Spatio-temporal Use of Oral Rabies Vaccines in Fox Rabies Elimination Programmes in Europe

In Europe, the elimination of wildlife rabies using oral rabies vaccination [ORV] of foxes for more than 30 years has been a success story. Since a comprehensive review on the scope of the different oral rabies vaccine baits distributed across Europe has not been available yet, we evaluated the use of different vaccine baits over the entire period of ORV [1978–2014]. Our findings provide valuable insights into the complexity of ORV programs in terms of vaccine related issues. More than 10 oral vaccines against rabies were used over the past four decades. Depending on many factors, the extent to which oral rabies virus vaccines were used varied considerably resulting in huge differences in the number of vaccine doses disseminated in ORV campaigns as well as in large spatial and temporal overlaps. Although vaccine virus strains derived from the SAD rabies virus isolate were the most widely used, the success of ORV campaigns in Europe cannot be assigned to a single oral rabies virus vaccine alone. Rather, the successful elimination of fox rabies is the result of an interaction of different key components of ORV campaigns, i.e. vaccine strain, vaccine bait and strategy of distribution.     

Landscape-Genetic Analysis of Population Structure in the Texas Gray Fox Oral Rabies Vaccination Zone

ABSTRACT In west-central Texas, USA, abatement efforts for the gray fox (Urocyon cinereoargenteus) rabies epizootic illustrate the difficulties inherent in large-scale management of wildlife disease. The rabies epizootic has been managed through a cooperative oral rabies vaccination program (ORV) since 1996. Millions of edible baits containing a rabies vaccine have been distributed annually in a 16-km to 24-km zone around the perimeter of the epizootic, which encompasses a geographic area >4 × 10⁵ km². The ORV program successfully halted expansion of the epizootic into metropolitan areas but has not achieved the ultimate goal of eradication. Rabies activity in gray fox continues to occur periodically outside the ORV zone, preventing ORV zone contraction and dissipation of the epizootic. We employed a landscape-genetic approach to assess gray fox population structure and dispersal in the affected area, with the aim of assisting rabies management efforts. No unique genetic clusters or population boundaries were detected. Instead, foxes were weakly structured over the entire region in an isolation by distance pattern. Local subpopulations appeared to be genetically non-independent over distances >30 km, implying that long-distance movements or dispersal may have been common in the region. We concluded that gray foxes in west-central Texas have a high potential for long-distance rabies virus trafficking. Thus, a 16-km to 24-km ORV zone may be too narrow to contain the fox rabies epizootic. Continued expansion of the ORV zone, although costly, may be critical to the long-term goal of eliminating the Texas fox rabies virus variant from the United States.     

An Epidemic of Sylvatic Rabies in Finland

When rabies reappeared in Finland in April 1988, the country had been rabies free since 1959. Soon a picture of sylvatic rabies become evident, its main vector and victim being the raccoon dog (Nyctereutes procyonoides). Between 8 April 1988 and 16 February 1989, 66 virologically verified cases were recorded (48 raccoon dogs, 12 red foxes, 2 badgers, 2 cats, l dog and 1 dairy bull) in an area estimated at 1700 km2 in south-eastern Finland. The greatest distance between recorded cases was 67 km. A positive reaction with monoclonal antibody p-41 indicated that the virus was an arctic-type strain. A field trial on oral immunization of small predators was initiated in September 1988 using Tübingen fox baits according to the Bavarian model of bait distribution. Each bait contained 5*107 TCID50/ml modified live rabies virus (SAD-B19). The 6 months’ surveillance indicate a seroconversion rate of 72% (N=126) in the raccoon dog population, 67% (N=56) in the red foxes and 13% (N=16) in the badgers, when titers ≥1.0 IU/ml are considered seropositive. In the whole follow-up period, no statistically significant difference could be detected between the raccoon dogs and red foxes in the rate of seroconversion or in the uptake of tetracycline from the baits. Notably high antibody levels were recorded in both raccoon dogs and red foxes within 4–5 months after vaccination. Of the seropositive animals, the proportion of animals with titers 3.0 IU/ml or greater was higher in raccoon dogs (73%) than in red foxes (51%) (x2= 5.29, p< 0.05). The trial shows that raccoon dogs can be immunized against rabies in the field with vaccine baits originally developed for controlling sylvatic rabies in foxes.     

