Trichinellosis is helminthosis of humans, as well as many wild and domestic animals caused by roundworms of the genus Trichinella. This disease has relevant socioeconomic and sanitary implications, having a considerable impact on the international commerce of animals (principally pigs) in many countries. Several different species of Trichinella can cause human disease; the most common species are T. spiralis, T. nativa, and T. britovi.
The most important source of human infection worldwide is the domestic pig, but, e.g., in Europe, meat of horses and wild boars plays a significant role. Cooking of meat to a temperature of > 71°C for at least 1 min reliably inactivates Trichinella larvae, whereas such methods of meat preparation as drying, smoking, or freezing are not regarded as completely safe.
After exposure of ingested meat to gastric acid and pepsin, the larvae are released from cysts, invade epithelium of small bowel, and induce the formation of syncytium. Larvae undergo four molts, thus developing into the adult stage (female 2.2 mm in length, males 1.2 mm) within a very short time of 2 days. Males and females copulate, and 5 -7 days postinfection, the females start to deliver new generations of larvae. Female lifelong productivity in different hosts ranges from hundreds to thousands larvae. The lifespan of females is around 4 weeks. Newborn larvae migrate via bloodstream to the striated muscles where they penetrate muscle cells. Within muscle cells, larvae develop without molting to infective encysted larvae. This maturation terminates within approximately 15 days. Some of the encysted larvae undergo calcification by the host, but some could remain infective for many years.
The main pathogenic features of trichinellosis include modification of cells of intestinal epithelium and striated muscles by adult worms and larvae respectively, as well as immune response of the host. In the gastrointestinal stage of infection, i.e. first several weeks, the villi could become deformed, and there is a proliferation of enterocytes at the villus margins, hyperplasia of the crypts of Lieberkühn and the presence of massive cellular infiltrates in the mucosal sublayer. The penetration and permanent presence of larvae in the cells of the striated skeletal muscles cause following cell modifications: the disappearance of sarcomere myofibrils, the encapsulation of the larvae, and the development of a capillary network surrounding the infected cell.
Eosinophilia is a common characteristic in most cases of trichinellosis. The release of histamine, serotonin, bradykinin, and prostaglandins results in an augmented permeability of capillaries and a leakage of fluids, electrolytes, albumins, and cell elements into the surrounding tissue. Another consequence of these inflammatory processes is vasculitis and fine intravascular thrombi, which represent the principal pathology in the acute stage of trichinellosis.
The length of incubation period, as well as severity of symptoms relate to the number of infectious larvae consumed in the meat. The first symptoms of trichinellosis are gastrointestinal (nausea, diarrhea, abdominal pain), usually occurring 1-2 days after infection. These symptoms are often weakly pronounced. The classic trichinellosis symptoms typically occur 3-4 weeks after eating contaminated meat, and can last up to 8 weeks. Main symptoms in this stage are myalgia, fever, and facial oedema. The most important complications of trichinellosis, sometimes leading to death, are cardiovascular (myocarditis and thromboembolic disease).
It is estimated that worldwide, around 10,000 clinical cases of trichinellosis occur every year. A large number of cases are diagnosed inSouth-Eastern Europe, Argentina, and China.
Cultural factors such as consumption of traditional dishes based on raw or undercooked meat play an important role in the transmission of the disease. In other countries, such as U.S., where pork production is under strict sanitary control, eating undercooked wild game (e.g. bear, wild boar) puts one at risk for acquiring trichinellosis.
The existence of immunity to Trichinella is demonstrated by the fact that in some endemic populations who regularly eat infected meat trichinellosis manifests itself as a chronic diarrhoeal syndrome. The latter is due to the strong gastrointestinal immune reaction consisting of the expulsion of adult worms from the intestine, thus preventing the muscular phase from occurring.
Trichinella antigens induce relatively few specific antibodies during gastrointestinal stage of the infection. By contrast, during the muscle phase of the parasite, there is abundant antibody induction. Sometimes the serodiagnosis is negative during the first days of the febrile phase, in which case a second assay performed a few days later is advisable. IgM and IgG antibodies are likely to be detected simultaneously, onwards from the 4th week after infection. During the acute phase of trichinosis antibody concentrations rise very rapidly, and the peak occurs around the 60th and 90th day after infection for IgM and IgG respectively. Thereafter antibody concentrations fall quite rapidly for IgM, and much more slowly in the IgG. IgG antibodies may be detectable for 10 years or more following infection, but in many patients who received treatment IgG becomes negative already 1 year postinfection.
An increase in total and specific IgE levels is not a consistent phenomenon found in trichinellosis, and thus, it is not possible to exclude trichinellosis on the basis of its absence.
The peculiarity of human immune response against Trichinella is the high percentage (around 50%) of antibodies directed against carbohydrate epitopes containing tyvelose, sugar specific for Trichinella. The main Trichinella epitope is a tetrasaccharide with tyvelose at its end, a component of many glycoproteins.
Following antigens can be used for serological diagnosis:
a)A crude antigen prepared from muscle larvae;
b)An excretory/secretory (ES) antigen produced in vitro after 18 h of cultivation of the muscle larvae;
c)Synthetic oligosaccharide chains containing tyvelose, which are covalently bound to an immunologically neutral protein.
ES proteins originate from the secretory granules of so called stichocyte cells present in larvae. These proteins are free of cross-reactive epitopes, and the percentage of tyvelose epitopes in ES proteins is around 90%. Therefore, ES proteins are currently the most popular antigen for immunoassays.
Since the signs and symptoms of trichinellosis are not pathognomic, misdiagnosis is quite common. For example, people with high fever and myalgia are often misdiagnosed with flu, particularly in winter. Therefore, signs and symptoms play an auxiliary role in the diagnostics of trichinellosis i.e. whereas the infection can be suspected on clinical grounds, definitive diagnosis of trichinellosis can only be made with highly specific methods (immunodiagnostic tests/muscle biopsy).
Examination of a muscle biopsy can be performed through artificial digestion or histological analysis upon hematoxylin-eosin staining. This technique is highly specific. The disadvantage of this method is that it requires a significant surgical intervention in the affected person, and insufficient sensitivity. The number of larvae in heavy cases could amount to several thousands per gramm of muscle tissue whereas under light infections, only single larvae could hardly be detected. Therefore, muscle biopsies are performed rarely on human patients.
Over the last decades many techniques have been adapted for detecting antibodies against Trichinella antigens, such as latex agglutination, indirect immunofluorescence (IIF), counterimmunoelectrophoresis, immunoblotting, and enzyme-linked immunosorbent assay (ELISA). At present, ELISA is the most highly recommended technique able to detect even asymptomatic infections. ELISA is best used in combination with immunoblotting to confirm ELISA-positive samples. In patients with suspected trichinellosis whose initial results are negative or weakly positive, at least two serum specimens should be drawn and tested to demonstrate seroconversion.