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The unwanted guests
Parasites are often defined as organisms that live at the expense of other organisms (hosts). This definition, however, implies that parasites are harmful, which is not always the case.
Some parasites are very beneficial. For instance, ciliates living in the rumen of the cattle produce enzymes to digest food. These parasites are called symbionts, that is they have a mutually advantageous relationship with the host. This relationship was probably established millions of years ago, during the course of evolution.
The first mammals were apparently carnivores, with a simple digestive tract. Because of the shortage of meat, and other factors, some were forced to eat plants, but had no enzymes to break down cellulose. This was achieved when ciliates and bacteria from the soil and water made rumen their home. Without these symbionts, the herbivores cannot survive.
A fascinating example of symbiotic relationship is seen in the case of leaf-cutter ants. These ants grow a mushroom-like fungus in underground gardens, which is their main food source. The fungus in turn is dependent on ants for its food in the form of leaves. However, the fungus can be destroyed by a green mould if it enters the garden. To prevent this from happening the ants carry a bacterium which produces an antibiotic lethal to the green mould!
Many parasites are commensals. That is they are neither harmful nor beneficial to the host. A number of protozoa in the intestinal tract fall into this group.
A few parasites are certainly “unwanted guests” and cause various health problems. These are called pathogens. A good example of a pathogen is Plasmodium, the malarial parasite.
Parasites are divisible into two main groups — protozoa and helminths. In a simplified classification, protozoa which are of medical importance, can be further divided into five phyla:
— Sarcodina or amoebae;
— Mastigophora or flagellates;
— Ciliphora or ciliates;
— Apicomplexa or organisms which possess a structure known as the “apical complex” at the anterior (front) end, and;
— Microspora or minute tissue-dwelling organisms which form spores.
The helminths are divided into three major phyla:
— Nematoda or roundworms;
— Platyhelminthes or flatworms, and;
— Acanthocephala or thorny-headed worms.
The platyhelminthes are further divided into two classes:
— Cestoda or tapeworms, and;
— Trematoda or flukes.
Unlike bacteria and viruses, parasites usually have a complex life cycle and may have one or more intermediate hosts. This makes their survival difficult, but they have devised some ingenious ways of overcoming the problem and passing from one host to another.
Parasites employ a number of methods to effect transmission. These include ingestion, direct penetration of the skin and the mucous membranes, inoculation by arthropod vectors, sexual intercourse (venereal transmission), and vertical or congenital (mother-to-foetus) transmission. Occasionally, parasites get transmitted by blood transfusion and transplantation of tissues.
The protozoan parasites are unicellular organisms which are widely distributed in nature and each protozoan cell is able to perform all the vital functions of life.
The following genera are amongst some of the medically important protozoan parasites:
— Entamoeba (an intestinal amoeba);
— Giardia (an intestinal flagellate);
— Trichomonas (a genital flagellate);
— Leishmania (a tissue flagellate);
— Trypanosoma (a blood flagellate);
— Plasmodium (a blood parasite);
— Toxoplasma (a tissue parasite);
— Balantidium (an intestinal ciliate);
— Cryptosporidium (an intestinal parasite), and;
— Enterocytozoon (an intestinal parasite).
In the genus Entamoeba, only one species, E. histolytica, is pathogenic, which is the causative agent of amoebic dysentery and is a very common infection of the third world countries. It is acquired by the ingestion of food and water contaminated by the cysts of E. histolytica, and passed in the faeces of the human carriers.
Giardia is another common intestinal parasite, acquired by ingesting contaminated food and water. The parasite has a characteristic shape with a disc-like structure on its ventral surface, by which it attaches itself to the mucous membrane of the small intestine.
In heavy infection, large parts of the small intestine may be covered with the parasites, preventing absorption of nutrients, especially fats. The genus Trichomonas has three species, of which only one is pathogenic (T. vaginalis).
This lives in the genital tract of the female and is transmitted by sexual intercourse. The male acts as the carrier of the parasite.
