Mission Critical

Abstract

The reports for Ebola virus Zaire (EBOV), Nipah virus, and Machupo virus (MACV) pathogenesis, in this issue of Veterinary Pathology, are timely considering recent events, both nationally and internationally. EBOV, Nipah virus, and MACV cause highly lethal infections for which no Food and Drug Administration (FDA) licensed vaccines or therapies exist. Not only are there concerns that these agents could be used by those with malicious intent, but shifts in ecological distribution of viral reservoirs due to climate change or globalization could lead to more frequent infections within remote regions than previously seen as well as outbreaks in more populous areas. The current EBOV epidemic shows no sign of abating across 3 West African nations (as of October 2014), including densely populated areas, far outpacing infection rates of previous outbreaks. A limited number of cases have also arisen in the United States and Europe. With few treatment options for these deadly viruses, development of animal models reflective of human disease is paramount to combat these diseases. As an example of this potential, a new treatment compound, ZMapp, that had demonstrated efficacy against EBOV infection in nonhuman primates (NHPs) received an emergency compassionate use exception from the FDA for the treatment of 2 American medical workers infected with EBOV, and they are currently virus free and recovering.

The reports for Ebola virus Zaire (EBOV), Nipah virus, and Machupo virus (MACV) pathogenesis, in this issue of Veterinary Pathology, are timely considering recent events, both nationally and internationally. ^1,2,10 EBOV, Nipah virus, and MACV cause highly lethal infections for which no Food and Drug Administration (FDA) licensed vaccines or therapies exist. Not only are there concerns that these agents could be used by those with malicious intent, but shifts in ecological distribution of viral reservoirs due to climate change or globalization could lead to more frequent infections within remote regions than previously seen as well as outbreaks in more populous areas. The current EBOV epidemic shows no sign of abating across 3 West African nations (as of October 2014), including densely populated areas, far outpacing infection rates of previous outbreaks. ³ A limited number of cases have also arisen in the United States and Europe. With few treatment options for these deadly viruses, development of animal models reflective of human disease is paramount to combat these diseases. As an example of this potential, a new treatment compound, ZMapp, that had demonstrated efficacy against EBOV infection in nonhuman primates (NHPs) received an emergency compassionate use exception from the FDA for treatment of 2 American medical workers infected with EBOV, and they are currently virus free and recovering. ⁸

The models reported in this issue for EBOV infection of guinea pigs, MACV infection of cynomolgus macaques, and Nipah virus infection of hamsters each cause lethal diseases. ^1,2,10,11 This is a beneficial attribute for efficacy studies of treatments against severe/lethal disease as seen in humans and as a clinically relevant end point for evaluation of candidate compounds. As the disease process may vary by route of transmission, animal models should recapitulate human transmission. For example, MACV may be transmitted to humans through mucosal contact, ingestion, or inhalation after sweeping dried urine of the carrier rodent. Alternatively, consumption of fruit bats or contaminated fruit may be the source of index infections with EBOV and Nipah virus. Importantly, these 3 infections are spread person to person by close physical contact, such as caretaker exposure to bodily secretions of patients (eg, EBOV). As such, in the laboratory, these viruses also pose high risks for infection (eg, needle sticks, etc) and thus require specialized personal protective equipment, safety protocols, and biocontainment facilities, which limit the number of laboratories able to study these diseases. From a biodefense standpoint, these pathogens could be aerosolized, with weapons that produce small or large virus-containing droplets affecting the deep lung or upper airways, respectively. ^7,9 With regard to the current EBOV epidemic, the high number of infections among health care workers despite protective gear to prevent exposure to a patient's bodily fluids, along with previous studies in nonhuman primate models, raises concerns about the possibility of aerosol transmission.^{12,13,14,15,16} Thus, there is also a need to understand the diseases induced by alternate exposure routes and evaluate therapies in this context.

Humans and NHPs that succumb to natural EBOV infection display marked pathology in liver and spleen but generally lack severe pulmonary complications. ⁶ However, if EBOV was used as a bioterrorism agent, inhalation of aerosols would be a possible exposure route. To study this, Twenhafel et al ¹⁰ developed a small-particle aerosol model of EBOV infection of guinea pigs that was characterized by lethal pneumonia, which has not been reported in aerosol-exposed NHPs. EBOV in guinea pigs does not cause a rash as characteristically seen in natural infection of humans and NHPs and thus could potentially indicate important differences in pathogenesis that must be elucidated. It is critical to understand EBOV pathogenesis and countermeasure efficacy in this model, should aerosol exposure of humans be found to result in a similar disease.

The viral dose administered to animals should mimic that of natural or potentially weaponized exposures. Interestingly, Nipah virus infection of humans can result in 2 distinct disease manifestations and Baseler et al ¹ reveal that this may be dose related. In Malaysian Nipah virus infections, humans acquire infection from pigs and develop neurologic disease. In Bangladesh, less is known about the pathology, but humans go on to develop respiratory disease after exposure to infected fruit bat secretions. High-dose intranasal Nipah virus infection of hamsters results in severe respiratory disease, and low-dose infection results in mild respiratory disease with neurologic signs that develop later, with Malaysian and Bangladesh strains behaving similarly. ¹ Therefore, differences in human Nipah infection in Malaysia and Bangladesh may be due to dose differences following different exposure modalities. While NHPs infected with Nipah virus present with disease similar to humans, ⁵ the hamster model, which also recapitulates the human disease, has the advantage of ease of procurement and reduced animal housing requirements, thus enabling fulfillment of the required statistical power in future drug studies.

Despite the difficulties of working with NHPs, cynomolgus macaques are a model of choice for critical studies since the immunologic and physiologic responses are thought to be similar to humans. ⁴ Bell et al ² compare MACV infection of cynomolgus macaques by intramuscular and small-particle aerosol routes. Intramuscular infection, representing needle-stick injuries, has historically been used in biodefense studies for ease of administration despite a lack of biodefense relevance. Aerosol administration and dosing is technically demanding, but these routes are successfully bridged in this study. Unlike previous reports that use animal-passaged strains, this study employs a nonadapted strain of MACV (Chicava), thus resembling a human clinical specimen, which is also lethal in a guinea pig model, enabling bridging between small animal and primate models. Clinical signs and pathology between intramuscular and aerosol groups were similar, aside from accelerated progression after intramuscular infection. Importantly, the authors determine that the clinical picture of the disease caused by MACV in cynomolgus macaques resembles reports of human cases, which is ideal for exploration of countermeasures.

Research on emerging agents such as EBOV, Nipah virus, and MACV does not garner much interest from pharmaceutical industries for drug development and is therefore heavily dependent on limited defense funding. Thus, despite years of research and a solid panel of promising vaccine and therapeutic candidates, there is no readily available response for situations that arise such as the current EBOV epidemic raging in West Africa. Mobilization of these resources will be dependent on further research and development and the use of well-characterized animal models like those reported by Bell et al, ² Twenhafel et al, ¹⁰ and Baseler et al ¹ in this issue of Veterinary Pathology.