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A pathogen is an organism that causes disease. Pathogens may be bacteria, viruses, prions (such as the causative agent of 'mad cow disease') fungi, or unicellular or multicellular parasites.

A disease that is caused by a pathogenic organism affects performance of the animal's vital functions, and usually gives diagnostic symptoms. An infectious disease in one that is communicable, liable to be transmitted to other animals. A non-infectious disease affects only one animal and is not transmissible.

The infectious diseases of wildlife are numerous, and can affect local groups or entire populations. Some diseases can infect any species of animal (e.g. Salmonella), while others are specific to only small number of host species (e.g. squirrel pox). A number of diseases are zoonotic: they are transmissible from animal to human (e.g. anthrax, rabies, salmonella). Some diseases are reverse zoonotic: they are transmissible from human to animal (e.g. Mycobacteria tuberculosis can infect deer, dogs and elephants, and cause cutaneous granulomas in birds).

Bacteria can be transmitted via inhalation of contaminated matter (e.g. M. tuberculosis), secretions (e.g. M. gallisepticum, an avian conjunctivitis), ingesting of feces or foods/water contaminated with feces (e.g. E. coli), ingesting contaminated soil (e.g. B. anthracis, which causes anthrax), direct contact with an infected animal (e.g. Listeria monocytogenes) and scratches/bites (e.g. Pasteurella multocida, prevalent in the mouths of cats).

Fungal infections can be contracted via inhalation of spores (e.g. aspergillosis) or by contact (e.g. candidiasis). Some organisms, like Candida, occur normally in animals, and disease occurs only when the organisms multiply rapidly because the animal is stressed, malnourished, ill, or treated with antibacterial medications (antibiotics). Affected birds can infect other birds (e.g. parents feeding their young; affected prey ingested by avian predator).

Viruses can be spread via blood or the bites of insects (e.g. rabies, West Nile virus,), ingesting scabs with active virus (e.g. avian pox), and via feces and/or respiratory discharges (e.g. paramyxovirus, herpes virus).

Parasites are usually contracted when an animal ingests contaminated feces or food, water or soil that has been contaminated with feces. Some of the more common parasites that infect wildlife include:

• Cestodes (e.g. tapeworm) are segmented flatworms that live in the intestines of the host. Animals are infected when they ingest infected earthworms or other prey, or fleas, or feces contaminated with eggs or segments.
• Nematodes (e.g. threadworms, roundworms, gapeworms) infect animals that ingest infected prey or feces.
• Protozoa infect animals that ingest eggs via contaminated food, water and substrates. There are many taxa that include common pathogens such as trichomonas, coccidia and giardia.

Contagion is a grave concern in the rehabilitation setting, where infectious diseases can spread like wildfire. It is crucial that newly admitted animals be quarantined and that their feeding implements, dishes, syringes, bedding, housing, pools, and other equipment not be used for other animals. Excellent sanitation, sterilization and husbandry protocols are an absolute necessity. A newly admitted animal should be examined, tested (fecal tests, mouth/throat swabs, and blood tests when indicated), treated, monitored, and given a clean bill of health before it is introduced to other animals.

Non-infected animals that are recovering from injuries, emaciation, illnesses, etc. or that are stressed are in a weakened state, and thus are susceptible to pathogens. Young animals in particular, who have little immunity and are perhaps weakened by malnutrition, must be protected from infection with pathogens.

Not all diseases are obvious initially. A non-symptomatic animal that is harbouring rabies and that has not been isolated will jeopardize the lives of animals and rehabilitator alike. In wildlife rehabilitation, we must, for the sakes of animals, volunteers, family members and ourselves, always err on the side of caution.

To learn more about wildlife diseases, choose a topic from the following selections, or from the right-hand menu.

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