Principles of Disease

• Host and microbe evolution is a constant struggle between health and disease
• Host defenses against disease include:
  • Intact skin
  • Urine flow
  • Immune cells and antibodies
  • The complement system
  • Normal flora
• Microbe defenses include:
  • Capsules
  • Enzymes
  • Toxins
  • Adhesins

Principles of Disease

• Pathology is the study of disease.
  • Gross pathology is visible with the naked eye.
  • Histopathology requires microscopic examination, often from biopsy results.
• Etiology refers to the cause of a disease.
  • Streptococcus equi causes equine strangles.
• Pathogenesis is the manner in which a disease develops or progresses.
• Infection is the invasion and colonization of the body by pathogenic microorganisms.
  • E. coli can cause urinary tract infections.
  • Infections may or may not present clinical signs, like with HIV.
• Disease is any change from a state of health, infectious or not.
  • DJD, cancer and parvovirus are examples of disease.
  • Clinical signs may be present.
• Normal flora are resident bacteria that typically do not cause disease.
  • They often help maintain health in areas like the eyes, mouth, respiratory tract, GI tract, urethra, and vagina.
  • Normal flora can suppress pathogens via microbial antagonism.

Normal Flora and the Host

• Microbial antagonism occurs when normal flora prevent pathogen overgrowth.
• Vaginal bacterial flora maintain a pH of about 4.0.
  • An acidic environment prevents Candida albicans (yeast) from overgrowing.
  • Antibiotics can disrupt bacterial flora leading to pH changes and yeast infections.
• E. coli prevents pathogen colonization in the GI tract.
• Probiotic supplements contain live cultures of beneficial bacteria to repopulate the body.
• Prebiotics are nutrition sources for probiotics.
• Symbiosis is when normal flora living with a host may be beneficial or neutral.
• Commensalism is a symbiotic relationship.
  • One organism benefits, and the other is unaffected.
  • Skin flora live off dead skin cells.
• Mutualism is a symbiotic relationship.
  • Both the host and organism benefit.
  • Normal gut flora, E. coli, protects the host from pathogens and synthesizes vitamins as it lives off nutrients.
• Parasitism is a relationship where:
  • One organism benefits and the other is harmed.
  • Bacteria, viruses and GI worms can be disease-causing parasites.
• Opportunistic organisms:
  • Typically do not cause disease in healthy individuals.
  • Can become pathogenic when the host is weakened.
  • Pneumocystis pneumonia in AIDS patients is an example.
  • Can cause disease if allowed to invade other parts of the body, like E. coli causing UTIs.

Classifying Infectious Diseases

• Symptoms include changes in body function that the patient experiences.
  • These are subjective and not apparent to observers, such as aches and malaise.
• Signs are objective changes that can be seen by others.
  • Lesions, fever, and swelling are examples of signs.
• Syndrome is a specific group of symptoms and signs that always occur together.
  • E. coli 0157:H7 infection can result in enteritis, shock, and renal failure.
  • HIV infection leads to Acquired Immune Deficiency Syndrome.
• Communicable diseases spread from one host to another, usually via direct contact.
  • Measles, influenza, FIV, and COVID are communicable diseases.
• Contagious diseases are communicable and spread easily.
  • Chicken pox, canine distemper, equine strangles, and COVID are contagious diseases.
• Non-communicable diseases:
  • Do not spread from host to host.
  • Environmental or individual causes, like tetanus, coccidiodomycoses, and FIP.

Occurrence of Disease

• Sporadic diseases occur occasionally, such as:
  • Hanta virus in the SW USA.
• Endemic diseases are constantly present in a population.
  • The common cold is an example.
• Epidemic diseases occur when many patients in an area acquire a disease in a short time.
  • Influenza outbreaks, HIV, and the Ebola 2015 outbreak are examples.
• Pandemic diseases are epidemic diseases that are occurring worldwide.
  • HIV, the 1918 Spanish Flu, and COVID are examples.

Severity and Duration

• Acute diseases:
  • Develop rapidly and last a short time (days).
  • Influenza and the common cold are examples.
• Chronic diseases:
  • Develop slowly and last a long time (weeks).
  • Tuberculosis (TB) and systemic mycoses are examples.
• Subacute diseases fall between acute and chronic durations.
• Peracute diseases have:
  • Very rapid onset and progression (hours).
  • Enterotoxemia (Clostridium perfringens) in sheep and calves is an example.
• Latent infections occur when a disease agent becomes inactive for a time.
  • Herpes viruses, chicken pox, and shingles are examples.

