Re: How come that vultures can resist dangerous toxins when feeding on carcass

Good question – and I had to look through some of my dusty old notes to develop an answer!

Vultures, like many carrion eaters, display remarkable tolerance to microorganisms that would be pathogenic to many other species. The basis for this is the fact that they have evolved, i.e. been naturally selected to eat the food and live in the environment in which they do.

Vultures eat carrion that may be the corpse of an animal killed by a bacterial or viral infection, or which has developed sufficiently large numbers of pathogenic bacteria, naturally present in small numbers on the skin or in the guts of many animals, that would be lethal to a variety of animals “not used to it”. What must be remembered is that many diseases and viruses are species specific – so what kills one animal will not necessarily kill another.

Vulture digestive acid is so strong that it easily digests putrid substances without presenting the bird any trouble. These birds are credited with reducing the occurrence of disease because they eat diseased animals and prevent the causative organisms spreading in this way. Their digestive systems have the unique ability to kill any almost virus and bacteria present in their food as vulture droppings and dry pellets (bolus) are clean and do not carry disease. This was proven by the United States Department of Agriculture in tests performed during a hog cholera epidemic in the southern states (The Turkey Vulture Society).

The digestive acids are not the sole factor present that benefits their carrion-eating lifestyle. Vultures must also have an ability to destroy toxins produced by bacteria that may be present long after the bacteria have been killed. Some potent toxins may well be absorbed by the oesophageal (food-pipe) epithelium before ever reaching the stomach. Clostridium Botulinum, the causative agent of Botulism, produces five different toxins (named toxins A through F). Toxins A & E are principally found connected with human illness, and toxin C with animal illness. It is perhaps for this reason that Vultures have very high titres (amounts) of antibodies against this toxin, and also antibodies against the other Botulinum toxins. This is also observable in Coyotes and Crows who also have remarkable ways of preventing poisoning (Ohishi et al, 1979).

[Coyotes have a highly sensitive emetic response (vomit) receptor in the brain, which will detect the smallest amounts of anything ‘nasty’ in the coyote’s last meal and cause the animal to vomit before the bacteria / toxin has a chance to cause any real damage].

The adaptive mechanism for life as carrion eaters may therefore be the presence of a substantial immune system, which may have a partly genetic basis, but also ‘immune training’ from the parent (i.e. in mammals – from breast milk) and from encountering microorganisms / toxins themselves (Ohishi et al, 1979).

Interestingly, the vulture's urinary tract empties directly onto its legs, soaking them in a powerful uric acid that acts as a coolant by evaporating, but also as a sterilising agent. The chemicals in the vulture's stomach are so powerful that the animal's urine will kill whatever bacteria may be present on its legs, after walking through a carcass (The Turkey Vulture Society).

It is for the above reasons that the digestive system of vultures’ has been the topic of research. The ability to disinfect rodent carcasses carrying Hantavirus is currently being tested and could prove to be of great significance to human medical research. There may also be vital information to be discovered for use in the event of biological warfare, acts of terrorism, or world-wide epidemics (Shriner, 1998).

Thanks for the question

Jim Caryl
MAD Scientist

References:

Shriner, W. (1998). The Turkey Vulture
The Turkey Vulture Society
Ohishi et al (1979). Journal of Wildlife Diseases 15: 3-9