The Basics of Vectors and Insects

In the field of environmental health, a vector is any organism that conveys disease to another organism. Vectors are an important consideration in the design and installation of composting toilets, because they can be a carrier of excrement.

Many kinds of animals, flies, beetles, mites, and other arthropods will be attracted to fresh human excrement for feeding purposes, if they can access it. Crawling insects and mites are mostly just a nuisance. Of greater concern are excrement-frequenting arthropods, such as flies, and the issue here is whether they can come and go: crawl on excrement and then land on humans or food.

All insects, spiders, mites, crustaceans, (crabs, shrimp, wood lice, water fleas, barnacles), centipedes, millipedes and others are members of the phylum Arthropoda, the only major invertebrate phylum with members adapted for life on truly dry land.

Arthropods will be found in abundance on and in all healthy soils and on all healthy living organisms, including the human body. Their presence is indicative of health and ecological integrity, and their presence in a composting system should not be alarming. They only become a concern when they can leave the composting system, as they can potentially be vectors for disease transmission. For example, flies that get into a composting toilet reactor can walk on excrement, then fly out of the composter to a food source, and the excrement on their legs can be transmitted to food.

For that reason, restricting fly access to human excrement is an important aspect of the prevention of enteric disease, as well as an aesthetic issue.

(Keep in mind, though, that overall sanitation (handling feces, no washing) and water quality rank first for consideration of direct transmission risk, far outweighing flies. Flies are the next most likely cause of fecal ingestion, and you probably need a lot of flies to do it.)

However, when biologically processed excreta has been transformed to a stable and fully oxidized state (not just desiccated) -- in other words, it has been turned into humus -- it will no longer attract most of the arthropods of concern, such as flies, as humus offers them no food or suitable breeding ground.

The control of arthropods is critical to the safe operation of these technologies, and is usually not addressed adequately in the promotional materials or the operation and maintenance manuals of most composting systems.

Composting systems can be maintained so that these organisms are not allowed to become a health or nuisance issue (see Chapter 5 for details about how to prevent and get rid of lies).

Some management methods:

• Use a toilet stool with a water-seal trap.
• Use a toilet seat and lid that is gasketed, or a fitted lid for the toilet opening.
• Screen ventilation openings.
• Seal cracks and openings (a smoke test can reveal them.)
• Apply environmentally benign insect repellents, such as pyrethrins and diatomaceous earth.
• Avoid putting kitchen scraps into the composter.

In a study by the USEPA and California Health Department (Enferadi et al., 1980), samples were taken from a variety of dry toilets for which the owners were not taking adequate precautions, and the results were dramatic and predictable:

"A wide assortment of flies, beetles, mites, spiders and other arthropods were found to inhabit the dry toilets. Mites were the most abundant kind of arthropods collected, generally occurring in numbers of 1,000 to over 100,000 per liter of solids. Fly larvae frequently occurred in number of 10 to 1,000 per liter, along with numerous beetles, moths and various wingless insects. No spiders considered harmful to humans were found in the vaults. The cockroach traps were all negative for cockroaches.

"Insects, mites, and other arthropods can gain entrance into the vaults by several means. They can fly through an open toilet, kitchen or access port, or they will land on the vault and crawl through small cracks or inefficient seals. Many of the gnats and small flies can pass through ordinary window screens. Several kinds of mites attach to and 'hitch a ride' on flies and beetles. Another common access of importance is introduction of the eggs and larvae of flying insects, and all stages of flightless insects and mites in bedding material (e.g., forest litter and leaves) or kitchen wastes put into the vaults.

"Once in the vault, many kinds of insects and mites can pass their whole life cycle in the shelter of the vault chamber and increase their numbers rapidly and indefinitely. Examples here are the gnats and small flies and mites. Others, including the larger domestic flies, must leave the vault to mate and cannot continue to colonize the solids without continuous access to the vault by egg-bearing females.

"The medical or nuisance significance of the arthropods found in the dry toilet vaults will vary according to their habits and to different circumstances of their occurrence. The wingless insects and other unobtrusive arthropods which tend to say in the vault once they are introduced are of little or no medical or nuisance significance. However, flying insects commonly found in the dry toilet vaults may leave the vault at times and become a nuisance in the home if the dry toilet is installed within the home... The gnats, small flies (except Drosophila), and beetles are of minimal significance as vectors of disease because they do not ordinarily seek out human food, and if emanating from a detached privy house, are not likely to enter the home at all....

"The domestic flies will enter homes even if produced in a detached privy house and will frequent human food. The vector potential of the kinds found here is not considered as great as the common house fly (Musca domestica), which was not found to frequent or breed in the vaults. However, if enteric disease agents were shed into the vault, the domestic flies found in this study would be one definite way that such agents could be transmitted to other persons.

"It could not be confirmed that flies were breeding in the composter."

Flying Insects Found in Waterless Toilets in One Study (Enferadi et al., 1980) The following types of flies were found in improperly installed or maintained composting toilets in a 1980 study by the California Health Department and the USEPA.
Flies (Diptera)	Large Flies (Calyprerare Muscoid Flies)
Gnats (Nematociera)	Fannia
Psychodidae	Muscina*
Fungivoridae	Ophyra*
Scatopsidae	Calliphorida*
Small Flies (Acalyptcrare Muscoid Flies)	Beetles (Coleoptera)
Phoridae	Nitidulidae
Milichiidae	Staphylinidae
Sphaeroceridae	Histeridae
Heleomyziidae
Drosophila*	Moths (Lepidoptera)
	Pyralidea
*Domestic flies

References

J. Martin and D. Focht, Soils for the Management of Organic Wastes and Waste Waters (Wisconsin: SCCA, ASA, CSSA, 1977)

Clarence Golueke, Composting (Emaus, Pa.: Rodale Press, 1972)

Ron Crites and George Tchobanoglous, Small and Decentralized Wastewater Management Systems (WCB/McGraw-Hill, 1998)

Karol Enferadi, Robert Cooper, Storm Goranson, Adam Oliveiri, John Poorbaugh, Malcom Walker, Barbara Wilson, "Field Investigation of Biological Toilet Systems and Grey Water Systems" (Sate of California Health Department, Department of Health Services and the Water Engineering Laboratory, Office of Research and Development, U.S. Environmental Protection Agency. EPA/600/2-86/069, 1980)