Worms, amoebae and fungi, and how diseases are prevented

Amoebiasis

• Cause: Entamoeba histolytica, a protozoan that lives in the large intestine.
• Symptoms: constipation, pain and cramps in the abdomen, and stools with a lot of mucus and clots of blood. It is also called amoebic dysentery.
• How it spreads: the parasite leaves with the faeces of an infected person. Houseflies carry it from the faeces to food. We call them mechanical carriers, because the parasite does not grow in them. Drinking water and food that are contaminated also spread the disease.

Ascariasis

• Cause: Ascaris lumbricoides, the common roundworm, an intestinal parasite.
• Symptoms: bleeding inside the intestine, pain in the muscles, fever, anaemia and a blocked intestine.
• How it spreads: the eggs of the worm leave with the faeces of an infected person. They contaminate soil, water and plants. A healthy person swallows them with contaminated water, vegetables or fruits.

Filariasis

• Cause: the filarial worms Wuchereria bancrofti and Wuchereria malayi. The worm of the second name is now placed in the genus Brugia. NCERT still prints the old name.
• Where: the worms live for many years in the lymphatic vessels, usually of the lower limbs. They cause a slow, long lasting inflammation.
• Symptoms: swelling of the limbs, which gives the name elephantiasis. In some patients the genital organs are also deformed.
• How it spreads: by the bite of a female mosquito. In many places it is the Culex mosquito.

Ringworm

• Cause: fungi of the genera Microsporum, Trichophyton and Epidermophyton. There is no worm in ringworm. The name comes from the round, red patch that the infection can leave.
• Symptoms: dry, scaly patches on the skin, the nails and the scalp, with intense itching.
• Where: heat and moisture help the fungi grow. They thrive in the folds of the skin, such as the groin and the gaps between the toes.
• How it spreads: from the soil, and from the towels, clothes and combs of an infected person.

All the diseases at a glance

• Typhoid: Salmonella typhi, a bacterium. It spreads by contaminated food and water. It affects the small intestine and the blood.
• Pneumonia: Streptococcus pneumoniae and Haemophilus influenzae, bacteria. They spread by droplets. They affect the alveoli of the lungs.
• Common cold: rhinoviruses. They spread by droplets and soiled objects. They affect the nose and the respiratory passage.
• Dengue and chikungunya: viruses. They spread by the female Aedes. They affect the blood, the joints and the skin.
• Malaria: Plasmodium, a protozoan. It spreads by the female Anopheles. It affects the liver and the red blood cells.
• Amoebiasis: Entamoeba histolytica, a protozoan. It spreads by food, water and houseflies. It affects the large intestine.
• Ascariasis: Ascaris, a roundworm. Its eggs spread in soil and water. It affects the intestine.
• Filariasis: Wuchereria, a worm. It spreads by a female mosquito. It affects the lymphatic vessels.
• Ringworm: fungi. They spread by soiled clothes, towels and soil. They affect the skin, the nails and the scalp.

Prevention and control

A good rule is that prevention is better than cure. NCERT groups the steps into four.

1. Personal hygiene. Keep the body clean. Drink clean water. Wash vegetables and fruits before you eat them. Keep your food covered.
2. Public hygiene. Dispose of waste and excreta properly. Clean and disinfect water tanks, pools and cesspools at regular times. Keep public eating places clean.
3. Control of vectors. Do not allow water to stand in and around houses. Clean coolers every week. Use mosquito nets. Put larva eating fish, such as Gambusia, in ponds. Spray insecticides in ditches, drains and swamps. Fit wire mesh to doors and windows.
4. Vaccination. A vaccine trains the body before the pathogen arrives. Vaccines exist against typhoid and many other diseases. The next lesson explains how they work.

A weekly routine

A mosquito lays eggs in water. The eggs hatch into larvae, and the larvae grow into adults in about a week or more. If we empty or cover the containers every week, the larvae have no water to complete their growth. This is why a weekly routine works.

The model below is only a teaching model. It shows that cleaning a share of the sites is not enough unless the share is large. The growth stops only when the weekly factor falls to 1 or below.

You should find that the growth stops when the share cleaned reaches $1 - \dfrac{1}{r}$. For a growth factor of 5, that share is 80 per cent.

Try it in Python

The program multiplies the mosquitoes by the weekly factor.

Practise with new numbers

Common mistakes

• Saying that ringworm is caused by a worm. It is caused by fungi.
• Saying that houseflies are the host of the amoeba. They only carry Entamoeba histolytica on their bodies, so they are mechanical carriers.
• Saying that filarial worms live in the blood vessels. They live in the lymphatic vessels.
• Mixing up ascariasis and amoebiasis. Ascariasis is a roundworm. Amoebiasis is a protozoan.
• Forgetting that the eggs of the roundworm leave with the faeces. The eggs of Ascaris reach the next person through soil, water and food.

Remember these

• Amoebiasis: Entamoeba histolytica, large intestine, mucus and blood in the stools, houseflies carry it.
• Ascariasis: Ascaris, a roundworm, eggs in the faeces, contaminated soil, water and vegetables.
• Filariasis: Wuchereria, the lymphatic vessels of the lower limbs, elephantiasis, a female mosquito.
• Ringworm: fungi (Microsporum, Trichophyton, Epidermophyton), dry scaly patches and itching.
• Prevention: personal hygiene, public hygiene, control of vectors, vaccination.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What are the symptoms of amoebiasis, and how do houseflies help it to spread?

2. How does a person get ascariasis?

3. Name the pathogen of filariasis, the part of the body in which it lives and the effect on the limbs.

4. Why is the name ringworm misleading?

5. Give four measures of vector control.

References

• World Health Organization (2024). Fact sheets: Lymphatic filariasis; Soil-transmitted helminth infections; Amoebiasis in the Weekly Epidemiological Record.
• Pratt H. D. and Moore C. G. (1993). Mosquitoes of public health importance and their control, US Centers for Disease Control and Prevention.
• NCERT (2023). Biology, Textbook for Class XII, chapter 7.