Mycoplasma ovis (formerly Eperythrozoon ovis) causes the disease known as eperythrozoonosis in sheep flocks throughout Australia. A review of District Veterinarian field investigations involving Mycoplasma ovis on the north-west plains of NSW was conducted by examining the paper property files and computer files (where available) of the former Walgett, Narrabri and Moree Rural Lands Protection Boards (RLPB).
Examination of the Moree files revealed only two cases of eperythrozoonosis, both occurring in the early 1980's. The results for the Walgett and Narrabri RLPBs are summarised in Table 1. As figures were collated from field investigation reports, they are approximations and have not been statistically analysed.
|Number of M. ovis diagnoses||21||17|
|Number of years over which cases were recorded||33||18|
|M. ovis diagnoses per year||0.64||0.94|
|M. ovis diagnoses based on clinical assessment only||25%||75%|
|M. ovis diagnoses confirmed by laboratory assessment||75%||25%|
|Average flock size affected||1800||1100|
|Morbidity Range||1 - 40%||1 - 50%|
|Mortality Range||0 - 5%||0 - 5%|
|Proportion of cases affecting flocks less than 12 months of age||90%||80%|
|Proportion of cases where flocks were shorn, crutched, mulesed or vaccinated within 8 weeks prior to the investigation||60%||50%|
|Cases including anaemia (including pale mucous membranes) as a significant clinical sign||70%||50%|
|Cases including exercise intolerance as a significant clinical sign||60%||50%|
|Cases including ill-thrift as a significant clinical sign||50%||40%|
|Cases where haemonchosis was ruled out as a differential diagnosis||60%||70%|
The clinical course including anaemia, exercise intolerance and ill thrift in north western NSW is similar to that described in the literature (Radostits et al. 2007; Daddow 1979; Sheriff et al. 1966). Although clinical signs usually occur a few weeks after initial infection, parasitaemic episodes can recur every month or so for up to four cycles resulting in a chronic clinical course (Sheriff et al. 1966). District Veterinarian in the Narrabri area for the last 20 years, Shaun Slattery, suggests that the disease is highly reliant upon the host - parasite - environment interaction. Weaners in good condition, without concurrent stress (especially internal parasitism) and on adequate nutrition will withstand the rigors of the parasite more easily than those under stress and malnourished. Mortality outcomes in particular are dependent on this interaction.
Infection is transmitted by any activity that transfers infected erythrocytes from one sheep to another. Indeed the transfer of one infected erythrocyte may be sufficient (Kabay et al. 1991). Management procedures like mulesing, shearing (crutching), marking or vaccination can therefore transmit the disease between animals. Investigations within the region support this mode of transmission as significant. As it is common practice to shear adult sheep first and the lambs last on many properties, it is likely that chronically infected and premune adults are the source of infection for naive youngsters (Sheriff et al. 1966). Biting insect vectors can also be involved in transmission (Sheriff et al. 1966).
It can be difficult to definitively diagnose eperythrozoonosis. Parasitaemia occurs prior to the onset of anaemia and is often absent once clinical signs are apparent (Radostits et al. 2007). Consequently, blood smears can be unreliable as a diagnostic tool. Where blood smears are the only option, at least 10 animals from the flock should be sampled including those not showing signs of disease.
The CFT is likely to be more useful as affected animals give positive reactions on the third day after the onset of clinical signs and then remain positive for about two to three weeks. Chronic carriers of M. ovis are usually negative on this test (Radostits et al. 2007).
The NSW DPI Diagnostic and Analytical Laboratory offer a PCR test to detect M. ovis infection. Although clinical trials have not been conducted to study the detection of M. ovis organisms via PCR throughout the clinical course of disease (M. Hornitzky, EMAI, pers. comm.), in the author's limited experience, the test has proven to be more useful than microscopy in diagnosing eperythrozoonosis. PCR has proven capable of detecting Mycoplasma suis (Eperythrozoon suis) during acute stages of the disease and during latent stages of infection in pigs (Hoelzle et al. 2003).
Previous serological prevalence studies in Australia are summarised in Table 2. Prevalence studies in NSW have not been published, however the results in other sheep zones suggest that farm prevalence is expected to be high.
|Prevalence of farms with M. ovis present in adult sheep||Prevalence of farms with M. ovis present in weaner sheep|
When talking with local producers, many are familiar with eperythrozoonosis and believe that the condition occurs on their property. However, this review found that relatively few cases of the disease have been diagnosed over the last twenty to thirty years. At most, eperythrozoonosis is diagnosed about once per year in each of the Narrabri and Walgett areas. As haemonchosis is also a common cause of anaemia in young sheep in the region, it is possible that the two conditions are often confused.
It is likely that eperythrozoonosis will remain somewhat of an enigma in north western NSW. While producers and veterinarians generally regard it alike to be a common disease, there is very little conclusive evidence to support this view. However, this lack of evidence is possibly due to the difficulty in definitively diagnosing the condition. It is also difficult to gain an understanding of the effect of eperythrozoonosis on the local industry as the resultant ill thrift, exercise intolerance and anaemia are more likely to result in decreased production rather than overt clinical disease.