Trap-vaccinate-release and oral vaccination for rabies control in urban skunks, raccoons and foxes.

Two rabies control tactics, trap-vaccinate-release (T-V-R) and oral vaccination were used for the control of rabies in skunks (Mephitis mephitis), raccoons (Procyon lotor), and foxes (Vulpes vulpes) in metropolitan Toronto, Canada. Using T-V-R, a mean of 45% to 72% (95% confidence limits of 40% to 81%) of the skunks and a mean of 17% to 68% (95% confidence limits of 14% to 76%) of the raccoons in a 60 km2 area of Toronto were vaccinated against rabies between 1987 and 1991. The area has been free of skunk rabies from May 1989 to April 1992. Forty-five rabies cases were diagnosed during 1980 to 1986. In contrast, only three skunk cases have been reported since the vaccination program began in July 1987. The T-V-R area also remained rabies free during an epizootic of skunk rabies in metropolitan Toronto during 1991. Following distribution of rabies vaccine-baits throughout the ravines of metropolitan Toronto, June 1989 to December 1991, 46% to 80% of the Toronto fox population was immunized during 1989, 1990 and 1991. Only one case of fox rabies was reported in metropolitan Toronto since vaccination began, compared to 80 cases reported between 1982 and 1988. The area has been free of reported fox rabies from October 1990 to April 1992.     

The Italian experience in the control of rabies. III. Laboratory studies on SADB19 Tü vaccine used in the fox vaccination campaign

Results of laboratory investigations on the SADB19 Tü vaccine used for oral vaccination of fox show: 1) the need to check the vaccine efficacy before its application in the field; 2) the importance of monoclonal antibodies to distinguish sylvatic from vaccinal virus strains in the diagnosis of rabies during the vaccination campaigns; 3) the relevance of a careful evaluation of the epidemiological risk encountered when releasing baits containing activated vaccine.     

Use of recombinant vaccinia-rabies glycoprotein virus for oral vaccination of wildlife against rabies: innocuity to several non-target bait consuming species

The pathogenicity of a vaccinia recombinant virus expressing the rabies glycoprotein (VVTGgRAB) was tested in several wild animal species which could compete with the natural rabies host, the red fox (Vulpes vulpes) in consuming vaccine baits in Europe. The following species were included in this study: wild boar (Sus scrofa), Eurasian badger (Meles meles), wood mouse (Apodemus sylvaticus), yellow-necked mouse (Apodemus flavicollis), bank vole (Clethrionomys glareolus), common vole (Microtus arvalis), field vole (Microtus agrestis), water vole (Arvicola terrestris), common buzzard (Buteo buteo), kestrel (Falco tinnunculus), carrion crow (Corvus corone), magpie (Pica pica) and jay (Garrulus glandarius). During the observation period, the 107 animals given the VVTGgRAB vaccine orally did not show any clinical signs. Daily monitoring for 28 days and postmortem examination did not result in the detection of pox lesions in the oral mucosa or the skin in mammals or the unfeathered portions of birds. VVTGgRAB seems to multiply in the mammalian species tested, since rabies seroconversion was observed in all of them. Birds failed to develop demonstrable rabies virus-neutralizing antibody. A seroconversion against vaccinia virus was observed in two of four wild boars. Serological results obtained in badgers and wild boars also demonstrates the absence of direct or indirect horizontal transmission of the recombinant virus. The potential of the recombinant virus for the immunization of badgers against rabies also was investigated. Only 50% of the badgers orally administered with 1 x 10(8.3) TCID50 of this vaccine were protected against rabies.     

