Helminth Infections – The Great Neglected Tropical Diseases

From biotechnology to bioethics, Biology Topics have significant implications for society as a whole.

The Effect of Helminth Infections and Their Treatment on Metabolic Outcomes

Filariasis

Filariasis is a disease in man caused by a type of thread-like worm called Filaria (Wuchereria bancrofti). In this infection, the initial disease symptom is lymphangitis with the manifestation of high fever and subsequent swelling of feet, hand, scrotum, and mammary gland may occur resulting in elephantiasis. Filariasis is an immense problem throughout the world and about 250 million people are infected with filariasis. In tropical and subtropical countries about one billion are at risk of infection by Wuchereria bancrofti. Though the disease is caused by the nematode parasite, the infection may get spread by the species of mosquito of the genus Culex (Mainly Culex quinquefasciatus).

Causative Organism of Filariasis

Filariasis, in India, is caused mainly by the species of nematodes namely Wuchereria bancrofti. The adult worms live in the lymphatics and they produce the larvae called microfilariae which invade the bloodstream. The nematode species use two hosts to complete its life cycle and these are man and mosquito. Man is the definitive host in which the adult forms of the parasite harbour and mosquito (female) is the intermediate host as it carries the larval forms of the organism.

A. Life Cycle in Human Host:
1. As mentioned before, human is the definitive host of Wuchereria bancrofti. The infective forms of the parasite are larvae, known as microfilariae which get entry into the human body through a mosquito bite.

2. The infective stage of microfilaria is its third stage larval form which enters through the skin wound developed during a mosquito bite. The larva then migrates through the lymphatic vessels and reaches the groin, armpit region, and scrotum. In these regions, they grow within 5-18 months to become adults.

3. The adult is whitish in colour and long thread-like in appearance and such a form is called filaria. Both ends of the thread-like body are gradually tapering, but the anterior end contains swelling. The adult worms exhibit sexual dimorphism. The male measures about 2.5-4 cm × 0.1 mm in size with a curved tail end and the female is about 8-10 cm × 0.3 mm in measurement with a narrow pointed tail end. The life span of the adult worm is about 18 years or more.

4. Female worm is ovo-viviparous and they lay a well-developed embryonated egg. From such an egg, a microfilaria larva comes out and invades the blood circulation of the host. Each microfilaria measures about 250 µm × 6-7 mm and remains within a sheath formed by the prolongation of the egg membrane. The larva contains a well-developed cuticle which is composed of sub-cuticular cells. The central axis of the body remains filled with some granules.

B. Life Cycle in the Mosquito:

• During the sucking of blood, the female Culex mosquito takes the microfilariae form of larvae into its stomach, where shedding of its sheath occurs within 2-4 hours.
• The larva penetrates the stomach wall and reaches the thoracic muscle in 12 hours.
• Further development occurs within the thorax of the mosquito and the microfilaria develops into a first-stage larva measuring 124-250 µm × 10-17 pm within the next two days.
• With shedding cuticle in 3-7 days the first stage larva is transformed into the second stage larva when it measures about 220-330 µm × 15-30 pm. This larval stage contains a well-differentiated digestive tract and its tail gets atrophied.
• By the 10th-11th day another ecdysis occurs and the filiform elongated third-stage larva appears (measurement 1.4-2 mm × 18-23 µm). The infective third-stage larva migrates to the proboscis of the female mosquito and waits for the chance to invade the human host. The microfilaria takes about 10-12 days to complete its development in the mosquito.

Incubation Period of the Parasite
In case of Wuchereria bancrofti, the time from the entry of the third infective stage larva in the human body to the appearance of microfilariae in the bloodstream is considered a biological incubation period. It is usually one year or more.

Mode of Transmission of Filariasis

In the body of the infected person, microfilariae appear in the peripheral blood during the night (peak hour 10 pm to 2 pm). However, there are two forms of W. bancrofti periodic and subperiodic. The periodic form of the worm shows that its microfilariae mature more during the night. In Asia, the periodic (nocturnal) forms are more prevalent.

