Humbio 153: Parasites and Pestilence

Jenna Wixon-Genack

Dr. Scott Smith

HumBio 153: Parasites and Pestilence

9 March 2012.

Control of Mechanical Vectors of Diarrheal Disease in Humanitarian Crises

Background

Flies are key to the transmission of enteric diseases as mechanical vectors1. While the term “fly” encompasses many species of the order Diptera, the flies of special importance in refugee situations are Musca domestica (the housefly), blowflies, and fleshflies1. Flies act as mechanical vectors when they passively transmit pathogens1. This generally occurs when they feed on organic liquids or lay eggs on decomposing matter, and later land on food, regurgitate contaminated liquids, or defecate on or around food1. It should be noted that although diseases are frequently transmitted due to pathogens on a fly’s exterior, pathogens which are ingested and later regurgitated may be transmitted over a longer period of time, increasing the risk of successful pathogen transportation 2. Flies lay eggs on organic matter originating from humans and vegetables, and larvae develop there1. Of particular concern in refugee camps are open toilets, the mixture of garbage, organic matter, and dirt, and decomposing animal excrement and meat1. Incredibly, a liter or kilogram of organic matter can support the development of as many as 10,000 flies 1.

Enteric diseases transmitted by flies include typhoid, shigellosis, and diarrhea1. However, while flies have been indicated in the transmission of all these diseases, it is crucial to note that the fly control outlined in this document and the accompanying presentation are not always necessary. In their excellent manual Filth Flies: Significance, Surveillance and Control in Contingency Operations, the Armed Forces Pest Management Board of the Department of Defense notes “The presence of flies does not automatically initiate a recommendation for control. Thresholds are established to help predict when control measures are needed”3. Thresholds vary with the environment and species of fly. However, the report recommends first examining sites ofpotential infestations, then monitoring for both larval development and the number and presence of adult flies.3

Ultimately, flies are especially likely to transmit disease in places with compromised sanitation 1. In places with sufficient sanitation, personal hygiene is likely to be enough to prevent flies from acting as mechanical vectors. However, in the absence of sanitation, personal measures will not be sufficient 4. Specifically, a situation of great risk is during a humanitarian crisis. Refugee camps, particularly during a stage of emergency, tend to lack infrastructure for sanitation, leading to numerous breeding grounds for flies. Other factors include warm weather and closely packed people—conditions frequently seen in humanitarian crises 3.

Indeed, the report “Complex Disasters and Public Health” notes, “The most common reported causes of death among refugees during the early influx phase have been diarrheal diseases, measles, acute respiratory infections, malaria, and other infectious diseases. These diseases have been the most critical causes of morbidity and the focus of most public health interventions”5. In short, diarrheal diseases are of critical importance in refugee camps, and in these same situations flies play an important role as mechanical vectors.

Methods for control: Sanitation

Improved sanitation is an absolutely critical component of fly control, particularly in the long-term. Because of the fundamental ties between sanitation and the ability of fly populations to breed and transmit diarrheal disease, establishing sanitation from the get-go in a humanitarian crisis is the preferred method, although this is of course not always feasible. This section outlines three areas for controlling flies via sanitation: building proper latrines, managing garbage and waste, and minimizing contact between flies, people, and food.

Constructing latrines

The WHO notes, “Defecation in the field, other than in latrines and toilets, may provide breeding places for filth flies (Musca sorbens). This is a common problem where large groups of people, e.g. refugees, stay together in temporary camps. Installation of proper latrines should be given priority.” 2An excellent model of for fly control is the ventilated improved pit (VIP) latrine. “The VIP latrine is constructed more substantially, incorporating a tall vertical ventilation pipe that is topped with a tightly fitting fly screen. The ventilation pipe has proven extraordinarily effective in both diminishing odors and, more importantly, in effecting fly control. Fecal odors in the VIP latrine are funneled in such a way that they exit largely through the ventilation pipe. Flies in the vicinity that are attracted by the fecal odors aggregate around the top of the pipe, which they try to enter; however, the screen on the top of the vent prevents them from gaining entrance. Flies that manage to enter the pit (and their eventual progeny after breeding) ascend the ventilation pipe during daytime (attracted by daylight) but are prevented from escaping; hence, the pipe acts as a de facto trap” 6. This model was compared with a latrine lacking a vent, and the number of flies that escaped from the VIP latrine was 100-fold smaller 6.

When VIP latrines cannot be constructed, pit latrines should be fitted with wooden lids that fit snugly and cover the entire hole1. They should also be designed so that people can open the lids with their feet, thus improving hygiene1. Education is an important part of this effort 1. Oxfam notes, “The type of sanitation system used may have a marked effect on fly numbers and also on the types of fly that will breed. Unless particular measures are taken to prevent fly breeding then the introduction of a sanitation system may actually increase fly numbers by providing permanent, damp, warm environments that are suitable habitats for flies to breed in”7.