Immunization of arctic foxes (Alopex lagopus) with oral rabies vaccine

Arctic foxes (Alopex lagopus) were successfully immunized against rabies using an orally-administered, liquid SAD-BHK21 live virus vaccine in a sausage bait. Immunization was determined by serologic response and by resistance to challenge with an arctic rabies virus strain. Virus was not shed in saliva following oral vaccination, indicating that arctic foxes would not infect other foxes after ingesting this vaccine. High antibody levels were present in all experimental foxes 2 wk following initial vaccination. A booster vaccination at 56 wk induced a significant serologic response within 1 wk, suggesting an anamnestic response but titers began to decline within 8 wk in most foxes. Foxes were observed for 16 mo following the challenge and exhibited no symptoms of rabies. The SAD-BHK21 rabies vaccine in a sausage bait system has a strong potential for vaccinating wild populations of arctic fox.     

Documenting Freedom From Disease and Re-Establishing a Free Status After a Breakdown Rabies

Rabies reappeared in Finland in the spring of 1988 after a 29-year absence. This time rabies occurred in sylvatic form and the major species involved was the raccoon dog. During the outbreak 1988–89 66 animals were diagnosed rabid. Vaccination of cats, cattle and horses was strongly recommended and vaccination of dogs was compulsory in the outbreak area. A field trial was started on oral immunisation of raccoon dogs and foxes against rabies using baits containing rabies vaccine strain. The outbreak area and a wide buffer zone were baited three times. Finland was declared free of rabies again in 1991. Oral vaccination campaign with vaccine baits has been organised along the southeastern border once a year since the beginning of 90s. Continuous surveillance and epidemiological screening is necessary to detect any new outbreaks of rabies at an early stage.     

Practical implementation of oral vaccination of foxes in canton Tessin

The oral fox vaccination campaigns in 1984/85 in canton Tessin in Switzerland are described in detail. An area of 197 km2 was treated 4 times with 2260 baits in order to prevent the invasion of a rabies epizootic from abroad. A comparison is made to the rest of Switzerland, where from 1978 to 1986 a total of 545,399 baits were distributed. An evaluation of the campaigns in 10 cantons revealed that the total costs varied between 2.17 Sfr (1.54 $) and 5.84 Sfr (4.16 $) per bait.     

The induction of protective immune response in mice vaccinated by recombinant avian adenovirus CELO expressing glycoprotein G of the rabies virus

The recombinant avian adenovirus CELO-gpRb expressing glycoprotein G of rabies virus (strain TS-80, ARRIW&M, Pokrov, Russia) was used for mice vaccination against rabies. Double intramuscular immunization by recombinant CELO-gpRb adenovirus in a dose 10(9) pfu per mouse caused the induction of virus neutralizing antibodies (VNA) synthesis in 78% of mice, while twice repeated intradermal injections of the recombinant adenovirus failed to induce the VNA production. The protection level in groups of vaccinated mice after intracerebral injection of CVS rabies virus in a dose of 100 MLD50 was equal to 45% at single intramuscular immunization and to 91% after twice repeated intramuscular immunization. The recombinant adenoviral vaccine against rabies, based on CELO viral genome, has a good perspective for domestic and wild animal vaccination, not only due to rather high protection level, but also because the production of adenoviral CELO vaccine in chicken embryos is of high technology and inexpensive.     

