Wheat curl mite, Aceria tosichella, and transmitted viruses: an expanding pest complex affecting cereal crops

The wheat curl mite (WCM), Aceria tosichella, and the plant viruses it transmits represent an invasive mite-virus complex that has affected cereal crops worldwide. The main damage caused by WCM comes from its ability to transmit and spread multiple damaging viruses to cereal crops, with Wheat streak mosaic virus (WSMV) and Wheat mosaic virus (WMoV) being the most important. Although WCM and transmitted viruses have been of concern to cereal growers and researchers for at least six decades, they continue to represent a challenge. In older affected areas, for example in North America, this mite-virus complex still has significant economic impact. In Australia and South America, where this problem has only emerged in the last decade, it represents a new threat to winter cereal production. The difficulties encountered in making progress towards managing WCM and its transmitted viruses stem from the complexity of the pathosystem. The most effective methods for minimizing losses from WCM transmitted viruses in cereal crops have previously focused on cultural and plant resistance methods. This paper brings together information on biological and ecological aspects of WCM, including its taxonomic status, occurrence, host plant range, damage symptoms and economic impact. Information about the main viruses transmitted by WCM is also included and the epidemiological relationships involved in this vectored complex of viruses are also addressed. Management strategies that have been directed at this mite-virus complex are presented, including plant resistance, its history, difficulties and advances. Current research perspectives to address this invasive mite-virus complex and minimize cereal crop losses worldwide are also discussed.

Brevipalpus mites (Acari: Tenuipalpidae): vectors of invasive, non-systemic cytoplasmic and nuclear viruses in plants

Multi-directional interactions occur among plant hosts, Brevipalpus mites and the plant viruses they transmit. Such interactions should be considered when evaluating the severity of a disease such as citrus leprosis. The current understanding of Brevipalpus-transmitted viruses relies on the capability of the vector to transmit the disease, the persistence of the virus in the host plant and the ability of the disease to spread. Previously, we discussed the Citrus leprosis virus (CiLV) and its importance and spread over the past decade into new areas of South and Central America, most recently into southern Mexico and Belize. Here, we address key questions to better understand the biology of the mite vector, fitness costs, and the peculiarities of Brevipalpus mite reproduction, virus survival, transmissibility and spread, and the expansion of the host plant range of Brevipalpus species vectoring the disease.

Using the gravity model to estimate the spatial spread of vector-borne diseases

The gravity models are commonly used spatial interaction models. They have been widely applied in a large set of domains dealing with interactions amongst spatial entities. The spread of vector-borne diseases is also related to the intensity of interaction between spatial entities, namely, the physical habitat of pathogens’ vectors and/or hosts, and urban areas, thus humans. This study implements the concept behind gravity models in the spatial spread of two vector-borne diseases, nephropathia epidemica and Lyme borreliosis, based on current knowledge on the transmission mechanism of these diseases. Two sources of information on vegetated systems were tested: the CORINE land cover map and MODIS NDVI. The size of vegetated areas near urban centers and a local indicator of occupation-related exposure were found significant predictors of disease risk. Both the land cover map and the space-borne dataset were suited yet not equivalent input sources to locate and measure vegetated areas of importance for disease spread. The overall results point at the compatibility of the gravity model concept and the spatial spread of vector-borne diseases.

Tick cell lines for study of Crimean-Congo hemorrhagic fever virus and other arboviruses

Continuous cell lines derived from many of the vectors of tick-borne arboviruses of medical and veterinary importance are now available. Their role as tools in arbovirus research to date is reviewed and their potential application in studies of tick cell responses to virus infection is explored, by comparison with recent progress in understanding mosquito immunity to arbovirus infection. A preliminary study of propagation of the human pathogen Crimean-Congo hemorrhagic fever virus (CCHFV) in tick cell lines is reported; CCHFV replicated in seven cell lines derived from the ticks Hyalomma anatolicum (a known vector), Amblyomma variegatum, Rhipicephalus (Boophilus) decoloratus, Rhipicephalus (Boophilus) microplus, and Ixodes ricinus, but not in three cell lines derived from Rhipicephalus appendiculatus and Ornithodoros moubata. This indicates that tick cell lines can be used to study growth of CCHFV in arthropod cells and that there may be species-specific restriction in permissive CCHFV infection at the cellular level.

