An analysis of diseases


IN developing countries, there is an increasing demand for, and production of animal products, a trend that has come to be known as the livestock revolution.  

Intensification, with more animals kept in a more commercial, highly productive environment has been ongoing for many years. 

By contrast, in many developed countries, the demand for organic products and higher animal welfare is increasing as a counter-reaction; a process often referred to as ‘extensification’, with animals being kept more extensively and often producing less. 

Intensification can be associated with both increased and decreased disease risks.  

For example, increased indoor pig keeping caused a reduction in Toxoplasma gondii, a neglected but important parasite, an infection which free-ranging pig farms are showing a re-emergence of.  

In Japan, the number of pigs produced has been increasing because of increased industrialised pig keeping, whereas there has been a decreasing number of pig farms. 

The incidence of JEV has not been following the increasing numbers of pigs, but instead has decreased with the number of pig farms.

Similarly, for cattle, zero-grazing systems have shown a decrease in transmission of some parasites.

Historically, infectious diseases have had civilisation-altering consequences. 

During the Spanish flu pandemic in 1918–1920, an estimated 50-to-100 million humans worldwide succumbed to the infection.  

The potato blight, a fungal disease that caused the Irish potato famine, reduced the Irish population by 25 percent either through starvation or migration.

When rinderpest spread to Eastern Africa in the 19th Century, it caused massive death in livestock and the subsequent death, by starvation, of almost two-thirds of the East African Massai population.

The outbreak of foot-and-mouth disease in the UK at the beginning of the 21st Century led to the culling of four million animals for the purpose of disease control, and cost the nation more than £3 billion, not including losses from decreased tourism

For tropical infectious diseases such as malaria, socio-economic factors may be much more important than climate.  

Infectious diseases cause not only suffering and death, but also severe economic implications, which are not always immediately appreciated. 

Economic losses may in addition be caused by secondary effects. 

Death of bats in North America due to the infectious white-nose syndrome, caused by an emerging fungus, and other anthropogenic causes of death, may cause agricultural losses of at least US$3,7 billion per year.

To fully evaluate the economic and societal impact of a zoonotic disease, it is important to include all measurements. 

The combined impact of zoonotic diseases on human health, animal health, and livelihoods make them especially costly.  

The World Bank estimates that direct costs of zoonotic outbreaks during the last century have exceeded US$20 billion.

The number of emerging infections has been increasing over the last 100 years

Emerging Infectious Diseases (EID) have been reviewed extensively during the last two decades, and it is now generally accepted that most drivers of emerging diseases are ecological, and the majority of these are caused by anthropogenic influences.  

Some of these anthropogenic drivers are the increased movement and transport of animals and goods; changes in ecosystems; deforestation and reforestation; altered land use; increased irrigation and creation of water dams and reservoirs and urbanisation.

The EID which have received most publicity during the last decades have been viruses. 

Notable examples have been HIV, SARS and Ebola. 

It is estimated that 44 percent of the diseases considered emerging in humans are viral.

Emerging diseases, such as highly pathogenic avian influenza, can lead to industry decline or restructuring with negative effects on small-scale producers and value chain actors.  

Some areas of the world have a tendency to have more occurrences of EID.  

These ‘hot spots’ often have a rapid intensification of agricultural systems, especially of livestock keeping, and increasing interactions between animals, humans and eco-systems; often caused by rapidly changing habits and practices within societies

Vector-borne diseases, which I have been covering extensively in may articles during the recent past, constitute around 23 percent of the infections considered emerging.  

Although arboviruses can be transmitted by a wide range of arthropods, mosquitoes are the most important from a veterinary and medical point of view and may have been parasitising on mammalian blood for 100 million years.

Disease from vector-borne pathogens often occurs as spillover events, as the pathogens generally circulate between reservoir hosts and the invertebrate vectors without causing apparent disease.

The complex nature of vector-borne transmission makes it difficult to predict how changes will affect the incidence. Temperature affects both the longevity, the incubation period within the vector, abundance, behaviour and the reproduction cycles of the mosquito and thus warmer climates may lead both to increased transmission as well as reduced, when the lifespan of the mosquito is reduced below the time required for the virus to replicate.

The opportunistic behaviour of many vectors can cause them to change their feeding according to the host availability, and even mosquitoes with strong preferences for humans will feed on other hosts if they are abundant enough. 

Intensified animal production causes high rates of contact between many individual animals, many of which are genetically similar, bred for other purposes than disease resistance, and kept in stressful environments. 

This increases the risks of spreading diseases and high numbers of individuals potentially carrying a zoonotic pathogen increases the risk for a host jump.

Large amounts of manure and effluence cause problems of safe disposal and risks for contamination of crops and water.  

Even though efforts to increase biosecurity can be made, the requirements of intense animal keeping, such as high ventilation, also pose means of introductions of pathogens and vectors. 

More industrialised animal keeping causes an increased segregation/isolation of the animals and their vectors from humans. 

This has been one of the proposed explanations for the eradication of malaria in many developed countries.

Extensive animal keeping, backyard farming and mixed production systems have also been associated with disease risks. The outbreaks of highly pathogenic avian influenza in south-east Asia have been demonstrated to be dependent on rice production, duck densities and human population density. 

In addition to the traditional backyard poultry keeping in poor rural or urban areas, there are increasing trends of keeping small flocks of poultry in middle-and-high-income urban areas in many countries.  

In both cases, biosecurity measures and awareness of the importance thereof are often limited or non-existent, such as in Zimbabwe.

Extensive livestock keeping is often the only option for small holders but may also entail risks: increased exposures to pathogens in the environment, decreased biosecurity and more interactions between species increase risks for pathogen jumps.

In spite of the knowledge that has been gained, there are still gaps in the understanding of ecosystem disease regulation and how human actions may affect disease indirectly and in the long term. 

Top-down interventions may be counterproductive if the incentives of the local populations are not fully understood, and control measures may be devastating for the public health if the disease epidemiology is not fully grasped. 

Dr Tony Monda holds a PhD in Art Theory and Philosophy and a DBA (Doctorate in Business Administration) and Post-Colonial Heritage Studies. He is a writer, lecturer and a specialist post-colonial Scholar, Zimbabwean socio-economic analyst and researcher. 

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