Understanding Parasites: Types, Transmission, and Impact on Humans
Indirect Defecation
Indirect defecation occurs when there is a prior maturation in the environment of eggs or larvae of the parasite, and then a child playing on the ground ingests them.
Parasite Pathogenicity
Mechanisms of parasite pathogenicity:
- Expoliatory or subtractive
- Obstructive and mechanical (pressure atrophy)
- Toxic
- Immunoallergic
- Inoculated or infectious
- Traumatic
- Necrotic and/or tissue lytic
Propagation and Dissemination of Parasites
Forms of resistance to the environment: These are generally infectious forms. Transmission occurs through contact or via a vector.
- Protozoa: Cysts, oocysts
- Helminths: Eggs, larvae, metacercariae
- Arthropods: Larvae, pupae, etc.
Vehicles of Transmission
Inanimate elements (fomites): Water, food, soil
Living things:
- Mechanical vectors
- Biological vectors
- Natural reservoirs
- Hosts
Nematodes
Nematodes are helminths, cylindrical worms, unsegmented, elongated, of different sizes and thicknesses. They are invertebrates, reproduce by eggs, lack a circulatory system, have a highly developed reproductive system, and exhibit sexual dimorphism. Males are smaller than females, with a thinner body and a curled end. They have a complete digestive tract and a pseudocoelom cavity. The seed coat has three layers: a non-nucleated cuticle, hypodermis, and muscle fiber.
Digestive tract:
- Mouth (cutting plates, hooks, and chisels allow attachment to tissue)
- Oral cavity (wide and bulbous)
- Esophagus (distal end has a muscular bulb)
- Midgut (absorbs digested food)
- Rectum (opens to the outside via the anus)
In males, the intestine and genitalia flow together into the cloaca, while in females, they open separately.
Life cycle: Depends on the environment; can be monoxenic or heteroxenic.
Molting: Involves the formation of a new cuticle (not all nematodes molt), resizing, increased structural complexity, and increased antigenic complexity.
Platyhelminthes
Platyhelminthes are cestodes, segmented, flattened, and of uniform thickness. They are internal parasites (inhabiting the host’s small intestine). They are hermaphrodites, lack a digestive system, and feed by osmosis. They do not have a single tissue cavity. They have a complex life cycle with an intermediate host.
The scolex is circular and presents in larval form. It has a head (with a crown of suckers for attachment) and a neck. The body is called the strobila, which is divided into segments called proglottids. Proglottids contain eggs and are grouped according to maturity: immature at the top, mature in the middle, and gravid at the end.
Platyhelminthes: Trematodes
Trematodes have a leaf-like shape, a smooth and flattened body, and an incomplete digestive system (no anus or cloaca).
Types of Parasites
- Optional: Can live inside or outside the host.
- Stray: Parasite found in a host it does not normally infect.
- Erratic: Parasite found in the wrong location within the host.
- Temporary: For example, pig larvae, which then infect a host.
Types of Hosts
- Definitive
- Intermediate
- Accidental
- Paratenic
Trichinosis
Trichinosis is a zoonosis caused by nematodes of the genus Trichinella, primarily Trichinella spiralis. It mainly affects mammals. The parasite exhibits sexual dimorphism: the male is not coiled, while the female is viviparous, producing 1000 to 1500 larvae per day.
Autoxenic cycle: The definitive host and the intermediate host are the same.
Collagen in the capsule of the larva in muscle forms cysts. The larva does not fall to the ground and never becomes infective.
Wild cycle: Wild carnivores infect other wild carnivores, and then humans.
Domestic cycle: Rats (which are either sick or serve as reservoirs) infect pigs, which then infect humans (carnivores). Eating infected pork transmits the disease. The cyst enters the stomach, the larva is released and travels to the intestine, where it differentiates into males and females. After copulation, the larvae travel through the bloodstream and encyst in striated muscle fibers, particularly in the intercostal muscles and diaphragm.
Symptoms of Trichinosis
Incubation period (3-30 days): Nonspecific symptoms such as muscle aches, difficulty breathing, facial edema, and inflamed eyeballs.
Invasion period (7-11 days): Infectious syndrome with oculopalpebral symptoms.
State period: Gastrointestinal and muscular pain.
Diagnosis of Trichinosis
In humans, consider:
- Clinical history
- Epidemiological antecedents
- Laboratory tests (CBC, biochemical profile with elevated CPK (creatine kinase))
In pigs: Inspection in slaughterhouses, ELISA, trichinoscopy.
Epidemiology of Trichinosis
Trichinosis is present worldwide but is most commonly found in the Northern Hemisphere and Africa. In Latin America, it is prevalent in Mexico, the Bahamas, Argentina, Uruguay, and Chile, particularly during autumn and winter.
Factors Contributing to Transmission
- Lack of health education
- Raising and slaughtering animals at home
- Clandestine commercialization
- Consumption of meat and meat products without veterinary inspection
Prophylaxis
- Avoid infection in pigs
- Maintain hygienic conditions indoors
- Do not feed raw waste to animals
- Properly eliminate rodents
- Properly dispose of human excreta
- Avoid buying meat of dubious origin