Specific Diseases of the Liver Viral Diseases

Rift Valley Fever

This arthropod-borne virus disease of ruminants, camels, and humans is thus far limited to Africa and the Arabian peninsula. The disease is characterized by acute, severe hepatic necrosis, with abortion in pregnant dams and high mortality in neonatal animals.

Etiology

The causative agent is an enveloped, single-stranded RNA phlebovirus in the Bunyaviridae family. The virus is very stable near neutral pH in blood or serum and also in aero­sol. It is killed by 2% sodium orthophenylphenate and 4% sodium carbonate disinfectants. There is only a single sero­type of the infective agent, though differences in patho­genicity have been noted between isolates (Swanepoel et al. 1986). Phylogenetic analysis of the virus indicates two distinct lineages for the RVF virus, one Egyptian and the other sub-Saharan (Sall et al. 1997). However, there are two distinct groupings within the sub-Saharan viruses, one West African and the other central and southern African, now including the viruses from the Arabian peninsula.

Epidemiology

RVF was first identified in Kenya's Rift Valley in 1930, although there are earlier descriptions of what could well have been RVF. The disease was historically limited to East and Central sub-Saharan Africa and was first recognized in southern Africa in the 1950s. However, later outbreaks in Egypt, Senegal, and Mauritania indicated its potential for further spread (Ksiazek et al. 1989; Lancelot et al. 1989). It is now known to occur throughout Africa, including the island of Madagascar. In 2000, the disease was confirmed for the first time outside of Africa, on the Arabian penin­sula in Yemen and Saudi Arabia, and it affected large num­bers of livestock and humans, with more than 120 human deaths (Al-Afaleq et al.

The RVF virus is transmitted to susceptible hosts during feeding by infected mosquitoes in the genera Aedes, Culex, Mansonia, Eretmapodites, Coquillettidia, and Anopheles, and possibly by other insects. The Aedes mosquitoes are capable of transovarial transmission of the virus, and their eggs can survive desiccation well, thus sustaining infection even for decades. Viremic ruminants and humans can also contribute to the spread and amplification of infection. Direct transmission from infected animals to people can occur by aerosols of blood at the time of slaughter, as well as by ingestion of blood and milk.

Epizootics often follow periods of heavy rainfall; the rain facilitates the hatching of previously laid, dormant, infected mosquito eggs, resulting in sudden increases in infected Aedes mosquito populations. These initiate infection, which is then amplified markedly by Culex and other spe­cies of mosquitoes whose population increases lag some­what behind those of the Aedes mosquitoes.

Two additional factors that contribute to epizootics are heavy winds that blow infected insects into previously uninfected areas and transport of infected livestock into such areas. Places with extensive irrigation networks or standing water, warm climate, suitable arthropod popula­tions, and susceptible ruminant and human populations are most prone to epizootic occurrences (Wittmann 1989). An RVF epizootic that affected the contiguous areas of the Northeastern province of Kenya, the south of Somalia, and southern Ethiopia in 1997-1998 is estimated to have affected more than a half million small ruminants and sickened 89 000 people, leading to possibly 450 human deaths in Kenya alone. A similar, severe RVF outbreak occurred in Kenya and Tanzania in late 2006 through early 2007 and spread later in the year into Sudan. More recent outbreaks of RVF involving multiple human fatalities include Madagascar 2008-2009, Republic of South Africa 2010, Mauritania 2012, and Niger 2016.

It is now well documented that RVF epizootics in the Horn of Africa are associated with the El Nino-Southern Oscillation (ENSO) phenomenon. These warm ENSO events are known to increase precipitation in portions of East Africa. Retrospective studies on data from 1950 through 1998 demonstrated that an analytic model that included measurements of southern oscillation indices, equatorial sea surface temperatures in the Pacific and Indian Oceans, and satellite-derived vegetation indices in East Africa would have accurately predicted 100% of the RVF epizootics that occurred during that 48-year period (Linthicum et al. 1999). Such predictive epidemiologic tools can be extremely useful for instituting timely and effective disease control programs.

