A neglected aspect of the epidemiology of sleeping sickness: the propensity of the tsetse fly vector to enter houses

Our plain-English summary of A neglected aspect of the epidemiology of sleeping sickness: the propensity of the tsetse fly vector to enter houses, by G.A. Vale, A. Chamisa, C. Mangwiro, et al (PLoS Neglected Tropical Diseases, 2013).

The study

• Authors: G.A. Vale, A. Chamisa, C. Mangwiro, S.J. Torr
• Journal: PLoS Neglected Tropical Diseases
• Published: 2013;7(2):e2086
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: When taking a bloodmeal from humans, tsetse flies can transmit the trypanosomes responsible for sleeping sickness, or human African trypanosomiasis. While it is commonly assumed that humans must enter the normal woodland habitat of the tsetse in order to have much chance of contacting the flies, recent studies suggested that important contact can occur due to tsetse entering buildings. Hence, we need to know more about tsetse in buildings, and to understand why, when and how they enter such places.

Methodology/principal Findings: Buildings studied were single storied and comprised a large house with a thatched roof and smaller houses with roofs of metal or asbestos. Each building was unoccupied except for the few minutes of its inspection every two hours, so focusing on the responses of tsetse to the house itself, rather than to humans inside. The composition, and physiological condition of catches of tsetse flies, Glossina morsitans morsitans and G. pallidipes, in the houses and the diurnal and seasonal pattern of catches, were intermediate between these aspects of the catches from artificial refuges and a host-like trap. Several times more tsetse were caught in the large house, as against the smaller structures. Doors and windows seemed about equally effective as entry points. Many of the tsetse in houses were old enough to be potential vectors of sleeping sickness, and some of the flies alighted on the humans that inspected the houses.

Conclusion/significance: Houses are attractive in themselves. Some of the tsetse attracted seem to be in a host-seeking phase of behavior and others appear to be looking for shelter from high temperatures outside. The risk of contracting sleeping sickness in houses varies according to house design.

In plain English

This field study at Rekomitjie Research Station, Zimbabwe, examined why tsetse flies, which can transmit sleeping sickness, enter houses. It is not a study of asbestos health effects. Asbestos appears only as the roof material of one of three unoccupied buildings compared, alongside a large thatched house and a small tin-roofed building. Men inspected the houses every two hours from 0700 to 1700 and caught the flies inside. Catches were compared with a host-like trap and with artificial refuges. Closing doors and windows cut catches in the thatched house by an average of 88%. In one experiment, the small asbestos-roofed and tin-roofed houses caught 17% and 13%, respectively, of the thatched house's catch. At midday the thatched house was about two degrees cooler than outdoor screen temperatures. Some flies landed on the inspecting men. The authors concluded that a house is itself attractive to tsetse, and their results suggested that some flies entered seeking a host and others seeking shelter from heat. They noted that their methods gave only crude measures of the numbers entering and that they studied only three unoccupied houses.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Field experiment. Unoccupied houses, with doors and windows opened or shut, were inspected for tsetse every two hours from 0700 to 1700, and catches were compared with those from a trap and artificial refuges.
• Who was studied: Tsetse flies (Glossina morsitans morsitans and G. pallidipes) caught at Rekomitjie Research Station, Zambezi Valley, Zimbabwe. House 1 was sampled between Aug 2009 and Aug 2010.
• Exposure: Three unoccupied buildings with different roofs: a large thatched house (House 1), a small house with corrugated asbestos sheets (House 2) and a small house with galvanized iron ('tin', House 3). Windows and doors were opened or shut in different combinations. Asbestos is studied only as a roof type, not as a health exposure.
• Main results:
  • In House 1, closing both windows and doors (the 'Nil' treatment) reduced catches by an average of 88%, but catches were not zero.
  • G. pallidipes made up 91% of the trap catch and 36% of the refuge catch. In House 1 the share was intermediate, at 61–81% (average 76%).
  • In Experiment 1 (Jan–Apr 2010), mean catches in the small asbestos-roofed House 2 were 17%, and in the small tin-roofed House 3 13%, of those in the large thatched House 1.
  • At midday the thatched House 1 was about two degrees cooler than screen temperatures, unlike the asbestos-roofed and tin-roofed houses.
  • When the small houses' roofs were covered with grass, their mean catches increased slightly to 14–36% of the House 1 catch.
  • During hotter weather, window type (glass versus netting) had no gross effect on catches in the small houses.
  • Flies alighted on the inspecting men. In House 1 these were 23 male and 16 female G. m. morsitans and 2 male and 1 female G. pallidipes. In Houses 2 and 3 they were 24, 3, 2 and 0. Males of G. m. morsitans made up 53% of these flies in House 1 and 83% in the other houses, a significant difference.
  • Among females caught in the houses, the share with undigested blood was 4.4% (252) in months other than Sep–Nov and rose to 10.5% (N = 153) in Sep–Nov. The authors read this as consistent with refuge-seeking flies entering in those hot months.
• The authors' conclusion: The authors concluded that houses are attractive in themselves. They inferred that some of the flies attracted seemed to be host-seeking and others appeared to be looking for shelter from high outdoor temperatures.
• Limitations the authors state:
  • The methods counted only flies found in the houses at each inspection, so they gave only crude measures of how many flies entered.
  • Flies staying in the houses overnight may have been reduced by ants.
  • The three Box refuges might have substantially underestimated the number of refuge-seeking flies in the much larger houses.
  • Only three house types were studied, and all were unoccupied except during the brief inspections.
  • The window-type experiment coincided with the onset of the hot-dry season, which partly undermined its main purpose.

Topics

Medical terms in this study, explained in our Lexicon: Epidemiology.

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