Cryptosporidia are protozoan parasites capable of causing diarrhoea in a very wide range of animals. Usually different species or strains of Cryptosporidia affect different species of animals, but there is some overlap, and the most common species in calves, Cryptosporidium parvum, is zoonotic, that is, it can be transmitted from animals to humans.
Humans with a healthy immune system generally recover from diarrhoea caused by Cryptosporidium, but in people with weakened immune systems, such as those with AIDS or other serious chronic illness, the disease can become chronic and life-threatening.
The method of exposure of humans who have become infected with Cryptosporidium is often unclear. Knowing that Cryptosporidium can cause scouring in dairy calves, we developed a study to see if we could demonstrate transmission of Cryptosporidium from dairy calves to dairy workers, and from them into the general community.
It was also hoped that the study would reveal the main risk factors involved in calves developing diarrhoea due to Cryptosporidium.
Veterinarians from the Livestock Health and Pest Authority (LHPA) visited 12 dairy farms in the upper Hunter Valley and 8 farms around Tamworth, and took rectal samples from 10 calves under 2 months old on each farm. Farmers were asked to fill in a questionnaire about their management of their calves, how they were fed and kept, their water sources, how often they were handled, whether they washed their hands after handling calves before eating or smoking etc, and whether they drank fresh (unpasteurized) milk. They were also asked whether they had visited swimming pools or child care centres, locations that have been identified as risk factors for human infection in the past.
If Cryptosporidium was detected in any of the 10 calves sampled on a farm, the people who had been in contact with those calves would then also provide a faecal sample for analysis.
All faecal samples were sent to Murdoch University in Western Australia, which at that time had the only laboratory in Australia that was offering PCR testing that detects extremely small quantities of Cryptosporidium DNA, and had the techniques available to separate and identify the different species of Cryptosporidium and their sub-types.
Results were then statistically analysed to try to detect any association between management practices and prevalence of Cryptosporidium and diarrhoea.
Infection of calves and risk factors
Three species of Cryptosporidium were found in the calves, the majority being C. parvum only (60.3%) followed by C. bovis (21.3%) and C. ryanae (9.9%). Mixed infections were found in 8.5% of calves. The overall prevalence of the parasite in calves was unexpectedly high, with Cryptosporidium being detected in 73% of calves sampled, although only 38% were actually scouring at the time of sample collection. On individual farms the number of sampled calves that were infected varied from 30% to 100%.
The method used to test for Cryptosporidium in this study picks up very small amounts of DNA. This may give a higher reading of infection than some other types of tests. This test does not give any accurate indication of how many of the parasites calves have been exposed to, and apparently some had received only low doses that were not sufficient to cause scouring.
All participating dairy herds practised year round calving, which may help maintain a background level of infection in the environment. Continuous exposure of cows may also tend to produce colostrum that will protect against Cryptosporidium and so result in a lower level of scouring in their calves.
The situation may be different in beef herds that usually practise seasonal calving, where the level of exposure is likely to be higher in the calving season than at other times of the year. However, it is more difficult to obtain samples from beef calves, as a farmer will not want to muster a beef herd while the cows are calving unless there is a serious problem.
Because the presence of the parasite was so common, it was difficult to determine factors that increased the likelihood of infection.
Time before calf removal in our study ranged from 0.5 to 7 days (mean 2.1). This did not appear to affect incidence of Cryptosporidium positive calves on farm. This suggests that Cryptosporidium exposure if from the dam occurs almost immediately, and/or exposure occurs in the calf's general environment.
No association was established between feeds provided, (whole milk, colostrum, milk replacer, pellets, hay or grain), rate of feed changes (immediate, over several days or over a week or more), housing (individual, paddocks, pens or chains) and herd prevalence of Cryptosporidium or the number of calves scouring.
The water source (well, tanks, dam, bore, natural waterway, other), regularity of cleaning water systems (daily, weekly, when required), water access (piped to communal trough or drunk direct from source) did not appear to affect farm prevalence of Cryptosporidium or number of calves scouring. The two farms that reported that there was an opportunity for cattle to foul the water source with dung did not have a higher prevalence of scouring.
Chances for human infection
Most interviewees had daily or twice daily contact with calves. Only in one case was a child involved in regular contact. The majority of families interviewed drank unpasteurised milk, with 14 families drinking more than 5 litres per week. Three farms reported persons who worked with cattle that also smoked cigarettes. Of the persons that smoked cigarettes two out of three reported not washing their hands after smoking. Two interviewees reported that they ate on the job. It therefore appears that there is ample opportunity for workers to become contaminated with Cryptosporidium, if hygiene is not observed.
Two owners reported attending a swimming pool in the last month, and two had at least one family member attend a child care centre. These are locations where Cryptosporidium problems have been thought to 'originate' when the NSW Public Health Unit has investigated outbreaks in the past.
Foxes, kangaroos and hares were reported in the paddocks where cows grazed, but their potential to spread Cryptosporidium is not established. As discussed earlier, Cryptosporidium strains are normally species specific, or affect only a few species.
The overall detection of Cryptosporidium in humans was 23.8%. Of 14 samples where the DNA sequence was successfully worked out, 12 were C. parvum , and two were C. bovis. This is the first report of C.bovis in humans. It has previously only been described in cattle and sheep. However only 4 of the 15 humans infected with Cryptosporidium reported diarrhoea. All four were infected with C.parvum. The two infected with C. Bovis had no symptoms, even though one was only three years old, which may have made him more susceptible to illness.
The most common sub-type of C. parvum, (the IIa sub-type) found in 69% of calves sampled was also found in 6 of the 7 sub-types in humans that were successfully sub-typed. On the other hand the IId sub-type was isolated from two humans on the same farm, but not from calves on that farm, or any of the other farms. This sub-type is rare in cattle in other studies overseas, and has never been found in cattle in NSW. This is the first report of this sub-type in NSW. It would therefore appear that although zoonotic transmission may be occurring with some sub-types, it is likely that it does not account for all the cases of C. parvum found in humans, and that human to human transmission is also occurring.
This study reveals that Cryptosporidium is very common on dairy farms in the study area, and the potential is high for all calves born to become infected. The fact that the same sub-type of C. parvum is the most common sub-type in both calves and humans suggests that there is a strong chance that humans working on dairies are being infected by their exposure to calves. The chance of infection is increased by some workers taking insufficient care with hygiene when handling calves.
Exposed dairy personnel can then become a means of spreading infection to other humans, as they visit public places such as child care centres and schools.
It is important that farmers are aware of this potential for infection with a zoonotic parasite, so that they can take the appropriate steps to avoid catching and spreading this disease.
The support of the McGarvie-Smith Institute in providing the funding to make this research possible is gratefully acknowledged.