctic and Australian regions. Two additional non-encapsulated species, Trichinella papuae, detected in wild pigs and saltwater crocodiles of Papua New Guinea, and Trichinella zimbabwensis, detected in farmed Nile crocodiles of Zimbabwe, can complete their life cycle in both mammals and reptiles. To the best of our knowledge, T. papuae and T. zimbabwensis are the only two parasites known to complete their entire life cycle independently of whether the host is warm-blooded or cold-blooded. This suggests that these two Trichinella species are capable of activating different physiological mechanisms, according to the specific vertebrate class hosting them.

Muscle distribution of sylvatic and domestic Trichinella larvae in production animals and wildlife

Only a few studies have compared the muscle distribution of the different Trichinella genotypes. In this study, data were obtained from a series of experimental infections in pigs, wild boars, foxes and horses, with the aim of evaluating the predilection sites of nine well-defined genotypes of Trichinella. Necropsy was performed at 5, 10, 20 and 40 weeks post inoculation. From all host species, corresponding muscles/muscle groups were examined by artificial digestion. In foxes where all Trichinella species established in high numbers, the encapsulating species were found primarily in the tongue, extremities and diaphragm, whereas the non-encapsulating species were found primarily in the diaphragm. In pigs and wild boars, only Trichinella spiralis, Trichinella pseudospiralis and Trichinella nelsoni showed extended persistency of muscle larvae (ML), but for all genotypes the tongue and the diaphragm were found to be predilection sites. This tendency was most obvious in light infections. In the horses, T. spiralis, Trichinella britovi, and T. pseudospiralis all established at high levels with predilection sites in the tongue, the masseter and the diaphragm. For all host species, high ML burdens appeared to be more evenly distributed with less obvious predilection than in light infections; predilection site muscles harbored a relatively higher percent of the larval burden in light infections than in heavy infections. This probably reflects increasing occupation of available muscle fibers as larger numbers of worms accumulate. Predilection sites appear to be influenced primarily by host species and secondarily by the age and level of infection.

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.

Evaluation of ELISA and Western Blot Analysis using three antigens to detect anti-Trichinella IgG in horses

We assessed a serological method for detecting Trichinella infection in horses, specifically, an ELISA using three antigens to detect anti-Trichinella IgG (i.e. a synthetic tyvelose glycan-BSA (stg-BSA) antigen, an excretory/secretory (ES) antigen, and a crude worm extract (CWE) antigen). Serum samples were collected from 2502 horses (433 live horses from Romania and 2069 horses slaughtered in Italy and originating from Italy, Poland, Romania, and Serbia). Serum samples were also taken from horses experimentally infected with different doses of T. spiralis and T. murrelli larvae, as controls. The cut-off value of ELISA was determined on serum samples from 330 horses from Trichinella-free regions of Italy, which were also examined by artificial digestion of preferential-muscle samples. In the experimentally infected horses, the stg-BSA and ES antigens were less sensitive than the CWE antigen. Trichinella spiralis showed a higher immunogenicity than T. murrelli, and the IgG immunoresponse was dose-dependent. The kinetics of anti-Trichinella IgG were similar among all experimentally infected horses. No circulating antibodies were detected 4-5 months after experimental infection, although these horses still harbored infective larvae. Depending on the antigen used, for 4-7 of the 330 horses from Trichinella-free areas, the optical density (OD) of the serum sample was higher than the cut-off value, yet these samples were negative when subjected to Western Blot. Similar results were obtained for the 1739 horses slaughtered in Italy (originating from Italy, Poland, Romania, and Serbia) and the 433 live Romanian horses. Of the 4 horses with muscle larvae, only one was positive by ELISA and Western Blot. Because the anti-Trichinella IgG remain circulating for only a short period of time, whereas the larvae remain infective for longer periods, serology cannot be used for either diagnosing Trichinella infection in horses or estimating the prevalence of infection. Artificial digestion of at least 5 g of preferential-muscle tissue continues to be the method of choice at the slaughterhouse for preventing equine-borne trichinellosis in humans.

