Theileria are protozoal blood parasites transmitted between livestock by ticks (Radostits et al., 2007). Theileriosis has a similar pathogenesis to tick fevers caused by other closely related protozoa such as Babesia and Anaplasma; however, Theileriosis is generally considered a benign disease in Australia. This is in contrast to the Theileria spp. present overseas, which can cause significant disease and high mortalities. Theileria annulata causes Mediterranean Coast Fever or Tropical Theileriosis, a severe and debilitating disease in susceptible cattle where mortalities can be has great as 90% in introduced stock; and Theileria parva causes East Coast Fever, an even more serious and highly fatal disease, occurring in susceptible African cattle (Radostits et al., 2007).
In Australia, Theileria are transmitted to cattle by Haemaphysalis ticks. H. humerosa and H. bancrofti are proven tick vectors of Theileriosis which are found on wildlife such as bandicoots and wallabies (Stewart et al., 1992). The Australian variety of Haemaphysalis longicornis (the bush tick or NZ cattle tick) is widely believed to be capable of transmitting Theileria, however, this is yet to be proven experimentally (Stewart et al., 1992). Meanwhile, the Japanese strain of H. longicornis is known to be capable of transmitting Theileria (Fujisaki, 1991). Once ingested by the ticks, Theileria form sporozoites which are injected into the mammalian host with the tick saliva when it feeds again. Theileria then parasitise the red and white blood cells, forming schizonts in leukocytes and merozoites in erythrocytes (Radostits et al., 2007). This parasitism, combined with oxidative bursts from macrophages damages the host erythrocytes, with haemolysis the likely cause of the ensuing anaemia (Radostits et al., 2007).
Theileria vary in pathogenicity, dependent on the species and strain. Morphologically, Australian species of Theileria belong to the T. buffeli/sergenti/orientalis group and historically, were able to parasitise the red blood cells of cattle without causing significant disease (Uilenberg, 1985). Theileriosis in Australia has been documented as a mild to moderate anaemia in animals with significant parasitaemia, usually with no mortalities (Radostits et al., 2007). Cases of Theileriosis have occurred only occasionally and sporadically in the past 50 years and clinical signs are usually mild; these clinical cases were usually attributed to a failure of host immunity rather than an increase in pathogenicity of Theileria (Stewart et al., 1996).
However, since late 2006 outbreaks of Theileriosis have been occurring along Coastal NSW and regular cases are now reported, especially in the Mid North Coast and Hunter regions. A few cases have also occurred on the South Coast of NSW including in the Bega Valley. Outbreaks are usually associated with a history of introductions to the herd, and it has been postulated that this increase in clinical cases could be due to either importation of an exotic pathogenic strain from Japan, increased movement of stock from naive to endemic areas or mutation of Theileria into a more pathogenic organism (Izzo et al., 2010). Clinically, these cases presented with lethargy, depression, inappetance, pyrexia, jaundice, anaemia, tachypnoea, tachycardia, decreased milk production, abortion of late term calves, stillborn or weak calves or increased incidence of periparturient diseases such as metritis, ketosis or displaced abomasums (M. Izzo, 2009, pers. comm., 5 June). Most cases were seen in Coastal areas in heavily pregnant, recently introduced cattle.
Blood samples from clinical cases were sent to Japan for Theileria strain typing and the pathogenic Japanese 'Ikeda' strain of T. buffeli was found to be present, often in conjunction with the other non-pathogenic strains (M. Izzo, 2009, pers. comm., 5 June). This is the first time this strain has been identified in Australia, and it is likely that the importation of this pathogenic strain, combined with increased stock movement into endemic areas, is the reason for the marked increase in clinical cases of Theileriosis. It is not known if any of the isolates from recent outbreaks in the South Coast have been associated with the pathogenic 'Ikeda' strain.
The aim of this study was to investigate the herd and individual animal prevalence of Theileria infected animals in dairy and beef herds located within the Bega Valley. Prior to this study Theileria parasites had only occasionally been found as an incidental finding in a few herds and there was essentially no local knowledge of the prevalence of infection in this area. Around the time this study was commenced, there had been two cases of clinical disease associated with Theileria infection diagnosed in the valley. The impact of specific predictors (tick history, reproductive status, age and herd type) on the presence of Theileria in blood smears was examined in the study. The effect of Theileria presence on infected animals packed red cell volume (PCV) was also determined.