[Isolation, identification and characterization of SFTS bunyavirus from ticks collected on the surface of domestic animals]

To understand the maintenance and transmission of SFTS virus, the potential vector ticks were collected from sheep, cattle and dogs in the endemic areas of SFTSV in Shandong Province. Among the collected ticks, the dominant species was H. longicornis ticks. Real-time PCR for RNA detection, virus isolation and characterization, genomic sequencing, phylogenetic and antigenic analysis were performed in this investigation. The results showed that the SFTS viral RNA was detected in 2.14% H. longicornis, and a SFTS virus was isolated from one of viral RNA positive ticks collected from sheep. Whole genome analysis of the SFTSV isolates with 11 human-origin SFTS virus revealed a highly pairwise similarity, and the growth curve analysis showed nearly identical in virus yield and the dynamic of virus reproduction compared to human derived viral isolates. Immunofluorescence and neutralization test showed identical serological reaction character of the two different origin viral strains. In this study, the characters of a SFTSV isolate was firstly described, which suggested that the tick species H. longicornis acting important vector role in the transmission of SFTS virus.

Differential salivary gland transcript expression profile in Ixodes scapularis nymphs upon feeding or flavivirus infection

Ixodid ticks are vectors of human diseases such as Lyme disease, babesiosis, anaplasmosis, and tick-borne encephalitis. These diseases cause significant morbidity and mortality worldwide and are transmitted to humans during tick feeding. The tick-host-pathogen interface is a complex environment where host responses are modulated by the molecules in tick saliva to enable the acquisition of a blood meal. Disruption of host responses at the site of the tick bite may also provide an advantage for pathogens to survive and replicate. Thus, the molecules in tick saliva not only aid the tick in securing a nutrient-rich blood meal, but can also enhance the transmission and acquisition of pathogens. To investigate the effect of feeding and flavivirus infection on the salivary gland transcript expression profile in ticks, a first-generation microarray was developed using ESTs from a cDNA library derived from Ixodes scapularis salivary glands. When the salivary gland transcript profile in ticks feeding over the course of 3 days was compared to that in unfed ticks, a dramatic increase in transcripts related to metabolism was observed. Specifically, 578 transcripts were up-regulated compared to 151 down-regulated transcripts in response to feeding. When specific time points post attachment were analyzed, a temporal pattern of gene expression was observed. When Langat virus-infected ticks were compared to mock-infected ticks, transcript expression changes were observed at all 3 days of feeding. Differentially regulated transcripts include putative secreted proteins, lipocalins, Kunitz domain-containing proteins, anti-microbial peptides, and transcripts of unknown function. These studies identify salivary gland transcripts that are differentially regulated during feeding or in the context of flavivirus infection in Ixodes scapularis nymphs, a medically important disease vector. Further analysis of these transcripts may identify salivary factors that affect the transmission or replication of tick-borne flaviviruses.

[A case of Crimean-Congo hemorrhagic fever with long incubation period in Kocaeli, Turkey]

Crimean-Congo hemorrhagic fever (CCHF) is a viral hemorrhagic disease with high mortality rate. CCHF is endemic in Central Anatolia and East and Central Black Sea parts of Turkey, however sporadic cases have been detected in the other regions. The incubation period of the disease is between 1-3 days (maximum 12 days). In this report, a very rare CCHF case with a long incubation period of 30 days, was reported. A 40-year-old female patient living in a village of Kocaeli, Turkey was admitted to a health center in June 2010 with the complaints of headache, myalgia, nausea, vomiting, fatigue and fever. Since laboratory results revealed severe thrombocytopenia (18.300/mm3), the patient was referred to the university hospital in Kocaeli. It was learned from her history that she had been working in the garden and removed a tick from the skin of gluteal area a month ago without seeking any medical help. Physical examination of the patient revealed that her general condition was well, oriented and cooperative, body temperature was 36.6°C, pulse 82/minute, trombocyte count 69.400/mm3 and liver enzymes were elevated (ALT: 194 U/L, AST: 499 U/L, GGT: 384 U/L, LDH: 1290 U/L). Petecchial lesions were seen on hard palate and extremities and a hyperemic lesion was detected at the gluteal area where the tick had attached. In-house real-time polymerase chain reaction test for CCHF, performed at Refik Saydam National Public Health Agency, Virology Reference and Research Laboratory, revealed positive result. This case was presented to withdraw attention to a long incubation period CCHF and also of its epidemiological importance since it was the first case in Kocaeli province, Turkey.

