Sunday, January 24, 2010

Ascaris

Ascaris lumbricoides is the largest roundworm of man and mammals. It is a common human parasite in the tropics and subtropics, with a high prevalence in children. It prefers warm, moist conditions and is normally present together with another roundworm, Trichuris trichura.
These are large elongated and cylindrical worms, tapering at the ends. The posterior end of the male is curved ventrally. The male measures 12 - 31 cm long by 3 mm in diameter and the female measures 20 - 35 cm in length by 3 – 6 mm in diameter. A closely related form, Ascaris suum, infects pigs and occasionally humans.
Life cycle
The female worm is highly prolific producing more than 200000 eggs daily. These relatively large and ovoid eggs have thick transparent shells with coarse rough appearance. They are covered by a cuticle that is quite resistant to desiccation and chemicals but shows less resistance to heat, freezing and direct light. If kept in a 10 % formalin solution, the eggs remain viable for weeks.

Favourable conditions for the survival and persistence of viable eggs are moist, well-aerated shady soils, especially loamy soils. In some parts of the world, coffee and banana plants, which are grown around the dwellings, provide ideal conditions for Ascaris and other intestinal parasites.
Unembryonated eggs are passed out in faeces. Embryonation occurs in moist environment and normally takes about two to three weeks. Under favourable conditions of temperature, moisture and air, the eggs develop active embryos, which do not become infective until they have moulted to the second larval stage.
Following the ingestion of the embryonated infective eggs, the larvae are liberated in the duodenum. The larvae penetrate the intestinal wall, enter mesenteric blood veins and lymphatic vessels and reach the lungs, where they spend at least a week. Around the 10th day since hatching, the larvae, which are now quite large, leave the lungs via the tracheal passages to the throat, where they will be swallowed and taken to the small intestine. The third moult takes place in either the lungs or intestine. A final moult that occurs in the small intestine produces mature worms. It has taken at least 8 to 12 weeks since ingestion of the eggs for the worms to become sexually mature and begin mating. Adult worms remain in the small intestine for 9 to 12 months. Quite often, however, they are expelled spontaneously before that.
                                                                                                            
Pathogenesis and symptomatology
Migrating larvae cause a lot of damage to the liver and the lungs. However, this depends on the numbers of larvae involved. A few larvae will usually provoke no response from the host, especially if it is an initial infection. However, subsequent migrations cause intense tissue reaction in the liver and lungs, even when relatively few larvae are involved.
Headache, muscular pain, pneumonitis consisting of a dyspnoea often of the asthmatic type, a productive cough, wheezing and fever are common symptoms associated with Ascaris infection. During the period of the productive cough, the sputum may contain many eosinophils and larvae. Lesions due to larvae are also found in the spinal cord, brain, and heart muscle.
Infection in children is usually associated with abdominal pain, loss of appetite, listlessness and at times nervous symptoms.
Adult worms in the small intestine frequently provoke no symptoms. However, susceptible individual and people lacking proper nutrition tend to carry heavy parasite burdens and may experience mild abdominal discomfort, lack of appetite, diarrhoea and constipation. A heavy ascarial infection is likely to interfere with the host’s protein nutrition, something of great concern in areas of the world where a protein-rich diet is rare.
Sometimes when disturbed by the drugs or other factors, adult worms tend to migrate forwards or backwards and become entangled with one another and cause obstruction of the intestine, resulting in gangrene and even death. Some of the worms may enter the bile and pancreatic ducts causing jaundice or pancreatic haemorrhage. Worms may enter the appendix and cause appendicitis. But should the worms penetrate the intestine and reach the heart or brain, they can cause death.
An aspect that is more of an embarrassment than injury is when a worm leaves the intestine, wanders up the alimentary canal and creeps out of the nostrils. On rare occasions, larvae have been seen coming out of the lachrymal duct.
Epidemiology
Ascaris is maintained by faecal contamination of the soil. In rural and in congested centres in the developing countries human faeces are usually deposited by small children (and older children during the hours of darkness) in and around the home and since the eggs are quite resistant to desiccation and can remain viable in the soil for months, or even years, the numbers build up in the soil to considerable levels as the time passes. With time, the compound, house floors and even the utensils may be heavily polluted with the eggs of Ascaris and, if the picture is the same in the whole village, the rate of transmission will be high and many people will be infected. With such a high rate of transmission, the intensity of infection in children and other vulnerable groups can be very high indeed.
Human infection is possible in a number of ways. Children can ingestion dirt contaminated with eggs or they may transfer eggs to the mouth via unclean hands. Furthermore, infection may be contracted through eating raw food especially vegetables that have not be cleaned sufficiently, or through drinking water contaminated with faecal matter.
Control Methods
The maintenance of proper sanitary conditions considerably reduces the transmission of infection. This involves the regular use of toilets to prevent soil contamination with human faeces and adhering to personal hygiene by washing hands before eating and eating food that has been properly cooked. Salads that are usually eaten raw should be washed thoroughly.
Mebendazole, pyrantel pamoate and piperazine are effective against ascariasis. All have a wide margin of safety.
Other Ascrids
Ascaris suum is a common pig roundworm that is similar to A. lumbricoides in many ways and is of great economic importance to pig-keepers all over the world. It causes stunting of growth in piglets.
Ascaris suum probably constitutes an occasional source of intestinal ascariasis in man. In some instances, it will develop to adult stage in man. Likewise, A. lumbricoides larvae have been shown to develop in pigs to adult stage.
Other important ascarids include Toxocara canis in dogs, Toxocara cati in cats and Parascaris equorum in horses. The latter is an important parasite of horses throughout the world that inflicts considerable mechanical damage through its migrating larvae on the tissues and lungs of horses, causing pneumonia and even introducing bacteria and viral infections. Like the human Ascaris, adult worms may perforate the intestinal wall, damage the liver and bile ducts and cause general toxaemia from metabolic waste products.