Is there safety in numbers? The effect of cattle herding on biting risk from tsetse flies

Abstract

In sub-Saharan Africa, tsetse (Glossina spp.) transmit species of Trypanosoma which threaten 45-50 million cattle with trypanosomiasis. These livestock are subject to various herding practices which may affect biting rates on individual cattle and hence the probability of infection. In Zimbabwe, studies were made of the effect of herd size and composition on individual biting rates by capturing tsetse as they approached and departed from groups of one to 12 cattle. Flies were captured using a ring of electrocuting nets and bloodmeals were analysed using DNA markers to identify which individual cattle were bitten. Increasing the size of a herd from one to 12 adults increased the mean number of tsetse visiting the herd four-fold and the mean feeding probability from 54% to 71%; the increased probability with larger herds was probably a result of fewer flies per host, which, in turn, reduced the hosts' defensive behaviour. For adults and juveniles in groups of four to eight cattle, > 89% of bloodmeals were from the adults, even when these comprised just 13% of the herd. For groups comprising two oxen, four cows/heifers and two calves, a grouping that reflects the typical composition of communal herds in Zimbabwe, approximately 80% of bloodmeals were from the oxen. Simple models of entomological inoculation rates suggest that cattle herding practices may reduce individual trypanosomiasis risk by up to 90%. These results have several epidemiological and practical implications. First, the gregarious nature of hosts needs to be considered in estimating entomological inoculation rates. Secondly, heterogeneities in biting rates on different cattle may help to explain why disease prevalence is frequently lower in younger/smaller cattle. Thirdly, the cost and effectiveness of tsetse control using insecticide-treated cattle may be improved by treating older/larger hosts within a herd. In general, the patterns observed with tsetse appear to apply to other genera of cattle-feeding Diptera (Stomoxys, Anopheles, Tabanidae) and thus may be important for the development of strategies for controlling other diseases affecting livestock.

Olfaction in vector-host interactions

“…Of note, while this study considered stable fly populations, it did not collect detailed information on their feeding behaviour. The interplay suggested by Torr and Mangwiro (2002) found substantial support in a follow-up study (Torr et al 2007). In this case, per capita load of Stomoxys was associated with a significant and negative reduction in tsetse feeding success, from about 80% at a per capita load of 20 flies to about 40% at a per capita load of 1000 flies (Figure 2).…”

“…The results show that for tsetse feeding on cattle there is a marked preponderance of meals from larger/older individuals within a herd (Torr et al 2007b). This bias seems to be due to younger cattle having higher rates of defensive behaviour and producing low doses of kairomones, particularly carbon dioxide and phenols .…”

“…The example of tsetse research also shows how a better appreciation of the host-orientation behaviour of flies can enhance our understanding of host-parasite interactions, which in turn might lead to improved methods of control (e.g. insecticide-treated cattle to control tsetse, see Torr et al 2007). Whether or not this also will be the case for stable flies remains to be seen.…”

“…Holloway and Phelps (1991) found that F3 traps baited with 1-octen-3-ol (0.7 mg/hr) caught about twice as many Stomoxys spp. as unbaited traps, though subsequent work carried out in the same area (Schofield 1996) with the same type of trap was not able to Torr et al 2007). …”

“…Several de facto dose responses to CO 2 also have been obtained from Zimbabwe. In these experiments, Stomoxyinae were collected from electric targets baited with the odour from various sized groups of cattle , Torr et al 2007. For both studies, catch rose with bait mass in a logarithmic fashion (Figure 1).…”