The frequency is probably much higher, due to home preparation of raw or undercooked fish dishes. In Japan, more than 1,000 cases are reported annually. Development of better diagnostic tools and greater awareness has led to more frequent reporting of anisakiasis. Within a few hours of ingestion, the parasitic worm tries to burrow though the intestinal wall, but since it cannot penetrate it, it gets stuck and dies. The presence of the parasite triggers an immune response; immune cells surround the worms, forming a ball-like structure that can block the digestive system, causing severe abdominal pain, malnutrition, and vomiting. Occasionally, the larvae are regurgitated. If the larvae pass into the bowel or large intestine, a severe eosinophilic granulomatous response may also occur one to two weeks following infection, causing symptoms mimicking Crohn's disease. Diagnosis can be made by gastroscopic examination, during which the 2-cm larvae are visually observed and removed, or by histopathologic examination of tissue removed at biopsy or during surgery. Raising consumer and producer awareness about the existence of anisakid worms in fish is a critical and effective prevention strategy. Anisakiasis can be easily prevented by adequate cooking at temperatures greater than 60 °C or freezing. The FDA recommends all shellfish and fish intended for raw consumption be blast frozen to −35 °C or below for 15 hours or be regularly frozen to −20 °C or below for seven days. Salting and marinating will not necessarily kill the parasites, as in Italy where two-third of cases were attributed to anchovies marinated in lemon or vinegar. Humans are thought to be more at risk of anisakiasis from eating wild fish rather than farmed fish. Many countries require all types of fish with potential risk intended for raw consumption to be previously frozen to kill parasites. The mandate to freeze herring in the Netherlands has virtually eliminated human anisakiasis. Even when the fish is thoroughly cooked, "Anisakis" larvae pose a health risk to humans. Anisakids (and related species such as the sealworm, "Pseudoterranova" species, and the codworm "Hysterothylacium aduncum") release a number of biochemicals into the surrounding tissues when they infect a fish. They are also often consumed whole, accidentally, inside a fillet of fish. Acute allergic manifestations, such as urticaria and anaphylaxis, may occur with or without accompanying gastrointestinal symptoms.