Biology Notes For Class 12 Common Human Diseases Caused By Helminths

Biology Notes For Class 12 Common Human Diseases Caused By Helminths

Biology Notes For Class 12 Common Human Diseases Caused By Helminths

Biology Notes For Class 12 :-

  • ASCARIASIS: This disease is caused by roundworm Ascarislumbricoides. Infections have no symptoms in more than 85% of cases, especially if the number of worms is small. Symptoms increase with the number of worms present and may include shortness of breath and fever at the beginning of the disease. These may be followed by symptoms of abdominal swelling, abdominal pain, and diarrhoea. Children who are 3 to 8 years old are most likely to be infected because of their contact with soil while playing in the soil; hence they suffer from poor weight gain, malnutrition, and learning problems.

The source of infection is from objects which have been contaminated with faecal matter containing eggs. Ingestion of infective eggs from soil contaminated with human faeces or contaminated vegetables and water is the primary route of infection. Infectious eggs may occur on other objects such as hands, money, and furniture.

The eggs hatch in the intestines, burrow through the gut wall and migrate to the lungs via the blood. There they break into the alveoli and pass up the trachea, where they are coughed up and may be swallowed. The larvae then pass through the stomach for a second time into the intestine, where they become adult worms.

It lies free in the intestine, having no organs for attachment. It feeds on the semi-digested food of the host, the blood, and the fluid of the alimentary canal of the host. There is no secondary host in its life cycle.

The first appearance of eggs in stools is 60–70 days. In larval ascariasis, symptoms occur 4–16 days after infection. The final symptoms are gastrointestinal discomfort, colic and vomiting, fever, and observation of live worms in stools. Some patients may have pulmonary symptoms or neurological disorders during migration of the larvae.  Generally, there are few or no symptoms. A bolus of worms may obstruct the intestine; migrating larvae may cause pneumonitis and eosinophilia. Adult worms have a lifespan of 1–2 years which means that individuals may be infected all their lives as worms die and new worms are acquired.

Eggs can survive potentially for 15 years and a single worm may produce 200,000 eggs a day.

Biology Notes For Class 12 - Common Human Diseases Caused By Helminths

For Class 12 Biology classes, click CBSE Class 12 Biology.


Prevention is done by improved sanitation, which includes improving access to toilets and proper disposal of faeces. Hand washing with soap is very important. In areas where more than 20% of the population is affected, treating everyone at regular intervals is recommended. Reoccurring infections are common. There is no vaccine for ascariasis. Treatments recommended by the World Health Organization are the medications albendazole, mebendazole, levamisole, or pyrantel pamoate. Other effective agents include tribendimidine and nitazoxanide.


  • FILARIASIS: The other names for Filariasis are Elephantiasis and Lymphatic filariasis (LF), It is caused by the filarial worm, This disease is characterized by the swelling of legs, scrotum, and of other parts of the body. Therefore, it is commonly known as elephantiasis due to its resemblance to a leg of an elephant.Filariasis, a parasitic disease is transmitted by black flies and mosquitoes (Culex mosquito). Lymphatic Filariasis (LF) starts from the bite of an infected mosquito, or when a person comes into contact with water where infected mosquitoes breed. The parasitic worms that are transferred to humans burrow into the lymphatic system, the body’s network of vessels and nodes which carry lymph fluid to the tissues and blood. The lymphatic system forms a major part of our body’s immune system.These parasites are thin, round and worm-like structures and have white in color appearance. The lifespan of filariae would be around 5 to 7 years. During their lifespan, they produce millions of larvae which are creamy in color. The size of female larvae ranges from 2-50cm and the size of male larvae is twice that of the size of female larvae.

Although it is not fatal, filariasis is chronic and very painful. The disease causes an accumulation of fluid (hydrocoele); swelling (lymphoedema) of the subcutaneous layer of the skin which houses fat and connective tissues; passing of cloudy-colored urine (chyluria), and in its most extreme form, the skin and underlying tissues of the lower limbs and scrotum thicken and become distended.


During its life cycle, a mosquito bites an infected individual with larvae. This mosquito bite injects the larvae into a healthy person. The injected larva of a parasite enters the bloodstream of a healthy host and starts consuming the resources required for the larva to grow. The complete stage of larva to adult filarial worms is carried out within the lymphatic system. The matured and adult filarial worm starts to release smaller worms called microfilariae. The newly generated microfilariae again enter into the mosquito along with the host’s blood and the cycle repeats itself.



Watch animated video with detailed explanation on the same, click NCERT solutions for Class 12 Biology

Biology Notes For Class 12 - Common Human Diseases Caused By Helminths

The inflammation that starts within the skin is usually caused by the immune system’s reaction to the parasite. It can also be caused by bacteria which may have invaded the skin because of an already weakened immune system. At this point, the lymph nodes and lymphatic vessels will also start to swell.

These symptoms develop in a very slow manner, sometimes taking years. Those who are infected do not show any outward signs until the disease reaches its late phase. In the latter stage of the disease, affected persons are immediately identified because of their grossly swollen legs, arms, breasts or genitals with cracked, thickened skin that is rough and hard to the touch. LF can also cause damage to the kidneys and the entire lymphatic system. People who are affected by lymphatic filariasis are subjected to social stigma and are unable to live a normal life.

For more Biology Notes For Class 12 click 12 Science.

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