Pathology: the house fly, M. domestica, is closely
associated with humans, livestock, their buildings
and organic wastes. Although it may be of only
minor direct annoyance to animals, its potential
for transmission of viral and bacterial diseases
and protozoan and metazoan parasites is of sig-
nificance. However, its pathological importance
varies considerably, depending on the precise
circumstances in which it occurs. The free avail-
ability of livestock or human excrement and low
levels of hygiene provide sites in which flies can
breed and allow flies to act as vectors as they
move from site to site. Pathogens may be carried
either on the hairs of the feet and body or
regurgitated in the saliva during feeding. More
than 100 pathogens associated with the house fly
may cause disease in humans and animals,
including typhoid, cholera, tuberculosis, anthrax,
and conjunctivitis. Musca domestica are also
thought to be capable of harbouring and trans-
mitting Corynebacterium pseudotubercolosis, the
bacterium responsible for mastitis in dairy herds,
after feeding on lesions or contaminated milk.
When fed on contaminated milk, house flies
harboured the bacterium on their body surface
for the following 5 minutes, in their saliva for up
to 3 hours and in their faeces for between 1 and 4
hours after feeding. In humans, the house fly is
important in the spread of Shigella and other
enteric bacteria. House flies are suspected to be
mechanical vectors of the Escherichia coli patho-
gens, harbouring the bacteria in their intestine
and excreting it for at least 3 days after feeding.
Eggs and larvae of various nematodes which
affect horses, such as Habronema spp., may also
be carried. The latter, when deposited in wounds,
may give rise to skin lesions of habronemiasis,
commonly called `summer sores' in horses.
Granulomatous nodules, which contain the
nematode larvae, appear on the skin, especially
around the eye, ventral abdomen, prepuce and
lower limbs.
In northern Europe, the face fly, M. autumna-
lis, may often be the most numerous fly worrying
cattle in pasture. In Britain this species tends to be
found in more southerly areas and, in general, is
not found north of Yorkshire. Musca autumnalis
is also one of the most important livestock pests
to invade the United States in recent years. Its
introduction into North America from Europe
was first detected in 1951 in Nova Scotia. From
there it spread southward and, by 1959, many
cases were being reported on cattle. It now occurs
practically throughout the USA. Face flies are
generally found around the eyes and nose of
livestock or on wounds where the females feed.
The annoyance caused by the flies results in cattle
aggregating and bunching in the shade to escape
and contributes to reduced production rates. In
the USA, M. autumnalis is an important vector of
bovine keratoconjunctivitis caused by Moraxella
bovis. Face flies are also intermediate hosts of
Parafilaria bovicola, the causative agent of par-
afilariosis of cattle in northern Europe and else-
where, and the irritation of the eye arising from
their feeding can exacerbate the transmission of
pinkeye and other conditions such as eyeworm.
Adults are developmental hosts for Thelazia
(Spirurata: Thelaziidae) nematodes which live in
the conjunctival sac of cattle and horses, causing
conjunctivitis, keratitis, photophobia and epi-
phora. This disease is an increasing problem in
the USA.
Musca sorbens is thought to be the principal
insect vector of Chlamydia trachomatis, the cau-
sative agent of trachoma in parts of Africa.
In addition to dung feeding, adults of the bush
fly M. vetustissima will persistently attempt to
feed at the mouth, eyes and nose. As a result, they
are of considerable significance as a nuisance pest
in Australia for both livestock and humans. In the
absence of native Australian dung beetles capable
of disposing of the dung of introduced cattle, the
dung of these herbivores is slow to decompose,
allowing M. vetustissima ample opportunity to
breed and reach large and problematic popula-
tion densities.
Hydrotaea (sweat and head flies)
Species of the genus Hydrotaea closely resemble
species of Musca and are known as the sweat flies
and head flies. They feed on exudates of the eyes,
nose and mouth. They do not bite. The genus
contains one particularly important species,
94 Veterinary Ectoparasites: Biology, Pathology and Control
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