Ecology of Infectious Disease

Human diseases
Many disease organisms that threaten humans worldwide have complex life histories that are affected by both human and non-human attributes of the ecosystems in which they occur. For example,

• Lyme disease is carried by ticks that move about on mammal hosts such as deer and mice - environmental parameters that affect these non-human hosts have implications for human exposure to Lyme disease
• Rabies is a disease that is contracted by wildlife and can be passed on to humans. NCEAS researchers have assembled and analyzed an extensive database documenting rabid raccoons, refining predictions of rabies dynamics
• The bacterium causing the gastrointestinal disease cholera is waterborne and associated with microscopic crustaceans - climatic and environmental factors that affect hydrodynamics and the ecology of aquatic food webs can influence the dynamics of cholera
• The brain parasite that causes toxoplasmosis is passed among rats, cats, and humans; in humans, infection is associated with lifelong personality changes that may influence human culture
• NCEAS ecologists have formulated and applied cutting edge approaches in analysis and synthesis of human disease scenarios in recent years, improving our understanding of human disease risk

Disease and ecosystems
Ecologists have a growing awareness of the importance of pathogens and parasites in the evolution and ecology of natural systems.  Researchers at NCEAS have examined the evolutionary relationships between disease organisms and their hosts, as well as more modern alterations of pathogen and parasite dynamics by humans . For example:

• Introduced species have escaped many of the parasites of their native ranges ;
• The removal of predators that normally select sick individuals from prey groups may increase pathogen transmission among their populations, when diseased individuals continue living within groups ;
• Primate researchers have examined the role of social and feeding behavior in moderating infection by sexually transmitted diseases and parasites in non-human primates ;
• Disease is considered to be among the most significant causes of the modern coral reef decline, and warmer temperatures encourage some of the most common diseases on coral reefs ;
• Environmental warming and human activities, such as fishing, may have complex disease effects as warm temperatures seem to favor some pathogens and parasites, while decreasing the prevalence or severity of others , and human activities alter host abundance, behavior and environment

West Nile disease

What is it West Nile disease?

West Nile virus is spread through the bite of an infected mosquito. The mosquito can become infected after biting an infected bird and this can then be transmitted from the bird to humans.

There is no evidence to suggest that West Nile virus can be spread from person to person or animal to person.

Where am I at risk?

The West Nile virus was first discovered in the West Nile area of Uganda in 1937.

It is prevalent in North America, where two thirds of the East coast states have had cases. There has even been a case in California.

There have also been cases in Israel, Africa and Asia and parts of Eastern Europe.

There was a UK case of a bird with the antibodies of the West Nile virus, but there have been no human cases.

What are the symptoms?

The patient can incubate the disease for up to a fortnight after being bitten.

Many of the people who become infected will have either no symptoms or mild flu-like symptoms, which last a couple of days.

These can range from fever, headache and body aches to skin rashes on the trunk of the body and swollen lymph glands.

But more serious cases, less than 1% of those who become infected, can result in severe illness such as West Nile encephalitis or meningitis and sometimes lead to death.

Out of about 1,000 people who became infected in Europe recently there were about 40 deaths.

How do I protect myself?

Scientists have developed a vaccine for the disease and it was due to start human trials in late 2002.

But as yet there is no vaccine for travellers to use so the best way to protect yourself is to avoid being bitten by a mosquito.

The mosquitoes which carry this disease bite at dawn, dusk and early evening, so if you are travelling to an area where the disease is prevalent you should either stay indoors, or wear long sleeved shirts and long-trousers and wear insect repellent.

People travelling with young children should ensure that they use a specially prepared insect repellent, rather than the adult version.

Deer Disease

Aim

The aim of this guide is to describe “notifiable” diseases, for which there is a legal responsibility to act and to outline what steps need to be taken if such a disease is suspected. This guide also aims to provide information on diagnosing and preventing infections in order to safeguard the health of humans, livestock, the wild deer population and other wild animals.

Notifiable Diseases

The following table describes the main ‘notifiable’ diseases, for which there is a legal responsibility to act.

