Approximately two thirds of the well-recognised diseases of humans are zoonoses; however, many of the high profile and significant new and emerging human diseases of recent years have derived from animals. Zoonotic diseases are even more of a problem to immunocompromised persons. Pertinently, in Australia we have had the emergence of several viral zoonotic diseases from bats; namely Hendra virus (1994) and Australian bat lyssavirus (1996), as well as Menangle virus (1997).
For a pathogen to emerge from a wildlife reservoir and cause significant disease, four events are required (Wang et al 2005):
1. Interspecies contact;
2. Cross-species transmission of the agent (spillover event);
3. Sustained transmission;
4. Adaption of the agent within the spillover species.
So, the question is why have these diseases recently emerged? Most appear to be well-adapted to their original host. There has probably been a variety of factors including incursion of humans into the animals' habitation, incursion of animals into human habitation, translocation of species (deliberately or accidently or inadvertently), changes in population dynamics and social behaviour and structure, environmental stressors, habitat destruction and/or alteration, changes in pathogen distribution, and/or changes within the agents per se altering host range, infectivity, virulence and/or pathogenicity.
Bats belong to the Order Chiroptera (literally winged-hand). There are more than 1,000 species world-wide, divided into Microchiroptera (small insectivorous bats that emerge at dusk) and Macrochiroptera (flying foxes, also nocturnal) - there are modifications of this classification emerging from more detailed genetic studies: Australia has approximately 75 well-recognised species belonging to seven different Families. None of the Australian bats are sanguivorous (suck blood); in general terms, the microbats eat insects and the flying foxes eat fruit and blossom, but they all tend to be omnivorous; ghost bats (a large Microchiroptera) are carnivorous, and blossom bats are nectivorous (a small Macrochiroptera); golden-tipped bats eat spiders and large-footed myotis occasionally take fish, as well as water-associated insects.
The distribution of the different species is recognised. They tend to be locally nomadic, although some species range further and are thus locally migratory; intermingling of species is common. Grey-headed Flying Foxes congregate in camps which may exceed 100,000 individuals and they may range up to 50 km for nightly feeding; during winter they migrate hundreds of kilometres, but as small groups, not en-masse. Little red flying foxes congregate in large camps of up to a million individuals but these are often temporary until feed supply runs out, then the group will move. Flying foxes carry their young for the first month or so whilst they are foraging. Little red flying foxes cannot take off from the ground but require elevation before becoming airborne, i.e. they need to launch out from up a tree, hence they are very vulnerable when grounded.
This short presentation will focus on several diseases of importance in bats, and also mentions some other diseases and findings that may be of interest.
Australian bat lyssavirus
In 1996, the first evidence of what became known as Australian bat lyssavirus (ABLV) was found in north coastal NSW in a black flying fox that was unable to fly. Shortly thereafter, a wildlife carer in Queensland who had been bitten by a yellow-bellied sheath-tail bat died after a short severe neurological episode (less than two weeks after being bitten). At about the same time, a woman in central-coastal Queensland was bitten by a flying fox when she intervened between a bat and a child at a barbeque, and she died of ABLV-associated encephalitis in late 1998 (after a three week clinical episode that commenced two years later). Detailed investigation of other potential human cases retrospectively and since have not thus far elucidated any further cases. It has been shown that the standard human anti-rabies vaccine is protective, and veterinarians and other persons handling potentially infected bats should be protectively vaccinated.
ABLV is distinct from, but closely related to, classical rabies (Lyssavirus genotype 1), and has been classified as Lyssavirus genotype 7; however, the ABLV from microbats is a separate clade within that genotype from ABLV from flying foxes. Surveys have shown that apparently healthy bats have a very low incidence of active infection, i.e. <1%, but it appears that little red flying foxes have a higher prevalence of clinical disease than other species.
Serology using rapid focus fluorescent inhibition test (RFFIT) often is negative in bats with active infection as revealed by ABLV antigen detection in brains (Barrett 2004). Bats with positive antigen in brain may have a non-suppurative meningoencephalitis and ganglioneuritis, but the extent varies from extremely mild and focal (and thus easily overlooked or missed in sampling) to severe and widespread: eosinophilic cytoplasmic Negri bodies are absent from many affected cases, and usually rare when present (Hooper et al 1999).
The reservoirs of ABLV infection are the black flying fox, grey-headed flying fox, little red flying fox, spectacled flying fox and yellow-bellied sheath-tail bat. It is probable that the virus has been present in Australian bats for a long time. Seropositive bats range from Darwin to Melbourne, and extend someway inland.
