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Bull Vet Inst Pulawy 53, 651-655, 2009
Department of Parasitology and Invasive Diseases, Faculty of Veterinary Medicine,
University of Life Sciences in Lublin, 20-033 Lublin, Poland
[email protected]
Received for publication June 22, 2009
In 2006, 725 horses aged 8 months to 16 years from different farms from the southern-eastern part of Poland were
examined post-mortem for the presence of botfly larvae. One hundred and four horses (14.75%) demonstrated the presence of botfly
larvae. An average infection rate was 52 larvae but low rate dominated. The larvae found belonged to Gasterophilus intestinalis and
Gasterophilus nasalis. The 3rd instars dominated in the infected horses; however, more than 25% of the horses harboured both 2nd
and 3rd instars.
Key words: horses, Gasterophilus, infection, extensiveness, intensiveness, Poland.
The prevalence of botfly larvae in animals
poses a serious epizootic and economic problem in
several world areas. In Poland, studies involving the
extensiveness of Gasterophilus larvae infections in
horses are not numerous, although many more reviews
and casuistic reports have been published worldwide (6,
7, 10-14, 16, 18, 19, 21, 22, 25-28, 30, 32, 33). Reasons
for such a situation should be sought both in difficulties
in intravital diagnosis of the infection and a belief in the
low pathogenicity of Gasterophilus larvae.
Horseflies of the genus Gasterophilus belong to
the Oestridae family. In Poland, four species, including
haemorrhoidalis, and G. pecorum, were described.
Matured Gasterophilus horseflies are hairy, 9 mm to 16
mm in length, yellow-brown-black, dark-brown, yellowrust or black coloured tachina flies. They have a
regressive mouth apparatus and do not take feed from
the environment. Inseminated females lay eggs on horse
hair except for G. pecorum, which lay eggs on plants.
Larvae at the first stage reach the oral cavity of horses
passively (G. intestinalis, G. pecorum) or actively. The
larvae stay in the oral cavity for some time, and are then
followed by instars, and as larvae at the second stage
inhabit typical sites such as the stomach and duodenum
where they grow and transform into the third stage
larvae, which leave the host after a few months and then
metamorphose into chrysalis from which insects emerge
into the environment.
The aim of these studies was to evaluate the
extensiveness and intensiveness of the infection of
Gasterophilus botfly larvae in horses from the southern–
eastern part of Poland.
Material and Methods
In 2006, the stomach and small and large intestines
from 725 horses aged 8 months to 16 years were
collected for parasitological purposes. All the horses
originated from different farms in the southern–eastern
part of Poland and were killed at the slaughterhouse.
The collected organs were examined for the presence of
botfly larvae, their localisation, species, and instar
stages. Differential diagnosis was performed on the
basis of larva morphology and its developmental stages
described by Draber-Mońko (7).
The larvae of botflies were found in 104 (14.75%)
of the 725 horses. The rate of infection ranged from 1 to
905 botfly larvae (an average of 52 larvae). The larvae
were found only in the stomach and small intestine and
Gasterophilus nasalis (Figs 1 and 2). The data
concerning the distribution of the larvae in the
gastrointestinal tract of the horses are shown in Table 1.
The majority (88.88%) of Gasterophilus intestinalis
larvae were found to be attached to the mucous
membranes of the stomach. Markedly less larvae
(7.79%), which belonged to Gasterophilus nasalis, were
presented on the mucous membranes of the small
intestine. Few larvae were found in the contents of the
stomach (2.09%) or small intestine (1.23%).
Fig. 1. Individual botfly larvae on the stomach mucosal membrane.
Fig. 2. Clusters of numerous botfly larvae on the stomach mucosal membrane.
Fig. 3. Crater-like alterations in the stomach mucosal membrane.
Small intestine
Table 1
Distribution of botfly larvae in the gastrointestinal tract in 104 infected horses
Mucous membrane
Number of horses (%)
Mean (min-max) Number of horses (%)
Mean (min-max)
98 (94.23)
49.36 (1-723)
24 (23.08)
4.75 (1-13)
33 (31.73)
12.85 (1–171)
15 (14.42)
4.47 (1-11)
Table 2
Frequency of botfly larva occurrence in selected
brackets in infected horses (N=104)
Infected horses
Number of larvae
1 - 10
11 - 50
51 - 100
101 - 500
More than 500
Table 3
2nd and 3rd instars in infected horses (N=104)
Number of
Number of horses
with larvae (%)
II stage
28 (26.92)
III stage
103 (99.04)
The frequency of intensiveness of infection in
defined ranges is shown in Table 2. More than half the
infected horses demonstrated a few larvae (10 or less).
The highest rate of infection included 905 larvae; of
these larvae 723 were attached to the mucous membrane
of the stomach, 171 to the mucous membrane of the
small intestine, and 11 were found in the contents of the
The mucous membranes of the stomach
demonstrated crater-like alterations caused by botfly
larvae (Fig. 3). These alterations occurred in the spots
from which the larvae were removed mechanically and
in the spots showing the traces of their presence.
However, no visible macroscopic changes on the
mucous membranes of the small intestine were found at
the site of the larvae localisation.
