http://www.geo.umass.edu/courses/geo458/Readings/Epstein_2001.pdf

Climate change and emerging infectious diseases.

There is a consensus among climatologists that our planet is experiencing a progressive rise in surface temperature due to the increased production of "greenhouse" gases. Some of the possible consequences of elevated temperature on malaria transmission are examined in the present review. A simple mathematical model is first used to examine the effect of temperature on the ability of Anopheles maculipennis to transmit vivax malaria. This indicates that small increases in temperature at low temperatures may increase the risk of transmission substantially. This is important, since vulnerable communities, poorly protected by health services, in areas of unstable or no malaria are likely to be at increased risk of future outbreaks. In contrast, areas of stable transmission may be little affected by rising temperature. It is thought that global warming will lead to coastal flooding, changes in precipitation and, indirectly, changes in land use. Just how these changes will effect transmission at a regional level requires an understanding of the ecology of local vectors, since environmental changes which favour malaria transmission in one vector species may reduce it in another. Methods for predicting future changes in malaria in different regions are discussed, highlighting the need for further research in this area. Most importantly, there is a need for researchers to validate the accuracy of the models used for predicting malaria and to confirm the assumptions on which the models are based.

Climate change and malaria transmission

1. Activity in Ixodes ricinus was studied by laying down newly emerged ticks in natural clumps of vegetation from which all hosts (with the possible exception of ‘mice’) were excluded. After a period of quiescence the ticks climbed to the ‘active’ position at the vegetation tips. Activity was assessed either by simple observation (in adults which were marked individually) or by ‘brushing’ the vegetation with the hands (nymphs and larvae). 52 % of the adults and 44 % of the nymphs were recovered.2. The seasonal activity behaviour was followed in three series of ticks set out during May, July and October 1945. The results were used in interpreting the time relations of seasonal activity in ‘wild’ populations. The timing of the activity cycle appears to be largely determined by the availability of unfed ticks rather than by the prevailing meteorological conditions.3. The diurnal pattern of behaviour in active ticks was closely examined during a 24 hr. period. Many remained at the tips without moving. The others began or ended a phase of activity by night or by day; but among these, the proportion ending a phase at night was significantly greater. This is the main reason for the previously reported diurnal fluctuation of activity in a tick population.4. Two hundred and seven adult ticks spent, on an average, a total of 9 days at the vegetation tips (individual limits 1 and 54 days). This activity was spread over an ‘active period’ (interval between first and last appearances) averaging 30 days. Very active ticks rarely remained continuously at the tips. The mean number of visits to the tips was 4 (limits 1 and 19), each lasting for an average of 2·5 days. Where long spells of activity alternate with short periods of quiescence, the behaviour is probably regulated by the humidity reaction, the quiescent phase providing an opportunity for restoring the depleted water balance.5. Newly moulted adult ticks possess reserves of fat adequate for many months of quiescence. During activity these reserves are exhausted in a few weeks or even days. Duration of survival mainly depended on whether the onset of activity was immediate or delayed. Some adults remained quiescent for 1 year before becoming active for the first time.6. Several types of orientations were observed in the field. In attaining the position at the tips favourable for encountering a host, the gravity response (upward-turning near the tip) is of major importance. The tick avoids wind and direct sunlight by sheltering behind its supporting stem. In sensing the approach of a ‘host’ (the observer's finger) the perception of eddies of warm air is particularly significant. The response (questing, then orientation) is elicited much less readily if the stimulus is applied from the leeward.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0031182000084031

The seasonal and diurnal activities of individual sheep ticks (Ixodes ricinus L).

The ability for public/veterinary health agencies to assess the risks posed by tick-borne pathogens is reliant on an understanding of the main tick vector species. Crucially, the status, distribution, and changing trends in tick distribution and abundance are implicit requirements of any risk assessment; however, this is contingent on the quality of tick distribution data. Since 2005 the Health Protection Agency has promoted an enhanced tick surveillance program. Through engagement with a variety of public and veterinary health agencies and practitioners (e.g., clinicians and veterinarians), wildlife groups (deer society, zoos, animal refuge centers, and academics), and amateur entomologists, >4000 ticks from 900 separate records across Great Britain have been submitted, representing 14 tick species (Ixodes ricinus, Ixodes hexagonus, Ixodes acuminatus, Ixodes arboricola, Ixodes canisuga, Ixodes frontalis, Ixodes lividus, Ixodes trianguliceps, Ixodes ventalloi, Carios vespertilionis, Dermacentor reticulatus, Haemaphysalis punctata, Hyalomma marginatum, and Amblyomma species). The majority of ticks submitted were I. ricinus (81%), followed by I. hexagonus (10%) and I. frontalis (2.5%). Predominant host groups include companion animals (411 records), humans (198 records), wild birds (111 records), and large wild mammals (88 records), with records also from small/medium wild mammals, livestock, the environment and domestic/aviary birds. The scheme has elucidated the detection of two nonnative tick species, the expansion of previously geographically restricted D. reticulatus and produced ground data on the spread of I. ricinus in southwest England. It has also provided a forum for submission of ticks from the concerned public and particularly those infected with Lyme borreliosis, thus raising awareness among public health agencies of the increased peri-urban tick problem in Britain. Our results demonstrate that it is possible to run a cost-effective nationwide surveillance program to successfully monitor endemic tick species, identify subtle changes in their distribution, and detect the arrival and presence of exotic species.

