Resistance in cattle to Haemaphysalis (Kaiseriana) longicornis

Samples of each instar of the 3-host tick, Haemaphysalis (Kaiseriana) longicornis were collected from grazing Bos taurus cattle to investigate their survival on hosts with previous exposure to the tick and on previously unexposed ‘naive’ hosts. The ticks were allocated to categories based on their size to observe at what stage of development mortality occurred. Analysis of the age structure of the ticks showed that small but variable proportions of larvae, nymphs and females matured on previously exposed hosts and larger proportions on the naive hosts. This suggests that cattle acquire and express different levels of resistance against each instar of the tick. The technique used has potential for estimating levels of resistance of hosts to 3-host ticks under field conditions.     

Role of an aquaporin in the sheep tick Ixodes ricinus: Assessment as a potential control target

Ticks undergo tremendous osmoregulatory stress as they take on up to 100 times their body weight in blood, returning about 75% of the ingested water and ions via their saliva into the host. We postulated that water channels, or aquaporins, involved in this mass water transport might be good targets for acaricide development. An aquaporin (IrAQP1) identified in the sheep tick, Ixodes ricinus, was present only in tissues involved in mass water flux, namely the gut, rectal sac and especially abundant in the salivary glands. IrAQP1 was localised by in situ hybridisation in specific cell and acini types, possibly Type III acini, but absent from the type I acini that are responsible for rehydration of ticks in the non-feeding phase. Gene knockdown of IrAQP1 in isolated salivary glands completely inhibited dopamine-stimulated secretion. Further, IrAQP1 knockdown adult females had 50% reduced body weight gains over the first 5 days feeding on an artificial feeding apparatus and 21% at the point of engorgement on hosts. Haemolymph osmolarity was increased in the IrAQP1-knockdown ticks. Importantly, the blood volume ingested per body weight was reduced by 30%. Overall, it would appear that water passage from the gut to the saliva was disrupted and tick guts were simply too “full” to ingest more blood. However, double-stranded RNA interference of IrAQP1 did not affect mortality of the ticks which successfully fed to detachment at day 9. Overall, our data indicate that IrAQP1 plays a pivotal role in blood meal water handling through the gut and salivary gland, and although its disruption by double-stranded RNA interference dramatically affects feeding performance, ticks remained feeding on the host with subsequent potential pathogen transmission and, therefore, IrAQP1 is not a suitable candidate target for tick control.     

Rhipicephalus appendiculatus: Larval feeding sites in guinea pigs actively sensitized and receiving immune serum

Histological analyses of larval Rhipicephalus appendiculatus feeding sites in naive and actively sensitized guinea pigs were made at 6, 24, 48, 72, and 96 hr post-tick attachment. As primary feedings progressed the cavity at the entrance of the ticks mouthparts into the uppermost dermis, and the surrounding cellular infiltrate (lesion) both increased. Early (6 hr) lesions were dominated by eosinophils (46% of the infiltrate), neutrophils predominated at 24 (55%) and 48 hr (68%), eosinophils again predominated at 72 hr (44%), and finally basophils were dominant at 96 hr (78%). At sites of secondary feedings in animals expressing acquired resistance, lesion size increased as tick feeding progressed and at each observation time was at least twice that observed in primary feedings. Dermal cavities at the site of entrance of the ticks mouthparts were occasional in occurrence and were reduced in size indicating altered tick feeding. Basophils were dominant at all observation times ranging from 61 to 91% of the infiltrate. The second cell type of significance was the eosinophil, ranging in abundance from 7 to 21%. Recipients of immune serum had a smaller cellular infiltrate around feeding ticks, but basophils were also dominant. Basophils appear to be the principal host cell involved in acquired resistance to tick feeding as indicated by the profound cutaneous basophil reaction that characterized the immune response to larval ticks both in actively and passively sensitized hosts. The finding of significant eosinophil accumulations at tick feeding sites of both hosts indicates that these cells may also contribute to acquired resistance.     

Major surface protein 1a effects tick infection and transmission of Anaplasma marginale.