New patterns of Trichinella infection

Human and animal trichinellosis should be considered as both an emerging and reemerging disease. The reemergence of the domestic cycle has been due to an increased prevalence of Trichinella spiralis, which has been primarily related to a breakdown of government veterinary services and state farms (e.g., in countries of the former USSR, Bulgaria, Romania), economic problems and war (e.g., in countries of the former Yugoslavia), resulting in a sharp increase in the occurrence of this infection in swine herds in the 1990s, with a prevalence of up to 50% in villages in Byelorussia, Croatia, Latvia, Lithuania, Romania, Russia, Serbia, and the Ukraine, among other countries. The prevalence has also increased following an increase in the number of small farms (Argentina, China, Mexico, etc.) and due to the general belief that trichinellosis was a problem only until the 1960s. The sylvatic cycle has been studied in depth at both the epidemiological and biological level, showing the existence of different etiological agents (Trichinella nativa, Trichinella britovi, Trichinella murrelli, Trichinella nelsoni) in different regions and the existence of "new" transmission patterns. Furthermore, the role of game animals as a source of infection for humans has greatly increased both in developed and developing countries (Bulgaria, Canada, Lithuania, some EU countries, Russia, USA, etc.). The new emerging patterns are related to non-encapsulated species of Trichinella (Trichinella pseudospiralis, Trichinella papuae, Trichinella sp.), infecting a wide spectrum of hosts (humans, mammals including marsupials, birds and crocodiles) and to encapsulated species (T. spiralis, T. britovi, and T. murrelli) infecting herbivores (mainly horses). The existence of non-encapsulated species infecting mammals, birds and crocodiles had probably remained unknown because of the difficulties in detecting larvae in muscle tissues and for the lack of knowledge on the role of birds and crocodiles as a reservoir of Trichinella. On the other hand, it is not known whether horse and crocodile infections existed in the past, and their occurrence has been related to improper human behavior in breeding. The problem of horse-meat trichinellosis is restricted to France and Italy, the only two countries where horse-meat is eaten raw, whereas mutton and beef have been found to be infected with Trichinella sp. only in China.

Detection of Trichinella infection in food animals

The first part of this review article deals with classical methods used for the detection of Trichinella larvae in muscle samples of those animal species which are recognized as traditional sources of trichinellosis for human beings, as well as those species which are important for epidemiological reasons. Special consideration is given to the main applications of these methods (routine slaughter inspection, and epidemiological studies in reservoir animals), and to the major factors that may influence detection methods (sampling site, sample size). Historical, current and future aspects concerning national and EU legislation for Trichinella inspection are also presented. The latter part of this review is directed at serodiagnostic methods for the detection of Trichinella-specific antibodies in different animal species. Classical methods of serodiagnosis such as the complement fixation test and immunofluorescence antibody test are reviewed and the characteristics and performance of the ELISA are discussed. Factors dependent upon the animal species being tested or on components of the ELISA test system are considered. This paper also reviews systematic development of the ELISA in relation to improvements in test specificity and sensitivity. Additionally, remarks are made on implementing this test for surveillance and control programs in domestic pigs and wildlife.

Host diversity and biological characteristics of the Trichinella genotypes and their effect on transmission

The host spectra and biological diversity of the Trichinella genotypes are reviewed. While all genotypes appear to reproduce equally well in carnivore hosts, their infectivity and persistence in omnivores and herbivores show remarkable differences. Most of the genotypes found in wildlife have low infectivity for pigs and some persist only for a few weeks; in herbivores this tendency is even more profound, but malnourished, environmentally stressed, or otherwise immuno-suppressed hosts are likely to be more susceptible to Trichinella genotypes that would otherwise cause no, or only low level infection in that particular host species. In the domestic habitat (e.g. domestic pig farms), Trichinella spiralis is found almost exclusively, but in the sylvatic habitat the other Trichinella genotypes have found individual ecological niches. Thus, when environmental stress is limited in the domestic habitat, the high reproductive capacity of T. spiralis has a selective advantage, but in nature, the tolerance of other (sylvatic) genotypes to high and low temperatures and decomposition of host tissue might be more important. Parasite distribution according to muscle appear to be independent of the genotype of Trichinella and predilection sites are primarily determined by host species and secondarily by the age and level of infection. The biological diversity of the Trichinella genotypes should definitely be considered when planning experimental studies, as the uniform high infectivity of all genotypes in carnivores probably make them more suited for comparative studies than rodents.