Tick surveillance in Great Britain.

Experiments in sheep-tick control require the comparison of tick counts on sheep pastured on an infested grazing or on a blanket dragged over the grazing. The former should include only attached females on axillae, forearms, head, neck and chest.



The distribution of tick counts on a group of sheep is in general not fitted by the normal distribution, nor, although it is positively skewed, by the Poisson. There is evidence that the distribution could be derived from a population distributed according to a negative binomial. Unless the data are more than ordinarily skewed (in which case, a ‘normalizing’ square-root transformation may be necessary), the best available method for comparing mean tick counts on sheep is the direct t-test using actual tick numbers. Within the range 0.3–103.7 ticks per sheep, the regression of standard deviations on means is linear and was calculated as 5=0.477x+1.246



For estimating the significance of small differences between dips or ground population densities, tick counts on groups of five sheep are inadequate. Groups of twenty or more sheep should be employed according to the magnitude of the difference required to be proved significant. For showing the trend, only, of tick activity throughout a season on a particular section of land weekly counts on ten sheep are adequate provided the same ten sheep are always used. The sources of the large variation in the tick counts of individual sheep are pointed out, with suggestions as to how this variation may be reduced.



In blanket dragging, counts of nymphs are best for estimating population densities. Drags may be. limited to 25 yd. A differential equation is given whereby the nymphs lost in the course of a drag are taken into account. This does not surmount the difficulty that uniformity of vegetation surface influences the efficiency of the blanket, which should therefore be used for comparisons only when the vegetation surfaces are of similar uniformity. A worn blanket picks up fewer ticks than a less worn blanket. The distribution of nymphal blanket counts is similar to that of female tick counts on sheep in that it is not in agreement with the Poisson law although positively skewed. It is in closer, though far from satisfactory, agreement with a ‘contagious’ distribution.



Because of the effect of changing meteorological conditions on tick activity, the densities of tick population (tick activities) on different plots must be compared by dragging the plots simultaneously. A virgin stretch of ground is necessary for each drag in each season. In such comparisons, the precision can more easily be increased by increasing the number of ‘occasions’ (days) of simultaneous draggings than by increasing the number of drags per ‘occasion’. Two, or three, drags per plot are sufficient provided the number of, ‘occasions’ is not less than 20. The significance of plot differences in density may be calculated from the analysis of variance of drags.



For plots less than 3 acres, the blanket method is easier; for large areas, especially with varied vegetation cover, tick counts on sheep are preferable.

The comparison of sheep-tick populations (Ixodes ricinus L.)

1. Serial examinations of the degree of infestation of thirty-five flocks during a nine-year period in Argyllshire, Perthshire, Selkirk and Northumberland, showed that tick incidence is greatest in spring and decreases markedly in early summer. Evidence is given of a similar periodicity in North Wales.2. Evidence is given of the occurrence of a slight recrudescence of activity in autumn, and of the almost complete absence of activity in winter.3. The spring curve of activity terminated in early May in Argyllshire, in the middle of May in Perthshire, and towards the end of May in the Borders and Northumberland.4. The estimation of the relative annual infestation, e.g., in relation to the question of spread or increase of ticks, by single annual counts is shown to be quite unreliable.5. No evidence was obtained of an increase in density of ticks in any area during the period of observation.6. The seasonal periodicity in activity was found to bear no relation to the density of stocking of the ground by sheep.7. The seasonal activity of ticks in Argyllshire, Perthshire, Selkirkshire and Northumberland, was found to be related to the air temperature, expressed as the weekly average of maximum temperatures. The limits of the air-temperature range corresponding to active tick infestation are 45°F. and 60°F. (average weekly maximum).8. The curve of tick infestation during 1937 on a moor in North-east Scotland was found to differ markedly from the normal, and the possible explanations of the discrepancy are discussed.

J. MacLeod

Papers

The Seasonal and Annual Incidence of the Sheep Tick, Ixodes ricinus , in Britain