Page 265 - The Veterinary Laboratory and Field Manual 3rd Edition

234 Susan C. Cork and Roy Halliwell

A product with a narrow spectrum of activ- husbandry, that is, reduction of stocking rates,
ity is often the best choice where the organism better nutrition and preventative interventions
has already been cultured and a diagnosis con- such as vaccination.
firmed. However, broader spectrum antibiotics
are favoured when the diagnosis is pending. In
these cases, it is important to select an antibiotic 4.6 Mycology
that is likely to be effective against the suspect
pathogen but has a minimal effect on other Mycology is the study of fungi. Fungi may be
(beneficial) microorganisms. parasitic or saprophytic (feeding off dead organic
matter) and are abundant in nature. Fungi are a
duratIon and FEaSIbILIty common cause of damage to crops, food stuffs,
The selected antibiotic will need to be avail- fabric and building materials. In animals, fungi
able in a formulation that is easily administered are usually secondary invaders, rather than pri-
to the animal(s) undergoing treatment. The mary pathogens, and therefore fungal infections
route of administration of the available antimi- often imply poor immune function or disruption
crobial products must be considered carefully, to normal flora. For example, Candida sp. yeast
that is, prolonged oral use should be avoided. infections are more common in animals that are
This is because the development of resistance debilitated due to concurrent disease or nutri-
is often associated with the selection and trans- tional deficiencies; they are also more frequent
fer of genetically resistant bacteria that inhabit following antibiotic use. However, healthy ani-
the gut and also because prolonged oral treat- mals can be severely affected by mycotoxins
ment can destroy the beneficial microbial flora. produced by fungal contaminants in forage and
Insufficient duration of treatment, or the inabil- feed supplies (for example, aflatoxins produced
ity to administer the required dose, can lead to by Aspergillus sp.). Screening feed for aflatoxins is
resurgence of the infection. outlined in Chapter 7. Many of the most common
fungal diseases in animals are cutaneous and
druG WItHHoLdInG PErIod spread by direct contact (for example, ringworm)
Duration of drug withdrawal periods may also but airborne and other routes of transmission are
be a factor in choosing suitable products, that is, possible and can result in systemic disease (for
the milk from dairy animals, eggs from hens and example, blastomycosis, histoplasmosis). Some
the meat from meat animals should not be used fungal diseases (mycoses) of veterinary impor-
for human consumption within the withdrawal tance are discussed below.
period as the antibiotic concentration may
remain above acceptable limits in the product.
Moulds and yeast
aLtErnatIvES to antIbIotIcS
In conclusion, antibiotics can provide a valu- The term mould is generally used to describe a
able treatment option for a range of bacterial fungus that produces branching tubular struc-
diseases. However, their use should be part of, tures. These structures are normally divided
and not a replacement for, integrated disease into sections (septa) by a cell wall. Unlike most
control and prevention programmes. These bacterial colonies, fungal colonies tend to spread
programmes usually include recommendations radially on a culture plate due to their branching
for appropriate hygiene and disinfection proce- growth. The peripheral filaments that make up
dures, effective biosecurity measures, improved a fungus are called hyphae (see Figure 4.14a).

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