Transmission of tick-borne pathogens between co-feeding ticks: Milan Labuda's enduring paradigm

During the 1990s, Milan Labuda's experimental results established a new paradigm for the study of tick-borne viruses that has since been strengthened by its demonstrated effectiveness in explaining the epidemiology of tick-borne encephalitis (TBE). This brief review summarizes the essential features of the transmission of tick-borne pathogens such as TBE virus. Leukocytes migrate between tick feeding sites, bearing infective virions and providing a transport route for the virus between co-feeding ticks independent of a systemic viraemia. Such tick-borne pathogens are thus transmitted from tick to tick via vertebrates; the ticks are the reservoirs as well as the vectors, while the vertebrate is the transient bridge. The aim is to bring the related but non-synonymous terms (co-feeding and non-systemic) to the attention of workers who use simple PCR screening to identify additional vertebrate reservoir hosts of vector-borne pathogens that are not in fact maintained in nature through systemic transmission.

Detection of Brevipalpus-transmitted viruses in their mite vectors by RT-PCR

The diagnosis of plant diseases caused by Brevipalpus-transmitted viruses (BrTVs) has been done through the analyses of symptoms, transmission electron microscopy, and RT-PCR of infected plant tissues. Here, we report the detection of Citrus leprosis virus C, Orchid fleck virus, Clerodendrum chlorotic spot virus and Solanum violaefolium ringspot virus in their viruliferous vectors Brevipalpus spp. using specific primer pairs for each of the viruses. The efficiency of virus transmission by Brevipalpus mites is low, so the detection of these pathogens in their vectors could constitute an important tool for studies involving virus-vector relationships, transmission, and monitoring the pathogen prior to the appearance of symptoms in the field.

Evaluation of knowledge about protection against Crimean-Congo hemorrhagic fever

This study was conducted in order to evaluate individuals' knowledge about protection against Crimean Congo hemorrhagic fever (CCHF). This descriptive study was carried out among 478 persons, to whom a Family Health Center located within boundaries of Erzurum Metropolitan Municipality, provides health service. A questionnaire form developed by the researchers was used for collecting data. Seventy-one point eight percent of individuals who participated in the study stated they had knowledge about CCHF, 25.9% stated that region was risky in terms of being bitten by ticks, 61.3% stated they could recognize ticks and 56.1% stated that not all tick bites cause the disease. Seventy-seven point eight percent stated CCHF is a virulent disease, 33.1% stated it can be transmitted from human to human and 30.3% stated it can be transmitted from animals to humans. In terms of protection from tick bites, 45, 15.3 and 11.3% of individuals stated wearing clothes to cover the whole body, carefully inspecting the body, and not touching ticks with bare hands, respectively, were good methods. Ninety-two point one percent stated it is necessary to go to a healthcare organization immediately in case of tick bite, whereas 18% of individuals stated it is necessary to remove the tick with tweezers or forceps. The results of this study show most individuals are not well informed about methods for protecting against CCHF, for removing ticks and what precautions to take to protect against tick bites.

Microclimate and the zoonotic cycle of tick-borne encephalitis virus in Switzerland

The focal distribution of tick-borne encephalitis virus (TBEV; Flaviviridae, Flavivirus) appears to depend mainly on cofeeding transmission between infected Ixodes ricinus L. nymphs and uninfected larvae. To better understand the role of cofeeding ticks in the transmission of TBEV, we investigated tick infestation of rodents and the influence of microclimate on the seasonality of questing I. ricinus ticks. A 3-yr study was carried out at four sites, including two confirmed TBEV foci. Free-living ticks and rodents were collected monthly, and microclimatic data were recorded. A decrease in questing nymph density was observed in 2007, associated with low relative humidity and high temperatures in spring. One site, Thun, did not show this decrease, probably because of microclimatic conditions in spring that favored the questing nymph population. During the same year, the proportion of rodents carrying cofeeding ticks was lower at sites where the questing nymph density decreased, although the proportion of infested hosts was similar among years. TBEV was detected in 0.1% of questing ticks, and in 8.6 and 50.0% of larval ticks feeding on two rodents. TBEV was detected at all but one site, where the proportion of hosts with cofeeding ticks was the lowest. The proportion of hosts with cofeeding ticks seemed to be one of the factors that distinguished a TBEV focus from a non-TBEV focus. The enzootic cycle of TBEV might be disrupted when dry and hot springs occur during consecutive years.