Extent of Host Involvement

• Local infection is limited to a small area of the body.
  • An abscess is an example.
• Systemic infections spread via blood or the lymphatic system.
  • Bastard strangles (Streptococcus equi) is an example.
• Bacteremia is bacteria in the bloodstream.
  • A "bacteremic shower” can occur after dental prophylaxis.
• Septicemia (sepsis) is bacteria multiplying in the blood.
  • Anthrax provides an example.
• Toxemia is toxins in the blood.
  • Tetanus and endotoxic shock are examples.
• Viremia is a virus in the blood, active FeLV cases, WNV, and ΕΙΑ are examples.
• Primary infections are the initial acute infection causing an illness.
  • Feedlot pneumonia viruses (BRSV, PI3, IBR/BVD) set the stage for secondary bacterial infections.
• Secondary infections:
  • Occur when an opportunistic organism infects a host weakened by a primary infection.
  • Bovine respiratory complex (BRC) is an example.
• Clinical infections have signs or symptoms.
• Subclinical infections do not cause noticeable signs.
  • West Nile Virus, poliovirus, and hepatitis may be subclinical.
  • A virus can be carried by people without developing disease.

Infectious Agent Locations

• A reservoir is the source of an infectious agent.
  • It can be a living host or inanimate object.
• Human reservoirs:
  • May show signs of disease or be asymptomatic subclinical carriers.
  • They harbor the disease organism without showing clinical signs.
• Animal reservoirs:
  • Involved in zoonotic diseases that spread from animals to humans.
  • Plague, rabies, Lyme disease, toxoplasmosis, and dermatophytes can be zoonotic.
• Nonliving reservoirs:
  • Soil can harbor tetanus and fungal diseases.
  • Water can harbor Giardia.

Transmission of Disease

• Contact transmission involves:
  • Direct contact, person to person or animal to animal; requires physical contact, such as touching or kissing.
  • Indicated through indirect contact with a nonliving fomite, such as bedding, boots, or instruments.
• Droplet transmission occurs when:
  • Saliva or mucus is discharged, releasing around 20,000 droplets per sneeze.
  • The droplets travel less than 1 meter.
  • Equine, avian, equine and human influenza, canine kennel cough, and feline URI dzs are transmitted this way.
• Vehicle transmission is through:
  • Water transmission from contaminated water, can cause cholera, leptospirosis, and Giardia.
  • Foodborne transmission from incompletely cooked, poorly refrigerated, or unsanitary conditions, can cause food poisoning.
  • Airborne transmission occurs through droplets or contaminated particles traveling more than 1 meter, such as tuberculosis.
• Vectors are living carriers of pathogens from one host to another
  • Arthropods (insects) like mosquitoes, ticks and flies are examples.
  • Mechanical vectors passively carry the organism on feet or body parts, like a housefly
  • Biological vectors the infective agent undergoes changes within the vector, a stage of growth or reproduction increases the pathogen numbers.
    • Example: malaria, Lyme disease

Nosocomial (Hospital-acquired) Infections

• There is no evidence of the infection upon admission to the hospital.
• Lister and Semmelweis were early pioneers in control. 5-15% of human patients in the USA get nosocomial infections
  • Around 2 million cases per year with >100,000 deaths per year, usually UTI’s, surgical site infections, bacteremia, or pneumonia.
• Factors that come into play with Hospital Acquired Infections:
  • High concentration of microorganisms in the hospital with opportunistic and drug resistant organisms
  • Weakened state of patience due to broken skin/mucous membranes, Immunocompromised (because of the dz and/or tx), or from IVC and surgical sites.
• Nosocomial infections spread through:
  • Direct contact via staff or patient.
  • Indirect contact via fomites or airborne particles.
• IV catheters, urinary catheters, ET tubes.
  • Hand washing is the most effective method to avoid nosocomial infections.
• Common nosocomial organisms: -Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella, and Klebsiella.

Development of Disease

• Predisposing: something that makes the host more susceptible to disease
  • Poor nutrition, fatigue, chemotherapy, and steroid use can cause disease
• After the microorganism overcomes the host defense, the disease follows a predictable patter:
  • Incubation is the time between infection and the first sign.
    • Can be consistent or variable because it depends on the organisms virulence and quantity.
    • Host immune response is the other important factor.
  • Prodromal : a Normally short and not always seen. - The patient will have nonspecific or mild symptoms and malaise.
  • Period of Illness : When the patient shows Overt symptoms- fever, cough, muscle aches and pain, may end in two ways:
    • The patient's immune system overcomes the organism or
    • Pathogen overwhelms the immune system and patient dies or remains ill.
  • Decline Phase: When signs and symptoms subside, patient is vulnerable to secondary infections.
  • Convalescence: When the patient gains strength and recovery occurs but may also still be shedding the infectious organism.
  • Some diseases occur at the period of incubation and others at convalescence illness.