EDTA blood samples were collected from a conveniently selected group (usually consisting of 10 cows) ranging from 2 to 12 years of age, from each of 21 convenience-selected herds located predominantly within the Bega Valley. One herd was sampled in the Bodalla area. This sampling occurred between 10 July 2008 and 5 November 2008, with most sampling occurring in July, August and September. Subjects for sampling were all considered healthy at the time of sampling and were not selected based on the presence of clinical disease. These blood samples were then submitted to determine the PCV of the animal and for examination of a blood smear to look for the presence of Theileria. Smears and examination for parasites were conducted at Sydney University, Camden Campus. PCV data was largely generated through microhaematocrits conducted at the Bega Veterinary Practice, although some were performed at Sydney University. Data was also collected on the breed, age and reproductive status of each animal, liver fluke drenching history and also whether the owner/manager saw bush ticks on cattle from the property. The vaginal mucosae of the cattle were examined, as a clinical indicator of anaemia. Ticks were also looked for on the escutcheon of the cows as they were being bled from the tail. Funding for laboratory work was provided by NSW DPIs special surveillance funding.
To assess the effect of putative predictors on the presence of Theileria, each explanatory variable was analysed using univariable logistic regression with Theileria-positive (or negative) as the binary outcome variable. Twenty-one properties were sampled therefore 'property' was included as a random variable in the analysis. Predictive variables examined were: cow reproductive status (pregnant or empty, lactating or dry); age; herd type (dairy or beef) and; tick history of the property. Tick history was given a 1 or 0, based on whether farmers recalled seeing ticks on their animals (cattle with ticks did not have to be identified on the day of sampling to score a 1). The predictive variables with a significant effect on the presence of Theileria (p < 0.001) were then incorporated into a multivariable logistic regression model with property included as a random effect.
The effect of the presence of a Theileria-positive blood smear and herd type (dairy or beef) on PCV were assessed using a linear mixed model. Property was included in the model as a random effect, and liver fluke treatment was accounted for as a possible source of variation in PCV, because since fluke suck blood, cattle routinely treated may have higher PCVs. A two-sample T test was also used to determine if vaginal mucous membrane colour was a clinically accurate indicator of the actual PCV of the animals.
Of the 21 properties sampled in the survey, 7 of these were beef properties. 19 of 21 properties had at least one animal infected with Theileria on blood smear. 194 animals were sampled altogether and on an individual basis, 78 animals were Theileria-positive on blood sampling - this equated to 40.2% of the total animals sampled. The Theileria burden identified on blood smears was quite low. Of the animals sampled, 25% had very occasional Theileria (1 per 25-50 high powered field (hpf)), 8.3% had occasional Theileria (1 per 5-10 hpf), 5.2% had moderate numbers (1 per hpf) and 1.5% had frequent Theileria (1-2 per hpf). The only predictor with a significant effect on whether Theileria were present on blood smear was the tick history of the property, with an odds ratio of 4.7 and a p value of < 0.001 (Table 1). Ticks were observed at the time of blood collection on three properties. These have been identified as H. longicornis at Orange Agricultural Institute. The highest animal prevalence of Theileria infection was observed on these farms with 10/10, 8/10 and 7/10 animals found with Theileria present on smears.
Other suspected predictors such as pregnancy, lactation and age did not have a statistically significant effect, with p-values of 0.99, 0.51 and 0.38 respectively; however, they may be observed to have a greater effect in a larger sample and with more variation in age and reproductive status.
The presence of Theileria on blood smear (Theileria = 1) and Herd Type (dairy or beef) both had a significant effect on PCV, with p-values of 0.047 and 0.002 respectively (Table 2).
On average, the predicted mean PCV for Dairy herds was 3.5% lower than the predicted mean PCV for beef herds (Table 3), which was also statistically significant with a p-value of 0.002.
|Herd Type||Predicted Mean PCV|
Additionally, the predicted mean PCV for Theileria-positive animals was 1.1% lower than the predicted mean PCV for Theileria-negative animals (Table 4), with the analysis taking into account variations in PCV due to property and herd type. Additionally, 5% of infected animals were defined as clinically anaemic (<24% PCV).
|Blood Smear Result||Predicted Mean PCV|
The two sample T-test to compare the mean PCV for animals with normal coloured mucous membranes and those with pale mucous membranes yielded a p value of 0.211. Therefore, the mean PCV for normal mucous membranes and the mean PCV for pale mucous membranes are not significantly different in this population and with these observers.