A study of the investigatory behavior of the dairy cow

A commission to investigate the possible ingestion by dairy cattle of air-dropped wildlife baits for vaccination against sylvatic rabies provided an excellent opportunity for a detailed study of bovine investigatory behavior. Twelve unarmed baits (half of which were placed in a bag with liver slurry serving as a fox attractant) were placed in each of two pastures. A group of 9 cows was then allowed to graze freely on each of the pastures from approximately 08.00 to 15.00 h for 5 consecutive days. Continuous direct observation revealed that the tendency of the cows to approach the baits varied with the type of bait encountered, and habituated over the 5-day study. However, once an investigation was initiated, the likelihood of contacting the bait and the manner in which the bait was contacted was not readily influenced by prior exposure to the baits. The type of bait being investigated did not influence the likelihood of a cow making contact, but did influence the manner in which the bait was contacted. Flehmen, which only occurred following head contact with a bait, most commonly followed mouth or tongue contact and was less likely to be employed as cows habituated to the objects being investigated.     

Feasibility of oral rabies vaccination campaigns of young foxes (<Emphasis Type="Italic">Vulpes vulpes</Emphasis>) against rabies in summer

Oral vaccination of foxes (Vulpes vulpes) against rabies has been shown to be highly effective. Several baiting strategies to increase vaccination coverage of the target population have been developed. For example, to increase the vaccination coverage of the young fox population before dispersal an additional summer vaccination campaign has been suggested. The effectiveness of such a campaign was evaluated using field data on home range size and shape of young foxes and a simulation model for aerial bait distribution. The results indicated that the limited ranging behaviour of the young fox population during the summer months severely reduced bait accessibility and consequently such an additional vaccination campaign would be very cost-ineffective.     

Concluding Remarks

Rabies seems to persist throughout most arctic regions, and the northern parts of Norway, Sweden and Finland, is the only part of the Arctic where rabies has not been diagnosed in recent time. The arctic fox is the main host, and the same arctic virus variant seems to infect the arctic fox throughout the range of this species. The epidemiology of rabies seems to have certain common characteristics in arctic regions, but main questions such as the maintenance and spread of the disease remains largely unknown. The virus has spread and initiated new epidemics also in other species such as the red fox and the racoon dog. Large land areas and cold climate complicate the control of the disease, but experimental oral vaccination of arctic foxes has been successful. This article summarises the current knowledge and the typical characteristics of arctic rabies including its distribution and epidemiology.     

Immunogenicity and efficacy of two rabies vaccines in wild-caught, captive raccoons

The immunogenicity and efficacy of two rabies vaccines in wild-caught, captive raccoons (Procyon lotor) were investigated. Raccoons were fed Ontario Slim (OS) baits containing a recombinant vaccinia virus-rabies glycoprotein (VRG) oral rabies vaccine, or they were given an intramuscular (IM) injection of IMRAB(?) 3 rabies vaccine. Blood samples collected before treatment and from weeks 1 to 16 posttreatment were assessed for the presence of rabies virus antibody (RVA). There were significantly more positive responders in the group that received an IM injection of IMRAB 3 (18/27) than in the group that consumed VRG in OS baits (VRG-OS; 4/ 26). There were no significant associations among age, sex, and seroconversion. Of those animals that mounted a humoral immune response to vaccination, RVA was first detected between weeks 1 and 5, with the majority of initial seroconversions detectable at week 2. A subsample of 50 raccoons (19 VRG-OS, 18 IMRAB 3, and 13 controls) from the longitudinal serology study was challenged with live raccoon variant rabies virus 442 days after initial treatment. There were significantly more survivors in the group that received IMRAB 3 (13/18) than in the VRG-OS (5/19) or control (2/13) groups. All 15 raccoons that demonstrated a serologic response survived challenge regardless of treatment. Of the 35 raccoons with no detectable serologic response, 30 (86%) succumbed to rabies virus infection (14/15 VRG-OS, 5/7 IMRAB 3, and 11/13 controls).     