Leishmania is composed of many species, causing a variety of illnesses, ranging from the ulcers of the skin to systemic infection (visceral leishmaniasis), which can be fatal if left untreated. The parasite is transmitted by the sand-fly.
Trypanosoma has two species, one causing African trypanosomiasis, and the other causing South American trypanosomiasis. African trypanosomiasis is also known as sleeping sickness as it involves the brain and is transmitted by the tsetse fly. The South American trypanosomiasis is also called Chagas’ disease, in which the cardiac muscle is involved.
This parasite is transmitted by the assassin or cone-nosed bug. The infective stage of the parasite is passed in the faeces of the bug, and the parasite gets inoculated through the puncture wound by the rubbing of the bitten area.
Toxoplasma has a worldwide distribution, and infection occurs by ingestion of improperly cooked meat or fondling of cats, as one of the infective stages is passed in cat faeces. The majority of individuals infected by this parasite are asymptomatic or show only a mild form of the disease.
In the compromised host, however, infection gets reactivated and the parasites undergo unrestricted multiplication, leading to severe illness. Toxoplasma can also pass from the mother to the foetus and it is an important cause of abortion and congenital deformities in countries where the infection is common.
Plasmodium, which is the causative organism of malaria, has four species. One of the species, P. falciparum, is the most dangerous, because it can give rise to a number of complications, including brain involvement (cerebral malaria), which carries a high mortality in untreated cases.
Plasmodia are transmitted by the bite of the female Anopheline mosquito and the parasite undergoes asexual development in the human, inside the red blood cells, and sexual development in the mosquito, in its stomach.
This complicated life cycle, and the multiple steps involved, has made it difficult to produce an effective vaccine against Plasmodium. Multiple drug resistance malaria is now widespread and the danger is that we may run out of treatment options, unless new drugs are discovered soon.
Balantidium is the only ciliate which infects humans. Its body is covered with hair-like cilia which enables it to move actively. It lives in the large intestine and can produce dysentery. Its main reservoir is the pig and humans get infected from pigs.
Cryptosporidium is acquired by the ingestion of oocysts passed in human faeces. The organism multiplies in the superficial part of the intestinal epithelium. Normally it produces little or no disease, but in people living with AIDS it can produce severe protracted diarrhoea.
Microspora is a large and complex group of protozoa. The genus Enterocytozoon is the most common microspora infecting humans and is a major cause of AIDS-related diarrhoea. Human-to-human transmission via the faecal-oral route is probably the most important mode of transmission.
The most common intestinal nematode infecting man is Ascaris. It resembles the earthworm in size and shape but is yellowish-white in colour. It resides in the small intestine and releases approximately 250,000 eggs per day.
For this, it needs to obtain proteins from the host. An infected child may carry dozens of worms leading to nutritional problems. The eggs of Ascaris are extremely resistant to chemical treatment, and can develop even in formalin solution and can survive in the soil for months. Humans get infected by ingestion of the embryonated eggs from the soil.
Hookworms (Ancylostoma and Necator) are so-called because their anterior end is curved or hooked. This parasite also develops in the soil and the infective stage is a larva, which penetrates the skin when the individual is walking bare-footed.
The adults get attached to the mucous membrane of the small intestine by their teeth or cutting plates. After attachment, they continuously suck blood and an estimated 0.15ml of blood is lost per worm each day.
Hookworms are, therefore, a very important cause of iron-deficiency anaemia in the endemic regions of the world. Strongyloides stercoralis, like hookworms, develops in the soil and enters humans in the tissues by skin penetration. It is only the medically important nematode which can multiply tissues by parthenogenisis and can cause life-threatening disease in people with AIDS.
Enterobius vermicularis, also known as pinworm, is a small nematode measuring a few millimetres in length. The adults live in the large intestine of humans and the females emerge from the anus when the patient is at rest, laying eggs on the perianal region. This causes pruritis or itching of the anal region which can be very disturbing to children, among whom the infection is common.