Pathogenesis

After infection, there is a viremia with fever and leukope­nia. Despite the destruction of liver cells by rapidly multi­plying virus, localization of virus in tissues does not occur to any great extent, and blood always contains the highest levels of virus. A carrier state does not develop in rumi­nants. Lesions are limited to the liver and are characterized by focal hepatic necrosis. Experimental infections in goats have been described (Easterday et al. 1962).

Clinical Findings

The incubation period is as short as 12 hours in kids and as long as 48 hours in adult goats. Adult animals are always less severely affected than young stock. In epizootics involv­ing goats, the predominant effects are abortion in pregnant does and death in kids younger than 1 week of age. Many infections in non-gravid individuals are subclinical.

While peracute death with no prodromal signs is most likely in neonatal kids, some young kids may show fever up to 42.2 °C (108.0 °F), accompanied by listlessness and inap­petence. Death may occur within the next several days.

The acute form of the disease is more likely in older kids and some adults, though it is seen less frequently than in sheep. In addition to fever and depression, variable signs may include jaundice, vomiting, hemorrhagic diarrhea, unsteady gait, catarrhal stomatitis, and skin necrosis on the udder or scrotum, with death in one to four days.

Except for abortion in pregnant does, most affected adult goats are likely to show only subacute disease, with fever in the range of 40-41.2 °C (104-106.2 °F), dullness, and inap­petence for one to three days. Mortality rates are low in adult goats. Breeds of imported goats are more likely to show overt signs of disease than are indigenous breeds.

Clinical Pathology and Necropsy

Leukopenia is common in the early stages of disease. Virus isolation in cell culture is possible from blood of live, febrile goats; or liver, spleen, and brain of fatal cases; and organs of aborted fetuses. When immediate submission to the lab­oratory is not possible, samples should be frozen at -70 °C (-94 °F). Alternatively, immunofluorescence can be used to identify RVF virus antigen on impression smears of liver, spleen, and brain. Other techniques available to confirm infection include enzyme-linked immunoabsorbent assay (ELISA) for viral antigen detection and reverse tran­scriptase polymerase chain reaction (RT-PCR) for viral RNA detection (OIE 2018).

After outbreaks, seroconversion can be documented using indirect ELISA to detect antibody in serum samples taken three weeks apart. An immunoglobulin (Ig)M cap­ture ELISA can diagnose RVF on a single blood sample taken early in infection. The plaque-reduction neutraliza­tion test can also be used to assess antibody response (OIE 2018).

At necropsy, the liver may be friable and appear moder­ately enlarged, soft, and pale, with focal areas of subcapsu- lar hemorrhage. On cut section, 1-2 mm gray-yellow necrotic foci are distributed throughout the parenchyma. Other possible findings include visceral and serosal hemor­rhage, icterus, hemorrhagic gastritis, and enteritis. Similar lesions may be seen in aborted fetuses.

Histologically, the liver lesion is one of focal centrilobu­lar or midzonal necrosis of small groups of hepatocytes. As lesions progress, they may become confluent and involve a large portion of the lobule. Eosinophilic, intra­nuclear inclusion bodies surrounded by marginated chro­matin are common in degenerating hepatocytes in advanced lesions.

Diagnosis

Definitive diagnosis is by virus isolation or the demonstra­tion of virus genome and by confirmation of seroconver­sion in affected goats. In those parts of Africa where both diseases occur, RVF must be differentiated from Wesselsbron disease, a liver disease that is also described in this chapter. In sheep, bluetongue is considered in the dif­ferential diagnosis, but bluetongue is almost always asymp­tomatic in indigenous goats. When people handling livestock complain of influenza-like illness during epizoot­ics of animal disease, the diagnoses of RVF or Wesselsbron disease should be pursued vigorously.

Treatment

The antiviral drug ribavirin is the drug of choice for bunya- virus infections in humans, but no recommendations for use in animals are made. The intravenous or intraperito­neal administration of 10-30 mL of serum from convales­cent sheep to 1-3-day-old lambs reduced mortality, even when lambs were already showing signs of illness (Bennett et al. 1965).

