Milk spot liver (roundworm)

Occurrence: Worldwide.
Age affected: Weaners, growers / finishers, human risk.
Causes: Pig roundworm - Ascaris suum.
Effects: Condemnation of livers and lungs, reduced growth and feed conversion, cough, jaundice.

Causes

Milk spot liver mainly results from the migration of larval Ascaris suum worms. The adults occur in the small intestine and may be up to 40 cm long. Infection follows the ingestion of eggs containing second stage larvae. After ingestion, the eggs hatch and the second stage larvae enter the mucosa of the pig caecum within 3 hours of infection and migrate to the liver within 6 hours. There they moult and then migrate to the lungs, reaching them 5-8 days post-infection to enter the tracheobronchial tree after a further moult to fourth stage larvae. They are coughed up and swallowed to mature in the small intestine. Transient lesions develop in the lung and milk spots appear in liver but resolve within 40 days. The pre-patent period (time from infection to production of eggs) is 40-53 days. Immunity can develop but may take some time. In immune pigs larvae may be prevented from penetrating the gut wall or be stopped in the liver or lung. In some cases liver lesions may occur and appear as milk spots at slaughter.  

Mode of transmission

Ascaris suum transmission is indirect. Eggs are shed in the faeces. The development of the egg to the infective second larval stage depends on temperature and relative humidity and takes 2-4 weeks indoors. Infective eggs may remain viable for up to 10 years in suitable conditions and are sticky, adhering to surfaces. Transmission results from the ingestion of these infective eggs and cannot result from the consumption of fresh faeces. Transmission from farm to farm usually results from the movement of infected pigs but can be through contaminated transport, clothing or equipment.

White spots (milk spots) on the liver indicate infection with large roundworms. 
Photo credit: Anja Gruenewald, MSD Animal Health.

Clinical signs

Clinical signs of intestinal infection with A. suum are rarely observed even in heavy infections. Growth rate may be reduced by 2-10% and food conversion by 5-13% following experimental infections and reduction in the digestibility of food has been demonstrated. Occasional animals may develop jaundice and some of these may die. A moist cough, may occur during the migration stage, about a week after infection. The role of ascarids in this cough is not clear, but it can be reproduced experimentally. Affected pigs appear hairy, may develop congested extremities and even die. Coughing is an indicator that liver migration has occurred.

Coughing and an increase in respiratory rate one week following the stocking of a contaminated pen or house may suggest the occurrence of larval migration. Pigs may occasionally develop jaundice which may indicate that the bile ducts are stuffed with adult ascarids. Where patent infections are present, diagnosis can be confirmed by demonstrating the sticky round eggs in faeces.  

Postmortem lesions

Normally, lesions are only incidental findings at post-mortem and at slaughter. They consist of the ‘milk spot liver’ in which whitish fibrous or fleshy lesions up to 1 cm in diameter occur on the liver surface and local petechial haemorrhages (blood spots) in the lung. The lung lesions occur during migration and resemble petechiae but consist of worm tracks filled with debris and the occasional larva. Mild hyperaemia of the small intestinal mucosa may be noted where adult worms are located. Occasionally large clusters of worms plug the intestinal lumen.

Treatment and prevention

Treatment may have little direct effect on milk spot liver as the larvae can often enter the tissues when anthelmintic is not being given and cause the lesions. Adults can be treated and eliminated by using ivermectin, doramectin, cambendazole, dichlorvos, tetramisole, levamisole, febantel, thiophanate, flubendazole, fenbendazole, oxibendazole, piperazine and pyrantel tartrate where registered. The larvae can be treated using ivermectin, doramectin, dichlorvos (L4 only), tetramisole, febantel, thiophanate, flubendazole and fenbendazole. Control is difficult because of the adhesive nature of the egg and its longevity on the ground (up to 5 years). In premises where ascariosis is a continuous problem thorough cleaning of farrowing and finishing quarters with detergent or hot washing soda should be combined with monthly anthelmintic treatment of the sow stock. The use of a horticultural flame gun has given good results in reducing the numbers of ascarid ova on concrete floors and thorough drying may also be of value. Pressure washing without drying may enhance survival. Composting of manure may destroy eggs but requires long periods (30 days at 37°C) or temperatures of 55-60°C and they may be killed by some proprietary ammonia-based disinfectants. Even strenuous efforts may not prevent milk spot completely. 

Special note

Milk Spot Liver is a cause of downgrading or retention of pig liver at slaughter. Occasional infections occur in humans following the consumption of infective eggs.