EPIDEMIOLOGY

GEOGRAPHICAL DISTRIBUTION AND HOSTS, EPIDEMIOLOGICAL ROLE OF WILD ANIMALS

Worldwide, the main pathogenic species for livestock are: B. abortus (all biovars), responsible for bovine brucellosis; B.

Despite their respective host preferences, B. abortus and B. suis have also been isolated from a great variety of wild­life species, such as bison (Bison bison), red deer (Cervus elaphus), feral swine and wild boar (Sus scrofa), red fox (Vulpes vulpes), European brown hare (Lepus europeaus), African buffalo (Syncerus caffer), reindeer (Rangifer taran- dus tarandus) and caribou (Rangifer tarandus groenlandi- cus), and wildlife has thus to be considered as a potential reservoir for brucellosis in livestock. Brucella melitensis is rarely reported in wildlife.

Brucella ovis and B. canis are responsible for ram epidi­dymitis and canine brucellosis, respectively, and are not reported in wildlife in Europe. For B. neotomae, only strains isolated from desert rats in Utah, USA, have been reported.

Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.

Since the first description of an abortion due to Brucella spp. in a captive dolphin in California in 1994(5), several reports have described the isolation and characterization of Brucella spp. from a wide variety of marine mammals such as seals, porpoises, dolphins and whales worldwide, excluding the Antarctic (reviewed in(6)). The overall char­acteristics of these marine mammal strains are different to those of any of the six ‘classical’ Brucella species, and since 2007 B. ceti and B.

Experimental studies and epidemiological evidence suggest that birds are very resistant to Brucella spp. infec­tion. In fish, recent studies related to natural infections of Nile catfish demonstrated that B. melitensis biovar 3 could be cultured from visceral organs⁽⁷⁾.

Brucella Abortus

Because brucellosis eradication efforts in the EU and in the USA focused on bovine brucellosis, the emphasis was put on the identification of a possible reservoir of B. abortus in wildlife. When brucellosis was prevalent in cattle, numerous surveys identified occasional seropositive results in wild ungulates, particularly cervids(⁸). Under free-ranging conditions, the infection was considered to be self-limiting or as a spillover of the infection in cattle. For example, in 1995, B. abortus was isolated from 7/112 culled chamois (Rupicapra rupicapra), but brucellosis did not seem to be present in larger areas of the Western Italian Alps, where bovine brucellosis is absent⁽⁹⁾.

Nowadays, in countries where the bovine brucellosis eradication programmes are nearly completed there are few known sustainable reservoirs of B. abortus in wild species, other than bison and elk ( Cervus canadensis) in the American National Parks of the Greater Yellowstone Area (GYA) and in the Canadian Wood Buffalo National Park. In South Africa, the African buffalo is considered a reser­voir of B. abortus. However, recent studies in feral pigs on the Atlantic coast of South Carolina, USA, challenge these thoughts. Indeed, B. abortus wild-type and the B. abortus S19 and RB51 vaccine strains have been isolated from feral pigs on a property where no cattle had been kept in the area frequented by the feral swine since at least 1970(¹⁰). This is the first report of B. abortus in feral swine and of a wildlife reservoir of B. abortus outside the GYA in the USA.

A recent survey in Spain highlighted the fact that wild ruminants were not a significant brucellosis reservoir for livestock. Brucella abortus biovar 1 was isolated from only a single red deer. Thus, these results suggest that in Europe, wild ruminants occasionally acquire infection from brucel­losis transmitted from infected livestock, rather than being true reservoirs of infection for livestock(¹¹). The available epidemiological information suggests that wild ruminants are not able to sustain B. abortus infection without intro­duction from infected cattle. Given the progress in the eradication programme of bovine brucellosis in the EU member states, it is unlikely that B. abortus will become a threat to wildlife in Europe in the future.

Brucella Melitensis

The known ecological range of B. melitensis in wildlife is more restricted than those of B. abortus and B. suis, although it is still a major problem in domestic small ruminants in southern European states and an important veterinary public health issue. Spillover from infected small ruminants has been documented in a few wildlife species, such as chamois and ibex (Capra ibex) in the French and Italian Alps(^12,13) Recently B. melitensis has been isolated from one Iberian wild goat in Spain⁽¹¹⁾. These reports highlight the fact that B. melitensis infection in European wildlife is nowadays anecdotal and always linked to the domesticated small ruminant reservoir. It will most likely become less important in the future, given the eradi­cation of small ruminant brucellosis in Northern and Central Europe and the progress in the B. melitensis eradi­cation programmes, in some Mediterranean EU member states. However, important foci of B. melitensis infections in small ruminants persist in the Balkans, putting wild ruminants of this region potentially at risk.

