Symbiosis, Parasites, and Zoonotic Diseases: An In-Depth Look
Symbiotic Relationships
- Mutualism: Both species benefit.
- Commensalism: One species benefits, and the other is unaffected.
- Parasitism: One species benefits, and the other is harmed.
The parasite and host usually live in balance. When this balance breaks, disease occurs.
Types of Parasites
- Helminth: Multicellular worm.
- Protozoa: Unicellular eukaryotes.
- Arthropods: Multi-jointed appendages.
- Cestodes: Segmented flatworms.
- Trematodes: Unsegmented flatworms.
Types of Zoonotic Diseases
- Direct Zoonosis: The way it affects animal evolution is the same as it affects humans.
- Cyclical Zoonosis: Different evolutionary forms live in the animal and in humans.
- Metazoonosis: The evolutionary form can be the same or different in animals and humans, and arthropods are always involved.
- Saprozoonosis: No insect vector is involved, but the parasite needs to live part of its life cycle on earth.
Insect Anatomy and Physiology
Ecdysis: The shedding of the exoskeleton, made of chitin.
Insects have articulated appendages, antennae, mouthparts, and legs. Diptera have two pairs of wings.
Hemocoel: The internal cavity of the insect. Hemolymph: The blood of the insect. Insects have a tubular heart and an open, ganglionic brain located above the middle and posterior intestine.
One pair of membranous wings allows for flight. The other pair is atrophied (halteres) and helps to balance flight.
Insect Classification
- Nematocera: Mosquitoes; have hair-like or feathery antennae.
- Brachycera: Horseflies.
- Cyclorrhapha: Flies.
Specific Insect Groups
- Culicidae: Anopheles (transmits Plasmodium, causing malaria). Phlebotominae: Hairy flies (transmit Leishmania).
- Aedes: Transmits yellow fever virus. Culex: Common mosquito (causes irritation).
Mosquitoes have an oral apparatus suitable for piercing, compound eyes, and feathery or filiform antennae. The larvae develop in aquatic environments.
Mosquitoes have legs longer than their body, transparent wings, and a piercing oral apparatus (only females are hematophagous). The male fertilizes the female, who lays eggs in the water. Within 3-7 days, a vermiform (worm-like) larva is formed. After 3 weeks, it pupates, and in 2-5 days, it becomes an adult. The posterior part has a respiratory siphon. It feeds on plankton and has four pairs of sensory hairs.
Sandflies (transmit Leishmania) have a body covered with hair.
Horseflies have a large oral sucking apparatus and feed on the blood of animals. They have compound eyes.
Flies of Medical Importance
- Musca: 6-7 mm long. These arthropods are of great medical importance because they are mechanical vectors of diseases. Fly larvae produce myiasis. They have a complete life cycle.
The adult impregnates the female, who lays eggs. In 12-24 hours, a vermiform larva develops (stages 1, 2, 3, and 4) and then pupates. The imago (adult) emerges after 8-20 days. The complete cycle takes 3-4 days in still conditions (adults do not eat). With heat, it takes 4-7 days for the imago to emerge again. The hairs trap fungi, parasites, etc. (mechanical vector).
- Stomoxys calcitrans: Bites and feeds on the blood of animals, causing significant losses in milk and meat production.
- Glossina spp.: Tsetse fly (transmits Trypanosoma brucei, causing sleeping sickness).
Myiasis
Myiasis is an infection caused by fly larvae. It can be ophthalmic, otic, or cutaneous. There are three types:
- Mandatory or primary: The fly lays its eggs in animal or human flesh.
- Optional or secondary: The fly completes its life cycle on dead or decaying tissues.
- Accidental: Eggs are laid on the ground or in any inorganic tissue.
Lice
Lice: 2-4 mm long, blood-sucking ectoparasites. They have a segmented body, three pairs of legs emerging from the thorax, a flattened, squarish head, short antennae, and three pairs of legs adapted to grip hair. They exhibit sexual dimorphism. The female has a crack for laying eggs.
Nit: The egg has an operculum (where the nymph emerges). Females lay about 10 eggs per day. The eggs are attached to the hair with a sticky substance. Lice have an incomplete (hemimetabolous) life cycle, which they complete entirely on the host.