General Characteristics of Pathogens
Helminthes are worms with complex and developed organ systems: excretory, digestive, nervous, and reproductive. They can infect various human tissues and organs, including the blood, liver, and intestines. The infection can be completely asymptomatic or progress to a manifest disease known as helminthiasis.
For clinical purposes, all helminthes are divided into three main classes:
- Nematodes (roundworms).
- Trematodes (flukes).
- Cestodes (tapeworms).
Transmission Routes and Development in the Body
Human infection occurs through various mechanisms:
- Alimentary route: ingestion of water or food contaminated with eggs or larvae.
- Percutaneous route: active penetration of larvae directly through the skin from the soil.
- Vector-borne route: transmission via bites of vector insects.
Upon entering the human body as the definitive host, eggs or larvae undergo a specific developmental cycle, including tissue migration, and reach the adult stage. Adult worms release new eggs or larvae into the environment via the excretory and digestive systems. In addition, larvae can pass from humans into insects, where they regain pathogenicity, completing the epidemiological cycle.
Classification of Nematode Infections
Diseases caused by roundworms (Nematoda) are ubiquitous. They are generally classified according to their geographic distribution and the localization of the pathogen into two major groups:
- Ubiquitous nematode infections:
- Intestinal (small intestine): ascariasis, enterobiasis, strongyloidiasis.
- Intestinal (large intestine): trichuriasis.
- Extraintestinal: trichinellosis, which affects skeletal muscle.
- Tropical nematode infections (typical of tropical climate regions):
- Intestinal: ancylostomiasis, necatoriasis.
- Extraintestinal: dracunculiasis (subcutaneous tissue), filariasis (blood vessels), onchocerciasis (eyes and lymphatic vessels).