• Inform the Divisional Veterinary Manager1 immediately where any of the following diseases are suspected.
• The Tuberculosis (Deer) Order 1989 makes it a legal requirement to notify suspicions of TB to the Divisional Veterinary Manager2, and provides powers for the veterinary inspector to carry out an investigation and to restrict movements on affected or suspicious deer.

Confirmation of a Notifiable Disease in wild deer is unlikely to lead to legal restrictions on the management of wild deer, but it may be relevant to the control of domestic livestock or farmed deer in the area, so it is mandatory information for the Divisionary Veterinary Manager.1

Safeguarding human health

A small number of diseases are also transmissible to humans either through direct transmission (e.g. Bovine TB) or indirect transmission (e.g. Lyme disease** from ticks).
Specifically in relation to Bovine TB, transmission to humans through eating venison from infected deer is very unlikely and there is no recorded case. The meat from infected animals can even be passed as ‘fit for human consumption’ (after the removal of the affected tissue) unless the carcass is generally emaciated. Those coming in contact with tuberculosis should consult their own doctor immediately.

• Even apparently healthy deer may be infected with a disease transmissible to man so that care should always be taken to minimise the potential for disease spread by carrying out good hygiene practices when handling carcasses.***

Prevention of disease

Preventing introduction of disease from outside the UK

• If importing live animals, hunting trophies or foodstuffs (animal and human), ensure that all UK disease prevention laws are complied with.
• Take care to ensure that those who have been in contact with deer overseas or walking in infected areas do not arrive with clothes or boots contaminated with mud or blood (e.g. Chronic Wasting Disease could be introduced in this way from North America).

Reducing risk of disease in wild deer

Stress (for example, malnutrition) can lower an individual’s ability to resist disease and parasites, and deer found in higher densities and concentrations are more likely to transmit and contract infectious diseases and parasites.

• Maximise quality of forage and shelter available throughout year to reduce the risk of malnutrition.
• Avoid/ minimise situations where deer are artificially concentrated in close proximity to each other (such as feed sites) to prevent transmission.

Notifiable diseases and prevalence

Bovine Tuberculosis (Mycobacterium bovis)

As Bovine Tuberculosis (bTB) is potentially a serious risk to livestock and may pose a threat to human health, the disease is notifiable. Wild deer can contract bTB. Fortunately, there is currently a low incidence of bTB in domestic livestock in Scotland. Only four cases of bovine TB in wild deer have been recorded since 1989.
The symptoms and signs for bTB overlap with two related diseases which can also be contracted: Avian Tuberculosis (aTB) (Mycobacterium avium avium) and Johne’s disease (Mycobacterium avium paratuberculosis). aTB and Johne’s disease (not notifiable) are more commonly found in wild deer.
Lesions of bTB are indistinguishable from those of aTB. The two infections can only be separated by culturing the bacteria from lesions in the laboratory and characterising their DNA. This usually takes at least 6 weeks.

Chronic Wasting Disease (CWD)

Chronic Wasting Disease (CWD) has been reported in several deer species in the USA. Although not strictly listed as a Notifiable Disease, CWD is a member of a group of transmissible spongiform encephalopathies (TSEs) to which scrapie of sheep and bovine spongiform encephalopathy (which is a Notifiable Disease) belong. TSEs have not been diagnosed in British deer although Europe wide surveillance is in place.

Foot and Mouth Disease

Foot and Mouth Disease (FMD): experimental work has demonstrated that red, fallow, roe and sika can all contract FMD. During the 2001 and 2007 epidemics no deer were confirmed to have had FMD despite very close contact with infected cattle and sheep.

Bluetongue

Bluetongue and Epizootic Haemorrhagic Disease (EHD): Epizootic haemorrhagic disease which affects deer is clinically and pathologically similar to Bluetongue. Both diseases are caused by a related but different virus. Bluetongue is now present in the UK having spread to livestock in England in 2007. Further spread of the disease will depend on environmental conditions allowing the virus to multiply in midges that transmit the disease. Although there have been a small number of cases of Bluetongue in deer in Europe, there have been no confirmed cases in deer in the UK. EHD has never been recorded in the UK.