Zoonotic Diseases

Think Swine Flu, Avian Influenza, Hantavirus and Lyme Disease and what immediately pops into one’s mind are images of squalor, dirty, plague-infested rats, pig and tics and all the menacing pathogens they bring with them. Labeled as ‘zoonotic diseases’, they have grown in number and impact in the last decade affecting human populations across the globe. But can these diseases and their rapid growth in the last couple of years be blamed on the animals themselves?

WHO defines zoonoses as “diseases and infections that are naturally transmitted between vertebrate animals and humans. A zoonotic agent may be a bacterium, a virus, a fungus or other communicable disease agent. At least 61% of all human pathogens are zoonotic, and have represented 75% of all emerging pathogens during the past decade.” According to a study by the Washington Department of Health, nine new human pathogens have emerged from 2000-2010 of which six are zoonotic in nature.

While these figures may appear alarming, there are those whose expertise leaves them unperturbed. “I do not think that’s terribly surprising,” said Dr Aaron Bernstein from the Harvard School of Public Health. “Infectious diseases in humans have similar cousins in other animals. It’s a pool of disease-agents which are closely related to each other and can be closely transmitted.” Some scientists are still debating over the figures. Dr Walter J. Tabachnick, Director of the Florida Medical Entomology Laboratory, University of Florida, says that there is “no data” to prove an overall increase in zoonotic diseases. “Few like dengue have increased, but that’s just a personal opinion,” he adds.

Studies have proven several factors leading to the sharp increase in zoonotic and vector-borne diseases in the last few years. While most point towards climate change being the largest contributing factor, many experts have opposing opinions. A widely expressed opinion states that climate change can only affect vector-borne diseases i.e. diseases transferred by arthropods or insects, as they are cold-blooded hosts. Since zoonoses specifically relates to diseases transmitted between vertebrate animals and humans, the climate change argument fails to apply to the entire gamut of zoonotic diseases. However, some scientists argue that warmer temperatures are not good enough to even explain the increase in vector-borne diseases alone.  “In order to have something happen, a lot of factors contribute to it. Temperature alone cannot make a difference. Malaria epidemics have decreased over the last 100 years, but the earth’s temperature has gotten warmer,” argues Dr Aaron Bernstein. Echoing a similar thought Dr Tabachnick feels that most studies “glibly offer assurances.” He adds that “climate change will have an effect, just like any environmental factor, but no one can predict specific consequences with assurance.”

With globalization of food, growth of cities and populations and spread of industry, came the excessive desire to constantly meddle with the earth’s environment and to alter its face. “We are reconstructing nature and causing humans to come into contact with animals in new ways,” says Dr Bernstein. He supports his claim providing examples from the live and wild animal markets in China stating how animals which would normally coexist in nature would be brought together in constrained conditions. The deadly Severe Acute Respiratory Syndrome (SARS) virus which claimed hundreds of lives in 2003-2004 can trace its origins to civet cats, which were held captive to feed the exotic food market, in China. Butchers of these cats were the first to be infected with the SARS virus.

Like Dr Bernstein, most scientists believe that people are misled into believing that the issue is climate change. The main problem is the constant and repetitive transformation of ecosystems on a large scale which breaks down ecological barriers. The way people interact with their live stock, their hunting practices, and their dependence on the land for their needs and their interaction within their own communities all add to the development of an epidemic. Dr Kenneth Gage, Chief of the Flea-borne Disease Activity Unit of the Centre of Disease Control (CDC), said “vector-borne disease cycles are very complicated. Land-use patterns and human behavior vastly add to these systems.”

Alterations in land-use can be singled out for older zoonotic diseases too.  Some studies demonstrate a correlation between the growth of the Atlantic seafood fishing industry and the introduction of the HIV AIDS virus in Western Africa. Rapidly increasing industrial fishing practices in the 1950s brought the fish population down to 10 percent of its original catchment a decade later. This made coastal tribes of Western Africa turn to bush meat to supplement their protein intake. The virus, once inside the human genome, mutated and became the Human Immunodeficiency Virus which became globally pandemic, nested within its travelling human host. “Twenty years ago people would have laughed at this reasoning,” said Dr Bernstein.

Human behavior within different societies, sharing the same environment, can show contrasting results in the spread of zoonotic diseases. Hantavirus, a lung disorder spread by rodents, spread wildly in Mexico in 1993-94 but was hardly significant on the other side of the Rio Grande River in the state of Texas. Dr Gage from the CDC points out that most Americans stay indoors on hot evenings in the comfort of their air-conditioned homes, while Mexicans would go outside on their porches to cool off. “These little things are very important,” he added.

In the case of zoonotic diseases, the factors are several and the impact is always the result of their cumulative effect. Understanding them is extremely complex as it involves a host of organisms from the pathogen, to the host to the infected organism, as well as the mutations which occur in this process. Dr Donald Thea from Boston University’s School of Public Health states that “it’s hard to talk about zoonotic diseases generically as they are a complex heterogenous body of biological beings.” He specifies that each disease needs to be studied and understood locally taking into consideration the environment, animal and human populations, industrial development and climate of the area. Moreover, scientists commonly agree that there is a lack of understanding the specifics of any particular episystem and the complexity of the interacting environment on vector-pathogen-host interaction. Due to these complexities, scientists seem to falter in their predictions for the future of these diseases. Even those confident in their climate models are questioned by the rest of the scientific community. “Predicting the impact of climate change in the future is like trying to predict how the stock market will behave in three years based on the conditions and information we have today,” said Dr Tabachinick.

While uncertainty may taint the understanding of the future of these diseases, resolution can be found in simple steps like educating local and medical communities globally. Dr Gage from the CDC notes that 14 percent of all fatal plague cases in the US, in the last 30 years, have been due to delayed medical attention or misdiagnosis. He states that the CDC focuses most of its efforts on educating people on managing their pets, livestock and disposing garbage in their neighbourhoods. Awareness and surveillance is the key to controlling the outbreak and spread of these diseases. He adds that with Remote Sensing and satellite imagery, scientists have access to land use, precipitation and pollution figures across the world which could help determine the possible epidemics in certain regions. “Theoretically we could reach a point for predicting zoonoses,” he adds. “But these diseases are very hard to predict and penetrate, “ he concludes soon after.



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