16.5 Parasite Classification & Arthropods

Parasite Classification & Arthropods

Quick Answer: Parasite classification schemes sort organisms by host relationship, life cycle, and transmission route. Arthropods occupy a special role: they act as vectors (transmitting pathogens) and as ectoparasites (living on the host). PA-CAT items test vector-pathogen pairings and the distinction between biological and mechanical transmission.

Classification Schemes

Parasites can be classified several overlapping ways:

• By host relationship: obligate parasites require a host to complete their life cycle (e.g., Plasmodium); facultative parasites can live free or parasitize (e.g., Naegleria fowleri, the free-living amoeba that causes primary amoebic meningoencephalitis when it invades the nasal mucosa).
• By life cycle: direct life cycles have a single host (e.g., Enterobius, person-to-person); indirect life cycles require one or more intermediate hosts (e.g., Schistosoma with its snail intermediate host; Plasmodium with mosquito and human).
• By transmission route: fecal-oral (Giardia, Entamoeba), skin penetration (hookworm, Schistosoma), vector-borne (Plasmodium, Leishmania, Trypanosoma), congenital (Toxoplasma), and foodborne (Taenia, Trichinella).
• By body site: intestinal, blood/tissue, and intracellular parasites.

The definitive host is where the parasite reaches sexual maturity; the intermediate host supports larval or asexual stages. For Schistosoma, humans are the definitive host and snails the intermediate. For Plasmodium, the mosquito is the definitive host (sexual reproduction occurs there) and humans are the intermediate.

Arthropods as Vectors

Vectors are arthropods that transmit pathogens. Biological vectors are required for the pathogen's life cycle — the pathogen replicates or develops inside them (Anopheles for malaria; Aedes for dengue). Mechanical vectors passively carry pathogens on their surfaces or in their gut without replication (houseflies carrying Shigella or Salmonella to food).

Mosquitoes are the most important vectors globally:

• Anopheles — malaria (Plasmodium) and lymphatic filariasis (Wuchereria bancrofti in some regions).
• Aedes aegypti/albopictus — dengue, Zika, yellow fever, and chikungunya.
• Culex — West Nile virus and lymphatic filariasis.

Ticks transmit a large and growing U.S. disease burden:

• Ixodes scapularis (deer tick) — Lyme disease (Borrelia burgdorferi), anaplasmosis, and babesiosis.
• Dermacentor variabilis (American dog tick) — Rocky Mountain spotted fever (Rickettsia rickettsii) and tularemia.
• Amblyomma americanum (Lone Star tick) — ehrlichiosis and southern tick-associated rash illness (STARI).

Tick-borne diseases are often diagnosed clinically with serologic confirmation; early Lyme is treated with doxycycline, while RMSF requires prompt doxycycline even in children given its mortality.

Mites include the chigger mite (transmits scrub typhus, Orientia tsutsugamushi) and Sarcoptes scabiei, the scabies mite that burrows in skin causing intense nocturnal pruritus and burrows in finger webs; treatment is permethrin 5% cream or oral ivermectin.

Lice (Pediculus humanus capitis corporis and Pthirus pubis) are ectoparasites that infest hair and clothing; body lice transmit epidemic typhus (Rickettsia prowazekii), relapsing fever (Borrelia recurrentis), and trench fever (Bartonella quintana).

Fleas transmit plague (Yersinia pestis) from rodents via Xenopsylla cheopis and endemic murine typhus (Rickettsia typhi); cat fleas transmit Bartonella henselae (cat-scratch disease).

Ectoparasites and Direct Disease

Some arthropods cause direct disease rather than vector transmission: scabies (burrowing mite), pediculosis (lice infestation), and myiasis (fly larvae in tissue). Tick attachment itself can cause local reactions, and prolonged attachment increases transmission risk for tick-borne pathogens — a key reason prompt tick removal matters.

Vector-Pathogen Pairings to Memorize

Vector Biology and Transmission Efficiency

The extrinsic incubation period — the time a pathogen needs to develop inside the vector before it can be transmitted — governs outbreak timing. Malaria sporozoites require roughly 10–21 days in Anopheles before the mosquito becomes infectious; dengue requires about 8–12 days in Aedes. This is why vector control during an outbreak can take weeks to show benefit and why a single infected traveler rarely sparks immediate local cases.

Mechanical vs. biological transmission also determines control strategy. Mechanical transmission (houseflies on food) is interrupted by sanitation; biological transmission requires killing or avoiding the vector itself — insecticide-treated bed nets, repellents, and source reduction of standing water for mosquitoes, tick checks and protective clothing for ticks. Zoonotic reservoirs complicate eradication: plague persists in wild rodent populations, Lyme in mice and deer, and Chagas in wild mammals, so human vaccination alone cannot eliminate these infections.

Ectoparasite Management

Scabies is treated with permethrin 5% cream applied neck-to-toe and washed off after 8–14 hours, repeated in 1–2 weeks; oral ivermectin is an alternative in crusted (Norwegian) scabies seen in immunocompromised hosts. Head lice are treated with topical pediculicides (permethrin 1%, or ivermectin lotion); body lice are managed by washing clothing and bedding in hot water — the organisms live in seams, not on skin. Tick removal uses fine-tipped forceps grasping the mouthparts close to the skin with steady upward traction; twisting or smothering with petroleum jelly is not recommended.