Rabbit haemorrhagic disease: field epidemiology and the management of wild rabbit populations

Rabbit haemorrhagic disease (RHD) has become established in wild rabbit populations throughout Western Europe, Australia and New Zealand. The abundance of wild rabbits has been significantly reduced, particularly in drier areas of southern Spain, inland Australia and South Island New Zealand. A detailed knowledge of the epidemiology of RHD is essential for the management of the disease in natural rabbit populations, either to rebuild or to control populations. When RHD first spread among naive wild rabbits, epidemiological studies provided unique information on the rate of spread, the possible role of insect vectors in transmission, and the correlation between the impact of disease on populations and climatic variables. Current research shows a consistent pattern of epidemiology between Europe and Australasia. Typically, the most severe epizootics of RHD occur among young sub-adult rabbits which have lost age-related resilience and maternal antibodies. However, the timing of these outbreaks reflects climatic variables that determine the breeding season of the rabbits and the periods when RHD virus (RDHV) is most likely to persist and spread. Further factors that may complicate epidemiology include the possibility that non-pathogenic RHDV-like viruses are present in natural rabbit populations. Additionally, the question of how the virus persists from year to year remains unresolved; persistence in carrier rabbits is a possibility. Understanding of the epidemiology of RHD is now sufficiently advanced to consider the possibility of manipulating rabbit populations to alter the epidemiological pattern of RHD and thereby maximise or minimise the mortality caused by the disease. Altering the epidemiology of RHD in this manner would assist the management of wild rabbit populations either for conservation or pest control purposes.     

Viral haemorrhagic disease of rabbits and human health.

Viral haemorrhagic disease of rabbits (VHD), a potential biological control for wild rabbits in Australia and New Zealand, escaped from quarantined field trials on Wardang Island and spread to the mainland of Australia in October 1995. This study looked for any evidence of infection or illness in people occupationally exposed to the virus. Two hundred and sixty-nine people were interviewed and 259 blood samples were collected. Exposures to VHD-infected rabbits ranged from nil to very high. No VHD antibodies were detected in any of the 259 sera when tested by VHD competitive enzyme immunoassay, which had been validated with 1013 VHDV-specific antibody negative sera. A questionnaire designed to elicit symptoms of disease in a range of organ systems found no significant differences between illness in those exposed and those not exposed to VHD, nor could an association be found between exposure and subsequent episodes of illness. The findings are consistent with the view that exposure to VHD is not associated with infection or disease in humans.     

First field trial of a transmissible recombinant vaccine against myxomatosis and rabbit hemorrhagic disease

As a novel approach for immunisation of wild rabbits, we have recently developed a transmissible vaccine against myxomatosis and rabbit hemorrhagic disease (RHD) based on a recombinant myxoma virus (MV) expressing the RHDV capsid protein [J. Virol. 74 (2000) 1114]. The efficacy and safety of the vaccine have been extensively evaluated under laboratory conditions. In this study, we report the first limited field trial of the candidate vaccine that was undertaken in an island of 34 Has containing a population of around 300 rabbits. Following administration by the subcutaneous route to 76 rabbits, the vaccine induced specific antibody responses against both myxomatosis and RHDV in all the inoculated rabbits. Furthermore, the recombinant virus exhibited a limited horizontal transmission capacity, promoting seroconversion of around 50% of the uninoculated rabbit population. No evidence of undesirable effects due to the recombinant virus field release was detected.     

Notes on the distribution of mosquito species that could potentially transmit West Nile virus on Vancouver Island, British Columbia

We record the spatial and temporal distribution of 8 potential vectors of West Nile virus (WNV) on Vancouver Island in 2003 and 2004. Five species were widely distributed, but the other 3 were restricted to specific larval habitats. Adults were trapped from early April to September. The findings indicate a potential for WNV transmission if the virus arrives on the island. The results extend the published range of 5 mosquito species.     