Anaplasma marginale, an ehrlichial pathogen of cattle and wild ruminants, is transmitted biologically by ticks. A developmental cycle of A. marginale occurs in a tick that begins in gut cells followed by infection of salivary glands, which are the site of transmission to cattle. Geographic isolates of A. marginale vary in their ability to be transmitted by ticks. In these experiments we studied transmission of two recent field isolates of A. marginale, an Oklahoma isolate from Wetumka, OK, and a Florida isolate from Okeechobee, FL, by two populations of Dermacentor variabilis males obtained from the same regions. The Florida and Oklahoma tick populations transmitted the Oklahoma isolate, while both tick populations failed to transmit the Florida isolate. Gut and salivary gland infections of A. marginale, as determined by quantitative PCR and microscopy, were detected in ticks exposed to the Oklahoma isolate, while these tissues were not infected in ticks exposed to the Florida isolate. An adhesion-recovery assay was used to study adhesion of the A. marginale major surface protein (MSP) 1a to gut cells from both tick populations and cultured tick cells. We demonstrated that recombinant Escherichia coli expressing Oklahoma MSP1a adhered to cultured and native D. variabilis gut cells, while recombinant E. coli expressing the Florida MSP1a were not adherent to either tick cell population. The MSP1a of the Florida isolate of A. marginale, therefore, was unable to mediate attachment to tick gut cells, thus inhibiting salivary gland infection and transmission to cattle. This is the first report of MSP1a being responsible for effecting infection and transmission of A. marginale by Dermacentor spp. ticks. The mechanism of tick infection and transmission of A. marginale is important in formulating control strategies and development of improved vaccines for anaplasmosis.     

Observations on Antricola ticks: small nymphs feed on mammalian hosts and have a salivary gland structure similar to ixodid ticks

Ticks use bloodmeals as a source of nutrients and energy to molt and survive until the next meal and to oviposit, in the case of females. However, only the larvae of some tick species are known to feed upon bats; females are obligatorily autogenous, and nymphal stages are believed to not feed. We investigated the presence of blood in a natural population of nymphal Antricola delacruzi ticks collected from bat guano; their ability to feed upon laboratory hosts; and the microscopic structure of both salivary glands and gut. DNA amplification of gut contents of freshly collected material was positive for a mammal in 4 of 11 first instar nymphs, but we were unsuccessful in the amplification of host bloodmeal DNA from late instar nymphs. All early nymphal stages (n = 10) fed on rabbits, and host DNA was detected and sequenced from gut contents. However, all the large nymphs (n = 10) rejected feeding, and host DNA remained undetected in these ticks. All stages of A. delacruzi have salivary glands similar in morphology to the ixodid agranular Type I salivary gland acini and to granular Type II or Type B acini. All stages of A. delacruzi had a similar gut structure, consisting of digestive cells in the basal portion that contained hematin granules. Neither regenerative nor secretory cell traces were observed in the sections of gut.     

Theileria parva: variation in the infection rate of the vector tick, Rhipicephalus appendiculatus

The variation in Theileria parva infection rates of experimental batches of adult Rhipicephalus appendiculatus ticks, used during the course of several years, was examined. It was found that considerable variation occurred, but that this could not always be correlated with the piroplasm parasitaemia in the cattle on which the ticks engorged as nymphs. Statistical analysis showed that the infection rate of ticks fed on cattle with a parasitaemia of 41–50 per cent was significantly higher than that of ticks fed on cattle with lower parasitaemias. A number of experiments were then carried out in which one or several factors of this aspect of the host-parasite relationship remained constant whilst others were altered. None of these factors was seen to play a major part in the variation. Finally, randomly selected groups of 10 ticks which had dropped engorged as nymphs from the same animal on the same day were examined. The variation observed even in these groups was so great that it was concluded that the infection rate could depend on a factor such as the juxtaposition of possibly-infected gut epithelial cells and developing salivary glands during the nymphal moult.     

Biological Parameters of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) Fed on Rabbits,Sheep, and Cattle

In order to determine the effect of various hosts on feeding performance of Rhipicephalus (Boophilus) microplus, we used 3 mammalian species as hosts, cattle (Qinchuan), sheep (T an), and rabbits (Japanese white rabbit) for infest-ing ticks. Five hundreds of R. microplus larvae were exposed to each animal (3 animals/host species). Tick recoveries were 11.0%, 0.47%, and 5.5% from cattle, sheep, and rabbits, respectively. The averages of tick feeding periods were not significantly different on cattle, sheep, and rabbits, 28.8, 25.3, and 26.7 days, respectively. The average weights of individual engorged female from cattle, sheep, and rabbits were 312.5, 219.1, and 130.2 mg, respectively and those of egg mass weights each to 85.0, 96.6, and 17.8 mg. The highest egg hatching rate was in the ticks from cattle (96.0%), fol-lowed by those from rabbits (83.0%) and sheep (19.2%). These data suggest that rabbits could be as an alternative host to cultivate R. microplus for evaluating vaccines and chemical and biological medicines against the tick in the laboratory, although the biological parameters of ticks were less than those from cattle.     

