Common Parasites Causing Human Diseases

The word parasite is composed of two Greek words: ‘para’ meaning beside and ‘sitos’ meaning food. In simple words, it means eating beside one another. Parasites can be defined as a group of species that live at the expense of the host (other species) making them connected both biologically and economically throughout the life span. The association between parasites and their hosts is referred to as parasitism.

Types of parasites:

Parasites can be grouped under two groups

1. Ectoparasites are parasites that live on the external surface of their host’s body. Hydramoeba is an ectoparasite as it feeds on the ectodermal cells of hydra.
2. Endoparasites are parasites that live inside the body of the host mainly for food. Such parasites do not have special organs for digestion as they feed on already digested food.

The life cycle of parasites can be categorised into two subparts: monogenetic parasites and digenetic parasites. Monogenetic parasites are those that have only one single host during their life cycle whereas digenetic parasites are those parasites that require two hosts during their life cycle. The two hosts are referred to as primary host and secondary host or vectors.

The primary host is the one in which the parasite’s ancestors evolved and the secondary vector hosts are the transmitting agents and the transmission is known as Cyclical and mechanical transmission. Sometimes the presence of a reservoir host is also found that can infect other animals once they become infected by the parasites, capable of causing parasitic diseases. 

All parasites post-infection show and incubation period, i.e., the period between entry of the pathogen in the host body and the first appearance of symptoms of the parasitic disease. In general, there are two main classes of parasites that are capable of causing parasitic diseases in humans: 

• Protozoa
• Helminths

The parasitic diseases that are caused by protozoa are malaria, trypanosomiasis, trichomoniasis, leishmaniasis. The parasitic diseases that are caused by helminths are common roundworms, tapeworms, filaria.

Malaria

Malaria is caused by a protozoan Plasmodium vivax. The life cycle of plasmodium vivax is digenetic, completed in two cycles: man and mosquito.

1. Man: Asexual cycle is passed in two phases in man. The infective stage is transmitted via a bite of the female anopheles mosquito along with its saliva. 
2. Mosquito: In female anopheles, the sexual phase is completed in two cycles.

Asexual phage includes entry of the pathogen parasite in the liver cells of humans, known as Liver schizogony, site of multiplication happens here i.e; multiple fusion forming merozoites and this marks the end of the First asexual phase of the asexual cycle in man.

In the second phase, erythrocytic schizogony formation in the blood is completed which later forms the Gametocytes at the end.

The steps can be listed as follows:

• Infection
• Sporozoites
• Liver schizogony
• Erythrocytic Schizogony
• Post erythrocytic schizogony
• Formation of Gametocytes

The sexual cycle is completed in two phases as well, they are gametogony and sporogony.

Gametogony is the production and fusion of gametes. Sporogony is the postzygotic multiplication that forms the infective individual known as the Sporozoites.

The steps can be listed as follows:

• Ingestion by mosquito
• Gametogony
• Fertilization
• Ookinete
• Encystment
• Sporogony

The incubation period of the pathogen for the parasitic disease is three weeks, during this period the pathogen grows and multiplies in the red blood cells and destroys the RBCs in the process.

The incubation period is used by the pathogen to increase progeny.

Chills and Fever are repeated on the third and fourth days. Three successive stages are generally seen in each fever attack, they are:

1. cold stage
2. hot stage
3. sweating stage

Control measures include destruction of mosquitoes and their larvae, avoiding mosquito bites by using repellants, and pest control.

Amoebiasis

Entamoeba histolytica is the protozoa causing amoebic dysentery that is the destruction of the lining of the large intestine and diarrhoea accompanied by griping pain and discharge of mucus and bloody stool.

The infection is caused by the contamination of food.

The incubation period is about one week

Dengue

Caused by any one of the four viruses, i.e, Dengue virus 1, 2, 3, 4 that is spread by the primary bite of mosquito Aedes aegypti and Aedes albopictus.

The dengue virus (DENV) is a single-stranded RNA virus belonging to the family Flaviviridae.

The mosquito vectors pick up the virus when they feed on the blood of an infected human, as a result, they can further spread the disease to other healthy humans via a bite.

In other words, it can be concluded that the Aedes mosquitoes are passive transporters of the parasitic disease dengue

.The incubation period for Dengue is about two-eight days that follows 3 phases in general, they are:

• Febrile
• Critical
• Convalescent.

The dengue fever lasts for about Two to Seven days, in severe cases, the platelet count decreases causing conditions like breathing difficulty and vomiting.

Taeniasis

They are caused by helminths, tapeworms.

Ingestion of infected pork causes Taeniasis. The tapeworm remains in the intestine and absorbs digested food of the host and using that they grow in length.

The matured worms then break off the small segments from their tail and pass out with faeces containing eggs.

The eggs when ingested by pig, hatch, and larval stage are reached. When uncooked pig meat is ingested the worm archives the maturity in the intestine.

Filaria

It is also known as Elephantiasis, the characteristic feature of this disease is swollen legs caused by a parasite known as Wuchereria bancrofti. The parasite attacks the lymphatic system. The disease is primarily spread by the Culex Mosquito which acts as a passive transporter of the disease as it unknowingly picks up the parasite when it sucks on the blood of an infected human. The parasites multiply in the mosquito and when it bites a new host it is introduced to the new host.

This parasitic disease is mainly seen in the rural areas of Bihar and the eastern parts of Uttar Pradesh. 

Conclusion

Parasites are capable of causing parasitic diseases in humans ranging from mild to severe conditions.

Mostly in humans, protozoans, mosquitoes, viruses are capable of spreading the diseases; however, that can be controlled by maintaining good hygiene, proper destruction of mosquito breeding areas and their larvae, clearing of blocked drains, and water collecting containers to prevent the growth of disease-causing mosquito larva.

In countries like India, malaria and dengue are two main parasitic diseases that require attention and making the mass population aware of the symptoms and treatments.