1) Goats, like all other food animals, are plagued by numerous
infectious diseases which reduce their productivity and profitability.
Infectious diseases are ailments produced by microscopic organisms as
a result of their existence and replication in the tissues of the host.
These microorganisms are really parasites but the term ''parasite'' is
commonly reserved for larger multicellular organisms such as lice,
mites, flukes and various gastrointestinal worms. Thus, infectious
diseases are commonly distinguished from parasitic diseases but also
from metabolic diseases, nutritional diseases, toxic diseases,
neoplastic diseases, etc. Infectious diseases are not necessarily
communicable, that is, transmissible from animal to animal.
Tuberculosis for example, is caused by a bacterium, Mycobacterium
tuberculosis and is readily spread from man to man or animal to animal.
Tetanus (lockjaw), on the other hand, although caused by a bacterium,
Clostridium tetani, is not transmissible from animal to animal. It is
associated with contamination of deep, penetrating wounds and is caused
by a toxin elaborated by the organism.
2) Infectious diseases can be broadly subdivided as specific diseases
caused by specific microorganisms (e.g. brucellosis: Brucella
melitensis) and non-specific diseases such as mastitis, pneumonia, etc.
which can be caused by a variety of different kinds of microorganisms.
Mastitis for example, may be caused by staphylococci, streptococci,
enteric bacilli, yeast, corynebacteria, etc.
3) Infectious diseases can also be subdivided according to the type of
microorganism responsible for the infection, e.g.:
1. protozoal diseases (due to one-celled animals): coccidiosis, toxoplasmosis
2. fungal: ringworm
3. bacterial: tuberculosis, brucellosis, caseous lymphadenitis, arthritis, Johne's disease
4. mycoplasmal: pleuropneumonia, arthritis
5. rickettsial: pinkeye
6. chlamydial: abortion, arthritis
7. viral: contagious ecthyma (sore mouth), arthritis
4) Mycoplasma, chlamydiae, and rickettsia are bacteria-like organisms.
Mycoplasma will grow on special artificial media. Chlamydiae and
rickettsia are also bacteria-like but, like viruses, can only
replicate in living hosts.
5) No matter what causes a particular infectious disease, the eventual
outcome of that infection is influenced by a number of different
factors. Certain parameters of the host, the environment, and of the
infecting microorganism are important.
6) With regards to the host, the integrity and preparedness of the
immune system are critical. Some animals are born with defects of the
immune system which make them unable to combat infectious diseases.
Some animals are genetically endowed with superior resistance to
infection; others are not. The newborn animal which receives passive
immunity via maternal colostrum (first milk) is in an enviable position
since it has a temporary protection against the microorganisms in its
immediate environment at a most vulnerable time. Age of exposure is an
important host factor since young animals are almost always more
susceptible than older ones. In addition, poor nutrition can adversely
influence an animal's resistance as can the presence of a concurrent
illness or parasite infestation.
7) The nature of the environment can also have a profound effect on
the outcome of a disease process. Cleanliness and adequate ventilation
can reduce exposure to disease-producing organisms and prevent
contamination build-up. Population density is also important since
overcrowding almost invariably leads to disease problems. With
infectious diseases in particular, the interchange of populations of
animals is apt to be troublesome. Many cases of infectious disease
outbreaks can be traced to the introduction of new animals into a
herd. Such animals may appear healthy but may be incubating a disease
or may be carriers of microorganisms to which the main herd has not
been exposed. Of course, transfer of infection can also occur from the
herd to the new animals.
8) Finally, there are factors associated with the infectious
microorganisms themselves which can influence the nature of the disease
produced. The virulence is genetically determined so that within a
particular species of bacterium, there are a number of strains which
vary in their ability to cause severe, even fatal disease. The dose of
microorganisms involved is obviously important. In a contaminated
environment, exposure to many microorganisms is more likely to result
in a serious infection. A microorganism which can live peacefully if
applied to the skin might wreak havoc if introduced to the lungs or
mammary gland.
9) There are many infectious diseases of goats, even though goats as a
species have not been well-studied from the infectious disease point of
view. As goats are more intensively reared and investigated, new
disease problems will undoubtedly be discovered. A brief discussion of
some of the more important infectious disease problems of goats
follows.
10) Caseous Lymphadenitis Caseous lymphadenitis, also called pseudotuberculosis or merely
''abscesses'' has been referred to as the curse of the sheep and goat
industry throughout the world. It is considered by some to be the
major disease problem of dairy goats in the United States. The
causative agent, Corynebacterium ovis, also called C.
pseudotuberculosis, was first described in 1894 from the same disease
in sheep. It is a small rod-shaped bacterium which is colored blue
(Gram +) by the common differential stain used in bacteriology. C. ovis
grows readily on sheep blood agar and other bacteriological media
enriched with serum. The organism forms small, dry, white to yellow
colonies which are initially very tiny but grow to a pin-head size in
about 48 hours. If an abscess has not ruptured and is lanced in a
sterile fashion, pure cultures of C. ovis are commonly obtained from
the pus.