Potential impact of climate change on emerging vector-borne and other infections in the UK

Climate is one of several causes of disease emergence. Although half or more of infectious diseases are affected by climate it appears to be a relatively infrequent cause of human disease emergence. Climate mostly affects diseases caused by pathogens that spend part of their lifecycle outside of the host, exposed to the environment. The most important routes of transmission of climate sensitive diseases are by arthropod (insect and tick) vectors, in water and in food. Given the sensitivity of many diseases to climate, it is very likely that at least some will respond to future climate change. In the case of vector-borne diseases this response will include spread to new areas. Several vector-borne diseases have emerged in Europe in recent years; these include vivax malaria, West Nile fever, dengue fever, Chikungunya fever, leishmaniasis, Lyme disease and tick-borne encephalitis. The vectors of these diseases are mosquitoes, sand flies and ticks. The UK has endemic mosquito species capable of transmitting malaria and probably other pathogens, and ticks that transmit Lyme disease. The UK is also threatened by invasive mosquito species known to be able to transmit West Nile, dengue, chikungunya and Zika, and sand flies that spread leishmaniasis. Warmer temperatures in the future will increase the suitability of the UK’s climate for these invasive species, and increase the risk that they may spread disease. While much attention is on invasive species, it is important to recognize the threat presented by native species too. Proposed actions to reduce the future impact of emerging vector-borne diseases in the UK include insect control activity at points of entry of vehicles and certain goods, wider surveillance for mosquitoes and sand flies, research into the threat posed by native species, increased awareness of the medical profession of the threat posed by specific diseases, regular risk assessments, and increased preparedness for the occurrence of a disease emergency.     

Compound Chinese herbal medicinal ingredients can enhance immune response and efficacy of RHD vaccine in rabbit

In order to validate the immune-enhancement efficacy of compound Chinese herbal medicinal ingredients (cCHMIs), made with astragalus polysaccharide (APS), epimedium polysaccharide (EPS), propolis flavone (PF) and ginsenosides (GS), as immune potentiator or vaccine adjuvants for rabbits, the effects of two cCHMIs on rabbit lymphocyte proliferation and IFN-gamma and IL-10 mRNA expression of T lymphocyte in vitro were determined. At the same time, two cCHMIs were injected into 35-day-old rabbits after mixed with rabbit hemorrhagic disease (RHD) vaccine taking aluminum adjuvant and phosphate-buffered saline (PBS) as controls. On days 7, 14, 21, 35 and 49 after the vaccination, the dynamic changes of peripheral lymphocyte proliferation and serum antibody titers of the rabbits were analyzed. On day 63, all rabbits were challenged with RHD virus. The results showed that the two cCHMIs could significantly promote rabbit lymphocyte proliferation and IFN-gamma and IL-10 mRNA expression of T lymphocyte in vitro. In vivo, two cCHMIs could significantly enhance serum antibody titers and lymphocyte proliferation. Their adjuvanticity was slightly superior to aluminum adjuvant. All the rabbits vaccinated with the cCHMIs adjuvant vaccine were protected. These findings confirmed that two cCHMIs possessed better immune-enhancement efficacy and would be used as effective immune adjuvant of RHD vaccine.     

Detection of rabbit haemorrhagic disease virus (RHDV) in nonspecific vertebrate hosts sympatric to the European wild rabbit (Oryctolagus cuniculus)

Since its detection in China in 1984, rabbit haemorrhagic disease (RHD) has been the subject of numerous studies. Yet, the evolutionary origin of rabbit haemorrhagic disease virus (RHDV) is still under debate. For example, some aspects related to the epidemiology of the disease are still unknown, such as where the virus is hosted between RHD outbreaks. To detect the presence of RHDV in rabbit-sympatric micromammals, 51 rodents (29 Mus spretus and 22 Apodemus sylvaticus) and 31 rabbits (Oryctolagus cuniculus) from the same location in central Spain were analyzed. In those samples in which the virus was detected, a fragment of the VP60 protein gene from the RHDV capsid was sequenced and the phylogenetic relationships between them and other strains of RHDV in the Iberian Peninsula were analyzed. In total, five viral strains were identified in A. sylvaticus, M. spretus and O. cuniculus. All strains were found to be well supported within the clade of RHDV found in rabbits in the Iberian Peninsula. Moreover, one of the strains was found in all three species under study, which suggests the capability of RHDV to infect other mammals apart from the rabbit which have not yet been investigated. The transmission of the virus is discussed as well as its ecoepidemiological implications.     