The post mortem examination of the horses
revealed the occurrence of 2nd instars of Gasterophilus
sp. larvae in a little more than 25% of the infected
animals; however, the 3rd instars dominated. The 3rd
instars larvae were not found only in one infected horse.
The data are shown in Table 3.
The extensiveness of botfly larva infection in
horses in Poland since the fiftieth year of the 20th century
varied with regard to regions, and ranged from 33% to
88.4% (7, 12, 19, 30, 33). Moreover, gasterophilosis is
one of the most abundant parasitosis, affecting from
8.4% to 52.8% of the horse population (16, 18, 21, 22).
Our results indicate that the prevalence of
nearly 15% gasterophilosis reported in horses from the
south-eastern part of Poland is similar to that found in
our earlier examinations (16), including horses from the
same region and twice as high as that reported by
Kornaś et al. (18) in horses from the Voivodeship of
Malopolska. It should be also stressed that a much
higher extensiveness of botfly larva infection,
amounting up to 40%-50%, was evidenced in horses
from the central and north-eastern parts of Poland.
In other countries, the extensiveness of botfly
infection ranged from 9% to 99% including 53% in
England and Wales (9), 58% in Belgium (1), 34% in
France (2), 43% in Ireland (31), 9% in Germany (24),
65% in Switzerland (3), 12.3% in Sweden (17), 82.2%
in Italy (20), and 98.7% in Kentucky, USA (8).
It seems that the extensiveness of infection is
affected by several factors. However, it is difficult to
determine those which cause a clear differentiation in
the rate of infection between the horses from the
northern and southern areas of Poland. It may be
suggested that the maintenance of horses is one of the
main factors influencing the prevalence of
gasterophilosis (12, 16, 18). In the south-eastern part of
Poland, alcove breeding, involving limited access to
pastures and short-term keeping in individual paddocks,
dominated until recently. This breeding decreased the
extensiveness of infection by limited contact with
female botflies.
The climate of the region seems to be another
factor creating the prevalence of Gasterophilus sp.
However, it is difficult to assess unambiguously the
climate influences on the various extensiveness of
Gasterophilus sp. infection in horses from south-eastern,
central, and the north-eastern part of Poland (7, 26). It is
well known that botflies are considered as relatively
stenothermal parasites and matured females need warm,
sunny, and windless weather. There is no doubt that a
higher extensiveness of Gasterophilus sp. infection is
reported in warmer countries (4, 8, 20, 23). Moreover,
the extensiveness of the infection may be also
influenced by the age and sex of examined horses;
however, results reported by several authors are not
consistent (1, 7, 20, 26).
It appears that the preparations containing
macrocyclic lactons (ivermectin and moxidectin),
applied commonly and successively over decades for
deworming in horses, may have decreased the rate of
botfly larva infections. The above suggestion is
supported by recent observations showing a decreasing
extensiveness of the infection in comparison to that in
the post-war period. The problem was not stressed in
previous publications.
The number of botfly larvae in horses seems
interesting. Our examinations revealed that the intensity
of infection in horses from the south-eastern part of
Poland was low - from 1 to 10 larvae (about 50% of
infected horses) whereas 15% of horses suffered a high
infection - from 100 to 905 larvae. The mean larva
intensity (52 specimen) was similar to that (46–0 larvae)
reported by other Polish investigators (12, 18, 22, 30).
The data reported by researchers from other countries
show a higher intensity of infection, which may result
from different climate conditions. For example, the
mean intensity of infection in Italy was 108 larvae but in
Kentucky (USA) 168 larvae. On the other hand, in
Belgium the intensity of infection ranged from 1 to more
than 200 larvae, whereas the majority of horses were
infected with 1 to 50 larvae (67%) (1).
Our examinations revealed that the large
majority of larvae were attached to the mucous
membrane of the stomach and duodenum as a result of
the various species statuses of the parasites (5, 9, 29).
Few larvae were also found in the contents of the
stomach. Because the post-mortem examinations were
carried out shortly after slaughter it may be assumed that
the larvae found were migrating. Low-rate infections
frequently noted in our examinations had a small
negative effect on the functioning of the gastrointestinal
tract. However, the literature data and our observations
indicate that the rate of infection involving hundreds of
larvae may exclude a vital part of the stomach or small
intestine from normal functioning and lead to
disturbances in the digestive processes.
Moreover, it should be stressed that in some
cases individual larvae may injure the stomach or
intestinal wall at the site of attachment, producing a
gateway for the penetration of viral, bacterial or mycotic
pathogens (5, 15, 22, 29).
More than 25% of infected horses from the
south–eastern part of Poland had 2nd and 3rd instars of
Gasterophilus sp. and the presented results seem to be
seasonally dependent. Similar data were reported by
Pawlas et al. (22).
Our results confirm that gasterophilosis in
horses remains a problem from the invasiological point
of view. Although clear clinical signs are rarely seen
during the course of the disease, the parasites may
directly or indirectly influence the health and
performance of horses. Hence, this infection should be
considered in the programmes including plans for
parasite eradication
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