Nature of lesions
Buffalo fly feeding can lead to the development of lesions that are of significant welfare concern. Lesions can range from small hairless areas to scab encrusted or severe open ulcerated sores. They are most common beneath the corners of eyes but are also found on the neck, dewlap, belly, shoulders and flanks of more susceptible cattle. In some northern areas up to 95% of cattle are affected whereas further south, although eye lesions are common, lesions on the body are seen less frequently. Lesions are usually not common in younger cattle but become more frequent and severe in older animals.
In some instances, these lesions are associated with a nematode (Stephanofilaria sp.) transmitted by buffalo fly, which infests the skin. The nematode is not always found, however, and the lesions can also result from a hypersensitive response to the effects of fly feeding, or possibly bacterial infection. Some cattle appear to be ‘allergic’ to buffalo fly bites and are intensely irritated by only a few flies, leading to excessive rubbing that can also result in open lesions.
Figure 1. Buffalo fly lesions on the brisket of an animal. Image courtesy of Jess Morgan
Impact of lesions
The lesions can reduce hide value and the presence of lesions can make cattle less acceptable for market, in particular for the live export market where animals with lesions are generally rejected. The presence of lesions is also said to reduce the price paid for cattle in saleyards. The lesions can penetrate the hide and render affected areas unusable, although the majority are in areas where they can be readily trimmed. Often the key concern of growers is lesions is their appearance, the irritation associated with them, and the welfare issues they represent. The lesions also present a focus for fly feeding (Figure 1), for potential infection and, potentially, for secondary parasites such as screw-worm flies, which represent a major biosecurity risk in northern cattle production areas and which are present in a number of Australia’s nearest northern neighbouring countries.
Stephanofilaria life cycle
Stephanofilaria are small nematode worms, transmitted by buffalo flies and often found associated with buffalo fly lesions. The adult worms occur in the skin, often in the hair follicles about 1 to 2 mm below the epidermis. The adult females are 3.8 - 6.4 mm in length and about 50 - 80 µm in diameter, while the males are somewhat smaller 2.3 - 3.2 mm in length (Figure 2). The microfilariae (the buffalo fly infecting stage) form in the uterus of the female worm and are released onto the skin surface or superficial tissue from where they are ingested by feeding buffalo flies. They develop through 3 larval stages in the fly, which takes 2 - 3 weeks. The 3rd stage larvae then move to the salivary glands of the fly from where they are released onto cattle during buffalo fly feeding. The third stage larvae then develop through two more stages and mature to adults to complete the life cycle.
Figure 2. Adult female Stephanofilaria recovered from a skin lesion. Image courtesy of Muhammad Noman Naseem
Once a lesion has developed on an animal, the likelihood of it reoccurring in subsequent years is high, and typically the lesions get worse over time. This can be a problem if severe lesions occur on young animals. Some cattle producers cull badly affected cattle and claim this has led to reduced genetic susceptibility in their herds.
There are currently no products registered for the treatment of Stephanofilaria in Australia. Treatments focussed on directly controlling Stephanofilaria have had limited success and frequent application of a macrocyclic lactone compound, prescribed by a veterinarian, appears to be the most effective method. In one study, prolonged retreatment with injected abamectin at approximately monthly intervals gave a significant regression in lesion size and oxfendazole appeared to provide some reduction, but single applications of abamectin, levamisole and morantel were shown to be ineffective. Daily treatment with a topically applied macrocyclic lactone or fly repellent can assist healing but is seldom cost efficient except possibly in very high value stock. Overall, implementation of an effective buffalo fly control program and culling severely affected animals appears to be the best method to minimise lesions.