Trichinella in horses: a low frequency infection with high human risk

After the initial report in 1976 of a trichinellosis epidemic caused by the consumption of infected horsemeat, 12 other outbreaks have been described in Europe. Since the first serious human outbreak several experiments have confirmed the susceptibility of horses to Trichinella species and the rapid disappearance of specific antibodies in this host that prevents the use of serological methods for routine screening. A review of the distribution of parasite burdens in muscles of naturally or experimentally infected horses indicates that the tongue is the most likely sample to contain detectable numbers of Trichinella larvae in low level infections. Requirements for testing of horsemeat are specified in legislation of the European Union, and other recommendations are published elsewhere. The EEC directives have evolved into very specific requirements which specify the testing of at least 5g of tongue, masseter or diaphragm per horse using a pooled digestion assay. More recently, France has revised the requirement for sample size to 10g for horsemeat originating from countries with high prevalence of Trichinella. To address the continuing outbreaks of human trichinellosis due to infected horsemeat, the development and implementation of a quality assurance system for testing is being considered.

Influence of temperature on the survival and infectivity of Trichinella spiralis larvae in Sarcophaga argyrostoma (Diptera, Sarcophagidae) maggots

To investigate the role of fleshfly maggots as a paratenic host for Trichinella spiralis larvae, maggots of Sarcophaga argyrostoma (Muscidae, Sarcophagidae) kept at different temperatures (26, 22, 20, 16, 12, 8, and 4 C) were allowed to feed on T. spiralis-infected mouse meat. Trichinella larvae found in maggots kept at 8-26 C were able to cause infection when inoculated in mice. Infective larvae survived in maggots up to 5 days postinfection at 8 C and for shorter periods of time at higher temperatures. The survival time in maggots was negatively related to the temperature of maggot breeding. The results suggest that the role of S. argyrostoma in the dissemination of Trichinella larvae in nature is limited in comparison to the role played by mammals with scavenger and cannibalistic behavior.

Predilection sites of Trichinella spiralis larvae in naturally infected horses

A total of 120 muscle tissues from three horses naturally infected with Trichinella spiralis were examined. The head was the most infected site. In particular, the muscles harbouring the highest number of larvae were: musculus buccinator (12, 411 and 1183 larvae g-1), the tongue (11, 615 and 1749 larvae g-1), m. levator labii maxillaris (17,582 and 1676 larvae g-1), and the masseter (4.9, 289 and 821 larvae g-1). Compared with the diaphragm, the number of larvae per gram was from 3.5 to 6.8 times higher in the tongue, from 3.5 to 6.5 higher in m. levator labii maxillaris, and from 2.5 to 4.6 higher in m. buccinator. Of the examined muscles, the diaphragm had from the 6th to the 15th highest level of infection (3.1, 166 and 256 larvae g-1). Published data from experimentally infected horses confirm these results, suggesting that efforts to detect predilection sites should focus on the head muscles.

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

To determine Trichinella infection in a selected group of farm raised pigs, 4078 pigs from 156 farms in New England and New Jersey, employing various management styles, were selected based on feed type (grain, regulated waste, non-regulated waste). The number of pigs bled from each farm were based on detecting infection assuming a 0.05 prevalence rate. Serum was tested by enzyme-linked immunoassay for antibodies to Trichinella spiralis. Seropositive pigs were tested by digestion at slaughter (when possible) for the presence of Trichinella larvae. Questionnaires completed at the time of serum collection were used to develop descriptive statistics on farms tested and to determine measures of association for risk factors for the presence of Trichinella-seropositive pigs. A total of 15 seropositive pigs on 10 farms were identified, representing a prevalence rate of 0.37% and a herd prevalence rate of 6.4%. A total of nine seropositive pigs and one suspect pig from six farms were tested by digestion; four pigs (representing three farms) harbored Trichinella larvae at densities of 0.003-0.021 larvae per gram (LPG) of tissue; no larvae were found in six pigs. Risk factors which were significantly associated with seropositivity included access of pigs to live wildlife and wildlife carcasses on the farm; waste feeding had no statistically significant association with seropositivity for Trichinella infection in pigs. The presence of Trichinella infection in pigs in New England and New Jersey has declined during the past 12 years when compared with previous prevalence studies.