The subperiodic forms show minor fluctuation and the microfilariae are found in the blood for the whole 24 hours showing a higher count during the afternoon and the evening. The female of Culex quinquefasciatus during the sucking of blood at night gets microfilariae which develop inside the body of the mosquito. The third stage larva after their development resides in the proboscis of the female mosquito. Such a mosquito when bites a healthy man, the larva (infective third stage) of W. bancrofti may enter the body of a man to develop a fresh infection.

Clinical Manifestation of Filariasis
The following disorders may be observed during filaria infection:

• Lymphangitis is accompanied by high fever (103°-104°F), chill, and rigor. Main affected area is the lymphatics of the testis and epididymis.
• Lymphangitis at groin and axilla.
• Lymphangiovarix, i.e., dilation of lymphatics; dilated lymphatics may rupture causing hydroxide, lymph scrotum, and lymphuria.
• Hyperplasia of connective tissue and skin resulting in lymphoedematous condition. In extreme cases elephantiasis results.
• Secondary infection by bacteria causing septic lymphangitis.

Vectors of Filaria Transmission
Though filaria is transmitted chiefly by Culex quinquefasciatus, some other species of mosquito have also been reported to be involved in transmitting the disease.

Vectors of W. bancrofti:

Name of the Species	Reported
1. Culex quinquefasciatus	India
2. Aedes Polynesiensis	South Pacific
3. Aedes harinasutai	Thailand
4. Anopheles & other mosquito	Malaysia and Indonesia

Control of Filaria

By proper treatment of the diseased person, preventing vector biting, and controlling the population of vector species filariasis may be controlled in the population.
1. Prevention of the incidence of filaria infection:

• Prophylactic measures:
  • Use of mosquito net to keep the individual away from mosquito bites.
  • Use mosquito repellants to prevent biting. Some repellants are Odomos, All-Out, mosquito coil, etc.
• Treatment of the infected person:
  • The infected person must be kept away from mosquito bites.
  • The treatment of the infected person should be done without delay as per the advice of the medical practitioner.
  • WHO recommends the administration of the drug diethyl carbamazepine in the population mass in the infected area.

2. Control of the Vector Population:
Proper knowledge of the breeding and behaviour of the vector species may help to control them easily. C. quinquefasciatus takes shelter in cowsheds, in dark corners of the dwelling houses, behind the dumped clothes and cupboards. Their preferred breeding sites are ground pools, ditches, cement pots, masonry tanks, domestic collection of water, catch basins, sewage farms, and other small collections of water. Therefore, some measures to control the population growth of this mosquito or other vector species may be enumerated as under:

• Reduction of breeding grounds: Accumulation of water in domestic areas should not be allowed. Houses and surroundings should be kept clean.
• Chemical control: The larviciding effect may come from spraying kerosene or light diesel oil (malarial) in the stagnant water bodies and the population of the vector species may be brought to depletion. Adulticides like pyrethrum may be sprayed indoors for killing mosquitoes. Larvicide fenthion spraying in water bodies may give an effective block to the rise in vector population.
• Cultivating larvivorous fishes: To destroy the larvae in small ditches and water-logged zones cultivation of larvivorous fishes such as Gambusia, Guppy, and Panchax may be practised.

Ascariasis

Ascariasis is a parasitic disease in men when the infected person suffers from abdominal disorders with other associated symptoms. The infected person also suffers from severe malnutrition. Ascariasis is very common in third-world countries like India, Bangladesh, Korea, the Philippines, and also in China and the Pacific Islands. In the whole world, about 644 million individuals are affected by ascariasis and among them, about 488 million people are from Asian countries. Usually, people inhabiting the region having poor sanitation suffer from this disease. The disease is caused by the attack of a nematode parasite in man, commonly known as roundworm.

