This study described the clinical outcomes of natural infections with an isolate of LSDV in Bos indicus in the tropical region of Indonesia, as well as seroconversion and the viral DNA load in circulating blood. Bos indicus has been known to be more resistant than Bos taurus to infection with Classical strains of LSD36. However, the LSDV that infected Indonesian cattle previously belongs to a different clade from LSDV that has been known to cause epidemics in Africa and Europe22 and there is a lack of information on the progression of clinical infection in local cattle with the Asian strain of LSD. Therefore, a study was needed to update such knowledge.
The clinical features of infection observed in this study were similar to those seen with classical strains of LSDV elsewhere, such as the presence of lumps, lameness, oedema of the limbs, fever, and reduced appetite12. Skin lesions were the first visible clinical signs, and it prompted the owners to report the cases to local veterinarians. Other clinical signs were detectable only in a few animals on Day 1 of the study.
Nodules developed only for a short period in this study. After two observations the number and size of nodules no longer increased, and on Day 8 nodules became smaller in size or, indurated and necrotic. The proportion of body surfaces suffered with nodules varied, between one-third to two-thirds of the skin area. In previous experiments with Classical strains of LSDV in Bos taurus, the lumps began to appear on Days 6 -10 post-infections and in some animals, lumps continued to develop both in number and size until Days 10 to 15 and could cover up to half of skin surfaces. Subsequently, some lumps were reduced in size or became inapparent during the second week after infection and, some other lumps became indurated or necrotic by two to three weeks post-infection26,37. This study indicates that lump progression due to infection with the Kulonprogo isolate of LSDV in Bos indicus might be similar, in terms of skin surface coverage and duration, to these with classical strains in Bos taurus..
The second most frequently observed clinical feature was lameness, observed in nine animals. Lameness tended to develop later than nodules but, has a strong correlation with the course of nodules in individual animals. On the other hand, anorexia and oedema of limbs were moderately correlated to each other and observed in just above half of the animals. Lameness, anorexia and oedema seemed to last longer in young animals than older ones. Lameness was observed in one to two time-point observations in animals younger than three years old, but only in one observation in older animals. Anorexia was visible in two to three observations in young animals compared to one observation in older animals, while oedema was visible in three observations in young animals versus one to three observations in older animals. The presence of anorexia may vary from 20 to 100% in Bos taurus showing nodular lesions in experimental LSD15,26,27. It occurred for four days in Bos taurus calves27. Oedema was reported in 12 – 50% of animals showing nodular lesions in the experimental infections26,37. The duration of oedema and lameness, however, has not been recorded. Therefore, this study added new knowledge on the progression of lameness and oedema due to LSD in cattle.
The prolonged duration of illness from LSD in young animals, however, was not associated with increased mortality. This was in accordance with previous findings that the mortality rate from infection with a classical strain of LSD was similar between age groups in either Bos indicus or Bos taurus36. Severe lameness due to LSD however, could cause animals to become recumbent, leading to euthanasia26. It may be the severity of lameness per se, not the duration, that relates to mortality or culling in LSD cases on farms.
Fever was the least frequent clinical symptom, observed in less than half of the animals, and when it occurred, it lasted only one observation point in the study. In this study, fever was only weakly correlated with lameness and anorexia. In infections with classical strains of LSD in Bos taurus, fever affected 75 – 100% of animals and started to be detectable a week after virus inoculation; the body temperature could reach 41.4 °C, and the fever might persist for two weeks15,26,27,37,38. The course of pyrexia in this study was similar to that observed after vaccination with live-attenuated vaccines of classical strains of LSDV in Bos taurus bulls, where pyrexia lasted for two to four days, and the peak temperature reached 40.8°C15.
Statistically, the fever did not correlate with the presence of viral DNA in sera. However, viraemia was moderately correlated with nodular progression but only weakly correlated with anorexia and lameness. Viral DNA was observed only on a few observation time point and, intermittently in a few animals and, this study indicates that it might be more readily detectable in young animals than older ones. Previous studies reported that in mild clinical or sub-clinical infections with LSDV, viraemia may start to appear on Days 5—8 post-infection and intermittently detectable in a few animals in an interval of one or two weeks, while in severe infections viraemia started detectable from Days 3 – 7 post-infection and persisted of more than two weeks15,26,27,38. The current findings seemed to follow the pattern of intermittent viraemia in mild clinical cases. Further, a pathological study of LSD in India reported that a high viral load in blood was detected from early-stage infection39. In this Indian study, the highest viral load in skin nodules, trachea, tongue, and lymph nodes was significantly higher in the mid and late stages of the infection39. The recurrent viral load in sera detected in the current study might reflect the increased viral load in internal organs during the late phase of the infection and indicate a shift from clinical to subclinical state of infection with LSDV in the region, during a chronic phase.