11) The pus is thick, often dry, and greenish-white in color. Its
consistency is best likened to toothpaste or putty. The abscesses
formed by C. ovis are usually associated with lymph glands and may be
''external'' where they handily break to the outside or internal where
they are not at all visible. In the goat the external abscesses of C.
ovis are most often found around the head and neck, frequently below
the ear and behind the jaw. They are initially small but invariably
grow larger. Because the goat often manages to put a thick connective
tissue wall around them, they do not readily rupture until they reach
the size of walnuts or larger.
12) Internal lymph gland involvement often affects the mediastinal
(between lungs), gastrohepatic (between stomach and liver) and
mesenteric (intestinal suspensory) areas. Interference with organ
function in these vital areas produces unthrifty and weakened animals
which are frequently afflicted with difficult breathing and a chronic
cough.
13) Much of our knowledge of caseous lymphadenitis comes from the
experience of Australian workers with the disease in sheep. They found
that environment contamination with C. ovis was common in afflicted
herds and that the widespread distribution of abscesses in the species
could be related to contamination of shearing wounds. The distribution
of most external abscesses about the neck and head suggests that goats
are most commonly infected via ingestion of the organism. Frequently
goats are exposed as kids but abscesses don't become evident until the
animals are at least a year of age. The disease is insidious in its
development.
14) Clinical Signs To minimize environment contamination, encapsulated abscesses should
be drained before they rupture. The hair should be clipped away around
the abscess and its surface disinfected with tincture of iodine or other
suitable antiseptic. The abscess should be incised vertically to
promote drainage and pus should be squeezed out and collected for
destruction by incineration or exposure to strong disinfectant
solutions. Since C. ovis has been associated with infections in man,
care should be taken to avoid direct exposure to the pus.
15) Following drainage, the affected goat should be isolated from other
goats until healing is well-progressed. The wound should be irrigated
initially and on a daily basis with an antiseptic solution such as
chlorhexidine (''Nolvasan'') diluted 1:10 in hydrogen peroxide.
Intramuscular application of penicillin - streptomycin on a daily basis
for at least 3 days can minimize complications and continued shedding
of the organism. Because of the presence of veins, nerves, arteries,
esophagus, and glands in the throat region, abscesses in this area may
require professional assistance in lancing. ''Throatlatch'' abscesses
are especially serious and endanger the life of the affected
individual.
16) Once established in a herd, caseous lymphadenitis is difficult to
eliminate. Even goats in which abscesses are properly lanced and
treated will often have recurrences, and environmental contamination
leads to infection of kids. To remove caseous lymphadenitis as a herd
problem, it is best to cull chronically affected goats. Kids should be
separated from infected does at birth, given colostrum from clean does,
and raised in a clean area on ''clean'' milk or replacer. Some
experienced goat people have recommended the administration of bacterins
made from C. ovis isolates from the herd in question but this practice
remains controversial since no clearly definitive scientific studies
have been made. Dr. Sam Guss, the eminent goat veterinarian, recommends
initial application of an autogenous bacterin at 3 weeks of age, a
second dose at 5 weeks of age, and booster doses at 3 to 6 month
intervals thereafter. Lastly, the importance of cleaning and
disinfecting premises before repopulation must be emphasized.
17) A bacterin is a young broth culture of C. ovis which has been
inactivated with a dilute formalin solution. In this way the organism
and its exotoxin are destroyed while the constituents which serve to
stimulate the immune response are still active. Bacterins in theory
should cause previously unexposed animals to more effectively resist
natural infection or infected animals to more readily purge themselves
of infection. The difficulty with C. ovis is that infected goats seem
to have the ability to wall-off the organism temporarily but mobilize
an immune response inadequate to effectively destroy it. Accordingly,
recurrent abscessation is common.
18) Bacteria other than C. ovis may be responsible for abscessation as
a result of contamination of lacerations or punctures. These are
usually associated with poor sanitation. Corynebacterium pyogenes is
frequently responsible for abscesses containing yellowish pus of a
mayonnaise consistency. Streptococci often produce a watery discharge
while staphylococci cause a creamy exudate. Although the nature of the
pus can give clues to the cause of a particular abscess, only
laboratory cultural methods can give definite information. Commercial
bacterins against C. pyogenes and Pasteurella species are available and
have been used prophylactically against pneumonia (which often
accompanies the stress of shipping) and even against caseous
lymphadenitis. Varying degrees of success have accompanied their use
but, again, their real value is not well-established.
19) Pinkeye Infectious keratoconjunctivitis or pinkeye is a disease which
usually appears in hot dry weather and is spread by close contact and
flies. The cause is not definitely established in goats but rickettsia
are believed to be involved in some cases and mycoplasma in others. The
eyes are afflicted with excessive tearing, reddened mucous membranes,
then a white discoloration of the cornea which obscures vision. In
severe cases the cornea ulcerates and loss of the eye may result. In
most cases, when the goats are protected from sunlight and given good
nursing care, recovery is usual. Nevertheless, all goats, even those
not affected, should be treated with broad spectrum antibiotic
ophthalmic powders or ointments to minimize the spread of infection.
Resistant carrier animals may serve as the source of the organism when
dry, dusty, sunny days predispose a herd to the disease.