Exposure of rabbits to ultraviolet light-inactivated rabbit haemorrhagic disease virus (RHDV) and subsequent challenge with virulent virus

This study investigated whether exposure to inactivated rabbit haemorrhagic disease virus (RHDV) can produce an antigenic response in rabbits and protect them from a subsequent challenge with virulent virus. The aim was to determine if the spreading of baits containing RHDV, which is a common management practice in New Zealand to reduce rabbit numbers, could result in protective immunity in wild rabbits. RHDV was inactivated by ultraviolet (UV) light using an electronic UV crosslinker with a UV dose of 168.48 W-s/cm2 and a UV intensity of 0.0078 W/cm2. Two groups of four rabbits were then inoculated with inactivated virus via oral and intramuscular routes. Rabbits were monitored for 30 days post-inoculation and then challenged orally with virulent virus. No rabbit exposed to inactivated RHDV developed clinical signs of RHD or had antibodies at day 30 post-infection and all animals died within 82 h after challenge with virulent virus. No antibodies were detected at the time of death. These findings suggest that exposure to virus completely inactivated by UV light in the field or on baits will not protect rabbits against challenge with virulent virus.     

Genetic differentiation of Aedes aegypti mainland and island populations from southern Thailand

Isozymes of 5 wild-caught collections of Aedes aegypti were compared by using starch gel electrophoresis. All collections were collected from Surat-Thanee Province, southern Thailand, an area considered to be a hyperendemic zone for dengue virus. One collection was from Donsak Harbor, whereas the other 4 collections were from 4 districts of Samui Island. The percent polymorphic loci (24.2-33%) in the 4 island collections was lower than in the mainland collection (36.4%). This study revealed a large effective migration rate among all 5 collections. No fixed differences were detected. No significant differentiation was found among the 5 collections from Surat-Thanee Province.     

Seroepidemiology of arboviruses among seabirds and island residents of the Great Barrier Reef and Coral Sea.

Duplicate neutralization tests were done on 401 avian and 101 human sera from island residents collected in the Coral Sea and on Australia's Great Barrier Reef against 19 known arboviruses. Antibodies to a potentially harmful flavivirus, Gadget's Gully virus, were equally present (4%) in both avian and human sera. Antibodies to another flavivirus, Murray Valley Encephalitis, and an ungrouped isolate, CSIRO 1499, were also present in both populations with non-significantly different incidences. Antibodies to Upolu, Johnston Atoll, Lake Clarendon, Taggert, Saumarez Reef and CSIRO 264 viruses were restricted to seabirds. Island residents with antibodies to Ross River and Barmah Forest viruses are thought to have been exposed to these viruses on the mainland as antibody to both viruses was absent among seabirds. These results indicate that consideration should be given to tick-associated arboviruses as potential public health hazards on islands where both seabird and human activities interact.     

Emerging rabbit haemorrhagic disease virus 2 (RHDV2) at the gates of the African continent

