Parasitic Diseases Notes

Parasitic Diseases

Diseases caused by parasites (protozoa and helminths) are quite common and comprise a very large group of infestations and infections in human beings.

Parasites may cause disease due to their presence in the lumen of the intestine, due to infiltration into the bloodstream, or due to their presence inside the cells.

Read And Learn More: General Pathology Notes

A short list of parasitic diseases is given in the table. These diseases form a distinct subject of study called Parasitology; only a few conditions are briefly considered below.’

Amoebiasis

Amoebiasis is caused by Entamoeba histolytica, named for its lytic action on tissues. It is the most important intestinal infection of man. The condition is particularly more common in tropical and subtropical areas with poor sanitation.

The parasite occurs in 2 forms:

1. A trophozoite form which is the active adult form seen in the tissues and diarrhoeal stools; and
2. A cystic form is seen in formed stools but not in the tissues.

The trophozoite form can be stained positively with PAS stain in tissue sections while amoebic cysts having four nuclei can be identified in stools. The cysts are the infective stage of the parasite and are found in contaminated water or food.

The trophozoites are formed from the cyst stage in the intestine and colonise in the caecum and large bowel . The trophozoites as well as cysts are passed in stools but the trophozoites fail to survive outside or are destroyed by gastric secretions.

Diseases caused by parasites:

Morphologic Features:

The lesions of amoebiasis include amoebic colitis, amoeba, amoebic liver abscess and spread of lesions to other sites.

• Amoebic colitis:
  • The most common type of amoebic infection begins as a small area of necrosis of mucosa which may ulcerate.
  • These ulcerative lesions may enlarge, develop undermining of margins of the ulcer due to lytic action of the trophozoite and have a necrotic bed.
  • Such chronic amoebic ulcers are described as flask-shaped ulcers due to their shape.
  • The margin of the ulcer shows an inflammatory response consisting of admixture of polymorphonuclear as well as mononuclear cells besides the presence of trophozoites of Entamoeba histolytica.

• Amoeboma:  Amoeboma is the inflammatory thickening of the wall of the large bowel resembling carcinoma of the colon.
• Microscopically: The lesion consists of inflammatory granulation tissue, fibrosis and clusters of trophozoites at the margin of necrotic with viable tissue.
• Amoebic liver abscess: May be formed by invasion of the radicle of the portal vein by trophozoites. An amoebic liver abscess may be single or multiple .
  The amoebic abscess contains yellowish-grey amorphous liquid material in which trophozoites are identified at the junction of the viable and necrotic tissue.
• Other sites: Where the spread of amoebic infection may occur are peritonitis by perforation of amoebic ulcer of colon, extension to the lungs and pleura by rupture of amoebic liver abscess, haematogenous spread to cause amoebic carditis and cerebral lesions, cutaneous amoebiasis via the spread of rectal amoebiasis or from anal intercourse.

Malaria

Malaria is a protozoal disease caused by any or combination of four species of plasmodia:

• Plasmodium viva
• Plasmodium falciparum
• Plasmodium ovale and
• Plasmodium malaria.

While Plasmodium falciparum causes malignant malaria, the other three species produce benign forms of illness.

These parasites are transmitted by the bite of the female Anopheles mosquito. The disease is endemic in several parts of the world, especially in tropical Africa, parts of South and

Central America, India and South-East Asia. The life cycle of plasmodia is complex and is diagrammatically depicted in A. P.

Falciparum differs from other forms of plasmodial species in 4 respects:

1. It does not have an exo-erythrocytic stage.
2. Erythrocytes of any age are parasitised while other plasmodia parasitise juvenile red cells.
3. One red cell may contain more than one parasite.
4. The parasitised red cells are sticky causing obstruction of small blood vessels by thrombi, a feature which is responsible for the extraordinary virulence of P. falciparum.

The main clinical features of malaria are cyclic peaks of high fever accompanied by chills, anaemia and splenomegaly.

Morphologic Features:

• Parasitisation and destruction of erythrocytes are responsible for major pathologic changes as under ):
• Malarial pigment liberated by destroyed red cells accumulates in the phagocytic cells of the reticuloendothelial system resulting in enlargement of the spleen and liver (hepatosplenomegaly).
• In falciparum malaria, there is massive absorption of haemoglobin by the renal tubules producing blackwater fever (haemoglobinuric nephrosis).
• At autopsy, cerebral malaria is characterised by congestion and petechiae on the white matter.
• Parasitised erythrocytes in falciparum malaria are sticky and get attached to endothelial cells resulting in obstruction of capillaries of deep organs such as the brain leading to hypoxia and death. If the patient lives, microhemorrhages and microinfarcts may be seen in the brain.

