Ecology and phenology of ticks in Zambia: Seasonal dynamics on cattle

A study of the seasonality and infestation rates of ticks was carried out in 11 cattle herds in different ecological habitats in Zambia between 1980 and 1982. Wherever possible supplementary data were obtained from opportunistic collections from cattle and other hosts.Analysis of over 1000 tick collections from cattle indicated that infestation rates of the most important species,Amblyomma variegatum andRhipicephalus appendiculatus vary in different ecological habitats: (i) In Western Province, infestations are much lower than elsewhere; (ii) in Central and Southern Provinces, moderate to high infestations occur; and (iii) in Eastern Province,R. appendiculatus numbers are generally low andA. variegatum numbers are moderate.These two species, however, have similar life cycles throughout their range with one generation per year. Larvae occur mainly from March to May, nymphae from May to September, and adults ofA. variegatum from October to December and ofR. appendiculatus from December to April. Boophilus decoloratus appears to have two to four generations per year but is uncommon during the rainy season. In some areas in central ZambiaRhipicephalus compositus adults are seasonally common in September–October whereasRhipicephalus evertsi is more or less ubiquitous. Low to moderate infestations ofHyalomma truncatum andHyalomma rufipes occur in most areas.At least 14 other less common or rare species ofRhipicephalus, Amblyomma, Haemaphysalis andIxodes were taken infrequently from cattle. These and other host-specific species were also collected from dogs, sheep, various wildlife hosts and the environment.Infestation rates, seasonality and host-relationship of tick species are discussed in relation to their ecology. Relevant biosystematic and disease relationships are reviewed briefly. The baseline data derived from this study are adequate for integrated analysis with those from other ecological and economic investigations to formulate tick control strategies.     

Long-term studies on the economic impact of ticks on Sanga cattle in Zambia

Three different tick control policies were tested in groups of traditionally managed Sanga cattle in the Central Province of Zambia over a period of 3 years. One group was given strategic tick control using 12 pyrethroid acaricide spray applications between the onset and the end of the wet season (October to March). The productivity of this herd was compared with that of a group with no tick control and a group under an intensive tick control regimen of spraying every week in the wet season and every 2 weeks in the dry season (36 applications per year). The highest output was associated with intensive tick control, followed by strategic control and then no tick control policies. However, when the costs of tick control were taken into account, the strategic tick control policy produced the best economic result, followed by the intensive and then the no tick control policies. Neither the strategic nor the intensive tick control policy was sufficient to prevent the transmission of East Coast fever (ECF) infection when this disease was introduced to the area.     

A survey of the ixodid ticks parasitising cattle in the Eastern Province of Zambia

The results of tick surveys carried out in the Eastern province of Zambia between December 1982 and February 1996 were principally in agreement with the findings of earlier surveys conducted during the period 1965–72. Boophilus decoloratus has almost been replaced by Boophilus microplus . Hyalomma truncatum was found in small numbers throughout the province and Hyalomma marginatum rufipes was only rarely encountered in collections made from cattle. Rhipicephalus evertsi evertsi was largely confined to the southern part of the plateau and the valleys of the Zambezi tributaries. The main difference between the present survey and the previous one concerns the status of Rhipicephalus appendiculatus . This species is currently expanding its range in a westward direction, whereas it was virtually absent from the southern part of the province during the period 1965–72. The majority of specimens collected are morphologically intermediate between R. appendiculatus and the closely allied Rhipicephalus zambeziensis . The available evidence indicates that R. zambeziensis ( sensu stricto ) is absent from the province. The phenology of R. appendiculatus is aberrant in the province: at lower altitudes a second generation of adult ticks is recorded on the hosts at the start of the dry season.     

Studies on the economics of ticks in Zambia

Two herds of experimental Sanga cattle were maintained under traditional savanna grazing management for three years in the Central Province of Zambia. One herd was kept free of ticks by regular acaricide treatment, while the other was given no tick control. Milk production, growth rate, fertility and mortality were monitored throughout the trial. The tick-free herd performed significantly better than the tick-infested herd, but the value of the additional production was much less than the cost of the acaricide used. it is concluded that there is no economic justification for intensive tick control under these conditions. However, strategic tick control would be justified if the quantity of acaricide used could be reduced by 50% without any major reduction in benefits. The results indicate that treating calves below 45 days of age reduces their performance, as does treating cows during periods of very low tick challenge. Therefore, it seems likely that economically beneficial strategic tick control policies could be developed.     