Brucella Suis

Brucella suis (biovars 1, 2, 3) is still widely distributed in the world.

Brucella suis infection is mainly restricted to feral pigs in the USA and Australia (B. suis biovar 1) and wild boars and European brown hares in Europe (B. suis biovar 2), where B. suis has been eradicated in the domestic pig population for decades.

Brucella suis biovar 2 very rarely causes disease in humans; when this does occur it is usually associated with immunodeficiency. In Croatia, during the period 1980— 2003, B. suis biovar 1 was isolated from wild boars, pigs, brown hares and horses⁽¹⁵⁾. This is an important veteri­nary public health issue, because B. suis biovar 1 is a known zoonotic agent. However, to date, no human B. suis biovar 1 infection has been reported in Croatia⁽¹⁶⁾. In 1993, B. suis biovar 1 was isolated from a butcher han­dling imported feral pig meat in Belgium, where the last B. suis biovar 1 infection had been reported in a pig farmer in 1983⁽¹⁷⁾.

Rangiferine brucellosis (i.e. brucellosis in reindeer and caribou) is caused by B. suis biovar 4 throughout the Arctic region, Siberia, Canada and Alaska and constitutes a serious zoonosis. Brucella suis biovar 4 may also infect moose and occasionally different carnivore species. Bru­cella suis biovar 5, isolated in rodents in Eastern Europe and for which very few strains are known, is most probably misnamed.

Brucella Suis Biovar 2 In Eurasian Wild Boar

Brucellosis in wild boar is widely distributed throughout Europe. During the 1990s, the number of wild boar reached a historical peak in the southeastern part of Belgium, with an estimated population of more than 10,000 individuals (45 animals per 1,000 ha of forest).

In France and Germany, B. suis biovar 2 infections have been reported in outdoor-rearing pig farms during the last decade, and wild boars appear to be the most likely source of infection.

In the Iberian Peninsula, B. suis biovar 2-infected wild boars are considered to be an important threat for the Iberian pig population reared in outdoor breeding systems. Brucella suis biovar 2 infection in wild boars could be of major concern should brucellosis control programmes in domestic pigs be implemented in the EU⁽¹¹⁾.

Brucella Suis Biovar 2 In European Brown Hare

Information regarding brown hare populations (and hence B. suis biovar 2 infection) is still very fragmented in Europe. In recent years, isolation of B. suis biovar 2 infection has been reported in central Europe and Spain. The occur­rence of B. suis biovar 2 or anti-Brucella antibodies in free- ranging hares may depend on the number of close contacts with infected wild boars or domestic pigs. Indeed, an increasing seroprevalence of brucellosis in wild boars preceding an epizootic of hare brucellosis was observed in the Czech Republic¹-¹⁸).

Molecular studies revealed that the B. suis biovar 2 haplotype found in hares was not found in pigs or wild boars in Spain⁽¹¹⁾ but was similar to the one described in domestic pigs from France and Croatia and also in wild boar from France, Italy and Switzerland¹-¹⁵). This raises important questions on the translocation of infected hares. However, hares may not contribute to the dissemination of brucellosis to the extent of wild boar (solitary, adult male boar may range very long distances if disturbed or when there is a food shortage), as hares have a non- migratory way of life and occupy small home ranges, which effectively reduces the interface with other wildlife.

In Denmark, during 1929—1999, 10 clinical outbreaks of B. suis biovar 2 infection in domestic pigs were recorded, and epidemiological evidence linked them to hares. Bru­cella suis biovar 2 infections in cattle have also been reported in Denmark¹-¹⁹) and the source of contamination is believed to have been hares, as there is no established population of free-range wild boar in Denmark.

Brucella Suis Biovar 4 In Caribou, Reindeer and Moose

Rangiferine brucellosis is enzootic in Siberia, Canada and Alaska in caribou and reindeer, and B. suis biovar 4 was isolated from both species in the 1960s. Human cases have been restricted to herders of diseased reindeer or caribou. It has been experimentally demonstrated that cattle exposed to B. suis biovar 4-infected reindeer can become infected¹-²⁰). In Canada, a natural B. suis 4 infection was confirmed in moose for the first time in 1993(²¹). In 1999, a serological study concluded that brucellosis was not present in reindeer in Finnmark, northern Norway(²²).

