Milky scours in calves are caused by unicellular (single celled) protozoan parasites of the genotype Cryptosoridium.
Milky scours is just one of the causes of debilitating neonatal diarrhoea of calves aged between 4 days and 6 weeks. It is common in calf rearing facilities that practise year-round calving. The overall prevalence of infection is put at about 75% but not all calves show signs of scouring.
Figure 1. Immunofluorescence image of Cryptosporidium parvum oocysts, purified from faecal material. Image courtesy of H.D.A. Lindquist EPA Wikimedia Commons
Cryptosoridium parvum is a zoonotic disease, that is, it can be transmitted from animals to humans.
Workers in calf rearing facilities are advised to take extra care with hygiene to avoid infection, as the oocysts in the dung are immediately infective when shed.
Cryptosoridium parvum primarily inhabits the posterior small intestine down to the distal colon.
Scours are profuse and creamy-yellow in colour but non-haemorrhagic (without blood).
The consistency varies from mucoid to watery and often contains flecks of fibrin or sloughed off mucosa from the wall of the gut. Calves become depressed a few days following birth with watery diarrhoea starting 24 hours later. A mild fever, dehydration, anorexia and weight loss can develop but the disease is self-limiting as exposure confers immunity. However, the disease can cause deaths if neonatal calves are infected before their immune system can respond.
Cryptosoridium in the posterior intestine excrete infective oocysts (type of egg) containing infective stages into the dung. Some oocysts excyst (break open) during passage out with the dung and consequently autoinfection (infection cycling within the one host) is possible. Unlike Eimeria oocysts, the oocysts of ‘crypto’ are infective when shed and do not require a period of development in the environment.
Oocysts can remain viable for months under cool, moist conditions. While contamination occurs mainly through drinking water contaminated with dung, immediate reinfection and autoinfection is possible. When ingested by calves, or people, oocysts excyst and release the viable infective stages which proliferate at the surface of the epithelial cells that line the lower half of the small intestine.
The pre-patent period (the time interval between ingestion of infective oocysts and release of the next generation of oocysts in the dung) ranges from 2 to 7 days. Oocysts are shed for about 3-12 days starting two to three days after the onset of diarrhoea.
Diagnosis is not simple and is based on clinical signs and confirmed on laboratory examination of faecal smears, or increasingly with DNA techniques. Histological examination is usually performed on post mortem material.
Cryptosporidia need to be differentiated from enteric viral and bacterial infections such as Salmonella, E. coli and Campylobacter. Combined infections are uncommon but when they do occur together, the diarrhoea is severe.
Affected calves should be isolated and given fluid replacement therapy to alleviate dehydration. Most clinically affected calves recover spontaneously. Few drugs are very active against clinical infections, but veterinary assistance for diagnosis anthelmintic treatment should be sought with severe outbreaks.
Maintenance of high standards of hygiene and adequate nutrition is important to control environmental contamination. All newborn calves should receive colostrum for the early development of immunity.