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wikitox:handbook_epidemiology_and_clinical_manifestations

Epidemiology and clinical manifestations

RUSSELL’S VIPER

The Russell’s viper (Daboia russelii russelii) has a well developed dentition and venom apparatus suitable for inflicting a deadly bite. Additionally, it has many reserve fangs, which remain dormant at different stages of development and maturity. It causes severe systemic envenoming causing even death and is responsible for 48% and 14% of snake bite admissions to tertiary care hospitals and peripheral hospitals respectively. These snakes frequently bite paddy farmers living in the dry zone of Sri Lanka and it appears to be an occupational hazard to farming community. It is a prolific breeder giving birth from five to thirty and hence the reason for its abundance. The Russell’s viper is widely distributed in the country. It is a nocturnal terrestrial snake. A majority of bites are inflicted at dusk or night on foot paths or on main roads. A fair number of bites occur while harvesting during the day time.

It can inflict a very effective bite causing an envenoming rate of 98%. Most of the time it escapes after bite and the offending snake is available only in 21% of cases for identification at the hospital. Incidence of bite is common among young males especially in the 4th decade of their life. Russell’s viper bites occur throughout the year with the two peaks corresponding to agricultural activities. The highest peak is noticed between the months of March-April during the paddy harvesting time and the next peak is from October- November at the time of sowing. Of the 336 Russell’s viper bite victims observed, 84% of bites were below the ankle and in 5% on the arm while harvesting paddy.

Clinical picture: local and systemic effects

Multisystem involvement occurs as local effects of the bite site in 92%, haematotoxicity (coagulopathy) in 77%, neurotoxicity 78%, nephrotoxicity 18%, cardiac involvement in 3- 12 %, myotoxicity 14% and hyperkalaemia in 6.5%. Involvement of more than one system is common, but in some patients, primary monosystemic effects are seen. Clinical diagnosis of the offending snake species in monosystemic presentation is difficult.

Local effects causing mild swelling at the site of bite is seen in 50% of patients, and severe local necrosis in 9%. Mild and moderate local swellings recede in 4 to 7 days. Severe cases need surgical intervention. Some patients who have bites on toes may be left with excruciating pain and numbness of the toe that may persists for more than six months after recovery.

Haematotoxicity
Positive WBCT20 (coagulopathy) is the marker of haematotoxicity and some patients develop haematuria and haemoglobinuria. Spontaneous bleeding, such as bleeding gums, haematemesis, melaena, and ecchymosis manifest only in a few patients. Development of coagulopathy occurs from 30min to 12hrs after the bite, mostly within the first six hours. It can be corrected over a span of 1h to 48hrs (mode 20h) with polyvalent antivenom therapy in the majority of patients. However, about 6% of patients may not respond to antivenom therapy and the coagulopathy may continue for more than 48hrs suggestive of DIC.

Neurotoxicity
Commonest neurological manifestation is ptosis and the rest include external ophthalmoplegia, weakness of bulbar group of muscles, and neck flexors. Weakness of limb muscles is not a significant feature and paralysis of the respiratory muscles is rare. Antivenom serum is not effective in reversing neurotoxicity and the recovery is spontaneous and slow from one to five days (mode 3 days).

Renal and metabolic effects
Renal involvement is seen in about 18% of patients and 8% might develop definitive acute renal failure as suggested by oliguria, high blood urea and elevated serum creatinine levels. Some patients develop hyperkalaemia and it is caused by either hypercatabolic state with normal renal functions or established acute renal failure. Development of silent hyperkalaemia is a problem and it is recognized as the cause of sudden deterioration in clinically stable patients on the 2nd and 3rd day after envenoming.

Cardiac effects
Reversible repolarization abnormalities in the ECG are seen in a significant number of patients. These are tall “T” waves or “T” inversion in anterior leads. Hypotension, shock, angina or myocardial infarction could occur.

Myotoxicity
Generalized severe muscle pain and tenderness are suggestive of myotoxic effects of the venom. Myotoxicity could cause hyperkalaemia and acute renal failure.

Effects on pregnancy
Abortion or intrauterine death of foetus could occur.

Complications and outcome
Development of complications can be seen in about 11% of patients. Severe disseminated intravascular coagulation (DIC) manifests as profuse persistent spontaneous bleeding, thrombocytopenia and multiorgan dysfunction. Hypercatabolic hyperkalaemia, acute renal failure, intracranial bleeding leading to coma, hemiplegia, acute respiratory distress syndrome (ARDS), hepatic failure with icterus, tender hepatomegaly, elevated ALT, AST, alkaline phosphatase and bilirubin levels and early hepatic encephalopathy are also recognised.

Case fatality rate varies between 4% and 1.6% as a result of acute severe DIC leading to multiorgan failure, silent hyperkalaemia and intracranial bleeding.

Existence of subspecies of Russell’s viper based on their morphology and the composition of the venom has been established. This fact is testified by the marked absence of neurotoxicity in the Indian species of Russell’s viper (Daboia russelii russelii). Similarly, Burmese species of Russell’s viper (Vipera russelli siamensis) is free of neurotoxicity, but it produces fatal coagulopathy, vascular damage, shock and disseminated intravascular coagulation. Old native texts in Sri Lanka describe 15 subspecies of vipers. Rural folk of central dry zone of Sri Lanka have a strong belief that a snake called “Gata polonga” is the most poisonous viper.