Causative Agent in Ascariasis
Ascariasis is caused by the round worm, Ascaris lumbricoides. Roundworm resides in the small intestine of man as a parasite. In the intestine of man, about 500-5000 roundworms may live. During the early phase of the life cycle, the parasite may travel through the organs of the body like the liver, heart, and lungs. This parasite uses only a single host, that is man. From contaminated food materials, the parasite may get entry into the body of the host.

Anatomical Features of Ascaris

• Body long and cylindrical somewhat identical to earthworm.
• In the intestine body appears slightly pinkish, but outside it appears milky white in colour.
• Anterio-posteriorly the body is tapering and pointed.
• The body anteriorly contains a mouth aperture which is provided with three lips.
• Sexual dimorphism is prominent in Ascaris and male Ascaris is smaller than female Ascaris.
• A fully mature female measures about 25-40 cm long with a 5 mm diameter, in comparison to which a mature male measures 15-25 cm × 3-4 mm.
• The posterior end of a male Ascaris is more pointed and curved, whereas in a female the posterior end is straight.
• In male Ascaris, the genital duct opens into the cloaca and the cloacal region is provided with a pair of copulatory spicules.
• In female Ascaris the genital aperture is present ventrally at the anterior 1/3 of the body, the anal aperture, and genital apertures are different.

Life Cycle of Ascaris
1. Eggs of Ascaris are ingested by man through contaminated food and water (Eggs of Ascaris are liberated with faecal matter of infected man) or by dirty habits like eating food without washing hands etc.

2. With the action of the enzyme in the intestine the outer covering of the egg gets dissolved and an aperture is formed. Through this aperture, the larva comes out in the intestine.

3. The larva measures about 14 × 0.25 – 0.33 pm and it is called rhabditiform larva. The larva then penetrates the mucous membrane of the intestine and enters the capillary vessels.
4. From there the larva migrates to the liver through portal circulation and in the liver, it may stay for 3-4 days.
5. Following this the larva reaches the heart through post caval vein and subsequently it goes to the lung via the right ventricle and the pulmonary aorta. In the lung, it stays for a few days and grows to a length of about 0.2-2 mm.
6. Ecdysis occurs in the larval body, thereafter the 3rd and 4th stages, the rhabditiform larva develops. Such larval form moves to the stomach and intestine of the host through the pharynx and oesophagus.
7. In the intestine the larva moults for the 4th time and forms the 5th stage rhabditiform and after 6-10 weeks maturity is attained by the larva.
8. Within two months of infection the female Ascaris starts laying eggs. The eggs may be liberated outside the body of an infected man through faeces.

Clinical Symptoms of Ascariasis
Some clinical symptoms that become prominent in men during infection by Ascaris lumbricoides are the following:

• Abdominal pain.
• Vomiting tendency.
• Loss of appetite.
• Regurgitation of water.
• Dysentery and liberation of worms with faecal material.
• Sometimes nematodes also come out through the mouth and nose.
• Salivation during sleep.
• Crushing of teeth at sleep and the act of speaking during sleep.
• Intestinal obstruction.
• Peritonitis.
• Appendicitis.
• Loffler’s syndrome or Ascaris pneumonia.

Mode of Infection of Ascariasis

The primary source of infection is contaminated food and drink. However, the ways through which Ascaris infection may occur are

• Entry of Rhabditiform larva through vegetables.
• Ingestion of the eggs of Ascaris with water.
• Taking of food by untidy hands.
• Inhalation of contaminated air (in air containing dried eggs & tiny larvae).

Control of Ascariasis

Treatment of the infected person, maintenance of proper sanitation, and practice of personal hygiene may rescue a man from ascariasis.

• Treatment: A person found to be infected with Ascaris must be treated without delay as per medical advice.
• There are drugs called nematicides which are given to the patients to destroy the nematodes.
• Maintenance of proper sanitation: This may be achieved by
  • The water supply of the locality must be protected from contamination by faecal material.
  • Water used for washing utensils as well as for bathing should be devoid of contamination.
  • The surroundings of the dwelling places should be cleaned.
• Practice of personal hygiene: They are
  • Always taking off well-cooked food.
  • Intake of filtered water.
  • Washing of hands with soap before taking food.