Until the beginning of this decade, the genetic characterization of rabbit haemorrhagic disease virus (RHDV) from Iberian Peninsula had revealed the existence of two genogroups, G1 and sporadically G6. In 2010, the new emerging rabbit haemorrhagic disease variant, RHDV2 or RHDVb, was described in France, from where it has rapidly spread throughout Europe, including Iberian Peninsula countries. Nevertheless, although cases of rabbit haemorrhagic disease (RHD) have been reported in the Canary Islands, a Spanish archipelago located 100 km off the coast of Morocco, no genetic characterization of RHDV had been carried out. Consequently, in order to identify the circulating RHDV strains in this archipelago, liver samples of six farm rabbits and fifteen wild rabbits were collected from several areas of the largest island, Tenerife, and analyzed for the presence of RHDV by antigen capture double antibody sandwich ELISA. In case of positive ELISA result, we amplified and sequenced two fragments of the vp60 gene, which were concatenated for phylogenetic purposes. The sequences analysis revealed the presence of RHDV2 in both farm and wild rabbits from several areas of Tenerife. This result constitutes the first finding of RHDV2 in the Canary Islands. These RHDV2 strains found in Tenerife shared two exclusive SNPs that have not been observed in the rest of RHDV2 strains. The identification of RHDV2 and the absence of classic RHDV strains in this study suggest that RHDV2 may be replacing classic strains in Tenerife, as has been also proposed in Iberian Peninsula, France and Azores. Given the proximity of the Canary Islands to the African continent, this result should raise awareness about a possible dispersal of RHDV2 from the Canary Islands to the North of Africa.     

中国大陆首例输入性寨卡病毒病病例调查分析

目的 分析2016年2月9日中国大陆首例确诊寨卡病毒病病例的流行病学特征,为防控寨卡病毒病提供参考依据。方法 对中国大陆首例确诊寨卡病毒病病例进行流行病学调查并临床观察病例同行者与接触者。采集病例血清、尿液等标本,应用荧光定量RT-PCR进行核酸检测。结果 该病例确诊为寨卡病毒病,于第18病日解除隔离、痊愈出院。病例第10病日血清标本和第11~13病日尿液标本检测均为寨卡病毒核酸阳性。该病例系江西省赣县籍外派工作回国人员,发病前在寨卡病毒流行地区有蚊虫叮咬史,同行者与接触者医学观察期内均未出现寨卡病毒病症状。结论 该病例为中国大陆首例寨卡病毒感染确诊病例,系境外输入性病例,感染来源与在寨卡疫源地委内瑞拉蚊虫叮咬有关。     

Preliminary evidence that synanthropic flies contribute to the transmission of trachoma- causing Chlamydia trachomatis in Latin America

Synanthropic flies have been shown to be important mechanical vectors of Chlamydia trachomatis, which causes trachoma. However entomological studies have not been forthcoming in Latin America. This study assesses the relationship between household dipteran fly densities and active childhood trachoma in a village on Marajó Island, Pará state, Brazil. For 78 households, members were examined for signs of trachoma, relative abundance of potential trachoma vectors (Diptera, Chloropidae and Diptera, Muscidae) was quantified by trap counts, and additional measures of household hygiene associated with C. trachomatis transmission were assessed. Active childhood trachoma prevalence was 24.1% (45/187), present in 46.2% of sampled households with evidence of case clustering. Childhood prevalence was positively associated with increased fly densities, whereas indirect measures of sanitary conditions (latrine ownership and perceived importance of flies) showed a protective effect. This study indicates that C. trachomatis can be transmitted by synanthropic flies in this region of Latin America.     

Transmission ecology of the fly Musca sorbens, a putative vector of trachoma

Recent evidence suggests that eye-seeking flies are important trachoma vectors. We conducted a series of investigations to identify which species of synanthropic flies are potential vector(s) of this blinding disease in The Gambia. Several species of fly were caught in fish-baited attractant traps placed in villages throughout the year (1997/98) but only 2 species, Musca sorbens and M. domestica, were caught from the eyes of children. M. sorbens comprised < 10% of the total number of flies caught with attractant traps but was responsible for > 90% of fly-eye contacts, the remainder were made by M. domestica. All fly species were more numerous in the wet season than the dry season. Eyes of young children are considered to be the main reservoir of Chlamydia trachomatis, the causative agent of trachoma. Collections of eye-seeking flies from children showed frequent fly-eye contacts (median [interquartile range], 3 [1.5-7] every 15 min). Children with potentially infective ocular or nasal discharge had twice as many fly-eye contacts than children with no discharge (P < 0.001). There was no difference in exposure to fly-eye contacts if a child sat inside or outside a house (P = 0.273). Female flies were more commonly caught from eyes than male (P < 0.001). The presence of Chlamydia DNA was demonstrated by PCR on 2 of 395 flies caught from the eyes of children with a current active trachoma infection. Both positive flies were M. sorbens, one male and the other female. Further elucidation of M. sorbens behavioural ecology and the development of sustainable strategies to control these flies should be a priority. It is likely that M. sorbens is the principal insect vector of trachoma in The Gambia.     