Bats actively infected with ABLV may display clinical signs of paralysis, paresis and inability to fly; recumbency and aggression are less common, change of voice, tremors or twitching and self mutilation are relatively rare. There are no characteristic lesions on necropsy, although evidence of trauma is not uncommon. Spread of infection is presumed to be via bites and deposition of saliva into damaged tissue.
Dogs and cats have been experimentally infected with ABLV via peripheral routes and seroconvert. The dogs showed mild transient clinical signs 2-3 weeks post-inoculation. These studies did not investigate if the dogs or cats were capable of producing sufficient virus to deliver an infectious to another animal (McColl et al 2007). To date, there has been no evidence of infection of other wildlife species.
In NSW, DPI SVDL investigates bats for ABLV where there is a history of biting or scratching a person. These are Category 3 events and are treated as urgent. Advice to the person at risk should be available within 48 hours so that appropriate prophylactic action can commence (Ewald and Durrheim 2008). The bats are necropsied and brain removed and cut sagittally: half is sent to the Australian Animal Health Laboratory (AAHL), Geelong, for antigen testing and/or isolation and the other half fixed for histology; the parotid salivary gland is also sent to AAHL, and the cervical spinal cord is fixed in-situ for histology.
Bats showing neurological signs (Category 2) need to have evidence of potential human health risk to be processed; handling of bats that have been in contact with dogs and/or cats is more problematic (usually bat bitten by dog!), since the investigations are expensive. In the event of a bite or scratch, the area should not be scrubbed; rather, wash thoroughly with soap and water, apply antiviral disinfectant such as iodine-based disinfectants or ethanol and immediately contact a medical health professional (Anonymous 2010).
In September 1994, there was a sudden outbreak of severe respiratory disease in thoroughbred stable at Hendra on the outskirts of Brisbane. Thirteen horses died acutely; four horses developed respiratory signs but survived, two of these were left with mild neurological signs and there were a further four horses which did not develop overt clinical signs but seroconverted. The trainer, Vic Rail, and a stable hand developed flu-like disease, and Vic Rail subsequently died after a short period of hospitalisation. A second focus, involving two horses, was retrospectively diagnosed in Mackay preceding the Hendra cases but brought to attention by the death of the stud owner from Hendra virus encephalitic disease in October 1995.
Hendra virus was found to be a member of Paramyxoviridae, and had some similarities to Morbillivirus, but was later allocated to sub-family Paramyxovirinae and a newly named genus Henipavirus. Seropositivity was found in all four flying fox species and is widespread around the coast from Darwin to Melbourne, and also in Papua New Guinea and Western Australia. Antibodies have been confirmed in Australian bats from serum archives dating back to 1982. However, antibodies in little red flying foxes appear to decline more rapidly than other species and thus may create a bias in interpreting results of surveys.
Outbreaks in horses and thence human cases have tended to coincide with breeding season of flying foxes. Experimental transmission has been demonstrated in horses, cats and guinea pigs, where the disease is respiratory and encephalitic; in flying foxes, virus has been isolated from urine and saliva, along with uterine fluids of natural and experimental cases. It is postulated that spill-over from flying foxes to horses occurs as a result of pasture contamination, i.e. ingestion, with foetal fluids or aborted material. Transmission from horse to horse was mechanical, via heavily contaminated material. Wildlife workers who are in regular contact with bat excreta do not appear to have become infected, suggesting this is not a potent source of infectious virus. However, the high prevalence of seropositivity amongst flying foxes indicates that the virus is readily transmissible amongst the host species (Field et al 2001; Weingartl et al 2009).
In 1998-1999, a major disease outbreak occurred in the Malaysian Peninsula, causing the death of >100 people and leading to the culling of approximately one million pigs. The disease in pigs was highly infectious between pigs, with acute pyrexia and respiratory signs, and sometimes neurological signs (tremors, spasms). Spread between pigs was presumed to be by respiratory route. The predominant clinical signs in affected humans were pyrexia and an acute encephalitis, leading to coma within 2 days. Most affected humans had a history of direct contact with pigs. Nipah virus was found to belong to Paramyxoviridae and was closely related to Hendra virus. Dogs in the immediate vicinity had a high prevalence of seropositivity: antibody has also been found in cats, goats and horses. Antibody to Nipah virus was found in four species of fruit bats, including two species of flying foxes, and a species of microbat.