Taeniasis

Taeniasis is a parasitic disease in which the infected person suffers from an abdominal disorder and in severe cases disorder of the brain. The disease is caused by a tapeworm named Taenia solium (Pork) tapeworm) or Taenia saginata (Beef tapeworm). Taeniasis is very common in third-world countries and in men, who take the meat of pig or beef as food, therefore, the disease may be developed through taking meat from pigs or cattle. From this, it becomes clear that the parasite of this disease also harbors the body of a pig or cow/buffalo.

Causative Organism of Taeniasis and its Life Cycle
As mentioned the causative organism of Taeniasis in man is either Taenia solium or Taenia saginata. Taenia solium uses a pig as one of its hosts and is known as a pig tapeworm whereas T. saginata uses a cow as one of its hosts and is known as a beef tapeworm. Both species of tapeworms are the same with respect to their systematic position. Tapeworm lives as an internal parasite in the body of their host.

Hosts of Taenia

Two hosts are used by the tapeworm to complete its life cycle and therefore, Taenia may be called a digenetic parasite. In the case of Taenia solium the hosts are man and pig but in the case of T. saginata the hosts are man and cattle.

(a) Man as the host:
Man is used by tapeworm as its primary host of definitive host because the adult life period of the parasite is maintained in the body of man. The parasite lives in the intestine of man and remains anchored with its hooks in the scolex at the wall of the digestive tract. From there the parasite absorbs the digested food of the host and thereby, the host suffers from malnutrition and many abdominal disorders. Occasionally the larval stage of Taenia invades the tissues of the gut wall and may be carried to the brain of the host. As a result of this brain of the host is affected.

(b) Pig/Cattle as host:
As mentioned earlier the pig is the host of T. solium and the cattle is the host of T. saginata. These sub-human hosts are the secondary or intermediate hosts because the larval period of their life cycle is passed in the body of these hosts. The larval stage of the parasite may reside in the muscle of the secondary host and when pork or beef of the infected host is taken by a man, infection develops there.

Structure of Taenia

The body of a tapeworm is elongated and ribbon-like and it may be several meters in length. The body of Taenia may be divisible into scolex, week, and strobila. The scolex is the anteriormost region of the body adapted for adhering to the host and it is comparable to the head. Next to the scolex, the narrow portion is the neck which gives rise to the main body portion called the strobila.

The tip of the scolex contains an elevated portion which is called rastellum which is provided with 28 to 32 hooks. With the help of these hooks, the parasite may be anchored at the gut wall of the host. The scolex contains four round suckers. The small tubular region posterior to the head is the neck and posterior to this, the strobila is present in the form of about 850 segments. Each segment of the strobila represents a sexual unit called proglottid. The proglottids exhibit a gradual increase in size toward the posterior extremity. Strobila may be of three types immature proglottid, mature proglottid, and gravid proglottid. The neck region is followed by the immature proglottids and the gravid proglottids remain at the last. All the proglottids are encircled by a thick cuticle that gives protection to the parasite.

In the matured proglottid, well-developed male and female reproductive systems may be observed. Besides the reproductive system, a pair of lateral nerves and two ventral nerve cords are present. The proglottid also contains a pair of longitudinal excretory ducts. A gravid proglottid measures about 10-12 × 5-6 mm. It remains filled with fertilized eggs. From the posterior side, 5-6 gravid proglottids are released from the parasitic body along with the faecal material. This process of detachment of the gravid proglottid is known as apolysis. Taenia is a hermaphrodite animal and may perform self-fertilization. However, it is protandrous in nature because in tape worms male reproductive system is developed first. Because of its parasitic adaptation, tapeworm completely lacks a digestive system and respiratory system.