Crimean Congo hemorrhagic fever: requires vigilance and not panic

Crimean Congo hemorrhagic fever (CCHF) has been in the news with reports of its outbreak in India from Gujarat. CCHF is caused by a virus which is a member of the Nairovirus genus of the family Bunyaviridae. All of these viruses are transmitted by either ixodid or argasid ticks. Humans get this infection after a bite of an infected tick or from one infected human to another by contact with infectious blood or body fluids. Workers in livestock and agriculture industry, slaughterhouses, and veterinary practice are most prone to this infection. In severe cases after 3-6 days of the onset of symptoms hemorrhagic manifestations occur. IgG and IgM antibodies may be detected in serum by ELISA from about the sixth day of the illness. The mainstay of treatment in CCHF is supportive. Management of DIC, sepsis, shock and MODS should be undertaken. The antiviral drug Ribavirin has shown benefits. Benefits of treatment with ribavirin outweigh the fatal risks, and ribavirin may therefore be recommended. People at risk should use effective personal protective measures against tick bites. Acaricide treatment of livestock in CCHF virus endemic areas is effective in reducing the population of infected ticks.

Species diversity of ixodid ticks feeding on humans in Amasya, Turkey: seasonal abundance and presence of Crimean-Congo hemorrhagic fever virus

Ticks (Acari:Ixodidae) are important pests transmitting tick-borne diseases such as Crimean-Congo hemorrhagic fever (CCHF) to humans. Between 2002 and 2009, numerous CCHF cases were reported in Turkey, including Amasya province. In the current study, species diversity, seasonal abundance of ticks, and presence of CCHF virus (CCHFV) in ticks infesting humans in several districts of Amasya province were determined. In the survey, a total of 2,528 ixodid ticks were collected from humans with tick bite from April to November 2008 and identified to species. Hyalomma marginatum (18.6%), Rhipicephalus bursa (10.3%), Rhipicephalus sanguineus (5.7%), Rhipicephalus (Boophilus) annulatus (2.2%), Dermacentor marginatus (2.5%), Haemaphysalis parva (3.6%), and Ixodes ricinus (1.6%) were the most prevalent species among 26 ixodid tick species infesting humans in Amasya province. Hyalomma franchinii Tonelli & Rondelli, 1932, was a new record for the tick fauna of Turkey. The most abundant species were the members of Hyalomma and Rhipicephalus through summer and declined in fall, whereas relative abundances of Ixodes and Dermacentor ticks were always low on humans in the province. Of 25 Hyalomma tick pools tested, seven pools were CCHFV positive by reverse transcription-polymerase chain reaction. Results indicated diversity of ixodid tick species infesting humans was very high, abundance of ticks changed by season, and ticks infesting humans had potential for transmitting CCHFV.

Molecular epidemiology of Crimean- Congo hemorrhagic fever virus genome isolated from ticks of Hamadan province of Iran

BACKGROUND & OBJECTIVES

Crimean-Congo hemorrhagic fever (CCHF) virus is a tick-borne member of the genus Nairovirus, family Bunyaviridae. CCHFV has been isolated from at least 31 different tick species. The virus is transmitted through the bite of an infected tick, or by direct contact with CCHFV-infected patients or the products of infected livestock. This study was undertaken to study the genetic relationship and distribution of CCHFV in the tick population of Hamadan province of Iran.

METHOD

In this study, RT-PCR has been used for detection of the CCHFV genome.

RESULTS

This genome was detected in 19.2% of the ticks collected from livestock of different regions of the Hamadan province in western Iran. The infected species belonged to Hyalomma detritum, H. anatolicum, Rhipicephalus sanguineus and Argas reflexus. With one exception, genetic analysis of the virus genome isolates showed high sequence identity to each other. Even though they clustered in the same group with the strain circulating in Iran, they had a closer relationship to the Matin strain.