Detection of Trichinella infection in slaughter horses by ELISA and western blot analysis

In order to determine the presence of Trichinella infections in horses slaughtered at an abattoir in Mexico, 147 serum samples were examined by two immunoenzymatic methods. Specific antibodies were detected by ELISA in 7% of the serum samples at a dilution 1:400 and in 10% at lower dilutions (1:20, 1:40) using Trichinella spiralis muscle larvae (ML) excretory/secretory (E/S) products. Serum samples from four naturally infected horses (confirmed by direct methods) gave negative O.D. values in an ELISA at a 1:400 dilution and only two of them were positive at a 1:20 and 1:40 dilutions. Serum samples from experimentally infected horses reacted by Western blotting with ML components with molecular weights of 47, 52, 59, 67, 72 and 105 kDa which correspond to the TSL-1 antigens. Serum samples from the four naturally infected horses and from the abattoir horses that were positive in ELISA using E/S antigens recognized several ML components, some of them reacted with all the TSL-1 antigens mentioned above and others recognized preferentially two or three of these molecules. Since the serologic assays may not offer the sensitivity required in the diagnosis of horses trichinellosis and the direct methods had not always been useful in the detection of larvae in horsemeat related to trichinellosis outbreaks in Europe, it is proposed that additional assays are performed to determine Trichinella infection in horses. These can include detection of parasite antigens by ELISA and Dot ELISA or PCR, which in turn may also help to determine the presence of the parasite in early and late infections of horses.

Epidemiological, clinical and serological studies on trichinellosis in Henan Province, China

The epidemiological, clinical and serological studies on human trichinellosis in Henan Province were carried out from January 1992 to December 1996. The results showed that 467 patients attending our department came from 12 administrative areas and cities of the Province and they acquired the infection mainly by eating undercooked dumplings, tasting the raw pork filling for dumplings, ingesting scalded pork or mutton. There was the high incidence season of trichinellosis during the winter. Most of the patients were workers, officers and merchants aged 20-49 years, and the incidence was higher in males than in females. The main clinical manifestations of trichinellosis were fever, general myalgia, muscle tiredness and eosinophilia. Most of the patients had not any gastrointestinal symptoms and rash. Eyelid edema was only seen in the early courses of the disease. Eosinophil count increased with time, reached to the peak 3 weeks and decreased markedly 6 weeks after the onset of the disease. Eosinophil levels elevated obviously 1 week after therapy, began to drop rapidly 2 weeks after therapy, and decreased to a near normal levels 4 weeks after therapy. Anti-Trichinella antibodies were detected by IFAT using the frozen sections of the purified T. spiralis larvae as antigen. The specific antibodies were observed in only 70.2% of patients one week after onset of disease, and increased to 91, 94.3, and 100%; 2, 3, and 4 weeks, respectively after the onset. The antibody positive rate was evidently increased to 100% 1 week after therapy with abendazole, then decreased to 25% 4 months after therapy.

Distribution of Trichinella spiralis larvae in muscles from a naturally infected horse

Epidemiological investigations conducted during 10 trichinellosis outbreaks between 1975 and 1994 showed that horse-meat was the probable source of infection. Though hundreds of thousands of horses have been examined at abattoirs in America and Europe to detect Trichinella infection by artificial digestion or trichinelloscopy, an infected horse has never been detected during routine analysis, which consists of examining 1 g of tissue muscle from the diaphragm. In November 1996, a naturally infected horse imported from Romania was detected in Southern Italy. The parasite was identified as Trichinella spiralis by random amplified polymorphic DNA analysis. Artificial digestion of tissue samples from 60 different muscles from 13 different sites of the infected horse carcass showed that M. levator Labii maxillaris, M. hyoideus transversus, and M. buccinator were the 3 most infected muscles. Muscles from the tongue, the masseter, and the diaphragm, which have normally been considered the muscles of choice for diagnosis, were the 4th, 6th and 13th most infected muscles, respectively. When comparing body sites, muscle tissues from the head showed the highest level of infection, followed by muscles from the neck. This finding may explain the negative results that have been obtained in the past during routine examination of the diaphragm of horses.

