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Vectors and emerging disease: a growing dilemma

Vectors — mosquitoes, ticks, midges, flies — have always been a major reason for the spread of disease among animals and people. Their importance has been magnified in recent years, confounded by things like new pathogens, climate change, global travel, antibiotic and pesticide resistance and the decline of public sector investment in disease surveillance.

The multitude of vectors and the diseases they spread is daunting. They are extremely mobile and easily transported — locally and internationally — by humans, aircraft, trucks, animals, migrating birds, ships and the wind. They are potentially transported in and on goods, material and living creatures that travel the globe. Climate, once a significant barrier to their establishment, has moderated, creating environments where both pathogens and vectors become ensconced where they never existed previously.

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Vector-borne diseases (VBDs) currently account for 17 per cent of the global burden of all infectious diseases. The most deadly of which, malaria, causes an estimated 627,000 deaths annually. Conditions that affect livestock can also have a significant economic impact. For example trypanosomiasis accounts for losses in cattle production of up to $1.2 billion a year. The prevalence of tick-transmitted diseases like anaplasmosis and Lyme disease steadily grows. The range of blue tongue virus infections followed the spread of midges capable of transmitting the virus across Europe from the Mediterranean basin. Diseases like dengue fever, chickengunya, rift valley fever sit at the doorsteps of susceptible populations far from where they ever existed before. New species of mosquitoes capable of transmitting new diseases are being found in places like California and Florida. West Nile virus jumped ship in New York City and is now established across North America, carried by migratory birds.

The International Federation for Animal Health (IFAH) commissioned an “industry first” white paper entitled: The growing threat of vector-borne disease in humans and animals. The white paper was produced through support of the Bill & Melinda Gates Foundation. Infographics produced by the IFAH press office complement the white paper. This material should be mandatory reading for all veterinarians and livestock producers.

Combating vector-borne diseases is an ever-changing and complex challenge. As the representative body for companies engaged in the research, development and manufacturing of animal medicines and health products, IFAH is uniquely positioned to address the challenges and help explore intrinsic links between human and animal health.

Key points made in the white paper include:

  • Arthropod vectors and the pathogens they transmit exert a major impact on the health and welfare of companion animals and livestock. The economic impact of VBD is measured in billions of dollars annually.
  • The relative significance of most VBDs is extremely difficult to quantify because many countries have no formal surveillance systems or methods to assess the prevalence of disease and diagnostic systems in many countries are underfunded and in a state of disarray. Without these tools, estimates of economic impact remain elusive.
  • Many VBDs currently lack effective medicines for treatment and prevention. Vaccination has the potential to be one of the most effective methods of preventing disease, but pharmaceutical sector activity in combating VBDs is often constrained by the lack of a market.
  • Combating VBDs is likely to face an increasingly serious combination of challenges in the coming years. The potential impact of climate change on vector distribution and VBD incidence remains an unknown; habitat change introduced by humans will influence establishment of vectors; the increased movement of goods, humans, livestock and companion animals worldwide is insatiable; and insecticide resistance, considered by some to be the single greatest threat to traditional disease control, steadily increases.
  • VBD management must be based on realistic, achievable objectives, and, in general, the eradication of vectors or pathogens from a system is not one of these. The white paper delves into the importance of reducing emphasis on pathogen-targeted research, with more research into the qualities of healthy animals — physiological and behavioural — and management systems that foster better health. Improvement of basic husbandry practices in developing countries could mean a significant contribution in this field.
  • Effective surveillance systems need to be established for most insect-borne diseases. Risks related to the transmission of disease need to be determined including evaluation of the potential spread to new areas, or the introduction of exotic species or diseases. Further surveillance is then needed to see if controls implemented were effective.
  • Awareness of the need to manage the development of resistance must be paramount in any attempt to use insecticides to control vector populations. Simple approaches are needed to incorporate the use of physical barriers to protect hosts from vector feeding. These may include bed nets, and mesh netting covering windows and doors. Such techniques may often be used in conjunction with an insecticide.
  • Training and education is paramount. Local governments and local staff need to be directly involved in developing strategies to manage and control VBDs. Communities need to be shown examples of success stories, and assisted in undertaking cost-benefit analysis of VBD management and control versus inaction. Improving levels of relevant expertise is particularly important for farmers in poorer countries, so that they can implement appropriate management and control of VBDs. Donor-funded information campaigns for farmers so they know when it is appropriate to use insecticides, which ones to use and how to use them are desperately needed.
  • The concept of One Health must move beyond an abstract idea into logical and intuitive application. It’s critical for One Health to become a collaborative, multidisciplinary and holistic force, capable of optimizing the interdependent skills necessary for animal, environmental and human health. While approaches to the control of VBDs in animals can be very different to approaches to VBDs in humans, there is common ground. It must be found. Progress depends on animal and human health sectors adopting a One Health mindset. The thrust to move forward will come with collaboration.

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent via email to Canadian Cattlemen or WCABP.

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Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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