Dr. James Evermann
Lecture 3: Overview of the Microbial World Affecting Livestock
Microorganisms may be classified as saprophytes or parasites on the basis of their habitat and mode of persisting in
the environment. Some important parasitic
states are defined below.
Saprophytism – Organisms survive and persist in non-living or decaying organic
matter. They are not usually parasitic, although may cause disease under certain
circumstances – e.g. Aspergillus fumigatus and bovine abortion (fungal abortion).
Parasitism – Denotes organisms which survive and persist on or within another living
organism. The parasite usually does not harm the animal. The most successful parasites
achieve a balance whereby both the parasite and host animal survive. Among the parasites
found on or within animals are bacteria, protozoans, fungi, mycoplasmas, rickettsia, and
viruses.
Commensalism – Parasitic state in which the parasite lives with the animal without
causing disease. Be cautious with this type of parasite.
Symbiosis – Parasitic state in which the parasite and animal survive in a beneficial
relationship – e.g. bacteria within the rumen.
Opportunistic pathogen - Parasites that are usually harmless commensals in their normal
habitat, but can cause disease when they gain access to other body sites – e.g.
Pasteurella haemolytica (pneumonia) and Staphylococcus aureus (mastitis). *Impairment of animal’s immune defense is the principle factor leading to
opportunistic infections.
Obligate pathogen – Parasite that generally will cause disease – e.g. Brucella
abortus (Brucellosis); Yersinia pestis (Plague); and Mycobacterium bovis (Tuberculosis).
Infectious Disease Terms
Pathogen/Pathogenicity – Capacity of a parasite to produce disease
Virulence – Usually used to define the degree of pathogenicity – e.g. E. coli strains with K99 pili have greater virulence than those strains with F41 pili. Pili are filamentous extensions on the surface of bacteria, which are important in motility, attachment to host cells, and invasion of cells.
Infectivity/Infectiousness – Capacity of parasite to become established in the animal’s tissues – e.g. rotavirus and coronavirus in calf scours, are carried in the gastrointestinal tract of calves (> 4 weeks of age) and older cattle without clinical signs of disease.
Transmissibility/Contagiousness – Capacity of parasite to spread between animals – e.g. Salmonella dublin is regarded as highly contagious between heifers and calves via feces.
Toxemia – Capacity of parasite to produce toxins (endotoxins or exotoxins). Be cautious about differentiating this from a toxic substance, such as lead poisoning from old batteries, or toxic plants.
The two principal determinants of the outcome of the parasite/animal relationship are
the virulence of parasite and the resistance of the animal. The immune system is the major
factor in determining the animal’s resistance to infection and susceptibility to
disease.
Koch’s Postulates – Used in infectious diseases to determine if the parasite will cause disease. The traditional criteria used are as follows:
Problems with Koch’s Postulates in 1999 – 2000.
Complex disease processes known to have multiple infectious agents – e.g. shipping
fever in cattle and calf scours (that are caused by a combination of bacteria and virus)
Diseases where the agent has not been cultured outside the animal’s body – e.g.
prions (scrapie in sheep and bovine spongiform encephalopathy).
Host Resistance and Susceptibility to Disease
Host Resistance (Figure 1)
Nonspecific mechanisms (innate immunity)
Specific mechanisms of Immunity
Immunity can be passively acquired, e.g. colostral antibodies passed from dam to off spring. Passive immunity is short lived and actively acquired immunity, i sacquired from an active infection and/or vaccination and will stimulate an immune response with long lived memory cells.
Figure 1. Course of Infection (From Cater, 1995)