Factors influencing the toxicity of salivary gland extracts of Ixodes holocyclus Neumann

A bioassay using mice was developed to compare the toxin content of extracts of salivary glands of I. holocyclus at various stages of feeding. The quantity of toxin increased rapidly from the third day of feeding. Toxin production continued and increased in ticks removed after 3–5 days on mice and held at 30°C at 92% RH for 24 h, whereas no toxin was detected in the salivary glands of ticks fed for 3 days and treated similarly. It is suggested that major physiological changes occur in the salivary glands of I. holocyclus on the third day, which once stimulated continue independently of feeding. Toxin production in ticks was not suppressed by passively immunizing mice with anti-tick toxin but was in ticks fed upon hosts with a previous experience of tick feeding.Thus, to obtain salivary glands containing high concentrations of toxin for chemical analysis, it is necessary for salivary glands to develop 5 days from the initial attachment of the tick to a host with no previous experience of tick feeding. This can be achieved by passively immunizing mice against toxin, thus enabling the tick to feed 5 days without killing the mouse or by keeping the tick for 24 h at 30°C at 92% RH following the death of the mouse on the fourth day.     

Amblyomma americanum (L): Tick macrophage migration inhibitory factor peptide immunization lengthens lone star tick feeding intervals in vivo

Immunizations of New Zealand White rabbits with specific macrophage migration inhibitory factor (MIF) tick peptide (PEP) produced circulating anti-tick PEP antibodies in the hosts. Antibody titers of greater than 1:5000 to tick MIF peptide were observed for crude sera from PEP-immunized rabbits. PEP- and BSA-vaccinated rabbits were infested with Amblyomma americanum adults. Feeding intervals, female weights, egg masses and percent egg hatch were measured for ticks feeding on control and immunized hosts. Feeding intervals were significantly lengthened to 13.3 days for PEP-vaccinated hosts compared to BSA-vaccinated controls at 12.4 days, while female engorgement weights and egg masses were unchanged. By immunizing hosts using specific tick PEP, we were able to alter the length of time the ticks fed on their hosts.     

Prevalence of hard tick infestations in cattle of West Bengal,India

The present study was conducted to determine the prevalence of hard tick infestations in cattle of West Bengal from July 2015 to June 2016. The prevalence of hard tick infestations was studied in relation to sex and age of animals and seasonal changes in a year. Cattle of selected places were examined carefully for the presence of ticks and in positive cases ticks were collected manually and identified on the basis of morphological characters. A total of 310 cattle were examined and out of which, 130 (41.93%) cattle were found to be infested with hard ticks and the prevalent species were Rhipicephalus (Boophilus) sp., Hyalomma sp. and Haemaphysalis sp. of ticks. A significantly (p < 0.01) higher infestation was observed in female cattle (43.30%) than males (35.71%). Age-wise highest prevalence of tick infestations was found in <1 year (65%) age group followed by >3 years age group (36.8%) and 1–3 years (35.63%) age group, respectively. Seasonally, the prevalence of hard ticks was highest (p < 0.01) in monsoon (59.25%) and lowest in winter (27.09%). The study revealed that the prevalence of Rhipicephalus (Boophilus) sp. (32.25%) was significantly (p < 0.01) higher compared to Hyalomma (12.58%) and Haemaphysalis sp. (3.22%). The observations of the present study would provide a basis for evolving effective control strategy for the management of ticks in bovines of West Bengal.     

Histology of digestion in nymphs of Rhipicephalus appendiculatus fed on rabbits and cattle naive and resistant to the ticks

Nymphs feeding on ears of four rabbits and four calves (Bos taurus) were examined during first and third or fourth infestations and also during the moulting period. The gut caecae were removed and examined by histochemistry and light and electron microscopy. Attachment sites of the nymphs were biopsied from all hosts and cell counts made by light microscopy. Resistance was expressed by reduction in numbers of ticks completing engorgement and reduced engorgement weights. The gut was comprised of a proliferative stem cell; a digestive cell that differentiated into a sessile type ingesting by pinocytosis and a motile type ingesting by phagocytosis; and a cell secreting a glycoprotein with acid phosphatase activity into the lumen. The gut grew during the early stages of feeding to accommodate the expansion during engorgement. On rabbits and cattle resistant to ticks the stem cells were damaged, with moribund nuclei and poorly differentiated cytoplasm. Thus there were fewer digestive and secretory cells and the gut did not expand to accommodate a full blood meal. The attachment sites were dominated by mononuclear cells and neutrophils in both host species at the first infestations but at the third or fourth infestations there was a considerable increase in proportions of eosinophils and basophils. Host granulocytes were traced to the lumen of the tick gut and to motile digestive cells which destroyed them by phagocytosis.     

