Zoonoses - diseases of animals and humans
The real determinants of disease mortality…are….for….better or for worse – the gun, axe, fire - Aldo Leopold
It is said that the human population in 1000 BC was about 100 million, at the beginning of the 21st century it was about 6 billion and by 2050 will be 10 billion. While these figures may not be accurate, it seems clear that our planet is becoming somewhat crowded. As any aircraft passenger or commuter in an overcrowded train will know, it is increasingly difficult to avoid other people’s germs. In addition to overcrowding, air travel enables microbes to cross continents at speed in the company of unsuspecting passengers: a matter of concern for many health authorities. This problem is exacerbated if the transported microbe passed on to others causes a new disease, as happened with HIV & AIDS, which have only been with us on a large enough scale to atract attention for the last 20 years. There are, however, other less well known infections that have also arisen recently.
Recognising that our highly populated towns and cities provide an ideal environment for new infectious agents or pathogens to spread and perhaps initiate epidemics, the question arises, where do these new infections come from in the first place? In a recent report on the study of emerging diseases it has been estimated that between 1940 and 2004 over 300 pathogens have caused new diseases in humans; most of these were due to bacteria, but worms, protozoa, fungi and especially viruses all made a contribution. Although we usually think of infectious diseases being restricted to particular hosts, ie human infections being restricted to humans, canine infections only affecting dogs, and so on, whenever people come into close contact with animals, either in the wild or domestically, there is the potential for a pathogen to jump the species barrier. Thus, about 60% of new human infections originate in animals and are defined as zoonoses. Of the 1407 total species of pathogens that affect man, 59% are zoonotic.
A pathogen has an increased chance of infecting new hosts if it is assisted by insects such as a mosquito or other biting fly. An interesting example is Chicleros’ ulcer, also called “chewing gum collectors’ ear”, in which an ulcer forms on the ears of men who collect gum from forest trees in Central America. The ulceration results from invasion by a parasitic protozoon, Leishmania mexicana transmitted by sand flies that normally feed on rodents. Nearly twenty species of Leishmania, often responsible for severely disfiguring lesions in humans, are parasites of a variety of animals including sloths, gerbils, hyrax species and domestic dogs.
The consumption of “forest foods” such as bush meat provides ideal conditions for viruses to cross from animals to humans. This was probably the route originally followed by HIV-1 and HIV-2 from their previous home in non-human primates. Bush meat is still popular and continues to be responsible for occasional epidemics of other viral infections including monkeypox.
As their habitats are destroyed by slash and burn agriculture animals are forced to adapt and look for alternative food supplies. In 1998, many Malaysian pig farmers developed severe febrile encephalitis with a mortality of near 59%. The infection responsible for this disease, Nipah virus, originated in fruit bats. The bats had moved onto fruit trees planted amongst the pig farms; their droppings infected the pigs and these in turn passed it to the farmers.
Most new infections are reported in first-world countries, with the USA topping the list, although the microbe or parasite discovered may not have originated there. The implication is that the number of new diseases reported world-wide is probably a considerably underestimated. With advances in technology come greater chances of finding new infections. An example of such a finding occurred in Sarawak, Borneo, where the use of DNA techniques to supplement traditional microscopy in diagnosing diseases revealed that an outbreak of malaria in forest workers was due to a monkey species, P. knowlesi, and not a human malarial parasite, as first thought. Previous experimental research had indicated that P. knowlesi could infect humans, but the Sarawak epidemic was the first in which this occurred naturally.
In general terms, parasites with a long evolutionary association with humans, such as the protozoon Toxoplasma gondii (carried by many of us, especially cat lovers, in our brains for life) are much less lethal than recently-acquired infections. Thus Ebola virus, Lassa fever, Marburg, etc., all of recent origin, kill patients frequently and rapidly. Rabies is somewhat an exception in that it causes virtually 100% mortality even though it has been with us for thousands of years.
Because of the dramatic death rates and highly infectious nature of Ebola and other haemorraghic viruses, there is an urgent requirement to find the source of any new infection and identify the animal reservoir responsible for carrying the disease. This is not always as straightforward as it may seem. Marburg disease appeared in Germany in the 1960s and killed staff working on tissue culture obtained from African vervet monkeys. Despite many efforts to find the virus in these monkeys in the wild, the search was largely unsuccessful but there are recent reports that it has been found in fruit bats, Rousettus aegyptiacus. Clearly, finding the source of an infection involves the collection and transport of a variety of different species and inoculation of tissue and blood samples into laboratory animals, usually mice. Experimental animals are also used to raise antibodies, and to determine the nature and characteristics of the infectious agent and the type of pathology it may cause. Without this information it is obviously difficult to devise preventive measures and appropriate treatments.
Since 1980, on average, new human pathogens have been discovered at a rate of about 3 per year and there is no reason to assume this will come to an end. Many of these new infections have originated in animals, and continued world-wide surveillance and research to discover the causes and carriers of new diseases are essential. The continuing work to identify and understand new diseases as they emerge is vital, both for human health, and for the welfare and survival of wild and domestic animals.
- Greger, M. (2007) The human/animal interface:emergence and resurgence of zoonotic infectious diseases. Critical Reviews in Microbiology. 33, 243-299.
- Woolhouse, M. and Gaunt, E. (2007) Ecological origins of novel human pathogens. Critical Reviews in Microbiology. 33, 231-242.
- Jones, K.E., et al. (2008) Global trends in emerging infectious diseases. Nature 451, 990-993