Detection of Trichinella spiralis in a horse during routine examination in Italy

Routine examination for Trichinella infection by artificial digestion of 5-g samples of muscle tissue revealed the presence of muscle larvae in one out of 28 horses imported from Romania to an abattoir in Italy. The parasite, identified as Trichinella spiralis by the polymerase chain reaction, showed a reproductive capacity index of 68 in Swiss mice. Light microscope examination of 200 nurse cell-larva complexes showed that 22% of them were calcified and that the capsules of the non-calcified nurse cells were 17.5-27.5 microns (s = 22.67 microns) thick and had very few cellular infiltrates. The serum samples from the parasitologically positive horse and from three other horses of the same stock, from which Trichinella larvae were not recovered by digestion, showed a low level of positivity as determined by ELISA and Western blot analyses using a crude antigen, whereas negative results were observed in both tests when an excretory-secretory antigen was used. The results, together with data from the literature, suggest that the horse had acquired the infection 8-10 months previously and confirm earlier observation obtained from experimental infections, which showed that muscle worm burden and specific circulating antibodies were lost several months after infection.

Experimental trichinellosis in goats

The susceptibility and distribution of Trichinella spiralis infection in goats were examined in ten autochthonous kids, 2 months old and about 10 kg body weight. The animals were divided into two groups: one experimental group with eight animals, infected with 10,000 T. spiralis 'T1' encysted larvae and a control group with two non-infected animals. All the animals of the experimental group infected by the parasite showed that Trichinella larvae have a special affinity for the tongue, masseters, diaphragm, flexor-extensor muscles, intercostal muscles and myocardium in decreasing order. The ELISA test carried out showed the first increments of optical density (OD) on Day 16 postinfection (p.i), peaking on Days 37-44 p.i. and remaining elevated from this day on, with a slight fall at the end of the experiment (Day 90 p.i.). No alterations were observed in the OD obtained in control animals throughout the experiment. The great muscular establishment of T. spiralis larvae and the sigmoidal evolution of antibody levels confirm the host character of the goat to the parasite.

Trichinella infections in arctic foxes from Greenland: studies and reflections on predilection sites of muscle larvae

Studies were carried out to determine the predilection sites of Trichinella nativa muscle larvae in arctic foxes (Alopex lagopus) caught in Greenland. The highest number of larvae per gram of tissue was found in the muscles of the eyes and the legs. With regard to predilection sites no significant differences were demonstrated either between age groups or between foxes with high and low total parasite burdens. Predilection sites were comparable with those recorded earlier in experimentally infected caged foxes and in other carnivorous species. Hypotheses on predilection sites of Trichinella muscle larvae are discussed.

Experimental Trichinella spiralis infection in sheep

During susceptibility studies of non-specific hosts, three merino sheep were infected with 3000, 5000 or 7000 Trichinella spiralis larvae by gavage. Clinical, physiological and serological parameters were assessed during the experiment. On the 152nd day p.i., animals were necropsied and, using artificial digestion methods, numbers of Trichinella larvae in muscle tissues were determined. The most infected parts were masseters with 3122 larvae g-1 muscle, 5526 larvae g-1 muscle and 4058 larvae g-1 muscle and diaphragms with 2778 larvae g-1 muscle, 2725 larvae g-1 muscle and 2320 larvae g-1 muscle, for the 3000, 5000 and 7000 infection levels, respectively. A positive correlation between infective rate and circulating antibodies was observed using ELISA and latex agglutination (LA) test methods. Trichinella larvae from sheep applied by gavage to ICR mice developed to the muscle stage. No significant changes were found in the clinical and physiological parameters of infected animals. Our results confirm the high susceptibility of merino sheep to T. spiralis infection.