Major surface protein 1a effects tick infection and transmission of Anaplasma marginale

Anaplasma marginale, an ehrlichial pathogen of cattle and wild ruminants, is transmitted biologically by ticks. A developmental cycle of A. marginale occurs in a tick that begins in gut cells followed by infection of salivary glands, which are the site of transmission to cattle. Geographic isolates of A. marginale vary in their ability to be transmitted by ticks. In these experiments we studied transmission of two recent field isolates of A. marginale, an Oklahoma isolate from Wetumka, OK, and a Florida isolate from Okeechobee, FL, by two populations of Dermacentor variabilis males obtained from the same regions. The Florida and Oklahoma tick populations transmitted the Oklahoma isolate, while both tick populations failed to transmit the Florida isolate. Gut and salivary gland infections of A. marginale, as determined by quantitative PCR and microscopy, were detected in ticks exposed to the Oklahoma isolate, while these tissues were not infected in ticks exposed to the Florida isolate. An adhesion-recovery assay was used to study adhesion of the A. marginale major surface protein (MSP) 1a to gut cells from both tick populations and cultured tick cells. We demonstrated that recombinant Escherichia coli expressing Oklahoma MSP1a adhered to cultured and native D. variabilis gut cells, while recombinant E. coli expressing the Florida MSP1a were not adherent to either tick cell population. The MSP1a of the Florida isolate of A. marginale, therefore, was unable to mediate attachment to tick gut cells, thus inhibiting salivary gland infection and transmission to cattle. This is the first report of MSP1a being responsible for effecting infection and transmission of A. marginale by Dermacentor spp. ticks. The mechanism of tick infection and transmission of A. marginale is important in formulating control strategies and development of improved vaccines for anaplasmosis.     

Diclidophora merlangi: Sloughing and renewal of hematin cells

Electron microscopy and section autoradiography have been used in an attempt to find evidence of sloughing and renewal of hematin (digestive) cells in the gut of Diclidophora merlangi. Hematin cells are connected by septate desmosomes to a polymorphic, syncytial connecting tissue which supports and protects each cell from stresses imposed on the gut by body movements. The sloughing of hematin cells occurs only rarely, and evidence of the event is restricted to the occasional finding of a free and relatively undamaged cell in the lumen of the main ceca. The connecting syncytium of the main ceca leading to the foregut is attenuated and, in places, perforated by small pores. Small, undifferentiated cells can be found below the pores, and some of these cells may represent embryonic gut cells. Immature hematin cells, without pigment or Golgi stacks, border the lumen in this region of the gut and are connected to the syncytium by septate desmosomes. Pulse-chase experiments with tritiated thymidine indicate that any renewal of hematin cells takes place a a very low rate.     

Rhipicephalus appendiculatus feeding on rabbits and cattle: Salivary-gland responses to varying host resistance

AdultRhipicephalus appendiculatus ticks were fed as three sequential infestations on both rabbits and cattle. The feedings at first infestation on naive hosts were optimum for the ticks, whereas at third infestation the hosts were resistant, as expressed by reduced tick feeding performance. Ticks from naive and resistant hosts were examined for histological differences of salivary glands. In ticks fed on resistant rabbits there was a large increase in the synthesis of glycoprotein secretory granules in thec ₁ cells compared with ticks fed on naive rabbits. In ticks fed on naive and resistant cattle, the activity of thec ₁ cells was less than in ticks fed on naive and resistant rabbits. It was concluded that the salivary glands are able to respond selectively to conditions at the feeding site, and that this may be advantageous to the tick.     