INTERPRETATION & CONCLUSION

Vector control programs should be applied for reducing population density of potential tick vectors in this province. Further surveys are indicated in this region to provide a better view of the distribution and epidemiology of the virus.

[Crimean-Congo hemorrhagic fever: basics for general practitioners]

Crimean-Congo hemorrhagic fever (CCHF) is a tick-borne disease described in more than 30 countries in Europe, Asia and Africa. The causative agent is the Crimean-Congo hemorrhagic fever virus (CCHFV) that is a member of the genus Nairovirus of the family Bunyaviridae. CCHFV that is characterized by a high genetic variability is transmitted to humans by tick bites or contact with fluids from an infected individual or animal. The initial symptoms of CCHF are nonspecific and gradually progress to a hemorrhagic phase that can be lethal (case-fatality rate: 10 to 50%). Characteristic laboratory findings of CCHF are thrombocytopenia, elevated liver and muscle enzymes, and coagulation defects. The pathogenesis of CCHF remains unclear but might involve excessive pro-inflammatory cytokine production and dysfunction of the innate immune response. Diagnosis of CCHF is based mainly on isolation of the virus, identification of the viral genome by molecular techniques (RT-PCR), and serological detection of anti-CCHFV antibodies. There is currently no specific treatment for CCHFV infection and the efficacy of ribavirin is controversial. In absence of an effective vaccine, prevention is based mainly on vector control, protection measures, and information to increase the awareness of the population and of healthcare workers.

African swine fever

African swine fever (ASF) is a devastating haemorrhagic fever of pigs that causes up to 100% mortality, for which there is no vaccine. It is caused by a unique DNA virus that is maintained in an ancient cycle between warthogs and argasid ticks, making it the only known DNA arbovirus. ASF has a high potential for transboundary spread, and has twice been transported from Africa to other continents--Europe and subsequently the Caribbean and Brazil (1957, 1959) and the Caucasus (2007). It is also a devastating constraint for pig production in Africa. Research at Onderstepoort Veterinary Institute has made and is making important contributions to knowledge of this disease, focusing on the cycle in warthogs and tampans and transmission from that cycle to domestic pigs, resistance to its effects in domestic pigs, and the molecular genetic characterisation and epidemiology of the virus.

Dynamics of tick-borne disease systems: minor role of recent climate change

Tick-borne disease systems are very sensitive to climate through the impact of temperature and moisture stress on rates of the demographic processes of ticks. There is no a priori reason, however, to expect tick abundance or seasonal activity patterns to respond to climate change in ways that inevitably increase the risk of infection by the transmitted pathogens. Changing host availability may be more important than climate in determining tick abundance. The credibility of any (inherently untestable) predictions of future system-specific changes will be strengthened if based on satisfactory explanations of the past. Tick-borne encephalitis (TBE) in Europe is presented as a case study: observed patterns of climate change are too similar within and between countries to provide the sole explanation for the extreme spatio-temporal heterogeneity of the marked upsurges in TBE incidence over the past two decades. Instead, a nexus of interacting factors affecting both the risk of infection and exposure of humans to that risk, and each differing in force in space and time, is a more powerful model. Many of these factors are driven by socio-economic changes, and include climate, land cover, wildlife, agricultural practices, industrial activities, (un)employment and income. The same principle may apply to the periodic epidemics of Crimean-Congo haemorrhagic fever.

[Current characteristics of natural nidality of tick-borne encephalitis: new or well forgotten?]

A number specialists' opinion as to which the rise in the incidence of tick-borne encephalitis at the turn of the centuries is due to the new features of the epidemiology of this infection and to global climatic changes is analyzed. There are no objective evidence suggesting the ongoing expansion of a natural habitat of the major vectors--taiga (Ixodes persulcatus) and wood (I. ricinis) ticks and the noticeable increase in their size and virus infection rates. The notion of the recent penetration of ticks into the metropolises where natural focuses have emerged and human beings are infected is inconsistent with the multidescribed facts. There is no significant evidence for the expansion of a nosoarea of tickborne encephalitis. The impact of reformed anthropurgic foci and that of the proportion of town-dwellers in the general structure of morbidity on the epidemic situation have been evaluated. The intensity of an epidemic manifestation of natural foci is always determined by two most important parameters: 1) the loimopotential of foci and 2) the intensity of the population's contact with them. The nature of an interaction between these factors, which has caused a rapid surge of morbidity rates and their subsequent long-time reduction, is considered.

