St George, T. D.

University of Queensland eSpace

Birds 120828 
The occupation of an ecological niche by cattle egrets 
TD St George 35 White St, Graceville, Queensland 4075, Australia. 
Arbovirus Res Aus 11 pp?? 
 
 
INTRODUCTION 
In the course of the 20th century, the cattle egret has expanded to three more 
continents and many islands of the Pacific and Atlantic Oceans. (Fig 1) This bird is 
described as Ardea ibis ibis, in Africa, America and Europe and as Ardea ibis 
coromandus in eastern Asia, Australia and the Pacific. The cattle egret is closely 
associated with large grazing animals including domestic cattle and buffalo as they 
feed principally on insects disturbed from grasslands by the grazing animals. They 
also pick ticks attached to the skin of these animals. Colonies are established close to 
water as resting or breeding rookeries. Many of the bird populations have an annual 
migration pattern while others can be resident year round. 
 
THE EXPANSION 
The sub species Ardea ibis coromandus may have entered Australia as early as 
the 1920s (McKilligan 1985) although other reports cite a later date (1948) in the sub-
coastal plain east of Darwin. These birds spread around the continent much later and 
by the 1960s were visiting New Zealand. They migrate annually between northern 
New South Wales and New Zealand. Occasionally they are seen on Macquarie Island.  
In a southward movement the Ardea Ibis ibis reached Cape Province in South Africa in 
1908.They crossed the Atlantic from Africa and entered Suriname in 1933. Herons 
have been observed from planes, flying steadily across the mid-Atlantic (Martinez –
Vilata et al 1992). From northern South America they expanded via the Caribbean 
islands to Florida in 1941 and bred there in 1953 (Gilbert 1972). They were first 
reported in Texas in July 1948. They had reached their present limits in North and 
South America by 1970 as shown in Fig 1. In Europe prior to 1930, they were confined 
to southern Portugal and south-western Spain. They are now visiting most northern 
European countries. Migration from central Africa occurs. 
 
Figure 1. The present approximate distribution of cattle egrets. The dates shown for 
their arrival may be for either small or for large numbers. 
 
ARBOVIRUSES ASSOCIATED WITH CATTLE EGRETS 
 In Australia, three arboviruses have been isolated from Argas robertsi ticks that 
were collected in cattle egret heronries, namely Kao Shaun (Doherty et al 1976), Lake 
Clarendon virus, (St George et al 1984) and CSIRO 1499 (unidentified). Lake 
Clarendon virus was isolated from pools of 8 adult ticks or nymphs. It is an icosohedral 
virus 65-75 nm in size.  Neutralising antibodies to this virus were found in adults and 
chicks in the same colony. Humphrey-Smith et al (1986) reported links with seabirds 
for CSIRO 1499 virus.Quaranfil and Nyamnini viruses have been isolated from cattle 
egrets. Nyamnini virus was first isolated from cattle egrets in the Sudan in 1957 (Haig 
et al 1965) and subsequently Nigeria, Egypt, India and Thailand. It thus infects both 
subspecies. It has also been isolated from Argas walkerae ticks. (Mihindukulasuriya et 
al 2009) 
Experimental infections 
The experimental infections of cattle egrets were described by Standfast et al 
(1986).Many egret chicks are pushed out of their nests by other chicks. They would 
normally starve or be killed by predators.  Young egrets collected below heronries 
without tick infestations, near Brisbane, were infected with a range of arboviruses. 
Four bird associated viruses Thimiri (Simbu group) Lake Clarendon, Kununnura and 
CSIRO 1499 viruses were reisolated from blood 1-19 days post-inoculation. Some 
multiplication of virus as judged by the generation of neutralising antibodies occurred 
18 days after infection with BTV1 and EHDV 5. Only Thimiri and Lake Clarendon 
viruses were reisolated from Argas robertsi ticks infected experimentally.  
Four species of Ixodid ticks were infected with BTV8 by Van Bouwknegt et al 
(2010). The virus passed transovarially in the soft tick Ornithodorus savignyi. 
 
TICKS ASSOCIATED WITH CATTLE EGRETS 
 Amblyomma variegatum is an African origin tick that is considered to be spread 
from island to island in the Caribbean by cattle egrets (Barre et al 1995). Argas 
robertsi occurs in Thailand. It seems to be a relatively recent arrival in Australia as it is 
still spreading. Only one of four egret colonies near Brisbane colonised by Ar. robertsi 
(Standfast et al 1986). 
 
