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. 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.
|