Modeling control of rabies outbreaks in red fox populations to evaluate culling,vaccination, and vaccination combined with fertility control

A predictive model of spread and control of rabies in red fox (Vulpes vulpes) populations was used to evaluate efficacy of culling, oral vaccination, and oral vaccination and fertility control (V + FC) as rabies control strategies. In addition, effects of season, fox population density, and a delay in starting control were modeled. At fox densities of 0.5 fox families/km2 or greater, a single oral vaccination campaign with bait uptake rates of less than 50% resulted in ineffective rabies control. An uptake rate of at least 80% was required to give a better than 80% chance of eliminating rabies. Vaccination was least effective at controlling rabies if applied 1 or 2 mo before the foxes gave birth. Seasonal timing of poison or V + FC had little effect on efficacy, which was always more successful than the oral vaccination alone. The longer the delay between the simulated start of the rabies infection and the application of a single vaccination campaign, the less successful was the control, particularly at the higher fox densities tested. At a fox density of 0.25 families/km2, all the strategies were equally successful at eliminating rabies. At higher fox densities V + FC was slightly less successful than culling, whereas vaccination-only was considerably less successful. The sole use of vaccination is not considered a viable control method for areas with high fox densities. The model suggests that an area of culling centered on the disease focus, plus an outer ring of vaccine or V + FC, could be the best strategy to control a point-source wildlife rabies outbreak.     

Prevalence of tetracycline and rabies virus antibody in raccoons, skunks, and foxes following aerial distribution of V-RG baits to control raccoon rabies in Ontario, Canada

More than 3.6 million baits containing a recombinant vaccinia virus-rabies glycoprotein (V-RG) oral rabies vaccine were aerially or hand-distributed during 1999-2006 in an approximate 4,000-9,000 km(2) area of eastern Ontario, Canada, as part of a multitactic approach to control the raccoon variant of rabies. The efficacy of the program was assessed through the collection and testing of > 6,900 animals for bait acceptance and rabies virus-specific antibodies. Raccoon acceptance of rabies vaccine baits was significantly greater (71-83% ) in areas baited at a density of 150 baits/km(2) compared to areas baited at 75 baits/km(2) (26-58% ), and more raccoons consumed vaccine baits in areas baited with a flight line spacing of 0.75 km (45.3% [321/708]) than with a spacing of 1.5 km (33.8% [108/320]). In addition, greater numbers of raccoons consumed vaccine baits during a drop in September (52.7% [213/404]) as opposed to a June bait drop (34.6% [216/624]). Seropositivity rates for raccoons ranged between 7% and 28% in areas baited at 75/km(2) and 10% to 27% in areas baited at 150/km(2) with statistical differences varying among years and treatments. The last case of raccoon-variant rabies reported in Ontario was in September 2005. The control of raccoon rabies in Ontario has resulted in an estimated $6M to $10 M Cdn annual savings in rabies-associated costs.     

Newcastle disease virus-vectored rabies vaccine is safe, highly immunogenic, and provides long-lasting protection in dogs and cats

Effective, safe, and affordable rabies vaccines are still being sought. Newcastle disease virus (NDV), an avian paramyxovirus, has shown promise as a vaccine vector for mammals. Here, we generated a recombinant avirulent NDV La Sota strain expressing the rabies virus glycoprotein (RVG) and evaluated its potential to serve as a vaccine against rabies. The recombinant virus, rL-RVG, retained its high-growth property in chicken eggs, with titers of up to 10^9.8 50% egg infective doses (EID₅₀)/ml of allantoic fluid. RVG expression enabled rL-RVG to spread from cell to cell in a rabies virus-like manner, and RVG was incorporated on the surface of the rL-RVG viral particle. RVG incorporation did not alter the trypsin-dependent infectivity of the NDV vector in mammalian cells. rL-RVG and La Sota NDV showed similar levels of sensitivity to a neutralization antibody against NDV and similar levels of resistance to a neutralization antibody against rabies virus. Animal studies demonstrated that rL-RVG is safe in several species, including cats and dogs, when administered as multiple high doses of recombinant vaccine. Intramuscular vaccination with rL-RVG induced a substantial rabies virus neutralization antibody response and provided complete protection from challenge with circulating rabies virus strains. Most importantly, rL-RVG induced strong and long-lasting protective neutralization antibody responses to rabies virus in dogs and cats. A low vaccine dose of 10^8.3 EID₅₀ completely protected dogs from challenge with a circulating strain of rabies virus for more than a year. This is the first study to demonstrate that immunization with an NDV-vectored vaccine can induce long-lasting, systemic protective immunity against rabies.     

