Trichinella

Trichinellosis is caused by larvae of roundworms, mainly the species Trichinella (T.) spiralis. To date, four species of trichinella are known in Europe. These pathogens are called trichinella or trichina worms. The last known autochthonous outbreak of disease in humans caused by a domestic pig occurred in 1970 after the consumption of poorly smoked meat from a Trichinella-infected domestic pig that was fed with fox meat.

Occurence

Trichinellosis is a mammalian zoonosis which occurs worldwide independently of climatic conditions. In Central Europe, trichinellosis occurs only rarely. In some eastern EU countries, the incidence is higher, with most cases occurring in the member states Bulgaria and Romania. Infections are often caused by meat products from wild boar. In wild animals, T. britovi and T. pseudospiralis are found in addition to T. spiralis.

Host animals

Several animal species can be carriers of the parasites. Domestic pigs, wild boar and horses are host animals. Rodents (e.g. rats) and wild animals (e.g. foxes) are considered reservoirs. Humans represent a false host, as infection is not passed on.

Mode of transmission

Infection occurs through the consumption of raw or insufficiently heated meat containing encapsulated Trichinella larvae (except Trichinella pseudospiralis). Digestive enzymes release the larvae, which mature into small worms in the cells of the upper mucosa of the small intestine within a few days. Female worms begin depositing up to 1,500 larvae as early as four to seven days after ingestion by the host. The young larvae pass through the intestinal mucosa and enter the muscles via the bloodstream, where they can survive encapsulated in muscle cells for years. Oxygen-rich muscles, i.e. muscles with a good blood supply, such as the diaphragm, neck, chewing muscles, muscles of the shoulder girdle or upper arms are preferred.

Incubation period

The incubation period is 5 to 15 days and depends on the number of Trichinella larvae ingested. The number of ingested Trichinella larvae that cause clinical disease in humans varies - more than 70 ingested larvae are likely to cause disease. Transmission from person to person is not possible.

Symptoms

Trichinellosis is a mild to fatal foodborne disease in humans, who are considered highly susceptible hosts. The severity of the disease depends on the number of larvae ingested and the immune response of the individual. Disease symptoms in humans are characterized by fever, abdominal pain and diarrhea in the initial phase. In the later course of the disease, muscle and joint pain as well as typical edema in the facial area are the main symptoms.

Therapy

Mildly infected patients usually recover without complications by bed rest and with the help of a painkiller. Severe infections are treated with drug therapy against worm larvae infestation. Drug treatment is more successful the earlier it is given.

Prevention

Heating meat to over 70 °C is considered to kill larvae. Freezing at -15 °C reduces the infectivity of the parasite. Smoking, curing and drying are not sufficient to kill larvae.

In Austria, human cases of the disease are very rare: In the past 50 years, only so-called "imported" cases of trichinellosis have been registered by the health authorities in Austria. These persons became infected with Trichinella larvae during a stay abroad or brought infected meat products to Austria, usually in the course of a holiday at home, and became ill in Austria after eating them.

In 2019, one laboratory-confirmed human Trichinella disease was reported in Austria (EMS, as of 04.08.2020). According to information in the EMS, this person became infected abroad.

Figure 1: Human cases of trichinellosis in Austria

In Austria, 5,063,302 domestic pigs, 564 horses and 20,834 wild boars from the wild and 1,348 farmed wild boars were examined for Trichinella in 2019 within the framework of the official meat inspection. Trichinella (T. britovi) were detected in one wild boar from the wild in 2019.

In Austrian breeding and fattening pigs as well as horses, no trichinella case has been detected for decades.

Pigs kept indoors are considered free of Trichinella infestation, as the animals have no possibility of ingesting infested fresh meat. The EFSA critically questions the appropriateness of the statutory trichinella testing of domestic pigs. Wild boar, on the other hand, must generally be regarded as possible carriers of trichinosis.

Scientific studies have shown that the parasite is also present in the fox population in Austria, with a clear west-east divide in its distribution.

The suspected diagnosis can be confirmed by the detection of specific antibodies in the blood of the patient; in case of massive infestation, the larvae can be detected in the tissue by histological examination of muscle biopsy specimens.

Animals that may be carriers of Trichinella and are intended for human consumption must be examined for Trichinella larvae after slaughter or killing and before the meat is placed on the market (Implementing Regulation (EU) 2015/1375).

The examination is done by the so-called digestion method: A precisely defined amount of muscle by weight of the examined carcass (usually from the area of the diaphragm pillar) is dissolved by means of artificial digestion and the sediment of the digestion liquid is checked under microscopic observation for the presence of Trichinella larvae.

In case of a positive Trichinella detection, the entire carcass is confiscated by the competent veterinary authority and sent for verifiable disposal.