Wild Tiger Health Project
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Summary: Mortality and Disease in Wild tigers

The definition of disease used throughout this website includes “any impairment that interferes with or modifies the performance of normal functions, including responses to environmental factors such as nutrition, toxicants and climate; infectious agents; inherent or congenital defects, or combinations of these factors” (Wobeser, G., 1981). Therefore, a very wide range of problems is covered by the term disease, including traumatic injuries.

The tiger is endangered throughout its range and the major current threats to the species’ survival are:

    • Unsustainable poaching to meet an illegal trade in body parts for the Oriental medicine market.
    • Loss and fragmentation of habitat due to rapid development and a rising human population.
    • Ungulate prey depletion through human hunting and competition from domestic livestock.
    • Tiger attacks on livestock and people leading to local community intolerance, retaliatory killings, and government-sanctioned removal of problem individuals.

These are all non-infectious hazards associated with human activities (Dinerstein et al, 2007; Goodrich et al, 2008; Korotkova et al, 2012; Robinson et al, 2015; Sharma et al. 2014), although the relative importance of each type of hazard remains a matter of debate (Chapron et al. 2008). In one study of Amur tigers Goodrich et al. (2008) estimated that 83% of the mortalities in a radiocollared population were caused by humans through poaching, suspected poaching and vehicle collisions. Based on individual necropsy reports held by the National Tiger Conservation Authority and other information from its “TIGERNET” database, Nigam et al (2016) reviewed 145 tiger mortalities from different landscapes in India between 2011 and 2015. Human induced mortalities (poaching, accidents and legal elimination) were found to be directly responsible for 59 (40.7%) cases, and a further 42 (29%) were attributed to aggression between tigers. However, at least some of this apparently high rate of conspecific trauma was suggested to be a consequence of habitat shrinkage and fragmentation – again caused by human activities.

Adult tigers have few natural predators. Large packs of dholes (Cuon alpinus) have occasionally been known to attack and even kill adult tigers, and large Saltwater crocodiles (Crocodylus porosus), bears and even porcupines can pose real dangers. Injuries inflicted by large prey such as gaur (Bos gaurus) & wild boar (Sus scrofa) can also be fatal, even if not immediately so. Intra-specific fighting between tigers and incidences of infanticide, especially in high density tiger populations, often accompany changes in territory holders.

Other non-infectious hazards that tigers face include the impact of adverse weather events. Following severe winter conditions characterised by deep snow and a resultant ungulate die-off, starvation was identified as a significant cause of death in young tigers (< 1 yr) in the Russian Far East between 2008 and 2010 (Lyubchenko et al, 2012). Small and isolated populations are also locally vulnerable to natural disasters such as the catastrophic cyclone in 1988 in the Bangladesh Sundarbans, resulting in at least eight tiger deaths (Khan, 2004). Loss of genetic diversity is also recognised as a threat in declining populations (Kenney et al.2014).

Retrospective mortality surveys of the type conducted by Nigam et al. (2016) are undoubtedly extremely informative – especially with results broadly categorised into mortalities caused by “natural” and those caused by “unnatural” (i.e. human induced) hazards. The results of such studies can be an asset for management interventions aimed at conservation of the species, especially as they seek to quantify risks and thus reveal mortality patterns. However, such studies are unlikely to produce accurate mortality patterns owing to the inevitable sampling biases fully acknowledged by the authors. Biases include: mortality figures only reflect dead tigers found or reported, not those dying of all causes; detection of mortality events is easier in well-staffed tiger reserves as opposed to more remote or less monitored areas; diagnosis is hampered by variability in post mortem degeneration of carcasses; many tiger cubs could be removed by carrion eaters or even parent tigers before they were found; variability in the quality of necropsy protocols used; etc. The need for a standardised necropsy protocol in all cases of free-ranging tiger deaths, uniform field investigation techniques and reporting system, and a centralised database of information is emphasised by the authors.