Brucella Ceti and B. Pinnipedialis

Marine mammal brucellosis was first reported in 1994 from stranded harbour seals (Phoca vitulina), harbour por­poises (Phocoena phocoena) and common dolphins (Del­phinus delphisf on the Scottish coast(²³) and from an aborted fetus of a captive bottlenose dolphin ( Tursios truncatuf) in California¹-⁵). In the 1990s, strains of Brucella spp., biologi­cally and genetically different from the six classical Brucella species, were isolated from cetaceans and pinnipeds inhab­iting seas and oceans of Europe and North America or kept in captivity. Anti-Brucella antibodies have also been detected in serum samples from several species of marine mammals from the northern and southern hemispheres. It is noteworthy that no Brucella spp. has been isolated from marine mammals in the Antarctic waters, although anti­Brucella antibodies have been detected in some marine mammals⁽²⁴⁾.

The isolation of Brucella spp. from an aborted bot­tlenose dolphin fetus indicated that marine mammal Bru­cella spp. may cause abortion¹-⁵). If Brucella spp. infection leads to reproductive disorders in marine mammals, then brucellosis may play an important role in the population dynamics of these species¹-²⁵). The polar bear ( Ursus mariti- mus) is the apex predator in the arctic marine food chain, and in the Svalbard area ringed seals (Phoca hispida), bearded seals (Erignathus barbatus) and harp seals are the main prey. Anti-Brucella antibodies were found in ringed seals and harp seals in the Svalbard area(²⁶). A seropreva­lence of 5.4% of anti-Brucella antibodies was found in plasma samples from 297 polar bears from Svalbard and the Barents Sea⁽²⁷). To date, there is no indication of disease caused by Brucella spp. in the polar bear population at Svalbard. Therefore, the potential impacts of Brucella spp. exposure on individuals or the population is unknown.

An experimental inoculation of three pregnant cattle with a Brucella isolate from a Pacific harbour seal resulted in two of the animals aborting. This study indicated that marine mammal Brucella spp. is capable of producing seroconversion and abortion in cattle but is less pathogenic in that species than B. abortus'¹²⁸. Another experimental investigation demonstrated colonization, limited establish­ment of infection, and low pathogenicity of the three marine-mammal Brucella strains for sheep, and limited potential to transmit infection in sheep⁽²⁹⁾. A marine mammal strain isolated from a human patient was not able to establish an infection in piglets⁽³⁰⁾.

Brucella Microti

A systemic disease occurred in a wild population of the common vole (Microtus arvalis) in South Moravia (Czech Republic) during the years 1999—2003. A novel species within the genus Brucella was isolated from these voles and given the name Brucella microti⁸⁷’³⁸. Long-term survival of B. microti in soil was shown — thus, soil might act as a reservoir of infection. Brucella microti was also isolated from the mandibular lymph nodes of red foxes hunted in the region of Gmund, Lower Austria. Brucella microtf is thus prevalent in a larger geographic area covering the region of South Moravia and parts of Lower Austria, and foxes could have become infected by ingestion of infected common voles⁽³²⁾. No human or livestock infection with B. microti has been reported to date.

TRANSMISSION

When a heifer or a cow aborts, large numbers of organisms are excreted. The abortion is associated with the excretion of 10¹²-10¹³ bacteria. Taking into account that a quantity of 15 ? 10⁶ Brucella deposited on the conjunctiva of heifers results in infection of 95% of them, the quantity of bacteria excreted in the course of a brucellosis-induced abortion could theoretically infect 60,000 to 600,000 ges- tating females, which in turn could infect other animals. This is called the ‘multiplication relay of the infection’ ⁽³³). Assuming that this situation may also prevail in wildlife, the potential for infection is extremely high.

However, the prevalence of brucellosis in some wildlife species is very low and thus, besides classical factors con­sidered to be important in transmission (host susceptibil­ity, shedding, survival in the environment, etc.), behaviour of individuals and the interaction between wildlife and livestock at the interface may actually be the most impor­tant drivers for transmission. For instance, the reclusive calving behaviour of elk will minimize risk of B. abortus transmission to other animals and, in free-ranging settings, elk may be looked upon as a dead-end host. By contrast, where winter feeding is practised, the risk of infection increases dramatically by increasing density and enhancing the risk of exposure to an infectious abortion near the feeding ground, resulting in feed contamination. In the GYA, winter feeding changed the behaviour and density of free-ranging elk, probably converting what in nature is a dead-end host to a maintenance host of B. abortus®⁸.

As far as marine mammal brucellosis is concerned, the fact that B. melitensis is able to multiply in fish warrants further studies in order to assess whether fish can be hosts for B. ceti and B. pinnipedialis and play a role in the epi­demiology of the disease.