Accelerated phenology of blacklegged ticks under climate warming

The phenology of tick emergence has important implications for the transmission of tick-borne pathogens. A long lag between the emergence of tick nymphs in spring and larvae in summer should increase transmission of persistent pathogens by allowing infected nymphs to inoculate the population of naive hosts that can subsequently transmit the pathogen to larvae to complete the transmission cycle. In contrast, greater synchrony between nymphs and larvae should facilitate transmission of pathogens that do not produce long-lasting infections in hosts. Here, we use 19 years of data on blacklegged ticks attached to small-mammal hosts to quantify the relationship between climate warming and tick phenology. Warmer years through May and August were associated with a nearly three-week advance in the phenology of nymphal and larval ticks relative to colder years, with little evidence of increased synchrony. Warmer Octobers were associated with fewer larvae feeding concurrently with nymphs during the following spring. Projected warming by the 2050s is expected to advance the timing of average nymph and larva activity by 8–11 and 10–14 days, respectively. If these trends continue, climate warming should maintain or increase transmission of persistent pathogens, while it might inhibit pathogens that do not produce long-lasting infections.     

The impact of natural infestations of ticks in Zambia on the productivity of cattle and implications for tick control strategies in central Africa

Studies were carried out in Zambia using a farming systems approach to quantify the effects of tick control on traditionally managed Sanga cattle. In the first trial to investigate the effect of tick control, significant improvements in liveweight gain (LWG) occurred only in periods of medium to high challenge with adult Amblyomma variegatum. However, during the first 18 months of the study when animals were young, the acaricide used caused a suppression in weight gains. A supplementary experimental study indicated that the adverse effects of ticks on weight gain may be delayed when animals are on a low plane of nutrition. In both studies on the effects of ticks on LWG, estimates of weight loss per female A. variegatum were in the range of 46 to 69 g. In the second long term trial to measure the impact of tick control on overall heard productivity, outputs of milk and weaner calf per livestock unit carrying capacity were about 25% higher in a tick-free herd. The annual cost of control in 1988 at ZK286.26/livestock unit was greater than the increase in value of the products at ZK175.48/livestock unit carrying capacity. It is concluded from these studies that intensive tick control in the multipurpose livestock system in Zambia is not justified in the absence of serious tick-borne disease. However, strategic seasonal tick control of adult ticks from November to April would probably be economically viable. Calves should not be treated until they are 3 months of age.     

Telomere shortening and survival in free-living corvids

Evidence accumulates that telomere shortening reflects lifestyle and predicts remaining lifespan, but little is known of telomere dynamics and their relation to survival under natural conditions. We present longitudinal telomere data in free-living jackdaws (Corvus monedula) and test hypotheses on telomere shortening and survival. Telomeres in erythrocytes were measured using pulsed-field gel electrophoresis. Telomere shortening rates within individuals were twice as high as the population level slope, demonstrating that individuals with short telomeres are less likely to survive. Further analysis showed that shortening rate in particular predicted survival, because telomere shortening was much accelerated during a bird''s last year in the colony. Telomere shortening was also faster early in life, even after growth was completed. It was previously shown that the lengths of the shortest telomeres best predict cellular senescence, suggesting that shorter telomeres should be better protected. We test the latter hypothesis and show that, within individuals, long telomeres shorten faster than short telomeres in adults and nestlings, a result not previously shown in vivo. Moreover, survival selection in adults was most conspicuous on relatively long telomeres. In conclusion, our longitudinal data indicate that the shortening rate of long telomeres may be a measure of ‘life stress’ and hence holds promise as a biomarker of remaining lifespan.     

Toxoplasma gondii: uptake and survival of oocysts in free-living amoebae

Waterborne transmission of the oocyst stage of Toxoplasma gondii can cause outbreaks of clinical toxoplasmosis in humans and infection of marine mammals. In water-related environments and soil, free-living amoebae are considered potential carriers of various pathogens, but knowledge on interactions with parasitic protozoa remains elusive. In the present study, we assessed whether the free-living Acanthamoebacastellanii, due to its phagocytic activity, can interact with T. gondii oocysts. We report that amoebae can internalize T. gondii oocysts by active uptake. Intracellular oocysts in amoebae rarely underwent phagocytic lysis, retained viability and established infection in mice. Interaction of T. gondii with amoebae did not reduce the infectivity and pathogenicity of oocysts even after prolonged co-cultivation. Our results show that uptake of oocysts by A. castellanii does not restrain the transmission of T. gondii in a murine infection model.     