Trichuris trichiura is also known as the whipworm because of its whip-like shape. Infection occurs by the ingestion of eggs from the soil. Light infections are generally asymptomatic but heavy infections can cause dysentery and prolapse of the rectum.
Trichinella spiralis is essentially a parasite of pigs, infecting humans through the consumption of pork. The larvae are carried in blood to various tissues of the body where they encyst. Heavy infections can produce serious disease and this is probably the reason why pork was prohibited in Islam and Judaism.
There are many species of filarial parasites and all are transmitted by arthropods. Wuchereria is the most widely prevalent genus, and causes massive enlargement of the limbs, scrotum and breasts, known as elephantiasis. This happens because the adult blocks the lymphatic drainage.
Another filarial parasite, Onchocerca, is transmitted by the black fly, and is the most important cause of blindness in the sub-Saharan Savannah belt of Africa. Blindness occurs because the microfilaria (embryos), by the adult worm, migrate to the eye and damage it.
There are three important genera of tapeworms. These are Diphyllobothrium, Taenia and Echinococcus. Diphyllobothrium is also the fish tapeworm, as the infection is acquired by eating improperly cooked fish.
It is the largest human tapeworm and can extend up to 33 feet, producing one million eggs per day. It causes vitamin B12 deficiency, by absorbing as much as 80 to 100 per cent of orally taken vitamin B12, thus depriving the host of this vitamin, which leads to pernicious anaemia.
Taenia are acquired by eating improperly cooked pork or beef. The pork tapeworm (T. solium), is particularly dangerous, as its larval stage (Cysticercus celluosa) can get into the brain and give rise to epilepsy and other serious problems.
Echinococcus is acquired by ingesting eggs, passed in the dog faeces, which harbours the adult worm in its small intestine. The adult worm is attached to the dogs’ intestinal tissues by scolex, which is made up of rostellum with hooks and four suckers.
The larval stage of the parasite is known as the Hydatid Cyst which develops in herbivores and humans. The liver is often involved, followed by the lungs and brain. The cyst can grow to an enormous size and can become larger than a ball. Inside the cysts are thousands of immature worms, which become adults when the infected tissues from the herbivores are eaten by dogs and other carnivores.
Amongst the flukes, two parasites are particularly important. These are Clonorchis, also known as the Chinese liver fluke, and chistosoma, known as the blood fluke. Clonorchis is acquired by improperly cooked fish. It resides in the biliary passages of the liver, leading to cirrhosis (fibrosis) and occasionally to liver cancer.
Schistosoma are thin elongated parasites which live inside the blood vessels. They use snails as their intermediate host, and the infective stage (cercaria) penetrates the skin to initiate the infection.
The adult worms remain in copulation throughout their long life of 20-30 years. The eggs, released in blood vessels, find their way out of the body via the faeces and the urine. However, a significant number are unable to do so, and get trapped in the urinary bladder and the intestinal tissues.
The disease schistosomiasis is caused by these trapped eggs. This infection is common in Egypt and calcified eggs can still be detected in Egyptian mummies.
Parasites, unlike bacteria and viruses, produce a great deal of morbidity (chronic sickness), rather than high mortality, with a few exceptions.
The morbidity adversely affects the productivity of the population, thus causing economic loss. In affluent societies, many parasitic diseases have been reduced or eliminated. Similarly, affluent families and individuals in developing countries show a lower incidence of parasitic diseases when compared to economically depressed classes. Therefore, parasitic diseases are mostly diseases of poverty and ignorance.
Once poverty and disease get established, a vicious cycle develops. People become sick because they are poor and unable to get themselves treated properly. Because they are sick they cannot work and get even poorer. One accentuates the other, debilitating and destroying whole communities.
In view of the rapid spread of international travel and movement of people across the borders, parasitic infections have seen a resurgence all over the world. This is accentuated by the emergence of AIDS, which renders some relatively harmless parasites into life-threatening infections. It is for this reason that parasites now command a great deal of attention even in the developed countries.
This article has been published with permission from World Scientific, the publishers of Prof Zaman’s book Life sciences for the non-scientist
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