TRICHINELLA SPIRALIS (Trichinosis/Trichinellosis)

GEOGRAPHICAL DISTRIBUTION

The infection is seen in countries such as United States, countries of former Russian federation and Europe. It is rarely seen in tropics. It also occurs in east and west sub-Saharan African region.

MORPHOLOGY

The adult worms are small and measures 2-3mm (females) and 1-2mm (male).

adult Trichinella

LOCATION IN HOST

The worm is found embedded in the small intestine mucosa. The larval stages are also found in the same host. 9 example of man being both definitive and intermediate host)

LIFE CYCLE

Each female matures rapidly and following fertilization lays about 1500 larvae. The larvae make their way by way of lymphatics and the blood stream to reach striated muscles. Each larva enters a muscles fibre and converts it to a characteristic capsule called the ‘nurse cell’. Therefore the larvae can live for long periods. Later they usually get calcified. In humans the muscle phase is a dead end. When this happens in animals such as pigs and horses the meat becomes infective if not properly cooked. The muscle fibres entering the stomach are digested and the liberated larvae quickly gain entry into the small intestinal mucosa and mature rapidly into adult worms.

Life cycle

After about1 week, female worms release newborn larvae that migrate via the circulation to striated muscle. The larvae of all species except Trichinella pseudospiralis and Trichinella papuae then encyst by inducing a radical transformation in the muscle cell architecture. Although host immune responses may help to expel adult worms, they have little effect on muscle-dwelling larvae. Human trichinellosis is most often caused by the ingestion of infected pork products and thus can occur in almost any location where the meat of domestic or wild swine is eaten. Human trichinellosis also may be acquired from the meat of other animals, including dogs (in parts of Asia and Africa), horses (in Italy and France), and bears and walruses (in northern regions). Although cattle (being herbivores) are not natural hosts of Trichinella, beef has been implicated in outbreaks when contaminated or adulterated with trichinous pork.

Encysted larva

Encysted larvae

PATHOGENESIS

In mild infections, adult worms cause minimal pathology. They may cause inflammation and congestion of the mucosa with cell destruction. Major pathology is by larvae due to their presence in the muscle or due to their migration through the central nervous system and the heart.

CLINICAL FEATURES

Clinical symptoms of trichinellosis arise from the successive phases of parasite enteric phase, invasive phase (larval migration), and encystment phase (muscle encystment)

1. Enteric phase (invasion of the intestine): patients develop nausea, vomiting and abdominal pain resembling an attack of acute food poisoning.

2. Invasive phase (larval migration): This phase is characterized by severe myalgia, periobital edema and eosinophilia. CNS or myocardial complications may occur during this phase.

3. Encysment phase (Encysment of larvae in the muscle): Severe infection leads to cachexia, edema and dehydration. Gram negative septicemia due to organisms. introduced by larvae may occur.

DIAGNOSIS

History of ingesting undercooked pork is suggestive. Often groups of people are affected as following parties.

Blood eosinophilia develops in_90% of patients with symptomatic trichinellosis and may peak at a level of 50% between 2 and 4 weeks after infection. Serum levels of IgE and muscle enzymes, including creatine phosphokinase, are elevated in most symptomatic patients.

Muscle biopsy can be used to demonstrate the encysted larvae. A definitive diagnosis requires surgical biopsy of at least 1 g of involved muscle; the yields are highest near tendon insertions. The fresh muscle tissue should be compressed between glass slides and examined microscopically, because larvae may be overlooked by examination of routine histopathologic sections alone.

In early infection serology may be useful (ELISA, western blot, Enzyme immunoassay.)

EPIDEMIOLOGY

Eating of undercooked pork or sometime horse meat is responsible. Feeding pigs with dead or killed rats should be avoided. This is the major avenue of transmission of the infection to pigs by rats, which maintain the infection.

PREVENTION AND CONTROL

Proper cooking of all meat will eliminate the infection. Cooking of all garbage fed to pig will also help. Deep freezing pork at -18º to -15º C is effective.

TREATMENT

Current antihelmintic drugs are ineffective against Trichinella larvae in muscle. Fortunately, most lightly infected patients recover uneventfully with bed rest, antipyretics, and analgesics. Glucocorticoids like prednisone (1 mg/kg daily for 5 days) are beneficial for severe myositis and myocarditis. Mebendazole and albendazole, like thiabendazole, appear to be active against enteric stages of the parasite, but their efficacy against encysted larvae has not been conclusively demonstrated.