Where no latrines can be constructed, a special area should be established for defecation that is at least 500 m downwind from any dwelling or food supply and a least 30 m away from water2. Feces should not be covered with soil because this makes them dry slower, rendering them a better breeding ground for flies 2.

Managing Garbage

Garbage and organic waste are where flies breed must be managed properly for effective control. This can be difficult in a humanitarian crisis, and before destroying anything it is good to ask if it can be recycled or reused. A recommended method is disposing of solid waste in a pit that is positioned away from living areas. The pit should be 30 m from water sources and dwellings. Its volume can be estimated by calculated 6 m3 per 50 people per week. If possible, the pit should be lined with clay or some impermeable material to avoid leaching 8. Once the pit is constructed, following the procedures of “controlled dumping” minimizes the ability of flies to breed in the waste2. Each day, newly added garbage should be covered with 15 cm of earth and compacted2. Once the garbage reaches a level 30 cm below the surface, the pit has reached capacity and should be filled with earth and compacted 2. Medical waste should never be added to this pit but instead should be disposed of separately and burned daily8. Of note is that community involvement is key to managing waste (and all other interventions listed here)8. Community education on waste reduction and management methods is key, as is consulting leaders to determine who in the community is normally responsible for handling waste 8. Note: flies cannot breed or survive at a temperature above 35 C1. Covering garbage with plastic sheeting causes fermentation at temperatures of 47 C that are lethal to larvae, and this is another method for fly control 1.

Fly exclusion

The Army manual notes, “It cannot be overemphasized that fly control will be unsuccessful in the absence of satisfactory exclusion methods” 3. Reducing contact between flies and food is crucial. Screens should be installed on the windows and doors of all healthcare facilities and places where food is eaten and prepared3. Screen with 12 holes per cm2 are the right size to prevent entry so flies and mosquitoes 1. Another method for preventing flies from entering is to hang beads in the entryway, or to place insecticide treated nets around the doors and windows. Food and utensils should be placed in containers when not in use, to prevent flies from landing on them 2.

Traps

Traps are important as a targeted control measure around food distribution and health centers. A study looked at their efficacy as a solitary control measure for reducing fly density and diarrhea in children. Comparisons were made for both fly density and incidence of childhood diarrhea (Shigella) between a control village and a village with baited traps9. The result of this study was that there was no significant impact of the baited traps on fly density9. However, the authors note that this was likely due to the plethora of attractive breeding sites available9. In areas with fewer breeding sites, traps have been more useful9. This indicates the need for multiple methods of control. Types of traps include sticky tape, light traps, and baited traps. Light traps attract flies to UV light, but because they require electricity they may not be appropriate for humanitarian crises 3. Baited traps can be managed as follows: “The container should be half-filled with bait, which should be loose in texture and moist. There should be no water lying at the bottom. Decomposing moist waste from kitchens is suitable, such as green vegetables and cereals and overripe fruits. Chunks of decomposing meat or fish can be added. Where evaporation is rapid the bait has to be moistened on alternate days. After seven days the bait will contain a large number of maggots and needs to be destroyed and replaced. Flies entering the cage soon die and gradually fill it until the apex is reached and the cage has to be emptied. The trap should be placed in the open air in bright sunlight, away from shadows of trees” 2.

Chemical control

Chemical control of flies can be very effective in the short-term, but should only be used in emergency situations and in conjunction with sanitation 1. A major reason for this is the tendency of flies to develop resistance to insecticides. As an example, a 1953 study found that houseflies developed resistance to DDT after only 10 months, while other insecticides proved ineffective after only 3 months 10. The UNHCR points out that proper management of insecticides is important to reduce resistance: “Residual pyrethroids should not be used by themselves because they can quickly cause resistance in flies which are already resistant to organochlorines such as DDT. Therefore, pyrethroid treatments must be alternated with organophosphates such as pirimiphos-methyl or malathion every six months”1. However, chemical control can nevertheless be important. Oxfam notes that it is especially called for when an epidemic of diarrheal disease (or trachoma, another disease for which flies are the vector) is threatened or occurring7.

Toxic baits

Toxic baits are a mixture of sweet, fly-attracting liquid (sugar and water can be used) and toxic chemicals2. 1-2% formaldehyde solution can be used if needed. However, organophosphorus and carbamate compounds are both extremely toxic to flies and generally safe for humans, so they are the best option2. There are several forms of toxic baits: dry scatter, liquid sprinkle, liquid dispenser, and viscous paint-on2. Each has its benefits, but ultimately the success of toxic baits depends on what the local fly population finds attractive (and thus if its attracted to the bait), and competition from other attractive sources in the immediate vicinity 2. Some benefits of poison baits are that they are cheap and easy to use2. Flies are also less likely to develop resistance to insecticide in poison bait form, and may be susceptible to it even when resistant to the same insecticide when sprayed 2. However, the UNHCR explicitly does not recommend use of poison baits in refugee camps, for reasons undisclosed.