Nodules, lameness and viral DNA in sera were positively correlated with each other and mostly detectable on Days 1 and 4 of the observations and no additional lumps appeared from Day 8. On the other hand, seroconversion began to be detectable on Day 8 of the study in all animals, and after a short period of peak, declined sharply by 16 days of the study but, remained above the threshold in the vast majority of the animals. The LSD Seroconversion in this study along with clinical signs may justify the diagnosis of LSD in the absence of detectable viral DNA in serum samples. Previous studies in Bos taurus reported different onset of seroconversion from day four to three weeks post-infection but it may last for more than six weeks post-infection15,26. As the incubation period of LSD in experimental infections was 6 – 10 days26,37, the period from infection to seroconversion in the current study could be in-between that abovementioned, and the duration of antibody seroconversion from infection with LSDV in Bos taurus might apply to Kulonprogo isolate of LSDV in Bos indicus.
The phylogenetic tree of this study was able to correctly classify the reference strains of LSDVs into Clade 1 or 2, and the Kulonprogo isolates of LSDV were predicted to be closely related to Clade 2, which include Asian strains of Cluster 2.2 from Sumatra Indonesia, Thailand, Vietnam, China, vaccine strains of Cluster 2.1, and a wild strain of Serbian LSD viruses. The Bootstrap values supported that the presented clustering was reasonably good40. However, the tree could not differentiate Cluster 1.2 from 1.1 or, Cluster 2.1 from 2.2 in the Clades, as previously described11. This lower classification sensitivity was anticipated as only short sequences of 174 bp in a core gene region were used. In the previous classification, Cluster 2.1 LSDV consisted of vaccine viruses and one wild European strain11. The vaccine strains of LSD have never been reported to cause an outbreak. Even though Cluster 2.1 included a European wild strain, epidemiologically, its transmission to Indonesia was unlikely as there was no transmission pathway to exist from the country to Indonesia, such as through insect dispersion or importation of cattle9. Instead, recombinant vaccine-like strains characteristic of Cluster 2.2, have caused outbreaks in China, South, East and Southeast Asia including Sumatra11,22,41. Moreover, the transport of cattle between Sumatra and Java is frequent42. Hence, it is most likely that the LSDV Kulonprogo isolate closely related to Cluster 2.2 of the Asian recombinant strains.
Bos indicus are known to be more resistant than Bos taurus to infection with Classical strains of LSD36. This host factor might also be associated with milder clinical outcomes of the infection with LSDV in this study, such as shorter periods of fever and elevated antibody levels. The LSD virus of this study, however, belongs to a different clade from that used in experimental studies on Bos taurus15,26,27,38. Thus, whether the host factors, the low virulence of the Kulonprogo isolate of LSDV or the interaction of both factors, contributed to the mild clinical outcomes in this study, remains unclear and needs further investigation. One limitation of this study was that there was no clear definition for Day 1 of the course of the infection, as this relied on farmer’s vigilance. The clinical features that were recorded on the first day of observation might occur earlier than the initial report thus, infection might have had a longer duration than that was reported in this study.
In conclusion, this study is the first to describe the time series progression of clinical outcomes of infection with an Asian isolate of LSDV in local cattle in Indonesia. The infections with the Kulonprogo isolate of LSDV in PO breed cattle caused clinical and serological outcomes milder than infection with Classical strains of LSDV in Bos taurus. This was indicated by shorter periods of mild fever, transient lameness, intermittent viraemia and shorter periods of high levels of antibody in animals studied. However, it caused a longer duration of lameness, anorexia and oedema in young cattle of the PO breed of Bos indicus compared to older cattle in the same breed. Data obtained from this study could be the baseline for further studies on vaccination, clinical treatments and prognosis of the disease in the country.
