Occurrence: Worldwide, many herds free.
Age affected: All ages.
Causes: Pig louse - Haematopinus suis.
Effects: Irritation, hair loss, anaemia, disease spread.
The pig louse Haematopinus suis is a large, yellowish-brown louse (5 mm) which moves about amongst hairs on the pig's skin and may be seen most readily on white pigs. It occurs only on the pig and is most common on the folds of skin of the neck or around the base of the ears, inside the ears, on the insides of the legs and on the flanks. Eggs are laid on the bristles and appear as a yellow crust in heavily infected areas and are particularly obvious in black pigs. The eggs hatch into nymphs in 12-20 days. Following two further nymphal stages, the life cycle is completed in 29-33 days. The lice can only live away from the host for 2-3 days.
In severe infestations damage to the skin is caused by the constant irritation and itching wich results in scratching and rubbing against gates and rails. Localized ulceration is found inside the pinna of the ears where lice congregate to feed. Large numbers of lice may cause restlessness, a reduced growth rate and, eventually, anaemia. The pig louse may act as a vector for the swine pox virus, African Swine Fever virus and Mycoplasma (Eperythrozoon) suis and has been found to be infected by Borella burgdorferi.
Mode of transmission
Transmission is usually from pig to pig by skin contact. It is possible that lice may reach a new host after rubbing on posts or rails or by movement from one pig to another in bedding, as they can be quite active, but their short survival time off the host (2-3 days), makes this less likely. As the eggs are fixed to hairs, they, too, are unlikely to be transferred passively. Infestation is usually introduced to a new farm on domestic pigs, but contact with wild boar/feral pigs may also be a source.
Pigs with louse infestations may be seen to rub or scratch constantly. Their rubbing may be sufficiently severe to cause long scratch on the flanks and the bristles on the flank are often broken or rubbed smooth. Infested pigs may lose condition. The lice may be seen scuttling about on the skin of white pigs but are less obvious on dark-skinned pigs. On these, the pale egg cases may be the most obvious feature, and may be found attached to hairs on and behind the ear and on the neck. When the ears are examined closely, small ring-shaped areas may be seen inside the pinna (ear flap). Groups of lice may be seen feeding at these and larvae and nymphs may also be present. In bright sunlight the lice tend to spread out over the body and may be found near the feet or inside the ears.
Louse infestation is suggested by the presence of scratches on the flanks of sows in poor condition. The hair shafts are broken. Confirmation is by finding the lice. Lice may be seen as brown spots moving across the skin and may be seen within the ears. Egg cases can be found most easily on hairs behind the ears or on the ears, but finding them does not confirm that presence of louse infestation as they may be empty or remain from a period prior to successful treatment. If lice cannot be found easily, examine the feet and inside the ears, as they may be hiding on the extremities. Re-examination at a different time of day may yield results.
Lice - Haematopinus Suis.
Photo credit: Paul Leroy - Mites-and-Parasites.org.
Louse infection is rarely a cause of death. Post mortem evidence of infection may be provided by finding the lice on the carcase in the areas described above, but they may have left the carcase as it cools. Egg cases will still be present. The rubbed and shortened hairs, the thickened skin on the sides of the neck and the reddened feeding rings also suggest the past presence of lice.
Treatment and prevention
Treatment has been carried out using a wide variety of insecticides, but those most commonly used at present are the avermectins. Compounds such as phosmet and amitraz are effective, but are not currently licensed in the EU. Ivermectin can be given by subcutaneous injection and orally. Doramectin, (and amitraz spray and phosmet pour-on) can be used to eliminate lice from affected animals. Control can be carried out by the routine treatment of all sows entering farrowing accommodation, boar treatment every 4 weeks and weaner and grower treatment at weaning and during the finishing period together with mange control programmes. Eradication is relatively straightforward and is the preferred option. A whole herd treatment by injection or in feed (ivermectin) should be followed by a second course 18-21 days later. Breeding stock from infected herds should be treated on entry to isolation, treated 18-21 days later, then brought into the herd 7 days after the second treatment. Similar programmes can be carried out using the other agents.