Although the Theileria burdens were quite low on blood smear, the infection was widely distributed throughout the Bega Valley. The prevalence of Theileria infection in the Bega Valley is high, with 90.5% (77.9-100% CI 95%) of sampled herds, and 40% of individual animals infected with Theileria. This presents a significant potential disease risk to introduced animals, since the widespread distribution of the infection in the Bega Valley renders the exposure of susceptible animals highly likely. Animals that appear to be at particular risk of succumbing to Theileriosis are heavily pregnant cattle introduced from inland areas where they have little exposure to ticks.
The tick history of the property was the only factor that had a significant effect on increasing the likelihood that Theileria would be diagnosed on blood smear. This is the first study in Australia to suggest a link between the presence of H. longicornis and Theileria infection of cattle. A farm with ticks was 4.7 times more likely to have cattle infected with Theileria, which is a substantial increase in the odds of infection. Of course, this tick data is not an accurate reflection of the actual tick burdens on the animals. It is only a rough means of comparing the infection levels of animals that come from properties where ticks have been noticed and those where they haven't. Given the results from this study, an area for further research would be to determine whether a direct relationship exists between the number of ticks actually counted on the cattle at the time of sampling and the number of Theileria on blood smear, or the proportion of animals infected.
Since so many of the recent cases of Theileriosis affected heavily pregnant cattle, we expected that advanced pregnancy would increase the likelihood of Theileria infection on blood smear. However, pregnancy did not have a significant effect on Theileria infection in this study, possibly due to the low number of animals sampled and inaccurate recording of stage of pregnancy.
There was no significant difference between the mean PCV of animals with normal mucous membranes and the mean PCV of those with pale mucous membranes. Therefore, mucous membrane colour does not appear to be an accurate means of estimating the PCV of cattle when the anaemia is not severe. Interestingly, beef animals on average had a 3.5% higher PCV than dairy animals. This is most likely due to the metabolic stress placed on dairy cattle by the extreme demands of lactation and the lower availability of energy and nutrients for maintaining PCV.
Since infected animals on average had a 1.1% lower PCV than non-infected animals, and 5% of infected animals were clinically anaemic, it can be said that Theileria are causing some mild damage to the red blood cells, even if the animals are not overtly unwell. These results are consistent with 'benign Theileriosis' where animals with heavy Theileria burdens have only a mild to moderate anaemia, since the damage to the red blood cells caused by the parasite is not severe. This would indicate that most Bega Valley animals are or have been infected with a benign strain of T. buffeli, and have some immunity to local strains. The subclinical effect of Theileria infection in bush tick-prone areas may be one of the contributing factors for the illthrift often observed in cattle introduced to the coast from inland areas.
Limitations to this study were the small sample size and the restricted area of study and therefore the extent to which these results can be usefully extrapolated to the rest of NSW. However, the study did find that Theileria are widely distributed in the Bega Valley and common. The likelihood of infection is greatly increased by the presence of ticks on the farm and on the cattle. Therefore, tick control is likely important for suppressing outbreaks. Theileria infection resulted in a reduced PCV, indicating that some damage is occurring to parasitised red blood cells, even though in most cases, it is not sufficient to cause overt illness and anaemia. Clinical cases of disease are occurring sporadically on the South Coast and the distribution and location of the new pathogenic strain that has been imported into Australia is currently unknown. Producers should be aware of this disease and the risk factors predisposing animals to infection. Pregnant animals newly introduced to endemic bush tick areas are especially at risk, and should be monitored carefully after introduction for signs of Theileriosis. In herds where introductions are likely it is suggested that tick control in cattle should be implemented and the introductions initially run in paddocks less likely to harbour bush ticks. Producers should avoid introducing tick-infested animals from other coastal areas into their herds, as this can present a disease risk to their own cattle and such introductions should be treated to remove ticks before arrival.