Health surveillance
What is health surveillance and why should we bother?
The purpose of health surveillance is to detect changes in the prevalence, incidence and spatio-temporal distribution of potential pathogens and diseases of tigers associated with infectious and non-infectious hazards. When surveillance is conducted over prolonged periods it may be possible to discriminate between endemic (background) disease and new or emerging disease threats. With enough data early warning of dangerous viral epizootics or even congenital defects caused by loss of genetic diversity is possible.
There is very little health surveillance data for free-ranging tigers at present and much more information is urgently needed from all tiger habitats, including corridors between core areas.
Surveillance can be described as ‘scanning surveillance’ where an overview approach is taken with no definitive ideas of disease or pathogens in mind, or ‘targeted surveillance’ when focussed on specific pathogens or diseases.
Health surveillance is intrinsically interesting from a veterinary point of view, but its’ main practical value is to inform decisions about protecting wild tiger populations from diseases that may threaten their survival. In other words, comprehensive health surveillance is the first step in developing preventative or reactive strategies to mitigate the impact of disease on tiger populations.
How is surveillance conducted?
It is always more challenging to monitor disease patterns and pathogen presence or absence in free-living wild animals than in domesticated species or humans. To monitor and assess disease threats to free-ranging tigers, surveillance must be applied not only to tigers, but to other free-ranging wildlife, domesticated animals, humans and the wider environment as there is significant potential for pathogen transmission between species in an ecosystem, through direct or indirect contact. Diseases that affect prey species may have serious negative effects on tiger populations and therefore should not be ignored in surveillance efforts. The need to adopt a broad approach is emphasised by the observation that the most important driving factors behind newly emerging infectious diseases affecting wildlife result from human activities such as movement of animals outside their native range, global climate change, expansion of the wildlife-domestic animal interface and inadvertent transportation of vectors and pathogens (Walzer 2005, McKenzie et al, 2018, Yon et al 2019).
Information can be gathered about wild tigers, other wildlife, domesticated animals, humans and the wider environment by direct or indirect methods (see below). All types of data are valuable, but their value can only be fully realised if data is published. All available data then need to be collated and interpreted to create as complete a picture as possible (with the caveat that diagnostic tests in many wildlife species are not fully validated).
Direct methods of surveillance
Direct health surveillance involves the testing of samples taken directly from tigers (or other species) when handled for radio-collaring, conflict mitigation, translocation, rehabilitation, post mortem examination, etc. Details of what samples to take, how to collect, store and transport them, and which tests are appropriate are given in other sections of the website. Details of post mortem examination and sampling technique can be found here.
Every opportunity to collect such samples should be taken even if comprehensive testing is not possible due to practical or financial constraints. Samples can be stored for testing in the future when appropriate resources become available or simply to establish valuable sample banks. In all cases accurate and comprehensive labeling of samples is essential.
A very wide range of infectious and non-infectious diseases (and exposure to disease agents without clinical disease) can be tested for using small samples of blood, urine, hair, faeces, etc. In the case of post mortem examinations tissue samples are essential. A complete list will not be given here, but infectious disease agents of current interest include viruses such as Canine Distemper Virus, Feline Immunodeficiency Virus, and Feline & Canine Parvoviruses; intestinal parasites such as Diphyllobothrium and Ancylostoma; the lung fluke Paragonimus; and protozoa such as Toxoplasma. (See also Summary – Mortality and Disease in Wild Tigers). Negative test results are as useful as positive results in disease surveillance.
A few serological investigations of tiger populations have been published (Goodrich et al, 2012; McCauley et al, 2018; Naidenko et al , 2108), but interpretation of serological data must be made with caution. The absence of antibodies to, or antigens of, infectious disease agents indicate that the population investigated either did not have any recent contact with these agents, and/or possibly, that the species is highly susceptible to infection (i.e. infected individuals do not survive). In contrast, a high prevalence of antibodies or antigens relating to specific disease agents may indicate that such agents do not cause serious health problems in the species, especially if related mortality has not been observed.
It is important to carry out a thorough clinical examination of tigers (or other species) when collecting biological samples for disease testing to avoid missing the signs of significant non-infectious conditions such as congenital heart defects, diaphragmatic hernias and other physical abnormalities.
Indirect methods of surveillance
Valuable information about some conditions from which a tiger (or population of tigers) may be suffering, or disease hazards to which it might be exposed, can be gained from a variety of indirect, non-invasive methods. These can be relatively simple to perform and do not involve catching and immobilising tigers. They include:
- Remote monitoring using still and photos and video: Not only can individuals be identified this way by their unique stripe patterns, but an assessment of body condition can be made, the presence of skin lesions, injuries and abnormal masses identified, lameness’s detected, colour variations noted, tail length estimated etc. With high resolution camera traps it is sometimes possible to detect dental lesions and even some eye abnormalities. Long term camera trapping in an area coupled with systematic body condition scoring can help monitor both individual and population health. Examples of health and disease identified by camera trapping can be found here.
- Faecal samples left by tigers (and other carnivores) can be analysed to identify internal parasites, viruses, diet, and conduct genetic and endocrine analyses.This can be challenging as most faecal samples found in the field have been degraded by time and weather. Modern molecular techniques can markedly increase test sensitivity, and appropriate reagents can be used to stabilise samples. However, fresh samples are undoubtedly best. Collection of suitable samples is greatly facilitated by the use of dogs trained to find wildlife scats.
- Hair samples left on branches, fences, hair traps etc can be useful in genetic studies, identifying the presence of heavy metal pollutants, assessing nutritional status and even providing measures of chronic stress.
- Urine samples can be tested to assess urinary function and analysed for genetic investigations.
- In areas where tigers walk on snow and ice (eg the Russian Far East), blood samples are occasionally left from small cuts to footpads. These can be tested for a variety of disease agents using molecular techniques that require very low volumes, genetic analyses, etc.
- Saliva from bite wound swabs can be used in forensic investigations to identify specific individuals.
- The probing of questing ticks with sensitive PCR techniques can identify disease agents that have the potential to affect tigers in an area (Thomas, et al, 2016).
- Analysis of epidemiological data from state agricultural veterinary services can reveal diseases in domestic livestock that may impact on tigers or prey species living in interface zones.
- Examination of hunting bags can identify diseases of prey species.
- Health screening of feral domestic cat populations in urban-wild interface areas can be used to identify infectious diseases that have the potential to spread to tigers.
- Pollutants can be assessed by analysing water sources.
- Etc
A comprehensive description of the potential and range of indirect techniques to assess wildlife population health can be found in Bechert, 2012.