Life Cycle of Taenia Solium
Gravid proglottids containing fertilized eggs being liberated with the faeces come to the soil and the eggs may remain attached to grasses in the field. However, within the egg cell damage results in the formation of a metamere and a small embryonic cell. Due to further development, the embryonic cell forms the embryo properly, and the metamere forms a sheath surrounding the embryo. This sheath is called an embryophore. The embryo then at its posterior pole develops six hooks and at this stage, it is called a hexacanth embryo. The hexacanth embryo along with the embryophore and eggshell is known as the oncosphere. The eggs during detachment of the proglottids actually contain the oncospheres. Outside in the environment, the proglottid disintegrates, but the oncosphere containing the hexacanth embryo remains alive. Further development of the hexacanth embryo is not possible unless it enters the body of a secondary host.

Pigs habitually take dirty materials from the soil as food and the oncospheres from the soil may be swallowed by them. By the action of the digestive enzymes in the stomach of a pig, the embryophore and eggshell get digested, when the hexacanth embryo is released. With the help of the hooks, the embryo penetrates the gut wall and enters the bloodstream. The embryos then take shelter in the muscle and tongues of the pig. In the muscle tissue, the hexacanth embryo undergoes encystment and takes a bladder-like appearance. The bladder develops a fluid-filled cavity within which a proscolex is formed. Such a structure is called the bladder worm of cysticercus. Pork containing cysticercus is called measly pork. When a man takes measly pork he becomes infected Cysticercus of T. stigmata in the muscle of cattle is called Cysticercus bovis and meat is called measly beef.

In the stomach of a man, the bladder of cysticercus is digested and the proscolex undergoes evagination forming a scolex. The scolex with the help of its hooks gets attached to the gut wall and further development starts to form the neck and strobila region.

Clinical Manifestation of Taeniasis
Symptoms of Taenia infection are the following:

• Occlusion of the alimentary tract.
• Vomiting tendency
• Anaemia
• Gastric disturbance
• Indigestion
• Malnutrition
• Dysentry or Constipation
• Neural disorders
• In severe cases cerebral cysticercosis

Mode of Infection
Infection by Taenia in man usually occurs through ingestion of improperly boiled pork or beef.

Prevention of Taeniasis
The following measures may prevent the spread of taeniasis in the population:

• Infected persons should be treated without delay by the medical practitioner.
• People should be prevented from liberating night soil in exposed places.
• Cattle or pigs should not be allowed to ingest human faecal material.
• Beef or pork should have to be taken only after it is properly boiled.
• Personal hygiene must be maintained to keep oneself healthy.

Diseases Caused by Fungi – Ring Worm (Dermatophytes)

Ringworm or dermatophytosis is a type of fungal disease. It mostly affects people, with a lack of personal hygiene.

Pathogen for Ring Worm:
Fungus Trichahyton

Classification of Dermatophytes
Dermatophytes have been classified into three generations – Trichophyton, Microsporium, and Epidermophyton.

• Trichophyton: It infects skin, nails, and hair. T.rubrum is the common species infecting man.
• Microsporium: It infects skin and hair. M. Cavis is a common species of microsporangium.
• Epidermophyton: It infects the skin and nails. E. floccosum is a common species.

Clinically ringworm is also classified according to affected parts, like-

• Tinea pedis: (Athlete’s foot) is the ringworm of the foot.
• Tinea capitis: Ringworm of the scalp.
• Tinea cruris: Ringworm of groin and perineum.
• Tinea barbae: Affect the bearded area of the face and neck.

Symptoms of Dermatophytes

• Itching, the red colour of skin, raised, scaly patches on the skin that may blister and ooze.
• Red patches are often much redder around the outside with normal skin tone in the centre – looking like a ring.
• Bald patches in hair.

Diagnosis of Dermatophytes
Skin biopsy, Skin culture, KOH exam.

Control of Ring Worm
Use of drugs like ketoconazole, clotrimazole, etc.

Preventive Measures for Ring Worm

• Clean and dry skin and feet.
• Shampoo regularly after haircuts.
• Avoid sharing personal care.
• Avoid touching pets with bald spots.