Serologic and molecular evidence for circulation of Crimean-Congo hemorrhagic fever virus in ticks and cattle in Zambia

Crimean-Congo hemorrhagic fever (CCHF) is a tick-borne zoonosis with a high case fatality rate in humans. Although the disease is widely found in Africa, Europe, and Asia, the distribution and genetic diversity of CCHF virus (CCHFV) are poorly understood in African countries. To assess the risks of CCHF in Zambia, where CCHF has never been reported, epidemiologic studies in cattle and ticks were conducted. Through an indirect immunofluorescence assay, CCHFV nucleoprotein-specific serum IgG was detected in 8.4% (88/1,047) of cattle. Among 290 Hyalomma ticks, the principal vector of CCHFV, the viral genome was detected in 11 ticks. Phylogenetic analyses of the CCHFV S and M genome segments revealed that one of the detected viruses was a genetic reassortant between African and Asian strains. This study provides compelling evidence for the presence of CCHFV in Zambia and its transmission to vertebrate hosts.     

Prevalence of Ehrlichia ruminantium in adult Amblyomma variegatum collected from cattle in Cameroon

Ehrlichia ruminantium, the etiologic agent of the economically important disease heartwater, is an obligate intracellular bacterium transmitted by ticks of the genus Amblyomma, particularly A. hebraeum and A. variegatum. Although serologic and microscopic evidence of the presence of heartwater have been reported in ruminants in Cameroon, knowledge of E. ruminantium infection in the tick vector, A. variegatum, is lacking. In order to determine the infectivity of A. variegatum ticks by E. ruminantium, we analysed 500 un-engorged A. variegatum ticks collected by hand-picking from predilection sites from 182 cattle [115 ticks from 82 cattle at Société de Développement et d’Exploitation des Productions Animales (SODEPA) Dumbo ranch (SDR) and 385 ticks from 100 cattle at the Upper Farms ranch (UFR)] by amplification of the open reading frame (ORF) 2 of the pCS20 region of E. ruminantium. PCR amplification of the 279 bp fragment of the pCS20 region detected E. ruminantium DNA in 142 (28.4 %) of the 500 ticks with a higher infection rate (47/115; 40.9 %) observed in ticks from SDR and 24.7 % (95/385) of ticks collected from cattle at UFR. Twenty five randomly selected PCR products were sequenced and results indicated that some of the isolates shared homology with one another and to sequences of E. ruminantium in the GenBank. This report represents the first molecular evidence of E. ruminantium infection in A. variegatum ticks in Cameroon and suggests possible exposure of cattle to this pathogen in our environment.     

Influence of the Biotope on the Tick Infestation of Cattle and on the Tick-Borne Pathogen Repertoire of Cattle Ticks in Ethiopia

Background

The majority of vector-borne infections occur in the tropics, including Africa, but molecular eco-epidemiological studies are seldom reported from these regions. In particular, most previously published data on ticks in Ethiopia focus on species distribution, and only a few molecular studies on the occurrence of tick-borne pathogens or on ecological factors influencing these. The present study was undertaken to evaluate, if ticks collected from cattle in different Ethiopian biotopes harbour (had access to) different pathogens.

Methods

In South-Western Ethiopia 1032 hard ticks were removed from cattle grazing in three kinds of tick biotopes. DNA was individually extracted from one specimen of both sexes of each tick species per cattle. These samples were molecularly analysed for the presence of tick-borne pathogens.

Results

Amblyomma variegatum was significantly more abundant on mid highland, than on moist highland. Rhipicephalus decoloratus was absent from savannah lowland, where virtually only A. cohaerens was found. In the ticks Coxiella burnetii had the highest prevalence on savannah lowland. PCR positivity to Theileria spp. did not appear to depend on the biotope, but some genotypes were unique to certain tick species. Significantly more A. variegatum specimens were rickettsia-positive, than those of other tick species. The presence of rickettsiae (R. africae) appeared to be associated with mid highland in case of A. variegatum and A. cohaerens. The low level of haemoplasma positivity seemed to be equally distributed among the tick species, but was restricted to one biotope type.

Conclusions

The tick biotope, in which cattle are grazed, will influence not only the tick burden of these hosts, but also the spectrum of pathogens in their ticks. Thus, the presence of pathogens with alternative (non-tick-borne) transmission routes, with transstadial or with transovarial transmission by ticks appeared to be associated with the biotope type, with the tick species, or both, respectively.     

The effect of chemotherapy on Babesia bigemina in the tick vector Boophilus microplus

Percentages of feeding ticks in which Babesia bigemina could be detected (infection rates) were determined following treatment of bovine hosts with each of four babesicides. Infection rates were suppressed by imidocarb dipropionate, quinuronium sulphate and amicarbalide, reaching minimum levels 3–4 days after treatment, but imidocarb dihydrochloride had comparatively little effect. Total elimination of the parasite from ticks was not achieved. Treatment of tick infested hosts with imidocarb dipropionate or quinuronium sulphate failed to prevent transmission of B. bigemina by transovarian passage or by transfer of adult male ticks. These findings indicate that the use of babesicides for chemotherapy is unlikely to have a significant effect on the rate of transmission of B. bigemina.     