北滘口岸2006-2009年进境集装箱携带医学媒介生物情况分析

目的通过分析北滘口岸入境集装箱输入医学媒介生物的特点,探讨相应的卫生监管措施以阻断潜在的疫情疫病的传播。方法对2006-2009年入境集装箱进行卫生处理后开箱查验,记录截获医学媒介生物情况,收集数据并进行整理和分析。结果北滘口岸共截获载有医学媒介生物的集装箱5768箱,主要为蜚蠊类、蝇类、蚊类和鼠类;医学媒介生物的分布有明显的地域性和季节性,与货物的种类密切相关。结论根据医学媒介生物输入的特点来采取相应的监管措施,以提高输入性医学媒介生物监管的针对性和有效性。     

Rift Valley fever

Rift Valley Fever (RVF) is a zoonotic arbovirosis. Among animals, it mainly affects ruminants, causing abortions in gravid females and mortality among young animals. In humans, RVF virus infection is usually asymptomatic or characterized by a moderate fever. However, in 1 to 3% of cases, more severe forms of the disease (hepatitis, encephalitis, retinitis, hemorrhagic fever) can lead to the death of infected individuals or to major sequels. The RVF virus (Bunyaviridae, genus Phlebovirus) was identified for the first time in the 1930s in Kenya. It then spread over almost all African countries, sometimes causing major epizootics/epidemics. In 2000, the virus was carried out of Africa, in the Middle East Arabian Peninsula. In 2007-2008, Eastern-African countries, including Madagascar, reported significant episodes of RVF virus, this was also the case for the Comoros archipelago and the French island of Mayotte. This ability to spread associated with many vectors, including in Europe, and high viral loads in infected animals led the health authorities worldwide to warn about the potential emergence of RVF virus in areas with a temperate climate. The awareness has increased in recent years with climate changes, which may possibly modify the vector distribution and competence, and prompted many RVF virus-free countries to better prepare for a potential implantation of RVF.     

Safety evaluation of a recombinant myxoma-RHDV virus inducing horizontal transmissible protection against myxomatosis and rabbit haemorrhagic disease

We have recently developed a transmissible vaccine to immunize rabbits against myxomatosis and rabbit haemorrhagic disease based on a recombinant myxoma virus (MV) expressing the rabbit haemorrhagic disease virus (RHDV) capsid protein [Bárcena et al. Horizontal transmissible protection against myxomatosis and rabbit haemorragic disease using a recombinant myxoma virus. J. Virol. 2000;74:1114-23]. Administration of the recombinant virus protects rabbits against lethal RHDV and MV challenges. Furthermore, the recombinant virus is capable of horizontal spreading promoting protection of contact animals, thus providing the opportunity to immunize wild rabbit populations. However, potential risks must be extensively evaluated before considering its field use. In this study several safety issues concerning the proposed vaccine have been evaluated under laboratory conditions. Results indicated that vaccine administration is safe even at a 100-fold overdose. No undesirable effects were detected upon administration to immunosuppressed or pregnant rabbits. The recombinant virus maintained its attenuated phenotype after 10 passages in vivo.     