COMMON KRAIT

The common krait (Bungarus caeruleus) is a nocturnal terrestrial snake. It is a proteroglyphous elapid, which is commonly encountered in human habitations. It is found in Sri Lanka, Pakistan, Bangladesh and India. The highest incidence of its bites in Sri Lanka is reported from the North Central Province (NCP), where the vegetation and climate provide an ideal habitat for snakes. Common victims of Bangarus caeruleus are farmers who live in open wattle-and-daub houses and farmers sleeping in watch huts in agricultural fields. It creeps into houses over the ground or through the roof exhibiting arboreal tendencies. Currently, it accounts for 9% of all snake bite admissions to the General Hospital, Anuradhapura. Envenoming is usually severe in 48% of patients who need assisted ventilation, but 41% of patients develop only mild to moderate envenoming and do not need ventilation. Ten percent of krait bites are “dry bites” as venom is not produced despite having definite bite marks. Incidence of common krait bite increases with the onset of rains.

Abdominal pain is the first symptom to manifest within minutes to a few hours after the bite. Other common clinical features are weakness of limbs, inability to stand up, drooping of eye lids, double vision, difficulty in breathing, and changing sensorium; all may progresses rapidly to severe neuromuscular paralysis. Less commonly, myalgia, paraesthesia at the site of bite, decreased hearing and vision, and faintishness are observed. Often the site of bite and fang marks are indistinct with minimal local reactions.

Respiratory paralysis
It is the most challenging and difficult complication to manage in hospitals with poor facilities. This condition may progress so fast that the patient may need assisted ventilation within 30min after the bite. But in some cases paralysis build up slowly over a few hours. Recovery takes 3 to 4 days.

Coma
The level of consciousness varies from drowsiness to deep coma. Patients in deep coma haven’t brainstem and spinal reflexes; pupils remain fully dilated and with absent light reflex. The onset of a deep coma ranges from 2 hours after the bite up to 48 hours and it may persists from 6 hours to five days (mode 12 hours). They develop more complications than others do during assisted ventilation. However, the patients recover completely.

Autonomic dysfunction
Fluctuation of sympathetic and parasympathetic activity is observed in all grades of envenoming during the first 48 hours and include sweating, tearing, chemosis, fluctuation of heart rate and blood pressure.

Hypokalaemia
Significant hypokalaemia occurs in patients during the early stages, especially in the first 48 hours, but recovery is spontaneous.

Acute respiratory distress syndrome (ARDS)
This is more often an iatrogenic complication than a direct venom effect, commonly occurs due to nasal insuflation of herbal medicines, aspiration of gastric contents and barotrauma at assisted ventilation. It carries high mortality.

Delayed neuropathy
Delayed neurological deficits develop in some patients. The manifestations include sensory loss at the site of bite and nerve palsy with wasting of small muscles.

Currently, it is understood that krait venom has bungarotoxins, which block the transmission at the neuromuscular junction. However, venom may have other components, which act differently to produce alteration of sensorium of the patients.

COBRA

The cobra is widely distributed in the island from the coastal plains up to an elevation of about 1,500m. It is found commonly close to human dwellings. The cobra is a diurnal terrestrial snake. However, some nocturnal bites are not uncommon.

The bite produces severe tissue necrosis which spreads rapidly both in the superficial and deeper tissues. Local effects could be the only manifestation in most of the cases. However, the venom has neurotoxin which could produce rapid respiratory muscle paralysis leading to death.

HUMP-NOSED VIPER

It is a nocturnal terrestrial snake that hides under leaf litter, underneath rubble, stones and logs, and undergrowth during the day. It is widely distributed in Sri Lanka from the plains to high mountains. It is common in plantations such as rubber, coconut, coffee, cocoa and spices. It accounts for the highest incidence (28%) of venomous snake bites in Sri Lanka.

Being a small snake (average 300 mm) the most vulnerable anatomical parts of the body are below the ankle and fingers. Envenoming invariably causes gross swelling associated with haemorrhagic blister formation and necrosis of tissues. Severe pain is common and relieved by surgical intervention. Even after healing neuralgic pain may last for months. Less than 5% of patients develop systemic envenoming and its prediction is impossible. Changing composition of the venom among snakes may be responsible for unpredictable systemic envenoming. The commonest systemic effect is prolongation of coagulation (positive WBCT20), which may take few days to get corrected spontaneously. However, some patients may develop severe disseminated intravascular coagulation leading to multiple organ failure eventually leading to death. Reversible acute renal failure presenting with oliguria and elevated blood urea is another problem which needs only conservative care. Rarely, acute renal failure becomes irreversible due to extensive renal cortical necrosis causing high mortality.

CEYLON KRAIT

It is an endemic snake, found mainly in wet zone of the island. The snake creeps into human dwellings at night, but rarely bites. The venom is a neurotoxin called bungarotoxin which produces rapid neuromuscular paralysis. The victim could die due to paralysis of the respiratory muscles.

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