East coast fever: 60Co-irradiation of Theileria parva in its tick vector, Rhipicephalus appendiculatus

Three experiments were carried out in which Theileria parva was irradiated in its tick vector, Rhipicephalus appendiculatus. In the first experiment, infected unfed adult ticks were irradiated at doubling doses from 4 to 32 krad. Some of the ticks were then fed for 4, 5, 6, 7 and 8 days on rabbits, and the parasites in their salivary glands examined. Five male and 5 female ticks from each irradiation dose were put onto each of a pair of susceptible cattle, whose reactions were recorded. Increasing doses of irradiation resulted in progressive destruction of the parasites. All cattle receiving ticks irradiated at doses up to and including 16 krad died of East Coast fever (ECF), and one of the cattle receiving ticks irradiated at 32 krad died.In the second experiment, recently engorged nymphs were irradiated at 1, 2 or 4 krad, and moulting nymphs at doses of 2, 4, 8, 12, 16 or 32 krad. The salivary glands of the resultant adult ticks were examined after the ticks had fed for 4, 5, 6, 7 or 8 days on rabbits. Engorged nymphs irradiated at 4 krad failed to moult, whilst moulting nymphs irradiated at 32 krad moulted but failed to attach to rabbits. Doses of irradiation survived by the ticks had no apparent morphological effect on the parasites they contained.In the third experiment, infected unfed adult ticks were irradiated at 0, 5, 10, 15, 20 25 30, 35, 40, 45, 50 or 60 krad. The ticks were fed on rabbits for 5, 6 or 7 days. Some of them were then examined morphologically, whilst others were ground in MEM/BPA and aliquots of the supernatant used to inoculate groups of 5 cattle. The reaction of these cattle, together with the morphological examination of the parasites, suggested that increasing doses of irradiation destroyed increasing numbers of parasites.     

Could insecticide‐treated cattle reduce Afrotropical malaria transmission? Effects of deltamethrin‐treated Zebu on Anopheles arabiensis behaviour and survival in Ethiopia

Anopheles arabiensis Patton (Diptera: Culicidae) is the most widespread vector of malaria in the Afrotropical Region. Because An. arabiensis feeds readily on cattle as well as humans, the insecticide-treatment of cattle--as employed to control tsetse (Diptera: Glossinidae) and ticks (Acari: Ixodidae)--might simultaneously affect the malaria vectorial capacity of this mosquito. Therefore, we conducted field experiments in southern Ethiopia to establish whether Zebu cattle (Bos indicus L.) treated with a pour-on pyrethroid formulation of 1% deltamethrin, widely used to control ticks and tsetse, would be effective against An. arabiensis or cause the female mosquitoes to feed more frequently on humans, due to behavioural avoidance of insecticide-treated cattle. Contact bioassays (3 min exposure) showed that the insecticide remained effective for about 1 month (kill rate > 50%) against mosquitoes feeding on the flanks of treated cattle. A novel behavioural assay demonstrated that An. arabiensis readily fed on insecticide-treated cattle and were not deflected to human hosts in the presence of treated cattle. DNA-fingerprinting of bloodmeals revealed that An. arabiensis naturally feeds most frequently on older animals, consistent with the established practice of applying insecticide only to older cattle, while allowing younger untreated animals to gain immunity against infections transmitted by ticks. These encouraging results were tempered by finding that > 90% of An. arabiensis, An. pharoensis and An. tenebrosus females feed on the legs of cattle, farthest from the site of pour-on application along the animal's back and where the treatment may be least residual due to weathering. Observations of mosquitoes feeding naturally on insecticide-treated cattle showed that the majority of wild female anophelines alighted on the host animal for less than 1 min to feed, with significantly shorter mean duration of feeding bouts on insecticide-treated animals, and the effective life of the insecticide was only 1 week. Thus the monthly application of deltamethrin to cattle, typically used to control tsetse and ticks, is unlikely to be effective against An. arabiensis populations or their vectorial capacity. Even so, it seems likely that far greater impact on anopheline mosquitoes could be achieved by applying insecticide selectively to the legs of cattle.