House flies and cockroaches
- Parasite
- For completeness
Household insects that do not bite but walk from faeces and rubbish onto food and eyes, carrying germs on their bodies. Cockroach droppings also trigger asthma.
Also called: Musca domestica, Musca sorbens (eye-seeking fly), German cockroach (Blattella germanica), American cockroach (Periplaneta americana)
Quick facts
- Type
- Parasite · Arthropod vectors
- Gram / shape
- Grey fly with a sponging
- Spread
- Mechanical · Flies also regurgitate
- Causes
- Bacterial dysentery · Trachoma
- Treatment
- Does not apply to the insect
At a glance
- Shape
- Grey fly with a sponging, non-piercing mouthpart; flat, oval, brown cockroach with long antennae
- Family
- Muscidae (house flies); Blattidae and Ectobiidae (cockroaches)
- Type
- Insects: non-biting flies (Diptera, Muscidae) and cockroaches (Blattodea); mechanical vectors
- Size
- House fly under 1 cm; cockroaches from about 1 cm (German) to several centimetres (American)
Morphology
- House fly: sponging mouthparts that liquefy food with saliva, and hairy legs and pads that pick up dirt.
- Cockroach: flattened body that slips into cracks, chewing mouthparts and spiny legs.
- Neither has a stage in which a pathogen multiplies or develops: they only carry it on or through the body.
Transmission
- Mechanical: microbes and parasite eggs stick to legs, body hairs and mouthparts and are left on food, utensils or eyes.
- Flies also regurgitate and defecate while feeding, depositing what they picked up from faeces or rubbish.
- Cockroaches contaminate food and surfaces at night; their faeces, saliva and shed skins carry allergens.
Reservoir: Human and animal faeces, open latrines and manure; Rubbish, kitchen waste and drains; Warm kitchens, bathrooms and sewers (cockroaches)
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- House flies lay eggs in batches on moist decaying matter: faeces, manure and rubbish.
- Eggs hatch into legless maggots that feed in the same material and moult twice.
- Mature maggots crawl to drier places and pupate in a brown puparium.
- Adults emerge, mate within days and live a few weeks; in warm weather egg to adult takes about a week or two.
- Cockroaches lay eggs in a purse-like capsule (ootheca); the German cockroach carries it until hatching.
- Nymphs look like small wingless adults and moult several times; there is no larva or pupa (incomplete metamorphosis).
Virulence factors
- Constant movement between faeces and food makes flies efficient carriers of faecal-oral pathogens.
- Very large populations near open defecation, markets and rubbish dumps.
- Cockroach allergens persist in household dust and sensitise children, worsening asthma.
- Both breed fast and resist many insecticides, so infestations return quickly.
Diseases it causes
- Bacterial dysentery: flies carry Shigella, which needs very few organisms to infect Bacterial infectious diarrhoea and dysentery →
- Diarrhoeal disease from other enteric bacteria, viruses and protozoan cysts Acute (viral) gastroenteritis →
- Trachoma: eye-seeking flies carry Chlamydia trachomatis between children's eyes Trachoma →
- Allergic asthma triggered by cockroach allergens Asthma →
- Spread of helminth eggs (Ascaris, Trichuris) and protozoan cysts onto food
Clinical picture
- No lesion from the insect itself: disease appears as diarrhoea, dysentery, conjunctivitis or asthma.
- Fly density and childhood shigellosis can rise together in the warm season.
- Cockroach-sensitised children with asthma have more severe symptoms in infested homes.
Diagnosis
- Entomological: identifying the insect and estimating fly density with sticky or baited traps.
- Diagnosis of the patient is that of the carried disease (stool culture, eye examination).
- Cockroach allergy: skin prick test or specific IgE to cockroach allergen.
Treatment
Does not apply to the insect. Control relies on sanitation, baits and traps; for cockroach-allergic asthma, standard asthma care plus allergen reduction.
Drug class only; doses and regimens are set by the clinician from the current guideline.
Prevention
- Safe disposal of faeces (latrines and toilets) and of rubbish in closed bins.
- Covering food and water; handwashing before eating and cooking.
- Window screens and baited fly traps.
- Facial cleanliness and better water and sanitation against trachoma.
- Cockroaches: sealed food and rubbish, fixing leaks, sealing cracks and using bait stations rather than sprays.
Test yourself
Think of the answer, then tap the question to see it.
What is the difference between a mechanical and a biological vector?
A mechanical vector only carries the pathogen on or through its body; in a biological vector the pathogen multiplies or develops before transmission.
Which bacterial diarrhoea is classically linked to house flies?
Shigellosis (bacterial dysentery), because a very small number of Shigella can cause infection.
Which eye disease spreads partly through flies that feed on eye and nose discharge?
Trachoma, caused by Chlamydia trachomatis.
Which parts of the cockroach carry the allergens that worsen asthma?
Its faeces, saliva, eggs and shed skins, which build up in household dust.
Connections
← Back to the group: Arthropod vectors
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Housefly Population Density Correlates with Shigellosis among Children in Mirzapur, Bangladesh: A Time Series Analysis (PLoS Neglected Tropical Diseases, 2013); Trachoma (WHO, 2025); Dust Mites and Cockroaches | National Institute of Environmental Health Sciences (NIEHS (NIH), 2025).
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Representative model, for educational purposes only.