Eliminating rabies in Estonia

The compulsory vaccination of pets, the recommended vaccination of farm animals in grazing areas and the extermination of stray animals did not succeed in eliminating rabies in Estonia because the virus was maintained in two main wildlife reservoirs, foxes and raccoon dogs. These two species became a priority target therefore in order to control rabies. Supported by the European Community, successive oral vaccination (OV) campaigns were conducted twice a year using Rabigen® SAG2 baits, beginning in autumn 2005 in North Estonia. They were then extended to the whole territory from spring 2006. Following the vaccination campaigns, the incidence of rabies cases dramatically decreased, with 266 cases in 2005, 114 in 2006, four in 2007 and three in 2008. Since March 2008, no rabies cases have been detected in Estonia other than three cases reported in summer 2009 and one case in January 2011, all in areas close to the South-Eastern border with Russia. The bait uptake was satisfactory, with tetracycline positivity rates ranging from 85% to 93% in foxes and from 82% to 88% in raccoon dogs. Immunisation rates evaluated by ELISA ranged from 34% to 55% in foxes and from 38% to 55% in raccoon dogs. The rabies situation in Estonia was compared to that of the other two Baltic States, Latvia and Lithuania. Despite regular OV campaigns conducted throughout their territory since 2006, and an improvement in the epidemiological situation, rabies has still not been eradicated in these countries. An analysis of the number of baits distributed and the funding allocated by the European Commission showed that the strategy for rabies control is more cost-effective in Estonia than in Latvia and Lithuania.     

Preliminary evaluation of Raboral V-RG® oral rabies vaccine in Arctic foxes (Vulpes lagopus)

We tested the Raboral V-RG? recombinant oral rabies vaccine for its response in Arctic foxes (Vulpes lagopus), the reservoir of rabies virus in the circumpolar North. The vaccine, which is currently the only licensed oral rabies vaccine in the United States, induced a strong antibody response and protected foxes against a challenge of 500,000 mouse intracerebral lethal dose 50% of an Arctic rabies virus variant. However, one unvaccinated control fox survived challenge with rabies virus, either indicating a high resistance of Arctic foxes to rabies infection or a previous exposure that induced immunity. This preliminary study suggested that Raboral V-RG vaccine may be efficacious in Arctic foxes.     

The use of an E1-deleted, replication-defective adenovirus recombinant expressing the rabies virus glycoprotein for early vaccination of mice against rabies virus.

An E1-deleted, replication-defective adenovirus recombinant of the human strain 5 expressing the rabies virus glycoprotein, termed Adrab.gp, was tested in young mice. Mice immunized at birth with the Adrab.gp construct developed antibodies to rabies virus and cytokine-secreting lymphocytes and were protected against subsequent challenge. Maternal immunity to rabies virus strongly interferes with vaccination of the offspring with a traditional inactivated rabies virus vaccine. The immune response to the rabies virus glycoprotein, as presented by the Adrab.gp vaccine, on the other hand, was not impaired by maternal immunity. Even neonatal immunization of mice born to rabies virus-immune dams with Adrab.gp construct resulted in a long-lasting protective immune response to rabies virus, suggesting that this type of vaccine could be useful for immunization shortly after birth. Nevertheless, pups born to Adrab.gp virus-immune dams showed an impaired immune response to the rabies virus glycoprotein upon vaccination with the Adrab.gp virus, indicating that maternal immunity to the vaccine carrier affected the offspring's immune response to rabies virus.