Written by: Maryann Szalka
Whether your Beardie is racing through a tunnel, herding sheep, gaiting around the show ring, performing a perfect figure eight, having a therapy visit at a local hospital, or just playing in your yard, there are an incredible number of creatures trying to invade that hairy body – viruses, bacteria, fungi, insects and worms – just to name a few. These organisms are eager to enter a warm body so they can mature, multiply and/or live off the host’s body. Your Beardie’s body has the important job of keeping these intruders at bay. The three main causative agents of disease (biological, chemical and physical agents) can be spread in many ways. Disease can be transmitted by air, water, food, insects, fomites (inanimate objects such as your shoes that carry disease) and other animals. What can make your Beardie sick? The major causative agents of disease may be classified as:
- Biological agents
- Chemical agents
- Physical agents
- Hereditary disease
- Stress
- Too little of something
- Too much of something
- Diseases of unknown causes
Biological agents of disease are living things that cause disease because of their effect on characteristics associated with life. These agents (viruses, bacteria, fungi, protozoa and invertebrate worms) are living beings that induce a wide range of illnesses. Canine distemper is a highly contagious disease caused by a virus. Pathogenic bacteria can cause diseases such as Leptospirosis or Lyme disease. Other biological disease causing classifications are fungi (yeast, ringworm, molds and a lot of specific fungi that cause diseases ending in mycosis or just osis) and protozoa, which are one-celled animals that often cause disease such as Coccidia, and Giardia. Parasitic worms, such as tapeworms, roundworms, heartworms or hookworms, are multicellular organisms that can cause disease. Finally, external parasites can transmit diseases directly or damage the skin – sometimes by causing the dog to scratch the itch – allowing easier access for other pathogens.
Chemical agents of disease are substances that cause disease because of their mass or ability to engage in chemical reactions with molecules in the body. Most homes contain chemicals that can cause disease. Some common examples are cleaning agents, pesticides, and petroleum products. Chemicals may cause contact problems, or may be ingested. Some may induce genetic change. A subclass would include pharmaceutical products.
Physical agents of disease do their damage by transferring energy to the body, and damaging body cells. Radiation, excessive exposure to loud noise, and even the sunlight (ultraviolet radiation) can be harmful.
Hereditary can be a disease agent. For example, in diseases associated with altered recessive genes, both parents (though disease-free themselves) carry one normal allele and one altered allele. Each offspring has one chance in four of inheriting two altered alleles and developing the disorder; one chance in four of inheriting two normal alleles, and two chances in four of inheriting one normal and one altered allele, and being a carrier like both parents.
Stress can cause ill health and weaken the body making it more susceptible to a disease process.
Too little of, or total lack of, certain life supporting materials can cause disease. Without food or water, the body weakens and eventually dies. Lack of Vitamin A in puppies directly relates to low growth rates, muscle weakness, poor vision, and loss of hair coat.
Too much of some substances can also cause disease. Even an over consumption of something as essential and benign as water can prove to be fatal in some cases. Too much food may lead to obesity, and obesity carries risk factors for a myriad of illnesses.
Unknown, in many cases the exact cause of an illness may never be known. Organs can falter, neurotransmitters in the brain can misfire and long dormant genes may spark to life. Environmental factors may work synergistically with other factors, such as genetics, to facilitate the expression of auto immune disease.
In order for microorganisms to establish themselves, they must enter a host (your Beardie) through a portal of entry. Once inside, they obtain nutrients, reproduce and ultimately cause disease. The major portals of entry to your Beardie’s body are the skin, the respiratory tract, the reproductive organs, and the digestive tract. Not to worry, your dog has a terrific line of defense to try to prevent those aforementioned agents of disease from gaining entry to Beardieville.
