The Australasian Society of Clinical Immunology and Allergy

By Dr Robert Heddle 

Global Perspective

Life threatening allergic reactions to ant stings are a major public health problem in only two areas of the world. In southern United States the fire ant (Solenopsis invicta) is a major problem. The other problem area is south-eastern Australia, where there is increasing recognition of ant stings as a major cause of anaphylaxis and where the aggressive "jack jumper ant", "hopper ant", "jumper ant" (Myrmecia pilosula) appears to be the major culprit (photograph). 

Characteristics of the Myrmecia genus

The various Myrmecia are members of the hymenoptera order and therefore related to other stinging insects such as honey bees and wasps. They are perhaps the most primitive ants that survive. Although there is fossil evidence of a wider distribution in the past, the 89 species of Myrmecia are confined to Australia and New Caledonia.

Myrmecia pilosula is abundant in Tasmania but also occurs in bush land and semi-rural areas in Victoria, New South Wales and the ACT, the south east of South Australia and at lower latitudes in sandy soiled high country, and is a notable problem in the Adelaide Hills. There is also a very similar ant in the Darling range in Western Australia which appears to be related.

The ant is 10-12mm long, jet black, but with yellow/orange limb extremities and pincers.
It has a characteristic hopping motion. Nests are highly variable but are characterised by a pile of finely granular gravel near the nest. This may be a mound of 20-60cm in diameter but may also be inconspicuous such as under a rock.

Myrmecia pilosula is carnivorous and a scavenger. It grasps its victim with its pincers and stings with a modified ovipositor, analogous to that of other hymenoptera such as honey bees and wasps. It is an important part of the food chain for small mammals including the echidna.

There are many other Myrmecia in wide spread distribution and some of the larger species are colloquially known as "inch ants" and "bull ants" and will sting if provoked. Anaphylaxis to these stings has rarely been described. 

Recognition of medical importance of reactions to Myrmecia stings

The late Dr Paul Clarke first drew medical attention to the problem of M. pilosula stings in 1986 (Clarke, 1986). The Australasian Society of Clinical Immunology and Allergy (ASCIA) ran a register of reactions to ant stings in 1989 - 1994 and recorded 454 sting episodes in 224 subjects with just over 50% of these events involving reactions with respiratory and/or hypotensive involvement (Weiner, Baldo et al. 1995).

More recently, 600 residents of rural and semi-rural Victoria were studied with a random postal questionnaire (Douglas, Weiner et al. 1998). About 2/3 of recipients responded and 2.9% reported systemic reactions but less than half recalled receiving treatment with adrenaline. Confirmatory serological studies were not performed as part of that study but serological evidence from other sting victims in south-eastern Australia would suggest that at least 90% of the ant venom allergy in south-eastern Australia is caused by Myrmecia pilosula. There have been 3 deaths in Tasmania in the last 6 years attributed to allergic reactions to stings of Myrmecia pilosula. 

Myrmecia pilosula venom

The allergens of Myrmecia pilosula have been extensively characterised and appear to have no equivalent in the venoms of other hymenoptera including other members yet studied of the Myrmecia. At this stage, three distinct allergens have been identified, Myr p1, Myr p2 and Myr p3. Other components of the venom have been recognised including histamine, a heat sensitive haemolytic factor and an eicosanoid releasing factor. 

What we don't know about medical aspects of Myrmecia pilosula and other Myrmecia

The natural history of stinging and allergy is not known. In Clarke's original report (Clarke, 1986) of those reporting any systemic reaction to a sting, half had subsequently experienced an equally or more severe reaction to further stings but this type of study is likely to have significant reporting bias. It is likely that a trial will proceed in the near future involving skin testing and immunotherapy with Myrmecia pilosula venom. 

The management of individuals with history of anaphylaxis and other severe allergic reactions to Myrmecia stings 

Avoidance. Owing to the limited distribution of M. pilosula some individuals choose to move to areas where the species does not occur. Limited protection may be obtained by having nests destroyed.

Protective measures. Unfortunately, Myrmecia pilosula is able to sting through thick clothing but partial protection can be obtained by wearing heavy footwear, clothing and using gloves.

Prophylactic medication. Myrmecia pilosula are active during the warmer months, e.g. October - April and there may be limited and partial protection obtained by prophylactic use of H1 antagonists over that season.

Emergency treatment. Subjects with a history of systemic allergic reaction to ant stings should be provided with adrenaline for injection (e.g. Epipen device) and an emergency action plan and a Medic Alert bracelet should be considered. The plan should include making immediate arrangements for emergency medical care. A means of summoning emergency assistance, such as carrying a mobile phone, should also be considered.

Referral. Individuals with a history of anaphylaxis to insect stings should be referred to a specialist in Allergy and Clinical Immunology for further management. 

Practice Points 

1. Jumper ants are distributed in Tasmania, rural Victoria, NSW, ACT and the South East of South Australia.
2. Stinging ant allergy is relatively common in endemic areas and may result in anaphylaxis and even death.
3. There is no current supply of Myrmecia pilosula venom for skin testing or immunotherapy.
4. The mainstay of therapy is avoidance and secondary treatment of reactions with adrenaline and antihistamines. 

References: 

• Clarke, P.S. (1986). Med. J. Aust., 145(11-12): 564-566.
• Douglas, R., Weiner, J.M., et al., (1998). J. Allergy Clin. Immunol., 101(1): 129-131.
• Harvey, P., Sperber, S. et al. (1984). Med. J. Aust., 140: 209-211.