Ecology of the free living stages of cattle nematodes during summer contamination in Argentina Western Pampas

Development, migration and survival of infective larvae (L3) were studied in the Western Pampeana Region. Faeces of naturally nematode infected cattle were deposited as artificial pats on plots during mid-spring-summer of 1994/1995 and 1995/1996. Since the start and during 1995, the study coincided with a severe drought, rainfalls being 29% below the 45-year means. The predominant genera recovered were Cooperia, Ostertagia and Haemonchus. Initial and peak recovery of L3 from pats occurred 8-15 and 15-21 days later respectively. A low percentage of L3 survived from November (0.3% L3) and January (0.06% L3) to the following autumn and winter. The mean persistence of larvae detected in pats or herbage was around 200 days from deposition. The migration of L3 from faecal pats to herbage started 15 to 30 days after deposition according to rainfall occurrence. Maximum herbage recoveries of L3 from pats deposited in late summer occur during autumn rainfalls. Only, few L3 were occasionally recovered from soil. Summer conditions were associated with rapid development and translation of L3 to herbage, but also with low L3 detection after initial recoveries. Faecal pats deposited from mid-summer were the main source of autumn herbage contamination.     

Nippostrongylus brasiliensis and Ascaridia galli: mitochondrial respiration in free-living and parasitic stages

Aerobic respiratory pathways have been delineated and respiratory efficiency has been assessed in mitochondria isolated from embryonated eggs, infective larvae, and adult Nippostrongylus brasiliensis and Ascaridia galli. Mitochondrial respiration in free-living stages of N. brasiliensis is mediated mainly by a mammalian-like antimycin A- and cyanide-sensitive pathway; specific respiratory activity is high and oxidative phosphorylation efficient. In mitochondria of adult N. brasiliensis, antimycin A- and cyanide-sensitive respiration is decreased relative to respiration though an alternative pathway, and specific respiratory activity and mitochondrial efficiency are lower. Respiration in mitochondria from embryonated eggs and tissues of adult A. galli is comparable, and apparently mediated by an antimycin A- and cyanide-insensitive alternative respiratory pathway; no evidence for the presence of a mammalian-like respiratory pathway in embryonated eggs of A. galli was found. The results of this study are compared to mitochondrial respiration in eggs, larvae, and adult body wall muscle of Ascaris suum.     

Ecological aspects of cattle tick control in central Zambia

In ecological studies in central Zambia, both climate and ecotype affected population dynamics of tick species. Below average rainfall for several years caused a suppression in numbers of Rhipicephalus appendiculatus Neumann adults. Reduction in rainfall leading to changes in grazing patterns is thought to have been responsible for an increase in numbers of Amblyomma variegatum Fabricius adults in a grassland habitat. There were reasonable correlations between numbers of each tick species on individual hosts over 1 year old. However, there were no relationships between numbers of ticks and bovine lymphocyte antigens (BoLA).     

Comparison of acetylcholinesterase genes from cattle ticks

We describe the isolation and characterisation of two putatively new acetylcholinesterase genes from the African cattle ticks Boophilus decoloratus and Rhipicephalus appendiculatus. The nucleotide sequences of these genes had 93% homology to each other and 95% and 91% identity, respectively, to the acetylcholinesterase gene from an Australian strain of another cattle tick, Boophilus microplus. Translation of the nucleotide sequences revealed putative amino acids that are essential for acetylcholinesterase activity: the active site serine, and the histidine and glutamate residues that associate with this serine to form the catalytic triad. All known acetylcholinesterases have three sets of cysteines that form disulfide bonds; however, the acetylcholinesterase genes of these three species of ticks encode only two sets of cysteines. Acetylcholinesterases of B. microplus from South Africa, Zimbabwe, Kenya and Mexico had 98-99% identity with acetylcholinesterase from B. microplus from Australia, whereas acetylcholinesterase from B. microplus from Indonesia was identical to that from Australia. Preliminary phylogenetic analyses surprisingly indicate that the acetylcholinesterases of ticks are closer phylogenetically to acetylcholinesterases of vertebrates than they are to those of other arthropods.     

Micrometeorological factors involved in development and survival of free-living stages of the sheep nematodesHaemonchus contortus andTrichostrongylus colubriformis. A review

Temperature and soil moisture are the most important factors affecting the development and survival ofHaemonchus contortus andTrichostrongylus columbriformis eggs and larvae on pasture. More than half of the eggs develop into infective larvae in the laboratory, but a very low percentage (0.03% forH. contortus) does so on pasture. There is a marked difference betweenH. contortus andT. colubriformis in survival of infective larvae.H. contortus larvae survived in the winter at Urbana poorly, whereasT. colubriformis did well. The former survived better than the latter in the spring and worse in the summer, while both survived equally well in the fall. Technics for larval recovery from pasture are not very efficient. Meteorologic conditions at ground level where the larvae exist are quite different from those in a standard weather shelter 1.6 m above the ground. Bioclimatographs in which mean monthly maximum temperatures are plotted against total monthly precipitation are fairly good in predicting the type of nematode liable to be important in a given region, but they are too simplistic to be relied on for more than approximations.