Hookworm in the Military

Abstract

Soil-transmitted helminths such as hookworm are rarely a problem for military units unless they directly contact faecally contaminated soil during tropical operations. Localised epidemics incapacitating small infantry units have been recorded in the Solomon Islands and Bougainville during World War II and in Vietnam, Grenada and Sierra Leone more recently. Scattered hookworm infection cases and Strongyloides stercoralis were seen during the Regional Assistance Mission to the Solomon Islands (RAMSI). Cutaneous reactions such as ‘ground itch’ from skin penetrating parasites and gastrointestinal disturbances mimicking ulcers occur after massive exposures and may occur prior to eggs appearing in the stool. Post-deployment mass anti-helminth treatment with albendazole and ivermectin is safe but likely kills few worms in most low-risk situations. Consideration should be given to focusing post-deployment treatment on infantry units conducting wet jungle operations in faecally contaminated environments and minimising chemotherapy for lower-risk groups conducting other international engagements.

Keywords: hookworm, Melanesia, geohelminths, chemotherapy, disease casualties

Besides ‘ground itch’ and ‘foxhole cough’, the parasite may cause an acute duodenitis that clinically simulates an ulcer diathesis, except that eating aggravates the pain.¹

Enteric disease control depends on sanitary disposal of faecal material, which is less than universal in some of the Melanesian and Polynesian countries of the Indo-Pacific.² Geohelminths spread by soil contamination with worm eggs are now very uncommon in Australia except for some isolated Aboriginal communities but remain an issue in the Indo-Pacific, particularly on small islands.³ Soldier disgust with visible worms or obvious faecal material does not protect against skin contamination, especially during engineer operations or when muddy water leaks into boots. Skin penetrating parasites are part of the life cycle of hookworms and are seen as a circuitous pruritic rash caused by migrating larvae in cutaneous larva migrans.⁴ Eosinophilia is one of the few laboratory clues to the presence of hookworm infections, as faecal eggs are not seen early in the disease and are often not searched for until the diagnosis is suggested by eosinophilia. Despite such exposures in poorly sanitised areas, hookworm outbreaks that incapacitate small infantry units rarely occur unless there is massive cutaneous exposure to focally contaminated soil. This brief review looks at some of these historical outbreaks to gauge the actual risk to current operations and calibrate the need for post-deployment chemotherapy against helminths.

During World War II, malaria was the predominant parasitic infectious disease, creating casualties in the Southwest Pacific Theater, but filariasis and geohelminths remained tropical diseases of interest.^5,6 Initially the worry was from the large number of troops being massed in bare bases with rudimentary sanitation recently established in Queensland and the Northern Territory before being sent to Papua New Guinea and the Solomon Islands. Although sanitation was imperfect, the disease outbreaks primarily comprised dysentery and diarrhoea.⁷ Outbreaks specifically attributed to geohelminths were few in comparison, although the Medical Journal of Australia did warn in 1943 of hookworm as a cause of anaemia in returned soldiers.⁵ New Zealand’s 40th Battalion was deployed in the Solomon Islands and reported an April 1944 outbreak of gastrointestinal disease described as ‘malaise, upper abdominal pain, anorexia, nausea and vomiting, general apathy, and loss of energy’ on Nissan Island.⁸ A third of the battalion had eosinophilia, as did at least 5% of the entire force. Mass treatment with tetrachloroethylene was instituted, and many were repatriated to New Zealand. A similar event occurred in 58/59th Battalion of the 2nd Australian Imperial Force (AIF) on Bougainville in March 1945 when 117/418 soldiers were found to be excreting hookworm eggs.⁹ Mass treatment again usually resolved the general malaise and gastrointestinal symptoms associated with hookworm. Although mild disease symptoms were the rule, some individuals with heavy exposure to gardens fertilised by faeces were severely affected, including pulmonary signs indicative of mass larval migration.¹⁰ Investigations of Australian soldiers in Australia at least six months following tropical deployments showed that those with proven hookworm were not significantly more anaemic than soldiers without hookworm eggs.¹¹

The USA military also experienced hookworm outbreaks in the Solomon Islands (Guadalcanal, New Georgia) and northern Burma, particularly in the infantry, during combat operations in wet, tropical conditions.^12,13 Overall infection rates and worm burdens were low (<1/1000 hospitalised). High-risk exposures occurred in local villages and enemy encampments, lacking latrine disposal of human faecal material during combat operations. Logistics and support troops not involved in combat had a much lower risk of hookworm.^12,14,15 Eosinophilia detected by differential blood leucocyte counts was a common way of detecting hookworm and other geohelminth risk, given the laborious nature of stool examinations for parasite ova.¹⁵ Mass treatment of field infantry units based on eosinophilia was done to avoid introducing exotic parasites (Ancylostoma duodenale) into the USA.¹⁴ Vague gastrointestinal complaints and cross-over into neuropsychiatric diagnoses made eosinophilia a useful objective finding indicative of likely hookworm infection.¹⁶