The danger of infectious disease should not be ignored, and its significance is likely underestimated as mortality studies generally rely on data from dead tigers found by chance. It is important to recognise that as any wildlife population declines, the potential impact of infectious disease can increase (Gilbert et al, 2014). As human populations grow the interface between people, their livestock, and wildlife expands – a process that exposes tigers to ever greater risks from infectious and non-infectious disease risks alike.

Infectious diseases may reduce carnivore survival and reproduction (Woodroffe, 1999), and catastrophic die-offs of carnivores are frequently related to them (Young, 1994). Even small decreases in survival and reproduction in small populations of slowly reproducing species like tigers could increase extinction risk (Chapron et al. 2008). Until recently, lack of knowledge about pathogen diversity and susceptibility in wildlife was one of the main causes of under-estimating infectious disease as a driver of host population dynamics (McCallum, 2012).

Whilst captive tigers have been reported as being susceptible to disease caused by a wide range of infectious agents including avian influenza (Keawcharoen et al, (2004), salmonellosis (Shilpa et al, 2012), various feline viruses etc., it should not be assumed that free-ranging tigers would  necessarily suffer from the same conditions. Cases in captive tigers of infectious diseases demonstrate the species’ susceptibility but not likelihood of their wild counterparts being exposed to the same agent or developing disease as a result of exposure. The differences between captive tigers and free-ranging tigers are profound in many ways – environmental, social, behavioural, nutritional, degree of physical and psychological fitness, activity level, size and complexity of territory, interactions with other species, level of immune system competency, pattern of pathogen exposure, etc. The presence and dynamics of potential pathogens are also affected by differences in environment between the captive and free-ranging scenarios. Furthermore, captive tigers in responsible zoos are provided with preventative health measures (eg vaccines, parasite control schedules, regular health checks), therapeutic treatments when ill, and are protected from most anthropogenic causes of mortality. It is therefore predictable that the pattern of morbidity & mortality in captive and free-ranging tigers would be different.

The most significant infectious disease agent that threatens free-ranging tigers today is considered to be the Canine Distemper Virus (CDV), based on its expanding host-range, its pathogenicity in felid species, and its complex and ever-evolving epidemiology. Robinson et al, (2015) calculated that this virus may have caused a 5% increase in mortality of Amur tigers in the Sikhote-Alin Biosphere Zapovednik of the Russian Far East between 2005 and 2012, resulting in a significant reduction in annual survival rates. Fatal CDV infections have been recorded or suspected in several range countries in recent years, though its impact on population dynamics is not yet clear. Nigam et al. (2016) found that deaths due to infectious disease in their study were relatively uncommon (23/145 cases or ~ 16%), but these included 4 CDV cases.

Other infectious disease agents or diseases have been reported from free-ranging tigers of which parvo viruses, rabies, the intestinal parasites Diphyllobothrium, Anyclostoma, Eucoleus and Galoncus, and the lung fluke Paragonimus  are of concern. Of somewhat less concern at present are the Feline Corona Virus, the protozoon Toxoplasma and a range of parasites including Aelurostongylus, Gnathostoma, Physaloptera, Spirocerca and Strongyloides.

Many other parasites reported from wild tigers probably pose only a minor level of risk unless present in abnormally high numbers. These include the tapeworms Diplylidium, Spirometra, and Taenia species; the roundworms Aoncotheca, Toxascaris, Toxocara, and Uncinaria; and the flukes Nanophyetus, Platynosomum  and Metorchis / Opisthorchis spp.

NB: The mere presence of moderate numbers of parasites in wild tigers should be considered normal in most circumstances (Patton & Rabinowitz, 1994).

There are many other potentially pathogenic infectious agents that have not been detected or reported in free-ranging tigers, although this may simply be a result of inadequate disease surveillance. Several such agents are listed below, based on their potential pathogenicity, combined with the likely increased contact between tigers and domesticated species arising from habitat encroachment and degradation. These include:

NB: The significance of simultaneous infection of tigers with combinations of pathogens (co-infection) is currently under-investigated. Furthermore, it should be borne in mind that serious non-infectious disease events, such as physical trauma, can predispose an animal to infections which ultimately become the cause of death.