Ill-thrift

C G STEWART AND G W BURROUGHS

Introduction

Ill-thrift, or unthriftiness as it is often called in South Africa, is an ill-defined condition of young sheep. Affected animals show mild to severe depression of growth rate and anaemia. Morbidity and mortality rates may vary considerably.

The condition has been reported from a number of countries, including Australia,17 New Zealand,⁶ France,¹¹ and Norway.²⁶ In South Africa, the disease occurs in the coastal areas of the Eastern Cape Province.³

Aetiology and epidemiology

Many outbreaks of ill-thrift have been associated with Eperythrozoon ovis infection,^{3–5, 11, 13, 16, 17, 18, 19, 29, 33} but proof that E. ovis is the sole causative agent is lacking.³⁵

The persistence of ill-thrift among lambs and the presence of E. ovis on most properties where several other possible causes, such as deficiencies of copper, cobalt, manganese, zinc and iron, and verminosis have been eliminated^{20, 29} led Sheriff et al.³³ to believe that the majority of outbreaks of ill-thrift are caused primarily by E. ovis. It has been suggested that if E. ovis were to be proved to be associated with ill-thrift, affected properties would have to be shown to have a higher prevalence of E. ovis infections, ill-thrift would have to be reproduced by experimental infection, and a protective effect by specific therapy demonstrated. ³³ At present very few of these criteria have been met.

The fact that E. ovis has been isolated on a number of occasions during experimental inoculation of blood for purposes other than E. ovis transmission suggests that it is a common infection.^{9, 10, 23, 26} Neitz²² found that 15 to 20 per cent of sheep in South Africa are infected, and demonstrated E. ovis in sheep in the Karoo and in the Limpopo Province where ill-thrift does not occur. Similarly, evidence of E. ovis infection has been found in sheep from farms in other countries where ill-thrift does not occur.^{8, 9, 28}

Most lambs inoculated with either strains of E. ovis isolated from cases of ill-thrift, or with other strains of the parasite become subclinically infected. Some develop a mild anaemia,^{10, 14, 15, 17, 21, 30} while only those that develop a prolonged parasitaemia manifest clinical signs,¹² including loss in body weight.⁷ Under laboratory conditions mortality among infected lambs is rare.²⁵

A multifactorial aetiology for ill-thrift has been suggested. Eperythrozoon ovis may enhance the effect of other infectious agents which by themselves are mildly pathogenic, ³³ or perhaps E. ovis takes the upper hand during conditions which are unfavourable for the animal.²⁹ For instance, in one trial on South African farms on which illthrift occurs, the disease was reproduced in sheep by the inoculation of pooled blood specimens obtained from lambs suffering from ill-thrift into lambs originating from an area where ill-thrift does not occur. Eperythrozoon ovis infection was detected in all the affected lambs, but the most severe reaction was manifested by those which were concurrently infected with Anaplasma ovis.³

Neitz and his co-workers²⁴ isolated a number of potentially anaemia-producing pathogens, namely E. ovis, A. ovis, Cytoecetes phagocytophila, Theileria ovis, Borrelia theileri and Ehrlichia ovina from unthrifty lambs in the Eastern Cape Province. Non-splenectomized sheep infected successively over a period of 367 days with E. ovis, C. phagocytophila, A. ovis or E. ovina, showed only mild reactions to E. ovis and,em> C. phagocytophila. However, no significant gains in mass occurred over a period of a year even though the sheep had received a balanced ration. It was concluded that mixed infections with these pathogens were the main contributing factors to the unthrifty state of the animals.

Ill-thrift in Australia has been reported mainly from areas of high rainfall.¹⁶ Similarly, in South Africa ill-thrift occurs in high rainfall areas of the coastal regions of the Port Elizabeth and Humansdorp districts.³ Its prevalence on farms in these districts appears to increase in successive years until virtually all the lambs in a particular flock are affected after five or six years. For information on the transmission and other epidemiological aspects of E. ovis consult Eperythrozoonosis.

Pathogenesis and clinical signs

As the aetiology of ill-thrift and the role of E. ovis in this disease are still not fully understood, little is known about the pathogenesis.

It has been suggested that nutritional and management factors, as well as other infectious agents, may play a contributory role to E. ovis infections.⁵

In South Africa, mixed infections (of particularly E. ovis and A. ovis) appear to be important, as the disease that is produced is more severe than that produced by a single infection of either parasite.²⁴

The syndrome of ill-thrift varies from one characterized by a mild, moderate or severe depression of growth rate with relatively low morbidity and no mortality¹³ to one in which there is severe clinical disease with high morbidity and mortality.^{3, 29, 33}

Lambs generally grow well until they are two or three months of age, after which their growth becomes retarded.³ Affected lambs are listless, anorexic, pot-bellied, mildly anaemic, lag behind the flock, appear stiff, stagger, breathe rapidly and, in advanced cases, may collapse and die when exercised.^{3, 4} Scouring, icterus and haemoglobinuria may be evident in some lambs.^{3, 5, 18, 27, 29} The icterus may...