Skin is your Beardie’s largest organ — without it, hair would have no place to grow, and internal body parts would become external body parts. Skin is both tough and elastic. It is moist on the inside, relatively dry on the outside. It helps regulate your Beardie’s body temperature through the blood vessels and reduces exposure to extreme cold by muscular action that fluffs the hair and traps heated air next to the body. Skin has three layers: epidermis, dermis, and panniculus adiposus – or subcutaneous fat. The epidermis is a barrier against injury, disease, and damage from ultraviolet light. The ability to quickly replace a damaged epidermis is critical, particularly in cases where burns, abrasions, or cuts leave the body vulnerable to infection and dehydration. Skin protects dogs from ultraviolet rays of the sun by providing a foundation for the coat and by producing melanin to color hair and skin. Cuts or breaks in the epidermis weaken this protective layer, allowing microorganisms to enter. A blood sucking insect (like a tick or mosquito) feeds by puncturing the skin, and in the process, introduces a pathogen into the body. Lyme disease and heartworms are transmitted in this manner. Bacteria can enter through cuts and abrasions and cause an infection.
Your Beardie’s respiratory tract can be divided into two parts, the upper respiratory tract (nasal cavities, the nasopharynx, the larynx and the trachea) and lower respiratory tract (the bronchi and the lungs). The respiratory tract has a great safeguard against invading infectious agents. This safeguard consists of tiny hair-like structures called cilia, which protrude from the cells lining the respiratory tract, and are covered by a coat of mucus. Debris (including infectious agents) gets trapped in the sticky mucus and the cilia move the mucus upward towards the throat where the collection of debris and mucus may be coughed up swallowed. Foreign bodies irritating the nasal cavity can effectively be removed by a sneeze! Bordetella, distemper and parvoinfluenza are some of the diseases transmitted via the respiratory system.
A mucosal physical barrier also helps to protect the urinary and reproductive tracts. Good bacteria, called normal flora, prevent pathogenic organisms from overgrowth, taking over and invading the body. Brucellosis is usually the first canine sexually transmitted disease that comes to mind, and is one that most owners screen for prior to breeding. The disease enters through the mucus membranes of the reproductive system and spreads to the lymph nodes and the spleen, and then ultimately to the uterus and the prostate gland. The dog/bitch may show no clinical signs, but can still transmit the bacteria in semen or vaginal fluid. Another pathogen, the canine herpes virus (CHV) is found in the female’s vaginal secretions and the male’s semen. Although direct bodily contact is usually required for sexually transmitted disease, keep in mind that infected semen from the dog can infect the bitch during AI (artificial insemination). Herpes virus is most often transmitted by normal licking and sniffing behaviors.
The canine digestive tract consists of the mouth, esophagus, stomach, small and large intestine. Harmful microorganisms can enter the digestive tract in food or liquids, but your Beardie has a few protective mechanisms here as well. First, the pathogen must try and penetrate the mucus membrane that lines the digestive tract; and then the digestive tract secretes chemicals that are fatal to many microorganisms. In a healthy Beardie, the stomach contains high levels of hydrochloric acid and digestive enzymes; and an abundance of good bacteria in the intestinal tract make it difficult for many pathogens to survive. Giardiasis is an intestinal infection caused by a parasitic protozoan called Giardia lamblia. Giardia causes its unpleasant effects on your Beardie not by invading the tissues, but simply by being present. It multiplies to the point where it “paves” the lining of the intestine and blocks normal digestion (malabsorption). This results in partially digested food passing unabsorbed through to the large intestine, and thereby causing diarrhea (a symptom of the infection*).
Other body parts have special protective mechanisms against disease, the eyes produce tears which help to flush out harmful particles, and tears also contain an enzyme that dissolves the cell walls of certain bacteria. Ears secrete wax that keep out undesirable matter. Reflexes (involuntary movements in response to stimuli) also play a role in protecting your Beardie from disease. Think about what a simple sneeze or blink can do. And we all know that the profuse coat of a Beardie is a formidable barrier, and can trap any number of things!
Just in case any of the above mentioned lines of defense fail to block the entry of a causative agent of disease, your Beardie has some addition weapons at his or her disposal – phagocytosis, immunity, and veterinary medicine. Discussion of your Beardie’s on-going battle against disease will be continued in part two.
* In many cases, dogs may be infected, but show no symptoms. So being infected is not the same as having an active disease process.