Update on temporal and spatial abundance of dengue vectors in Penang, Malaysia

It is important to obtain frequent measurements of the abundance, distribution, and seasonality of mosquito vectors to determine the risk of disease transmission. The number of cases of dengue infection has increased in recent years on Penang Island, Malaysia, with recurring epidemics. However, ongoing control attempts are being critically hampered by the lack of up-to-date information regarding the vectors. To overcome this problem, we examined the current situation and distribution of dengue vectors on the island. Residences throughout the urban, suburban, and rural areas were inspected through wet and dry seasons between February 2009 and February 2010. Two vectors were encountered in the survey, with Aedes aegypti present in especially high numbers mostly in urban areas. Similar observations were noted for Ae. albopictus in rural areas. The former species was more abundant in outdoor containers, while the latter showed almost equivalent abundance both outdoors and indoors. The dengue virus was active in both urban and rural areas, and the number of cases of infection was higher in areas where Ae. aegypti was predominant. The abundance of immature Ae. albopictus was positively correlated with rainfall (r2 = 0.461; P < 0.05), but this was not the case for Ae. aegypti. For both species, the size of immature populations tended to increase with increasing intensity of rain, but heavy rains resulted in population loss. In addition to updating data regarding the larval habitats and locations (outdoors and indoors), this study highlighted the importance of spatial vector control stratification, which has the potential to reduce costs in control programs.     

The illegal introduction of rabbit haemorrhagic disease virus in New Zealand

In 1997, a group of pastoral farmers, frustrated by governmental and official responses to their problems of rabbit control, introduced and spread the rabbit haemorrhagic disease virus in a clandestine operation that succeeded in distributing infection over a large area of the South Island before the disease was detected by government officials. The government concluded that eradication was not technically or economically feasible and the disease was accepted as being endemic. The episode highlighted the inadequate decision-making environment that existed at the time, now improved by the passage of the Hazardous Substances and New Organisms Act. It also highlights the importance of having a comprehensive biosecurity detection and response capability, including the ability to conduct prompt risk assessments, since preventing entry of biological agents may be difficult to achieve in the face of a determined adversary.     

国境口岸媒介生物携带病原体的研究

〔目的〕研究国境口岸媒介生物携带病原体的情况,为媒介生物防治及疫情控制措施的制定提供基础数据。〔方法〕利用分子生物学技术对鼠类携带的汉坦病毒（流行性出血热病毒）和莫氏立克次体（斑疹伤寒立克次体）进行检测,对蚊类携带的流行性乙型脑炎病毒和西尼罗病毒进行检测,对蜱类携带的莱姆病螺旋体进行检测;利用细菌培养技术对蝇类、蜚蠊携带的致病菌进行检测;利用寄生虫学技术对蝇类、蜚蠊携带的寄生虫虫卵进行检测。〔结果〕2005年10月—2009年2月,共检测媒介生物样品3388份（输入性2023份、本底1365份）,检出阳性样品95份,阳性率2.8%。输入性媒介生物主要检测结果如下：2005年9月27日,国境口岸首次从输入性蝇类（大头金蝇,柬埔寨籍船舶“凯越”号上捕获）中检测到寄生虫虫卵（肝吸虫卵和蛔虫卵）;2006年4月24日,国境口岸首次从输入性蜚蠊（德国小蠊,某外轮上捕获）中培养出肺炎克雷伯菌;2009年2月27日,国境口岸首次从输入性鼠类（褐家鼠,柬埔寨籍船舶“诺斯”号上捕获）中检测到莫氏立克次体（斑疹伤寒立克次体）。本底媒介生物主要检测结果如下：2006年8月24日,从三带喙库蚊等蚊虫（辽宁营口港捕获）中检测到流行性乙型脑炎病毒;2006年8月,从长角血蜱（辽宁大连开发区捕获）中检测到莱姆病螺旋体;2008年10月15日,从褐家鼠（辽宁大连湾港捕获）中检测到汉城型流行性出血热病毒。〔结论〕媒介生物是国境口岸传染病的重要传播载体,应加强监测和控制。