[Detection of the West Nile Virus and its genetic typing in ixodid ticks (Parasitiformes: Ixodidae) in Tomsk City and its suburbs]

Four tick species, Ixodes persulcatus, I. pavlovskyi, I. trianguliceps, and Dermacentor reticulatus, were found in Tomsk and its suburbs in 2006. The species I. pavlovskyi was found to be dominant in the localities situated in Tomsk City, and I. persulcatus was dominant in its suburbs. Viral RNA and viral antigen of the West Nile virus (WNV) were detected in the ticks I. pavlovskyi and I. persulcatus collected in the city and its suburbs by the RT PCR method and enzyme immunoassay with monoclonal antibodies against protein E of the WNV. Average rate of the WNV infected ticks varied from 5.2 up to 11.7% in different localities. Identification of the nucleotide sequence of the protein E gene fragment allowed classifying the cDNA obtained as genotype Ia of the WNV. The sequences are proved similar to the strain LEIV-Vlg99-27889-human of the WNV isolated in Volgograd. The obtained data showed that natural foci of the WNV virus can appear in the city and its suburbs probably involving two dominant tick species. The WNV infected imagoes, larvae, and nymphs of I. persulcatus and I. pavlovskyi were collected from small mammals, lizards, and birds. Therefore we presume that these hosts can be involved in the circulation and distribution of WNV on the territory of Tomsk Region.

[Molecular and epidemiological characteristics of tick-borne encephalitis virus in the Sverdlovsk Region on the basis of genotype-specific RT-PCR]

Studies and complete awareness of the regional and epidemiological properties of tick-borne encephalitis virus (TBEV) allow one to improve methods for preventing, diagnosing, and treating its severe neurological infection. The authors have developed reverse-transcriptase polymerase chain reaction (RT-PCR) systems for the detection of RNA of TBEV and for the determination of its genotype in the ticks and clinical materials. RT-PTC was shown to have a higher sensitivity and specificity than the practically used enzyme immunoassay system. Despite significant variations in the spread of infected ticks in some districts of the Sverdlovsk Region (5-12%), the average regional value was 8% over the study period. The authors have studied more than a thousand of ticks collected from the nature and humans in the epidemic season of 2005-2006. There was a virtually complete predominance (more than 95%) of the Ural-Siberian genotype, with rare cases of the European genotype (slightly more than 4%) being detected. The Far-Eastern genotype was not detected.

Prevention of tick-borne diseases

Tick-borne diseases are on the rise. Lyme borreliosis is prevalent throughout the Northern Hemisphere, and the same Ixodes tick species transmitting the etiologic agents of this disease also serve as vectors of pathogens causing human babesiosis, human granulocytic anaplasmosis, and tick-borne encephalitis. Recently, several novel agents of rickettsial diseases have been described. Despite an explosion of knowledge in the fields of tick biology, genetics, molecular biology, and immunology, transitional research leading to widely applied public health measures to combat tick-borne diseases has not been successful. Except for the vaccine against tick-borne encephalitis virus, and a brief campaign to reduce this disease in the former Soviet Union through widespread application of DDT, success stories in the fight against tick-borne diseases are lacking. Both new approaches to tick and pathogen control and novel ways of translating research findings into practical control measures are needed to prevent tick-borne diseases in the twenty-first century.

[Development of amplification technology-based methods for tick-borne encephalitis virus detection and genotyping]

The development of methods for tick-borne encephalitis (TBE) virus genotyping, such as determination of nucleotide sequences, analysis of restriction fragment length polymorphism, polymerase chain reaction using genotype-specific probes with hybridization-fluorescence detection, is under consideration. The capacities and application area of varions typing techniques, such as TBE detection and genotyping in the field, autoptic, and cultural materials; study of circulating genotypes in different natural foci and in various viral vectors and carriers, are described. Each technique was tested using a large number of strains isolated from ill humans and ticks in different regions of the country and employed to investigate abundant materials.