Control

In endemic areas, an attenuated live vaccine, prepared from the mouse-brain-adapted Smithburn strain, has long been available. The vaccine is recommended for use in cattle, sheep, and goats, but should not be used in pregnant ani­mals, because viremia, abortion, fetal death, and fetal anom­alies occur, especially in sheep. Immunity lasts at least two years and is generally considered to be lifelong. A second live attenuated vaccine, the clone 13 RVF vaccine, also has become available. Clone 13 is a naturally attenuated strain of RVF virus characterized by a large deletion of the gene encoding for the main virulence factor, the non-structural NSs protein (OIE 2018). The vaccine is currently approved in South Africa for use in cattle, sheep, and goats, including pregnant animals. A recent vaccine safety trial of a Clone 13 vaccine prepared in the Netherlands in anticipation of the possible introduction of RVF into Europe demonstrated that in pregnant ewes the Clone 13 virus is able to spread to the fetus, resulting in malformations and stillbirths.

Vaccination should not be undertaken during an epizo­otic of RVF. If it is deemed necessary, then needles must be used on a single animal only and then replaced, because needle passage of the disease during periods of active transmission is quite likely and can further spread the dis­ease and extend the duration of the outbreak.

Formalin-inactivated vaccines are available for use in non-endemic areas, but they are less effective, especially in cattle. Inactivated RVF vaccines require a booster three to four weeks following initial vaccination, followed by yearly revaccination.

Mosquito control is problematic, but there are some indications that insecticidal fogging and the use of Bacillus Ihuringensis to kill mosquito larvae might have potential for control in the face of an epidemic. Movement of animals out of known mosquito breeding areas after heavy rainfall, for example into cooler highland areas, might reduce the incidence of disease. During epizootic outbreaks, move­ment of non-vaccinated animals should be restricted.

In recent years, the use of historical and real-time cli­mate data and vegetation indices obtained by satellite imaging have allowed for the reliable prediction of RVF epizootics two to five months in advance of the first clinical cases (Linthicum et al. 1999; Anyamba et al. 2002). In 2006, the Food and Agriculture Organization of the United Nations (FAO), through its Emergency Prevention System (EMPRES), was able to accurately predict the RVF epi­demic in Kenya and Tanzania about two months before it occurred (FAO 2006). It was demonstrated during the 2006-2007 Kenyan epidemic that participatory disease sur­veillance data can provide strong support to systems based on satellite imaging for predicting the emergence of epi­demic RVF in Kenya (Dr. Christine Jost, Research Scientist, International Livestock Research Institute, Nairobi, Kenya, personal communication 2007). These are very useful tools for allowing the implementation of disease response plans, including vaccination, before the onset of disease.

Most human cases can be related to direct contact with blood or tissues of infected animals, or inhalation of aero­sols associated with such tissues. While the disease in humans is usually inapparent or produces transient flu­like symptoms, in a subset of patients the disease can be fatal following signs of hemorrhage, encephalitis, and/or severe hepatic disease. Retinal damage and visual impair­ment are common sequelae to acute human infection. Veterinarians should take suitable precautions against infection when performing examinations or necropsies and advise against slaughter and consumption of meat from suspected cases (Davies 2006). Consumption of raw milk in areas undergoing an active outbreak is unwise. Vaccination with a killed vaccine should also be considered by people such as veterinarians and laboratory workers with a high risk of exposure.

Wesselsbron Disease

While serologic evidence indicates that this mosquito- borne virus disease of domestic livestock is enzootic in southern, central, and western Africa, clinical disease is reported mainly from small ruminants in South Africa. Largely asymptomatic in adult animals, it primarily causes abortion and neonatal mortality, especially in lambs and kids. It is important in the differential diagnosis of RVF. It occasionally infects humans.

Etiology and Epidemiology

The causative agent is an enveloped, single-stranded RNA, group B flavivirus in the Togaviridae family. The disease was first identified in the Wesselsbron district of the Orange Free State in South Africa in 1955. Aedes caballus, Aedes Circumluteolus, and Aedes lineatopennis mosquitoes are the primary disease transmitters, but contact and aerosol transmission have also been reported (Barnard 1986).