Space spraying

Space spraying can be conducted in and outdoors. Outdoors, the method is called ultra-low-volume spraying, or ULV2. Insecticide is released as very fine droplets in the air that make contact with the flies as they move through the air 2. It is highly effective at rapidly bringing down fly numbers, and the Army manual notes, “In situations where fly populations must be brought under control immediately, e.g., to reduce the incidence of diarrheal diseases in a refugee camp, ULV pesticide application is the only assured means of immediate control.” For space treatment, the insecticides of choice are deltamethrin at a dilution of 0.05 g/l (0.005%), permethrin 0.5 g/l (0.05%), pirimiphos-methyl 20 g/l (2%;), or malathion 50 g/l (5%) 1.

Residual insecticide

This method takes advantage of flies’ natural tendencies to rest in certain places. For example, flies are drawn to vertical resting spots such as strings and cords, which can be treated with a long-lasting insecticide. They also tend to rest or spend time around garbage, so residuals can be applied there 3. There are also species specific behaviors: “In the evenings and on cloudy days, Chrysomyia flies prefer to rest outside dwellings, on surfaces near latrines or garbage pits, whereas houseflies often remain inside dwellings” 1. To identify areas where flies prefer to rest, you can look for excrement. Residual insecticide should be applied only in places where flies are attracted, to minimize the risk of developing resistance 1. Upon landing, adult flies absorb the chemical through their feet 1. Garbage sites should be sprayed with residuals weekly; latrines should be sprayed every 2-3 months. The entire latrine should be sprayed to target newly emerging flies. Eating areas should not be treated with this method. 1

Zero-Fly Tarps

Zero-Fly Tarps, manufactured by Vestergaard-Frandsen, are an example of a novel method for chemical vector control. They are insecticide treated plastic tarpaulins (ITPS)11. The design of the tarps is to provide long-lasting vector control with constant low levels of insecticide11. The insecticide, deltamethrin, is incorporated into a woven piece of fabric that is laminated on both sides11. The lamination protects the insecticide from UV light, which destroys it11. The slow release of the insecticide is chemically controlled, so that it seeps to the surface of the tarp over time11. This ensures a slow release of insecticide. 11 A major benefit of ITPS is that it provides vector control as well as basic shelter11. This means it can be distributed immediately in a humanitarian crisis, and thus that vector control is implemented from the start while elementary needs are also addressed 11. This tool is designed specifically to target the Anopheles mosquito, which is the vector of malaria, a disease that is especially pertinent in refugee camps.

Studies of Efficacy

Multiple studies of similar technologies have been carried out to determine the efficacy of this tool. Onefound that when used in mass coverage of a community, the tarps had significant value in reducing the mosquito population and thus vector competency 12. Another found that lining a shelter with ITPS may have a similar protective effect to indoor residual spraying for A. gambiae control 13. A third found that ITPS was an effective tool in malaria prevention and could be especially useful in acute emergencies 14. A double-blind phase III field trial was conducted to determine the use of ITPS in the situation it was designed for: an acute emergency15. The purpose was to ascertain the effect of ITPS upon malaria incidence in young children in an area of high transmission15. The setting was two camps for Liberian refugees in Sierra Leone between 2003 and 200415. One camp was assigned full coverage (all walls and ceilings), while the other had partial coverage (ceilings only). The two conditions of ITPS and UPS (untreated polyethylene sheeting) were randomly distributed to sections of the camps. The study took place over 8 months. The most notable result was that where full coverage was used, the P. falciparum incidence rate in children less than 3 years old, cleared of the parasite and monitored for 8 months, was 163 per 100 person years with UPS, while with ITPS it was 63. This difference was not as substantial with partial coverage. The protective efficacy of ITPS under the roof-only condition was 15%, while with full-coverage it was 60%. Ultimately, this study indicates that ITPS is a viable and effective vector control strategy for malaria during the acute phase of emergency, and that it must be used as full-coverage in a dwelling. 15

The implication of these results for fly control is less clear. As noted previously, flies have a documented tendency to develop resistance to insecticides (although permethrin is an insecticide known to kill flies). The study “Rapid Development of High-level Permethrin Resistance in a Field-collected Strain of the House Fly (Diptera: Muscidae) Under Laboratory Selection” by Scott and Georghiou in 1985 showed that “under laboratory selection, resistance to permethrin developed rapidly, attaining a 5,945-fold level by generation F22” 16. Resistance to other insecticides was also seen to increase16. While lab conditions are different from the field, this nevertheless indicates the remarkable ability of flies to develop resistance.