DISCUSSION 
  The two subspecies of cattle egret made a massive expansion to three 
continents in the 20th century. Presumably individual birds had visited in the past but 
the local conditions were unsuitable for establishment. A major change in   was 
development of large herds of cattle in North and South America and Australia where 
formerly populations of large ruminants were low or non-existent.  It is more difficult to 
explain why the small egret population that was confined to a southwest strip of Spain 
and Portugal has invaded much of Europe, at least for summer months. 
 The association between cattle egrets and tick species bearing arboviruses is 
strong, as this bird is capable of spreading both. The adult Argas species ticks spend 
a very short time feeding on chicks or adults at night but larval ticks can be carried 
long distances in egret migrations. This has the potential to spread arboviruses within 
the tick larvae and, if the ticks establish, for the viruses to persist. At least five 
arboviruses have been isolated from egrets or ticks associated with them, namely, 
Quaranfil, Nyamnini, Kao Shaun, Lake Clarendon and CSIRO 1499 (unidentified). A 
new taxon has been proposed for Nyamnini and Midway viruses by Mihindukulasuriya 
et al (2009). The taxonomic status of other viruses is not finalised. 
Egrets arrived in the USA in 1941. Bluetongue as a new entity was first 
diagnosed in Texas in July 1948. Only about 20 sheep were found to have the disease 
at that time. The disease reappeared on a much larger scale in June 1951 (Cox 1954). 
Each of the three bluetongue epidemics originated along the Rio Grande and began in 
May or June. No cattle developed clinical bluetongue. The main proven Culicoides 
vector of bluetongue in the United States is C. sonorensis (formerly C. variipennis) and 
a second vector species in Florida (C. insignis) has a subtropical distribution (Gibbs et 
al 1989). Gibbs suggested that the possibility of BTV vectors being blown across the 
Atlantic to the West Indies could not be excluded though the transfer would take 6 
days.                    
 
Stott et al (1985) have shown that American bluetongue virus can multiply in 
Ornithodorus coriaceus.  Bouwknegt et al (2010) have raised the potential for ticks to 
account for the over-wintering of BTV in Europe. This leaves open the link between 
birds and cattle or sheep. There is a very close association between cattle egrets and 
cattle but the tick infestations are not usually shared. There are species of Culicoides 
that will feed on birds and cattle to provide a link for lateral spread. Standfast et al 
(1956) collected 12 species of mosquitoes and 8 species of Culicoides with light traps 
that were set close to cattle egret nests in trees (Table 1).  The adult ticks, larvae, 
mosquitoes and Culicoides can be feeding simultaneously, especially on the poorly 
feathered young chicks. Thus virus can be injected into the bloodstream or be present 
in the lymph and available to other blood feeding arthropods without any lag time for 
multiplication. 
 
Table 1 Species of insects in Egret heronry 
 
 Culicoides   Hosts 
C. austropalpalis  Birds 
C. brevitarsis  Cattle & horses 
C. bundyensis  Cattle & macropods 
C. dycei   Cattle, horses, rabbits, sheep.  Birds unlikely. 
C. marksi   General mammalian feeder, birds unlikely 
C. narrabeenensis  Not known, rare 
C. victoriae   General mammalian feeder, birds unlikely 
C. “Willi Willi”  Not known, rare 
 
Mosquitoes 
 
Anopheles annulipes General mammalian, probably occasional bird 
Aedes vittiger  Anything that moves 
Ae. theobaldi  General mammalian, probably occasional bird 
Coquillettidia xanthogaster General mammalian, probably occasional bird 
Culex annulirostris Cattle and birds 
Cx. australicus  Birds 
Cx. bitaeniorhynchus Cattle and birds 
Cx. orbostiensis  General mammalian, probably occasional bird 
Mansonia uniformis General mammalian, probably occasional bird 
Mimomyia elegans Birds & frogs 
Uranotaenia nivipes Frogs 
Aedeomyia catasticta Birds 
 
 Such a transfer of arboviruses has been demonstrated experimentally by two 
groups. Jones et al (1984) used ticks (Rhipicephalus appendiculatus) and Thogoto 
virus and guinea pigs. Thogoto virus was first isolated from ticks in Africa (Haig et al 
1965) and has been associated with cattle egrets. McGee et al (2009) were able to 
transfer West Nile virus from infected to uninfected mosquitoes by feeding them 
simultaneously on mice. Infection occurred at a maximum spacing of 40mm and 
persisted for up to 45 minutes after a donor feed. BTV is abundant in the dermis of the 
skin during experimental disease and thus available in blood and lymph flowing into 
the wound produced by a tick or a Culicoides. ( Darpel et al 2012) 
The discovery of bluetongue (BTV8) in Belgium and Holland in August 2006 
without a spread from adjacent countries to the south created a mystery. The virus 
was unusual in that it killed cattle and its closest antigenic relatives were south of the 
Sahara. BTV can multiply in egrets (Standfast, unpublished data). More recently, 
Schmallenberg virus in the Simbu group has arrived in approximately the same region 
of Europe (Hoffmann et al 2012). A Simbu group virus (Thimiri virus) that exists in a 
wide range from Africa, through Asia to Australia was shown to multiply in cattle egret 
chicks (Standfast et al 1986). It was first isolated in Australia from a bird associated 
species of Culicoides (C. histrio Standfast  et al 1984).   
It is possible for BTV8 and Schmallenberg viruses to have arrived in egrets, or in 
ticks or larvae associated with them, migrating from central Africa. The cattle egret is 
capable of crossing oceans or deserts and could be a factor in the long distance 
movements of arboviruses and other pathogens. Virus transfer, with or without a 
viraemia in the cattle egrets, from a tick or tick larvae to a flying vector on arrival from 
Africa at night when the birds are resting or nesting is possible. During the day, the 
very close proximity of cattle egrets to cattle, different species of vector may be 
involved to transfer arboviruses. 
 
  
References  
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role of ticks as vectors of bluetongue virus.  Exp Appl Acarol 52 183-192 
 
Cox HR (1954) Bluetongue Bacteriol Rev 18 239- 253 
 
Darpel KE et al (2012) Involvement of the skin during bluetongue virus infection and 
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The occupation of an ecological niche by cattle egrets

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The occupation of an ecological niche by cattle egrets

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