There have been relatively few reports of hookworm as a military problem since the Second World War. During the Vietnam War, 46 USA soldiers from a single unit of the 1st Cavalry Division were admitted to the hospital with severe gastrointestinal complaints found to be due to hookworm infection several weeks following initial exposure.¹ Other localised hookworm outbreaks were documented in combat units occasionally severe enough to mimic gastric ulcer disease. Five to seven weeks after the 1983 invasion of Grenada >20% of 684 USA Army soldiers of the 82nd Airborne Division reported gastrointestinal symptoms, with many also demonstrating eosinophilia of >10%.¹⁷ Thirty-two cases of confirmed hookworm infection were found on stool examination. In 1990, the UK’s Parachute Battalion encountered geohelminths during a very brief hostage rescue mission in the West African country of Sierra Leone. Half the company had both gastrointestinal symptoms and eosinophilia 6 weeks later and 26% were found to have hookworm infections. Disease risk was concentrated in one sub-unit that conducted a ground assault through the enemy camp area, including the latrines.¹⁸

Despite numerous recent tropical deployments for humanitarian and disaster relief missions, the ADF has had little recent experience with documented geohelminth infections, particularly hookworm. The decade-long deployment of the Regional Assistance Mission to the Solomon Islands (RAMSI) was a possible exception due to its size and duration. One hookworm (Ancylostoma ceylanicum) and 14 Strongyloidiasis infections were documented in RAMSI.^4,19 This low-level disease risk led to improved post-patrol cleaning of field equipment to minimise exposure to soil and the routine administration of albendazole and ivermectin post-deployment to eliminate residual infections. Given the variability of exposure to faecally contaminated soils, the likely small number of worms in most infections and the unknown compliance with any post-deployment medication, it is difficult to judge the extent of geohelminth risk, particularly hookworm to the modern ADF in the Indo-Pacific.

Is routine albendazole and ivermectin still appropriate post-exposure prophylaxis for ADF members returning from tropical deployments in the Indo-Pacific Region? Post-deployment medications aim to prevent illness in the individual, stop any cross-border introductions of exotic pathogens and minimise any relapses or chronic illness. The medications are extremely safe, having been given to a large proportion of the global population, but are they actually killing any worms when given to ADF members post-deployment in the Southwest Pacific?²⁰ The very low infection rate of ADF members and the decreasing disease burden make the utility of routine anthelminthic drugs in the ADF very marginal. The need for post-deployment medication likely depends on the exposure risk; estimates from the military-specific literature suggest that only soldiers deployed under arduous or combat conditions in poorly sanitised, tropical countries are likely at risk of hookworm infections. Improved sanitation across Southeast Asia and mass administration of anti-helminthic drugs throughout the Indo-Pacific have likely lowered the environmental risk of hookworm exposure.² Would risk-based medication use be practical knowing that most health support plans are copied and pasted from previous exercises and staff officers resist changing what they do not understand? Such an approach would suggest that an infantry company deployed to Wewak for jungle exercises (e.g. Olgetta Warrior) could continue to receive albendazole and ivermectin. In contrast, such medication could be omitted for those staying in hotels or barracks in Port Moresby and Lae. It is logical to either use both drugs for a synergistic effect in highly exposed groups or neither medication in the vast majority who have little exposure to contaminated soils during their deployment outside of Australia. Since some gastrointestinal complaints are nearly universal following such deployments, it is difficult to see how symptom screening has anything to contribute to such a decision. Eosinophilia has been a sensitive but non-specific indicator of helminth infection and could be used in recently returned ADF members requiring evaluation for medical complaints or generic symptoms. The COVID‑19 pandemic graphically demonstrated that getting an entire population to participate in public health measures is often problematic, especially when healthcare providers find it challenging to communicate the risk of the medication versus the anticipated benefit. Limitations on the admittedly very safe anti-helminthic drugs used post-deployment in the ADF need to be seen in this context and not as an economic measure. Rational use of any medication needs to focus on the populations most likely to benefit.

Author affiliations: Australian Defence Force Infectious Disease and Malaria Institute, Gallipoli Barracks, Enoggera, Queensland, Australia; University of Queensland, School of Public Health, Brisbane, Herston, Queensland, Australia

Funding: No specific funding was given for this work.

The author acknowledges Australian and Allied soldiers of previous generations who struggled against the enemy of hookworm and thanks the many un-named military officers, scientists, historians and medical librarians who have unselfishly provided data and ideas for this manuscript, especially the librarians at the Australian Defence Force Library at Gallipoli Barracks, Queensland.

Conflicts of interest: The author does not claim any conflicts of interest.