AS I write, communication from the Veterinary Department of South Africa has alerted livestock farmers in that country about the resurgence of theilerioses, also known as rift valley fever (RVF), cattle fever or as January disease in Zimbabwe and in the Southern African region where El-Nina has recurred following several years of El-Niño-induced droughts.
In Southern Africa, La-Niña results in wetter-than-normal conditions from December to February; whereas it results in drier-than-normal conditions over the same period, over equatorial East Africa.
East Coast fever is caused by theileria parva and is transmitted by ticks of the genus theileria.
There has been considerable naming and renaming of T. parva and the associated diseases in Africa.
‘Classic’ East Coast fever (ECF) occurs in East Africa and is associated with T. parva, transmitted from cattle-to-cattle by the brown ear tick (rhipicephalus appendiculatus).
East Coast fever also occurs either as corridor disease (recognised in 1934), in Eastern and Southern Africa or as January disease in Central Africa, including Zimbabwe, because of its seasonal occurrence.
Corridor disease is transmitted from buffalo to cattle by either R. appendiculatus or R. zambeziensis; the agent responsible used to be called T. parva lawrencei.
Close contact between buffalo, cattle and ticks is essential for the spread of disease.
The disease is more acute than classical ECF, but after serial passage in cattle, it is indistinguishable from classical ECF. January disease occurs mainly between January and March and the agent was named T. parva bovis.
The disease is also more acute than classical ECF, with death sometimes occurring within four days.
These three clinical diseases are otherwise indistinguishable from one another and hence the causative agents are currently referred to simply as T. parva.
Today, theileria parva is changing from seasonal to non-seasonal, with rapid spread and high mortality.
An apparently identical syndrome in the southern province of Zambia is regarded as ECF, for which the lowveld brown ear tick (rhipicephalus zambeziensis), first identified in 1981, is considered the main vector.
Originally a colonial disease, ECF was introduced in 1901/2 by settlers importing exotic stock via East African ports.
It was first recognised in Zimbabwe in 1936, as Zimbabwe theileriosis (January disease)
Though Corridor disease and Zimbabwe theileriosis persisted, ECF was eradicated in 1954, but is currently affecting the whole of Southern Africa – why?
Because of the inter-tropical convergence zone (ITCZ); the band of clouds, usually thunderstorms, that circle the globe near the equator.
The ITCZ, particularly in this season (December-March) of increased precipitation, affecting the Southern African region has given rise to a widespread resurgence of some hitherto unknown strains of the tick-borne disease, including ECF.
In Zimbabwe, the disease causes a significant number of deaths each year, necessitating the implementation of intensive dipping regulations to control its vector.
The disease also affects the whole of the Eastern hemisphere, particularly the southern tips of Asia and west Australia, where an increased occurrence of theileriosis in the Tamworth District and the Eastern Fall country around Walcha have recently been experienced.
Given the climatic zones that have traditionally been associated with the parasite, the recent outbreaks are said to be unusual.
Debate has surrounded the recent outbreaks as to whether they have been caused by the same species of theileria that has been in Queensland since the early 1900s or, in fact, whether it is a different species of the parasite.
Tropical theileriosis is a severe and fatal disease caused by the protozoan parasite theileria annulata, transmitted by species of the tick genus hyalomma.
Virulent strains of theileria mutans may also be involved in the ECF syndrome.
The pathogen is trans-stadially transmitted by at least nine-to-13 tick species.
However, the principal vector is the brown ear tick rhipicephalus appendiculatus.
Four species or types of bovine theilerias are known to occur in Zimbabwe, namely:
- Theileria parva lawrencei – the causal organism of corridor disease.
- Theileria parva bovis – a parasite apparently unique to Zimbabwe, causing a disease distinguishable from ECF.
- Theileria taurotragi – a non-pathogenic parasite transmitted by rhipicephalus appendiculatus, formerly known as theileria mutans.
- Theileria mutans proper – transmitted by amblyomma spp, not known to be pathogenic in Zimbabwe.
Globally, there are variations in diseases caused by the different species of the parasite, with the most severe being ECF, which is endemic in African countries but exotic elsewhere such as Australia.
It has long been considered that tropical and sub-tropical climates tend to favour the survival and transmission of the theileria.
The two most important species in cattle and water buffalo are T. parva, which causes ECF of East and Central Africa, and T. annulata, which causes Mediterranean or tropical theileriosis across North Africa and Central Asia.
The African buffalo (Synceruscaffer) is an important wildlife reservoir of T parva.
Theileria parva, called ECF, can cause 90-100 percent mortality in affected cattle.
After foot-and-mouth disease, it is the second most important disease transmitted from buffalo to cattle.
Other theileria spp, including T. mutans, T. velifera, T. taurotragi, T. buffeli, and T. sergenti, usually cause asymptomatic infection or can increase severity of ECF and tropical theileriosis.
A species of the parasite, theileria buffeli, has been known to be present in Queensland, Australia, since around 1910. It was introduced by the cattle tick from Japan.
This parasite has been relatively non-pathogenic pathologic. Recently, however, there have been reported cases of theileriosis in several states of Australia which resulted in severe production losses and deaths in dairy and beef herds.
Research regarding the outbreaks in Australia is currently underway in NSW.
The main clinical symptoms of theileriosis disease include:
- drop in milk production;
- weakness and in appetence;
- difficulty breathing, with rapid and shallow breaths;
- Increased heart rate.
Signs of anaemia associated with theileriosis are likely to be more evident around calving time; in calves (two-to-three months), animals with other health challenges or potentially at mating time.
Other less common clinical signs include; excessive salivation, diarrhoea, constipation, swelling of lymph nodes and brown urine.
The variation in the clinical syndromes is due to individual cow immunity and tolerance, and also variations due to the different strains of the parasite.
The disease can result in death of the affected cow after two-or-three weeks.
However, rapid recovery is also possible if treated in time.
A blood sample needs to be taken from the affected cow for diagnosis.
A blood smear is then made to examine for the parasite associated with red blood cells.
It is important to get a blood sample early in the stage of infection, as the spleen removes the parasitised blood cells, which can result in an apparently normal blood smear.
Both, ECF and tropical theileriosis are most severe when susceptible animals are introduced to endemic areas and mortalities can reach 100 percent and 90 percent, respectively. The outcome of exposure is largely determined by the susceptibility of the cattle and indigenous animals may have a morbidity rate of 100 percent.
The incubation period for either disease ranges between eight-21 days.
Dr Tony M. Monda is Zimbabwean socio-economic analyst and scholar. He is currently conducting veterinary epidemiology, agronomy and food security and agro-economic research in Zimbabwe and Southern Africa.
For views and comments, email: tonym.MONDA@gmail.com