Subsequent outbreaks of Nipah virus and Nipah virus-related disease have occurred in South East Asia, including Bangladesh and India, thus far resulting in several hundreds of human deaths. There appears to be a seasonal occurrence from January to April. Many species of bats have been indentified as seropositive. It is assumed that the mode of effect of infection within the bats has some similarities to Hendra virus.
In autumn-winter 1997, there was a sudden outbreak of severe reproductive failure in a large pig farm, with stillborn piglets, mummification and teratogenic effects, such as arthrogryposis, brachygnathia and kyphosis, with significant central nervous abiotrophy. Histopathologically, there was a non-suppurative encephalomyelitis with paramyxovirus-like inclusion bodies.
A paramyxovirus was isolated from the piglets and experimentally was able to infect foetal piglets. The virus was found to belong to Family Paramyxovirus, Genus Rubulavirus, and is closely related to Tioman virus from fruit bats in Malaysia. Virus-like particles were observed in faeces and urine collected into tarpaulins placed under the colony of grey-headed flying foxes and little red flying foxes that camped nearby (about 200 m from the piggery), and these animals were also strongly seropositive for Menangle virus. Antibody was also present in sera collected from flying foxes elsewhere in NSW and from Queensland, and also in pigs transferred from this unit and raised elsewhere. Two workers from the piggery developed flu-like symptoms and were seropositive.
It was proposed that spread from flying foxes to pigs was by access to dropped young or faeces/urine released from flying foxes as they passed over the piggery. The colony of flying foxes had been in proximity to the piggery for >30 years. Endemic infection occurred in the piggery for a few years whilst the piggery was under quarantine, and then the piggery was depopulated but not re-populated, effectively eradicating the disease from pigs. The effect of infection in affected flying foxes is unknown.
Severe acute respiratory disease
The outbreak of severe acute respiratory disease (SARS) in humans in 2002-2003 is estimated to have cost >$US50 billion. More than 8,000 human cases were confirmed and >750 persons died (a fatality rate approaching 10%). Initial investigations postulated on the origin of SARS-coronavirus from cattle and/or poultry but a positive link of SARS coronavirus was tracked to palm civets, a cat-like animal farmed for meat production in China, especially in Guangzhou Province. Further investigations revealed that horseshoe bats (Rhinopophus spp) were infected with SARS-like coronaviruses; those viruses were more genetically diverse than the virus in the civets and human SARS coronaviruses. Horseshoe bats are also host to a range of other non-SARS related coronaviruses (Wang et al 2006). Whether diseases are associated with these viruses in the bats is unknown. It is probable that a particular SARS-like coronavirus was transmitted by the faeco-oral and/or respiratory route from horseshoe bats to palm civets, and thence from civet to human, and then human to human, the latter inefficiently enough not to become an established cycle of infection.
Respiratory histoplasmosis due to Histoplasma capsulatum was reported in a group of cavers from near Wee Jasper NSW in the 1970s. The cave (Church Cave) was a well recognised maternity cave for greater bentwing bats and had a dense build up of guano, which is the material that supports fungus growth, producing the spores that induce disease. Of 16 infected cavers, three developed clinical disease. Presumably, the resident bats, especially their young, were also at risk from the same environmental source.
Toxoplasmosis is recognised as a panzootic disease and has been reported in bats overseas. Encephalitic toxoplasmosis induces neurological clinical signs in flying foxes but these are rare due to the necessity of access to infective cat faeces (Sangster and Gordon 2012).
The rat so-called 'lungworm' Angiostrongylus cantonensis has a complex lifecycle involving slugs and snails as intermediate host for the infective third-stage larvae (L3). Once ingested by the definitive host (a rat), the larvae migrate through the CNS, leave the capillaries and moult, and thence move into the subarachnoid space and, as L5 larvae, enter the venous system, where they are transported to the right ventricle and pulmonary arteries when they become established as adults. Ingestion of slugs and snails containing infective third-stage larvae by bats (and other species, including humans, dogs and birds) leads to invasion of the brain and/or spinal cord with development of a granulomatous or eosinophilic meningoencephalitis. Affected bats show neurological signs.
High concentrations of lead were reported in flying foxes in Australia in 1980s. The affected animals presented with neurological signs such as fasciculation, ataxia, diarrhoea and excess salivation. Histopathologically, there were perivascular microhaemorrhages in the brain and large intranuclear acid-fast inclusions in the proximal tubular cells of the kidney. The source of the lead was not determined, but there was high concentration in fur, suggesting environmental contamination; possibilities considered were build-up of lead in roadside eucalypt blossom, environmental sources (e.g. mining, smelting, refining), deliberate use of lead containing compounds by orchardists and/or malicious events.