The diagnosis of malaria is made by demonstration of malarial parasites in thin or thick blood films or sometimes in histologic sections.

Differential diagnosis has to be made from babesiosis which is caused by malaria-like protozoa, Babesia microti and B. divergens (see below) and transmitted by deer-tick which also causes Lyme disease.

Major complications occur in severe falciparum malaria which may have manifestations of:

1. Cerebral malaria (coma)
2. Hypoglycaemia
3. Renal impairment
4. Severe anaemia
5. Haemoglobinuria
6. Jaundice
7. Pulmonary oedema and
8. Acidosis followed by congestive heart failure and
9. Hypotensive shock.

Babesiosis

Babesiosis is a tick-born infectious disease caused by protozoa of the genus Babesia; the most common is Babesia microtia. It is caused by infected deer ticks of the Genus Ixodes; the other deer-tick-born disease is Lyme disease and both diseases have the same geographic distribution.

• The organisms, B.microtia, invade and lyse red blood cells, or the infection can be acquired by infected blood transfusion. The infection may remain asymptomatic and without long-term complications or may produce features such as malaise, fatigue, weakness and high-grade fever.
• The diagnosis is made by identification of the obligate parasite B.microti in the red cells as trophobioses, ring form or amoeboid forms, requiring distinction from malaria; however, extracellular parasites can also be seen if parasitaemia is high.

Filariasis

Wuchereria bancrofti and Brugia malayi are responsible for causing Bancroftian and Malayan filariasis in different geographic regions. The lymphatic vessels inhabit the adult worm, especially in the lymph nodes, testis and epididymis.

Microfilariae seen in the circulation is produced by the female worm Majority of infected patients remain asymptomatic. Symptomatic cases may have two forms of the disease — an acute form and a chronic form.

• Acute form of filariasis presents with fever, lymphangitis, lymphadenitis, epididymal-orchitis, urticaria, eosinophilia and microfilariaemia.
• Chronic form of filariasis is characterised by lymphadenopathy, lymphoedema, hydrocele and elephantiasis.

Morphologic Features:

• The most significant histologic changes are due to the presence of adult worms in the lymphatic vessels causing lymphatic obstruction and lymphoedema.
• The regional lymph nodes are enlarged, and their sinuses are distended with lymph.
• The tissues surrounding the blocked lymphatics are infiltrated by chronic inflammatory cell infiltrations consisting of lymphocytes, histiocytes, plasma cells and eosinophils.
• Chronicity of the process causes enormous thickening and induration of the skin of legs and scrotum resembling the hide of an elephant hence the name elephantiasis.
• Chylous ascites and chyluria may occur due to rupture of the abdominal lymphatics.

Cysticercosis

Cysticercosis is an infection by the larval stage of Taenia solium, the pork tapeworm. The adult tapeworm resides in the human intestines.

• The eggs are passed in human faeces which are ingested by pigs or they infect vegetables. These eggs then develop into larval stages in the host, spread by blood to any site in the body and form cystic larvae termed cysticercus cellulose.
• Human beings may acquire infection by the larval stage by eating undercooked pork (‘measly pork’), ingesting uncooked contaminated vegetables, and sometimes, by autoinfection.

Morphologic Features:

The cysticercus may be single or there may be multiple Mysticeti in the different tissues of the body.

• The cysts may occur virtually anywhere in the body and accordingly produce symptoms; the most common sites are the brain, skeletal muscle and skin.
• Cysticercus consists of a round to oval white cyst, about 1 cm in diameter, with hooklets. The cysticercus may remain viable for a long time and incite no inflammation.
• But when the embryo dies, it produces a granulomatous reaction with eosinophils. Later, the lesion may become scarred and calcified.

Parasitic Diseases:  

• Amoebiasis is caused by Entamoeba histolytica and the lesions produced are amoebic colitis, amoeboma, and amoebic liver abscess.
• Malaria is a protozoal disease caused by any one or combination of four species of plasmodia:
  • Plasmodium vivax
  • Plasmodium falciparum
  • Plasmodium ovale and
  • Plasmodium malaria.
• Babesiosis is a deer-tick-born parasitic disease in which the protozoa, Babesia microti, invade and lyse RBCs and is required to be distinguished from malaria
• Wuchereria bancrofti and Brugia malayi cause Bancroftian and Malayan filariasis.
• The lymphatic vessels inhabit the adult worm, especially in the lymph nodes, testis and epididymis, while microfilariae are seen in the circulation.
• Cysticercosis is an infection by the larval stage of Taenia sodium, the pork tapeworm, and produces cystic larvae in different tissues called cysticercosis cellulose.