Sheep, goats, and cattle are principally affected, but the mammalian host range is wide and includes humans. Despite the serologic evidence of infection throughout southern, central, and western Africa, disease incidence is low. The virus has also been isolated from mosquitoes in Thailand. Epizootic outbreaks occur in South Africa in association with heavy rainfall, which favors increased vec­tor mosquito activity, conditions similar to the emergence of RVF. Cattle may be a natural reservoir of the disease. The virus is isolated from some wild rodents and birds, including ostriches, but their role as reservoirs is debatable.

Pathogenesis

Infection is followed, after a short incubation period, by viremia with fever. The virus shows a predilection for the liver, and even when clinical disease is not observed, focal necrosis of the liver occurs, as demonstrated in experimen­tal infection of adult goats (Coetzer and Theodoridis 1982). Kids are more severely affected than adults (Theodoridis and Coetzer 1980). Increased levels of antibody are detect­able within three weeks of infection.

Clinical Findings

In epizootics of Wesselsbron disease, clinical signs in adults are usually limited to fever and abortion in pregnant does and ewes, with non-pregnant females and males showing no clinical effects. Kids and lambs, especially if under 4 weeks of age, can die suddenly or soon after showing some weakness, loss of appetite, increased respiratory rate, jaundice, and fever up to 41 °C (105.8 °F). Lamb mortality as high as 30% has been documented.

Clinical Pathology and Necropsy

Direct virus isolation can be performed in cell culture using Vero cells, baby hamster kidney cells, or primarily lamb kidney cells producing either plaque formation or cyto- pathic effect. Preferred specimens for submission include liver, spleen, and serum. Alternately, immunohistochemi­cal staining of formalin-fixed liver specimens using poly­clonal antibody against Wesselsbron virus also has been reported in diagnosis of field cases and experimental infec­tions (van der Lugt et al. 1995). In lieu of virus or antigen identification, seroconversion can be documented using the hemagglutination inhibition (HI) assay in acute and convalescent serum samples taken three weeks apart. An antibody-detection ELISA has also been developed. It is reported to be more sensitive than HI and less cross­reactive to other flaviviruses (Williams et al. 1997), but to date the HI test is still routinely used.

At necropsy, the liver may be slightly swollen and yellow to orange-brown in color. In some animals, slight hydrop­ericardium and ascites, lymphadenopathy, splenomegaly, serosal hemorrhage, and icterus may be seen. Histologically, the liver lesions are characterized by small, focal areas of necrosis and a pronounced, nodular Kupffer cell reaction (Coetzer and Theodoridis 1982). Eosinophilic intranuclear inclusions are sometimes seen. Lymphoid tissues may show pyknotic lymphocytes and lymphoid hyperplasia. Neutrophilic infiltration of the red pulp of the spleen may occur. Myocardial necrosis has been noted in some goats.

Diagnosis

Wesselsbron disease and RVF present a similar clinical pic­ture in goats. It is important to make a specific etiologic diag­nosis, because the zoonotic implications of RVF are much more profound. Virus isolation is the ideal method. Intraperitoneal inoculation of suckling or weaned mice or hamsters leads to death of the lab animals in three to four days when RVF virus is present in the inoculum, but not in the case of Wesselsbron disease virus. Serology and histopa­thology can also be helpful in differentiating the two diseases.

Treatment and Control

There is no treatment for this virus disease. An attenuated live virus vaccine produced in South Africa is in use in enzootic areas. Pregnant animals and those younger than 6 months of age should not be vaccinated, because the vac­cine itself can cause abortion or illness in young stock. It is recommended that flocks be vaccinated in the spring three to six weeks before mating, because disease usually occurs in late summer and autumn. Vaccine should not be used in the face of an outbreak. Immunity, which develops three weeks after vaccination, is considered to be lifelong. Removing animals to high ground after heavy rains in anticipation of increased mosquito activity in low, wet areas may reduce the risk of disease.